5Bulletin of the American Meteorological Society

1. Introduction

The FIRE [First ISCCP (International SatelliteCloud Climatology Project) Regional Experiment1] Arc-tic Clouds Experiment was conducted during April–July 1998 to study arctic cloud systems under springand summer conditions. The main goal of the experi-ment was to examine the effects of clouds on radiationexchange between the surface, atmosphere, and space,and to study how the surface influences the evolutionof boundary layer clouds. Observations collected dur-ing the field phase of the project will be used to evalu-ate and improve climate model parameterizations ofarctic cloud and radiation processes, satellite remote

FIRE Arctic Clouds Experiment

J. A. Curry,a P. V. Hobbs,b M. D. King,c D. A. Randall,d P. Minnise

G. A. Isaac,f J. O. Pinto,a T. Uttal,g A. Bucholtz,h D. G. Cripe,d H. Gerber,i C. W. Fairall,g

T. J. Garrett,b J. Hudson,j J. M. Intrieri,g C. Jakob,k T. Jensen,l P. Lawson,l

D. Marcotte,o L. Nguyen,e P. Pilewskie,m A. Rangno,b D. C. Rogers,d

K. B. Strawbridge,o F. P. J. Valero,h A. G. Williams,n D. Wyliep

aUniversity of Colorado, Boulder, Colorado.bUniversity of Washington, Seattle, Washington.cNASA Goddard Space Flight Center, Greenbelt, Maryland.dColorado State University, Fort Collins, Colorado.eNASA Langley Research Center, Hampton, Virginia.fAtmospheric Environment Service, Downsview, Ontario, Canada.gNOAA/Environmental Technology Laboratory, Boulder, Colorado.hScripps Institution for Oceanography, La Jolla, California.iGerber Scientific, Inc., Reston, Virginia.jDesert Research Institute, Reno, Nevada.kEuropean Centre for Medium-Range Weather Forecasts, Read-ing, United Kingdom.lSPEC, Inc., Boulder, Colorado.mNASA Ames Research Center, Moffett Field, California.nInstitute for Aerospace Research, National Research Council,Ottawa, Ontario, Canada.oAtmospheric Environment Service, Egbert, Ontario, Canada.pUniversity of Wisconsin—Madison, Madison, Wisconsin.Corresponding author address: Dr. J. A. Curry, Program in At-mospheric and Oceanic Sciences, University of Colorado, Box311, Boulder, CO 80309-0311.In final form 18 June 1999.©2000 American Meteorological Society

ABSTRACT

An overview is given of the First ISCCP Regional Experiment Arctic Clouds Experiment that was conducted dur-ing April–July 1998. The principal goal of the field experiment was to gather the data needed to examine the impact ofarctic clouds on the radiation exchange between the surface, atmosphere, and space, and to study how the surface influ-ences the evolution of boundary layer clouds. The observations will be used to evaluate and improve climate modelparameterizations of cloud and radiation processes, satellite remote sensing of cloud and surface characteristics, andunderstanding of cloud–radiation feedbacks in the Arctic. The experiment utilized four research aircraft that flew oversurface-based observational sites in the Arctic Ocean and at Barrow, Alaska. This paper describes the programmaticand scientific objectives of the project, the experimental design (including research platforms and instrumentation), theconditions that were encountered during the field experiment, and some highlights of preliminary observations, model-ing, and satellite remote sensing studies.

1FIRE is a U.S. national project that is funded primarily by theNational Aeronautics and Space Administration (NASA); it in-cludes participation of scientists from Canada, England, andNetherlands.

6 Vol. 81, No. 1, January 2000

sensing of cloud and surface characteristics, and under-standing of cloud–radiation feedbacks in the Arctic.

The strategy of the FIRE Arctic Clouds Experimentwas to use research aircraft to obtain remote and in situmeasurements of the properties of clouds and the seaice/ocean surface. The NASA ER-2 flew at an altitudeof 20 km with a suite of remote sensors that can beused to infer the characteristics of the surface andclouds below. Other aircraft, instrumented with in situand remote sensing instruments, were used to measureradiation fluxes and the physical, optical, and chemi-cal properties of the clouds. The aircraft observationswere made over surface-based observational sites inthe Arctic Ocean and at Barrow, Alaska.

FIRE’s arctic field program interacted closely withthe Surface Heat Budget of the Arctic Ocean (SHEBA)project2 (Perovich et al. 1999) and the AtmosphericRadiation Measurement (ARM)3 program (Stokes andSchwartz 1994). SHEBA, ARM, and FIRE share sci-entific objectives that focus on improving simulationsof arctic processes in global climate models and im-proving satellite retrievals of atmospheric state and seaice conditions in the Arctic. SHEBA emphasizes thesurface energy balance and the sea ice mass balance,while ARM is devoted to surface-based observationsand modeling of clouds and radiation. The field com-ponent of SHEBA focused on an icebreaker ship de-ployed in the Arctic Ocean and left to drift for a year,which served as a floating science station. ARM pro-vided a number of key surface-based radiometers andremote sensing instruments designed to provide mea-surements of clouds and radiation at the SHEBA icestation. ARM also operates a duplicate set of instru-ments at Barrow as part of a decade-long program tomonitor clouds and radiation on the north slope ofAlaska.

The FIRE program, which has been under waysince 1983, is aimed at improving the simulation ofclouds and radiation in large-scale models and enhanc-ing satellite cloud retrieval techniques (Randall et al.1995). FIRE Phase I (1984–89) was designed to ad-dress fundamental questions concerning the character-istics of cirrus and marine stratocumulus cloudsystems. FIRE Phase II (1989–94) focused on more

detailed questions concerning the formation, mainte-nance, and dissipation of cirrus and marine stratocu-mulus cloud systems.

FIRE Phase III commences an investigation of arc-tic cloud systems. The FIRE Arctic Clouds Experi-ment, in collaboration with SHEBA and ARM,represents an important broadening of the scientificscope of the FIRE program. This strategic step wasmotivated by the importance of the Arctic for the glo-bal climate system, and an appreciation of how poorlywe understand arctic clouds and the energy budget ofthe arctic surface. The purpose of this paper is to sum-marize the FIRE Arctic Clouds Experiment field op-erations and present some preliminary results. Section2 presents a more thorough description of the projectscience objectives. Section 3 describes the researchplatforms and instrumentation. Section 4 provides anoverview of the observations. Section 5 provides somepreliminary highlights, including some comparisonsof model results with the data.

2. Project goals and objectives

The overarching goals of the FIRE Arctic CloudsExperiment are to improve the satellite retrieval ofcloud and surface characteristics in the Arctic, and toimprove the representation of arctic clouds and radia-tion in general circulation models. The objective of theFIRE III field experiment is to produce an integrateddataset that

1) supports the analysis and interpretation of physi-cal processes that couple clouds, radiation, chem-istry, and the atmospheric boundary layer;

2) provides in situ data for testing of satellite andground-based remote sensing analyses; and

3) provides initial data, boundary conditions, forcingfunctions, and test data to support Arctic FIREmodeling efforts.

Motivated by the plans of SHEBA, FIRE, andARM to mount field experiments in the Arctic, Curryet al. (1996) prepared a review of research prior to 1995related to arctic clouds and radiation. While this re-view provides a background for the science objectivesof the FIRE Arctic Clouds Experiment, objectives rel-evant to large-scale numerical weather prediction andclimate modeling in the Arctic are reviewed byRandall et al. (1998). Some background on the scienceissues that motivated the FIRE Arctic Clouds Experi-

2SHEBA is a U.S. national program that is funded primarily bythe National Science Foundation (NSF) and the Office of NavalResearch (ONR); it includes the international participation ofCanada, Japan, and Russia.

3ARM is sponsored by the U.S. Department of Energy (DOE).

7Bulletin of the American Meteorological Society

ment is given below, as well as specific science ques-tions that focused the scientific investigations.

a. CloudsSubstantial uncertainties exist in our present under-

standing of arctic clouds. These uncertainties arisefrom difficulties in observing these clouds, both fromthe ground and from satellite, and from the unusualcloud types that form in the polar regions. These un-usual cloud types include ice crystal clouds that reachthe surface, commonly referred to as “diamond dust;”convective plumes associated with leads or polynyas(openings in the sea ice); persistent mixed-phaseclouds; and multiple layers of thin cloud decks thatoccur in the statically stable arctic environment. Somekey scientific issues relating to arctic clouds follow:

• What is the influence of leads and other open wa-ter on cloud properties when large surface–air tem-perature differences exist?

• How does the extreme static stability and low at-mospheric water vapor content of the lower tropo-sphere, especially during winter, affect the flow ofenergy across the air–sea interface?

• What is the mechanism that leads to the spectacu-lar multiple-layering of summertime cloud systemsover the Arctic Ocean?

• How does the transition of low clouds from liquidto crystalline depend on temperature and aerosolcharacteristics, and how does the springtime tran-sition differ from the autumnal transition?

b. RadiationClouds are the dominant atmospheric modulators

of the arctic radiation climate. Cloud radiative prop-erties depend on the amount of condensed water, thesize and shape of the cloud particles, and the phase ofthe particles (liquid or ice). Recent studies of arcticclouds suggest that over the course of the year cloudshave a net warming effect on the surface, while thetop-of-the-atmosphere cloud radiative forcing isdominated by the shortwave flux. The radiation en-vironment of the Arctic is complicated considerablyby the highly reflective and inhomogeneous snow/icesurface, the complex vertical structure associated withtemperature and humidity inversions, low tempera-tures and humidity, and high solar zenith angles. Keyscientific issues related to arctic radiation follow:

• What is the spectral distribution of longwave ra-diation? In particular, what is the role of the 20-µm

rotation-band “window” region in regulating thesurface and atmospheric temperature in the Arctic?

• What are the effects of springtime “arctic haze” onthe absorption of solar radiation in polar clouds?

• What is the role of diamond dust in determiningthe radiation fluxes?

• What are the shortwave radiative effects of thehorizontally inhomogeneous stratocumulus cloudsover the inhomogeneous, highly reflective snow/ice surface?

• How do the optical properties of the arctic surfacevary in response to changes in snow and ice char-acteristics (including meltponds)?

c. AerosolsIt has been hypothesized that the microphysical and

optical properties of arctic clouds are particularly sus-ceptible to influence by atmospheric aerosol. The con-centration and size distributions of cloud dropletsdepend in part on the aerosol in the atmosphere, spe-cifically on the cloud condensation nuclei (CCN). Theinteractions between clouds and aerosol are not sim-ply one way; scavenging by clouds depletes CCN, butaerosol may also be produced and enhanced within theclouds through chemical and physical processes. Iceparticle concentrations in clouds may also be affectedby aerosol, although this connection is complex andnot well understood. The concentration and composi-tion of the ice-forming nuclei (IFN) are hypothesizedto be important for determining the phase of arcticclouds, and therefore their impact on the radiationbalance. Key questions regarding the aerosols foundin the arctic atmosphere follow:

• What is the activity spectra of CCN and how doesit vary?

• What are the sizes and composition of CCN, andhow and why do they vary?

• How do the CCN distributions interact with thecloud droplet distributions?

• How do processes within arctic clouds modifyCCN?

• What is the nature and source of IFN?• Is it possible for ice particles to form at relative

humidities below water saturation, by depositionnucleation?

d. Remote sensingSatellite retrievals of cloud and surface character-

istics are hampered by the complex vertical structureof the atmosphere, including temperature and humid-

8 Vol. 81, No. 1, January 2000

ity inversions; low temperatures and low water vaporamounts; little visible, thermal, and microwave con-trast between the clouds and the underlying surface;heterogeneity of the underlying surface; and the pres-ence of complex cloud types (e.g., mixed-phase clouds,thin multilayered clouds). Specific questions relatedto satellite remote sensing of arctic clouds include thefollowing:

• How do the vertical variations of cloud and atmo-spheric properties affect the interpretation of sat-ellite measurements of clouds?

• How well can surface and atmospheric radiativefluxes be reconstructed from satellite-based obser-vations?

• What are the appropriate averaging periods forsurface-based cloud observations so they can bemeaningfully compared to satellite data? Do theseaveraging periods vary significantly as a functionof cloud height and/or cloud type?

• How accurately can satellites detect the presenceof clouds over ice and snow surfaces?

• What is the radiative significance of clouds that arenot accurately detected by satellite?

e. ModelingTo meet the objectives of the FIRE Arctic Clouds

Experiment, the following modeling activities are be-ing undertaken:

• A variety of 1D and 3D radiative transfer models arebeing tested using near-instantaneous measurementsof the radiation field and the atmospheric param-eters that determine the radiation field (e.g., clouds,temperature, aerosol, trace gas concentrations).

• Large eddy simulation (LES) models are beingused to develop parameterizations of cloud andboundary layer processes, which eventually can beincorporated into climate models.

• Improved parameterizations of physical processesfor climate models are being tested against field ob-servations using single-column models (SCMs).An SCM is a single vertical array of cells from a3D climate model, the forcing of which may behighly constrained to test individual parameteriza-tions (Randall et al. 1996).

• To help in providing forecast guidance for the Ca-nadian Convair-580 during the field project, theCanadian Mesoscale Compressible CommunityModel (MC2) was run at 35- and 10-km resolution(Benoit et al. 1997).

3. Experimental design

The FIRE Arctic Clouds Experiment was designedto produce an integrated dataset that

• supports the analysis and interpretation of physi-cal processes coupling clouds, radiation, chemis-try, and the atmospheric boundary layer;

• provides in situ data for the testing of satellite andground-based remote sensing analyses; and

• provides initial data, boundary conditions, forcingfunctions, and test data to support arctic modelingefforts.

The overall design of the experiment is to combinemeasurements at the surface, from research aircraft,and from space to address problems of arctic clouds,radiation, and aerosols, including their modeling andremote sensing.

The operational objectives of the FIRE ArcticClouds Experiment were to make aircraft observationsover the SHEBA ship and Barrow during severalmonths of spring and summer. The location and tim-ing of the FIRE Arctic Clouds Experiment were de-termined by the scheduled operations of the SHEBAexperimental site in the Beaufort Sea during October1997–11 October 1998. The Canadian Coast Guardicebreaker Des Groseilliers was deployed in a multi-year ice floe on 2 October 1997, at 75°16.3′N,142°41.2′W. The thickness of the undeformed multi-year ice at deployment was 1.7–2.0 m. Instrumenta-tion was fully deployed and operational at the site by30 October. The ARM site at Barrow was fully opera-tional by 19 March 1998, providing a secondary sur-face site for the experiment. Over the course of the fieldstudy, the SHEBA ice camp drifted considerablynorthwestward (Fig. 1); it was at 78.5°N, 166°W bythe end of July 1998.

A key aspect of the experimental design was todesign flight plans that economically used the aircraftresources to provide observations over the entire pe-riod of spring and summer transitions in arctic cloudand surface characteristics so as to address a multitudeof physical process studies and remote sensing andmodeling efforts. Four research aircraft were em-ployed during FIRE. Flights were conducted in theimmediate vicinity (∼ 50 km) of, and directly over, theSHEBA surface site and the ARM site at Barrow. Thethree medium-altitude aircraft made measurementsspanning the period 8 April–30 July 1998. In addition,during the period 18 May–6 June, the NASA ER-2

9Bulletin of the American Meteorological Society

flew at an altitude of 20 km above either the Univer-sity of Washington (UWA) Convair-580 or the NCARC-130 aircraft. The timing of the aircraft missions,spanning the period from spring to midsummer, wasdesigned to capture the transition between the winter-time boundary layer with predominantly ice clouds tothe summertime boundary layer with predominantlyliquid clouds, and to capture the onset of the sea icemelt season. This period was also selected as one forwhich both climate models and satellite remote sens-ing techniques are in particular need of improvement.

The flight plans of the three medium-altitude air-craft were designed to meet requirements for measur-ing both the horizontal variability and vertical structureof the atmosphere. Horizontal traverses of 20–200 kmwere made at various levels above, below, and withincloud; in the boundary layer; and at various altitudesto map the surface using aircraft remote-sensing in-struments. Additionally, slow ascents and descentswere made to obtain high-resolution vertical profilesusing in situ instruments.

The timing of the flights was coordinated with sat-ellite overpasses. The specific satellites of interest in-cluded the following:

• NOAA-12 and -14 Polar-orbiting Operational En-vironmental Satellites. Aboard these satellites arethe Advanced Very High Resolution Radiometer,which is a 4- or 5-channel radiometer measuringvisible and infrared wavelengths, and the Televi-sion Infrared Observation Satellite (TIROS) Op-erational Vertical Sounder (TOVS),which measures incoming radiationin the infrared and passive microwavewith 27 channels.

• Defense Meteorological SatelliteProgram F12 and F13. Aboard thesesatellites are the Special Sensor Mi-crowave/Imager (SSM/I), which is a7-channel, 4-frequency, linearly po-larized passive microwave radiomet-ric system that measures brightnesstemperatures at 19.35, 22.235, 27,and 85.5 GHz; and the SSM/T2,which measures the brightness tem-perature near the 183.3 GHz watervapor line and also at 90 and 150GHz.

• RADARSAT-1. Aboard this satellite isa Synthetic Aperture Radar (SAR),which sends pulsed microwave sig-

nals to Earth and processes the received reflectedpulses.

The general strategy for FIRE’s satellite remote-sensing research is to use the aircraft primarily toevaluate remote-sensing instruments located at the sur-face and to extend the point observations made at thesurface to the larger horizontal scale seen by satellites.The entire time series of surface-based remote-sensingobservations can then be used to evaluate and interpretsatellite retrievals of cloud and surface characteristics.Coincident surface-based, aircraft and satellite mea-surements are essential for some validation exercises.

To support the FIRE modeling activities, the FIREdataset must be readily accessible for modelers to use.To achieve this end, subsets of the data are being pack-aged so that they can serve as input files to models aswell as for model validation. Specific integrateddatasets that are being prepared (in collaboration withSHEBA and ARM) include the Integrated RadiativeFlux dataset, the Large-Eddy Simulation dataset, andthe Single-Column Model dataset.

Details of the surface-based and aircraft observa-tions are described in detail in the subsequent two sub-sections.

a. Surface observationsAn extensive array of instrumentation was deployed

at the SHEBA ice camp. Further information on theship, its track, and instrumentation can be found on theSHEBA Web site (http://sheba.apl.washington.edu).

FIG. 1. Map showing drift of the SHEBA ice station.

10 Vol. 81, No. 1, January 2000

Information regarding the instrumentation provided bythe ARM program can be found on the ARM Web site(http://www.arm.gov/docs/instruments.html). The

surface-based instrumentation of particular relevanceto the FIRE Arctic Clouds Experiment is summarizedin Table 1, providing measurements of cloud proper-

GPS rawinsondes pressure, temperature, humidity, winds 2–4 times per day

Tethered balloon profiles pressure, temperature, humidity, winds up to 1 km

Vaisala thermistor probe mean air temperature on tower, 0–15 m

Vaisala hygrometer probe mean air humidity on tower, 0–15 m

R.M. Young propellor, vane mean wind velocity on tower, 0–15 m

Setra barometer air pressure on tower, 0–15 m

Sonic anemometer/thermometer stress, sensible heat flux on tower, 0–15 m

Ophir rapid response hygrometer sensible heat flux on tower, 1 m

TABLE 1. Summary of surface-based measurements at SHEBA.

Radiation

Instrument Wavelengths Comments

Precision/infrared radiometer broadband (IR) upwelling and downwelling

Precision spectral pyranometer broadband (SW) upwelling and downwelling

Infrared thermometer 9.6–11.5 µm up- and down-looking

Normal incidence pyrheliometer broadband (SW) direct beam solar irradiance

Multifilter rotating shadowband radiometer 0.415, 0.5, 0.665, 0.862, 0.94 µm downwelling, direct/diffuse

Multifilter radiometer 0.415, 0.5, 0.665, 0.862, 0.94 µm upwelling

Net radiometer broadband (SW, IR)

4-π radiometer broadband (SW) deployed on balloon

Extended Range Atmospheric Emitted 4–20 µm, 1 cm−1 resolution upwellingRadiance Interferometer (AERI)

Solar Spectral Flux Radiometer (SSFR) 0.30–2.2 mm, 5–15-nm resolution zenith, nadir directions

Clouds

Instrument Wavelengths Comments

Cloud radar 35 GHz zenith, up to 13 km

Micropulse lidar (DABUL) 0.5235 mm, polarized up to 20 km

Ceilometer 0.925 mm up to 7.5 km

Whole sky imager cloud fraction

Microwave radiometer 23.8, 31.8 GHz uplooking

Cloud particle replicator — deployed on balloon

Meteorology and surface fluxes

Instrument Wavelengths Comments

11Bulletin of the American Meteorological Society

ties and radiation fluxes, meteorological conditions,and the near-surface atmospheric boundary layer.

Incident shortwave and longwave radiation fluxesat the surface were measured by the ARM program,as well as by SHEBA and FIRE investigators.Measurements of the direct solar beam and diffusesolar radiation were also made. Upwelling radiationfluxes were measured, as well as infrared measure-ments of the skin surface temperature. High-resolutionspectral measurements were made in both the solar andinfrared portions of the spectrum, which can be usedto retrieve atmospheric temperature and humidity pro-files, trace gases, aerosol characteristics, and cloudproperties.

Cloud properties were measured at the SHEBA siteusing ground-based remote sensors and a tethered bal-loon. Remote measurements were made almost con-tinuously for the duration of the SHEBA deployment.The cloud radar, lidar, and ceilometer are used to de-termine cloud layer boundaries and cloud fraction.Cloud microphysical properties such as liquid and icewater content and particle phase and size are deter-mined from retrievals using the cloud radar, lidar, mi-crowave radiometer, and other radiometers such as theAERI and SSFR. To obtain in situ measurements ofcloud properties, tethered balloons with instrumentsfor cloud microphysical and radiation measurementswere flown between March and June. Instrumentsflown on the balloons measured profiles in the lowest1 km of the atmosphere of cloud particle concentra-tions and size distributions, and mean radiative inten-sities (actinic flux).

Meteorological data were obtained from rawin-sondes, tethered balloon profiles up to 1 km, and mea-surements from several towers (10–20 m in height).Turbulent fluxes of sensible and latent heat and mo-

mentum were obtained at multiple levels from the20-m tower.

SHEBA investigators conducted detailed measure-ments of the optical and physical characteristics ofsnow and sea ice, including snow depth, density, andgrain size, as well as ice temperature, salinity, density,brine volume, and air volume. Studies were also con-ducted over lines ranging up to 20 km in length to as-sess the spatial variability of snow characteristics,surface albedo, and ice transmittance.

Observations of atmospheric state, cloud charac-teristics, and radiation fluxes using nearly identical in-struments to those at SHEBA were obtained at theARM site in Barrow, Alaska, beginning 19 March1998.

b. Aircraft observationsFour research aircraft were deployed during the

FIRE Arctic Clouds Experiment: the NASA ER-2, theNational Center for Atmospheric Research (NCAR)C-130Q, the UW Convair-580, and the Canadian Na-tional Research Council Convair-580 (Table 2). Sum-maries of the remote sensing and cloud microphysicalinstrumentation deployed on these aircraft are givenin Tables 3 and 4.

The NASA ER-2 is a single-engine, single-seat,high-altitude subsonic aircraft. Active and passiveremote-sensing instruments were deployed on the ER-2 during this period, to determine properties of clouds,aerosols, trace gases, surface temperature, cloud andsurface radiation characteristics, sea ice concentrationand age, precipitation, and profiles of temperature,water vapor, and ozone. A summary of the remotesensing instrument characteristics and derived dataproducts is given in Table 3. Details of the instrumentson the ER-2 can be found on the FIRE Arctic Clouds

NASA ER-2 60 000 Fairbanks 18 May–6 Jun 8 11

NCAR C-130Q 100–25 000 Fairbanks 4 May–27 May 8 08 Jul–30 Jul 8 0

UW Convair-580 50–32 000 Barrow 19 May–24 Jun 8 11

Canadian Convair-580 50–25 000 Inuvik 8 Apr–29 Apr 4 2

TABLE 2. Overview of aircraft platforms.

Flight Period No. of flights No. of flightsAircraft Altitude (ft) Base (1998) over SHEBA over Barrow

12 Vol. 81, No. 1, January 2000

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13Bulletin of the American Meteorological Society

Experiment Web site (http://eosweb.larc.nasa.gov/ACEDOCS/data/appen.d.2.html).

The three medium-altitude aircraft shared somecommon instrumentation. In particular, many of theinstruments to measure in situ cloud microphysicaland aerosol characteristics were common to all threeaircraft. Parameters measured by these aircraft includeaerosol concentration, composition and size distribu-tion, cloud particle concentration and size distribution,liquid water content, droplet effective radius, particleshape, cloud condensation nuclei spectra, and icenucleus concentration and composition. Specificcloud physics and aerosol instruments are listed inTable 4.

The NCAR C-130Q is a four-engine, medium-altitude research aircraft that is designed to carry apayload of up to 13 000 lbs with full fuel load. TheNCAR C-130Q measures atmospheric state param-eters, turbulent fluxes, cloud physics, and radiativefluxes, and also conducts remote sensing using scan-ning radiometers and video photography. In additionto the NCAR instruments, the C-130Q can support alarge user-supplied payload. Some of the major re-search instrumentation deployed on the C-130Q forthis experiment is described in Tables 3 and 4.

The UW Convair-580 is a two-engine, medium-altitude research aircraft, which flew research missionsfor the first time in this experiment but deployed in-strumentation that has been used frequently by the UWon other aircraft. The UW Convair-580 measures at-mospheric state parameters, cloud physics, radiativefluxes, and aerosol chemistry, and includes remote-sensing instrumentation (see Tables 3 and 4).

The Canadian Convair-580 aircraft is nearly iden-tical to the UW Convair-580. The Canadian aircraftmeasured atmospheric state parameters, turbulentfluxes, cloud physics, radiative fluxes, and aerosol andair chemistry. Trace gas measurements included O

3

SO2, and organohalogens, as well as fast response hu-

midity and CO2 measurements. Aerosol chemistry

included total organic carbon, speciated organics, in-organic and organic ions, black carbon, trace metals,and total organic carbon.

4. Overview of experiment

The operational objectives of the FIRE ArcticClouds Experiment were to make aircraft observationsover the SHEBA ship during several months of springand summer, using Fairbanks, Alaska; Barrow,U

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14 Vol. 81, No. 1, January 2000

Passive cavity aerosol spectrometer aerosol size distribution 0.1–3.0 µm x x xprobe PCASP-100 (diameter)

Condensation nucleus particle counter total aerosol concentration x x x

Forward scattering spectrometer probe size and concen of aerosol and 0.3–20 µm x x xFSSP-300 drops (diameter)

FSSP-100 cloud drop size distribution and 2–47 µm x x xconcentration (diameter)

Scanning humidigraph effect of RH on aerosol light 30% < RH < 85% xscattering

Laser aerosol spectrometer-200 aerosol size distribution 0.5–11 µm x

Differential mobility particle aerosol size distribution 0.01–0.6 µm xspectrometer

1D optical array probe (OAP) 260X drop and crystal size distribution 40–600 µm x x xcloud probe (length)

2D OAP cloud probe drop and crystal shape and size 25–800 µm x x xdistribution (length)

2D OAP precipitation probe rain and snow particle shape and 200–600 mm x xsize distribution (length)

2D OAP Grey probe particle shape and size distribution 25–1600 µm x

SPEC cloud particle imager (CPI) digital images of cloud particles 5–2300 µm x x x

Cloudscope mass and size distribution of ice x xparticles

DRI replicator images of cloud particles x

King Hot-wire Probe liquid water content 0.05–3 g m−3 x x

Johnson–Williams Probe liquid water content 0–2 g m−3 x x

Gerber PVM-100A liquid water content drop 0.05–3 g m−3 x xeffective radius

Icing detector supercooled liquid water content 0.001–1 g m−3 x x

Nevzerov probes liquid water content x

Counterflow virtual impactor (CVI) condensed water content impacts cloud part xin-cloud aerosol char d = 5−30 µm

Cloud condensation nucleus CCN spectra supersaturation x xspectrometer range 0.01%–2%

Continuous flow diffusion chamber concentration and composition of x(CFD) IFN

Integrating nephelometer aerosol backscattering coeff x x

Particle soot-absorption photometer light absorption, graphitic xcarbon

Cloud integrating nephelometer scattering/extinction coeff 635 nm xasymmetry parameter

TABLE 4. Cloud microphysical and aerosol instruments on the medium-altitude aircraft: NCAR C-130 (C-130), UW Convair 580,and Canadian Convair 580 (CAN).

Instrument Parameter Range C-130 UW CAN

15Bulletin of the American Meteorological Society

Alaska; and Inuvik, Northwest Territories, as basesof operation. The SHEBA ship, initially deployed at75.27°N, 142.69°W, was expected to drift to thewest-northwest about 320 n mi by the end of the ex-periment (based upon buoy drift over the past 20years). By the commencement of the CanadianConvair flights in April, the ship had moved to76.12°N, 164.64°W, which was beyond the range ofthe Canadian Convair based in Inuvik. Hence, it wasnecessary for the Canadian Convair to refuel and over-night in Barrow when making flights to the SHEBAship. During July, the ship was located in the vicinityof 78.11°N, 167.16°W, which required a one-wayferry time of 3 h for the C-130Q from Fairbanks, sig-nificantly diminishing flight time on station. The UWConvair, based in Barrow, was in the best position toreach the SHEBA ship; nevertheless the SHEBA shipwas 410 n mi from Barrow by the end of its flights inJune.

Communications from the ship to the mainlandwere handled by INMARSAT, which advertises com-munication only as far as 75°N. As theship continued to drift northward, con-cerns were raised that the ship would losesatellite communication, but this did nothappen. All four aircraft were able tocommunicate with the ship via high-frequency radio when they were withinabout 100 mi of the ship. This commu-nication was extremely valuable in con-ducting the flights, since updates fromthe surface-based observations and sat-ellite observations received on the shipvia TERASCAN allowed modificationof flight patterns to optimally sample thesituation.

An additional consequence of thelarge westward shift of the SHEBA shipwas that it left the Beaufort Sea (theoriginal targeted region for the measure-ments) by the end of winter and movedinto the Chukchi Sea (see Fig. 1). TheBeaufort Sea is characterized in summerand spring by a surface anticyclone anda predominance of stratus clouds. In theChukchi Sea, the weather situation wasmore dynamic with southerly flow pre-dominating during June and July, bring-ing in high clouds that were oftenassociated with frontal systems. Becausethe SHEBA ship was often on the edge

of the anticyclone or affected by small storms, fore-casting the weather for flight operations was difficult,particularly during July, with conditions often chang-ing rapidly. The difficulty in forecasting the weather,combined with the long ferry flight to the SHEBAship, made it difficult to plan flights prior to actuallyreceiving radio communication from the ship aboutan hour before arriving at the site.

Table 5 gives an overview of the conditionssampled by the three medium-altitude research aircraftduring the FIRE Arctic Clouds Experiment. A vari-ety of conditions were sampled, including clear skies,boundary layer clouds, mid- and upper-level clouds,and clouds in the presence of open leads. Various flightpatterns were flown, aimed at evaluating surface-based, satellite, and ER-2 remote sensing instruments,and providing input for evaluating and modelingcloud–radiative interactions, boundary layer clouds,surface albedo and radiation fluxes, and surface char-acteristics. Given the limited number of flight hoursand temporal coverage of the aircraft flights, we were

Cloud conditions

Clear 2 1 2

Open leads 0 0 8 8

Cloudy boundary layersliquid 4 10 3 17ice 1 1mixed phase 3 7 6 16

Cirrus and altostratus 6 4 9 15

Storm (precipitation) 2 1 3 6

Flight patterns

Mapping of surface features 15 15

Surface albedo and radiation 14 8 22fluxes

Validation of surface remote 13 14 6 33sensing instruments

Cloud radiative properties 14 8 4 28

Cloudy boundary layer 7 8 15

Bidirectional reflectance 15 15

TABLE 5. Distribution of cases (expressed as number of flights) for medium-altitude aircraft: NCAR C-130 (NCAR), UW Convair 580, and Canadian NRCConvair 580.

No. of Cases

NCAR UW CAN Total

16 Vol. 81, No. 1, January 2000

not able to sample with aircraft the entire range ofweather and cloud conditions that occur in this locale.Because of high winds during July, we obtained onlyone case of a statically stable summertime arctic stra-tus, and none with multiple cloud layers in the bound-ary layer. We also did not obtain as many cases wherethere was only ice (no liquid) in the atmosphere aswould have been optimal for both modeling andremote-sensing studies. Nevertheless, the casessampled provide a rich dataset with which to addressthe project objectives.

During the period of aircraft measurements (April–July 1998), cloud and surface conditions over the icechanged from near-winter conditions with boundarylayer ice clouds and a snow-covered surface, to thepeak of the summer melt season with abundantmeltponds and liquid clouds in the boundary layer. Anoverview is given below of the weather, cloud, andsurface characteristics at the SHEBA ship for each ofthese four months.

a. AprilAt the beginning of the Canadian Convair-580

flights on 8 April, the surface air temperature atSHEBA was −13°C; at the end of the flights on 29April it was −18°C. During mid-April there was a4-day period when the surface air temperatureremained unseasonably warm, above −10°C. Bound-ary layer clouds sampled in the vicinity of the shipwere entirely liquid on 17 April, when cloud tempera-tures were −5° to −10°C, and entirely crystalline on21 April when cloud temperatures were −15° to−20°C. The synoptic situation in April was dominatedby a surface high northeast of the ship with a broadweak cyclone in the lower Chukchi Sea and along theAlaskan coast. This combination produced easterlysurface winds most of the time with southerly or south-easterly winds aloft. Deviations occurred when a cy-clone moved north out of the Bering Strait, crossingthe SHEBA ship on 18–21 April, and a surface anti-cyclone invaded from the northwest during 24–26April. Some of the Canadian flights took place closeto the coast between Inuvik and Barrow, and includedsampling of open water regions associated with leadsand polynyas.

b. MayDuring May 1998, the C-130Q, ER-2, and UW

aircraft conducted research flights. May was domi-nated by an anticyclone to the east of the ship thatmoved north later in the month. A persistent cloud-

topped surface mixed layer was present from 30 Aprilthrough 19 May. Surface-based mixed layers arethought to be rare in the Arctic, although they are mostlikely to occur during May when the surface warmsrapidly. At the SHEBA ship, steady warming at thesurface was evident, with temperatures of −20°C at thebeginning of the month and 0° to 2°C at the end of themonth. Early in the month, two leads opened in thevicinity of the SHEBA ship, several hundred meterswide. These leads froze and reopened intermittentlyduring the month, but during the latter half of the monthvirtually no open water could be seen in the vicinity ofthe SHEBA ship. At the end of the month, snowmeltbegan, which was accelerated by several days of rain.

c. JuneJune weather was characterized by an anticyclone

east of the SHEBA ship, which moved into Canada onoccasion. The ship was generally under a southerlyflow caused by weak cyclones and upper-air troughsto the west. During the first week of June, a near bal-ance existed between upwelling and downwellinglongwave radiation at the surface; however, the netshortwave radiation of about 150 W m−2 near solarnoon contributed significantly to snowmelt and thedevelopment of meltponds. Clouds were clearlywarming the surface; during clear-sky periods the netradiation became negative and the surface changedfrom melting to freezing. The surface melting wassomewhat sporadic, interrupted by periods of surfacefreezing induced by clear skies and/or high surfacewinds. By the third week in June, most of the snowhad melted. During June, multilayered clouds weremost commonly encountered, with the bases and topsof a given layer varying appreciably over relativelyshort distances.

d. JulyJuly was a fairly stormy month, with persistent

southerly flow from the North Pacific Ocean and fre-quent mid- and high-level clouds. A strong surfacetemperature inversion characterized the region from 17to 30 July, the strength of the surface inversion reach-ing as high as 12°C. Periodically, a shallow, surface-based mixed layer less than 100 m deep would developunder conditions of high wind speeds or a surfacefog, especially during the first half of the month.Boundary layer clouds were entirely liquid in phase.Surface characteristics evolved over the course of themonth, with increased meltpond coverage and openwater, and corresponding decreases in surface albedo.

17Bulletin of the American Meteorological Society

5. Some highlights of preliminaryresults

In this section we present some examples of datacollected in the FIRE Arctic Clouds Experiment thatillustrate the technology used in the experiment andhow the datasets can be synthesized and integrated tohelp improve modeling and satellite remote sensingand to address key scientific questions.

a. CloudsAmong the numerous cloud situations observed

during the experiment, we describe here two cases thatillustrate the data and some pre-liminary findings on cloud par-ticle phase.

Measurements of cloud char-acteristics are illustrated for 4May using C-130Q and surface-based observations. The timeseries of cloud radar returns (seeFig. 2) shows a persistent bound-ary layer cloud (−10 dBZ) sur-mounted by altostratus clouds(−40 dBZ) that had almost dis-appeared by 2200 UTC whenthe C-130Q arrived on site. Theboundary layer was character-ized by a cloud-topped mixedlayer (Fig. 3), with cloud top at1080 m and base at 660 m. Thehumidity inversion above thecloud-topped mixed layer ap-pears to have contributed to thehomogeneity and persistence ofthe cloud deck by inhibitingevaporative cooling associatedwith entrainment mixing atcloud top. Such humidity inver-sions are rare outside the polarregions; they are hypothesizedto be associated with moistureadvection and precipitation dry-ing of the lower atmosphere (bydiamond dust, snowfall, anddrizzle). Profiles of liquid andice water content (Fig. 3c) showthat the cloud was mixed phase,with slightly more than half ofthe condensed water in the crys-talline phase. This is consistent

with the enhanced lidar backscatter values in Fig. 2bbetween 660 and 1100 m, indicating the presence ofliquid water. The ice water content in Fig. 3b was de-termined from the Cloud Particle Imager (CPI), usingimages such as those shown in Fig. 4. Also determinedfrom the CPI are size spectra of the ice particles(Fig. 4). CPI images taken above the cloud show ro-settes greater than 500 µm, columns with side planegrowth and small ice particles; presumably theseparticles have fallen from the dissipating altostratuscloud. These large particles accreted drops as they fellthrough the mixed-phase cloud and probably accountfor the presence of relatively large (up to 1 mm) rimed

FIG. 2. Time series of (a) cloud radar and (b) lidar depolarization observations from theSHEBA ship for 4 May 1998. In (a), the lidar reflectivity shows a boundary layer cloud ataltitudes below about 1 km with reflectivity of about −10 dBZ, and an altostratus cloud (about−40 dBZ) at higher levels that was gradually diminishing with time. In (b), the lidar is at-tenuated by liquid water at altitudes above about 800 m, with depolarization ratios less than0.2 indicating the presence of liquid water.

18 Vol. 81, No. 1, January 2000

ice particles in this shallow cloud. The regions withlarge rimed particles also account for the peaks in theice water content (Fig. 3c). Below cloud, precipitationin the form of mostly rimed ice particles was observed.Near the SHEBA ship, ice forming nucleus (IFN)

concentrations were quite small, less than 0.5 L−1 at−22°C at a water supersaturation of 3%. Above cloudtop the IFN concentration was ∼ 25 L−1 at −27°C.Condensation nuclei (CN) at low levels were fairly lowand uniform (∼ 200 cm−3), indicating relatively cleanair.

One of the unusual cloud types that occur over seaice during the cold portion of the year is convectiveclouds that emanate from open water in leads or polyn-yas. Figure 5 depicts a low-level transect by theCanadian Convair 580 aircraft across Cape BathurstPolynya in the Beaufort Sea (near 70°N, 135°W) on25 April 1998. The lowest panel shows output froma downward-looking multispectral Landsat spec-trometer in the wavelength interval 0.45–0.52 µm witha 15° field of view. Clearly evident is the sharp tran-sition from solid sea to open water on the southernedge of the polynya (point A), with thin ice and leads(between points B and C) characterizing the region tothe north of the polynya. The upper two panels showambient temperatures and humidities along thetransect and eddy correlation fluxes of the same quan-

tities. Advection of cold air overthe relatively warm open waterresulted in positive sensible heatand moisture fluxes betweenpoints A and B, leading to in-creases in potential temperaturesand specific humidities in thedownwind direction across thepolynya. The sensible and latentheat fluxes resulted in the for-mation of a shallow convectivecloud over the polynya that isadvected downwind (Fig. 6) on27 April.

Large variations in the rela-tionship between cloud tempera-ture and phase of the arcticclouds were seen in this experi-ment, consistent with previousobservations (e.g., Curry et al.1996; Hobbs and Rangno 1998).During May, liquid water wasobserved by the C-130Q inmixed-phase clouds at tempera-tures as low as −23°C, whileduring June the UW Convair-580 observed ice crystals inmixed-phase clouds at tempera-tures as high as −4°C. In the ab-

FIG. 3. Vertical profile of the cloudy boundary layer on 4 May1998 at 2300 UTC from the NCAR C-130Q. (a) Temperature; (b)water vapor mixing ratio; (c) liquid water content (solid line) de-termined by the King probe and ice water content (dashed line)determined from the Cloud Particle Imager (CPI).

FIG. 4. Observations of cloud particles from the CPI aboard the C-130Q on 4 May 1998.Profile segment heights, average temperatures, and average particle concentrations are listedat the left for above cloud top, just below cloud top, in middle of cloud, and in the precipita-tion region. Representative images of ice particles (middle left) and water drops (right) areshown for each profile segment. Size distributions for ice particles (middle right) and waterdrops (right) are averaged over a time period for each profile segment.

19Bulletin of the American Meteorological Society

sence of ice particles falling from above, such as thecase in Fig. 4, the occurrence of ice in the boundarylayer clouds appears to be related to maximum drop-let sizes. For example, ice crystals were present be-tween −4° and −6°C when the cloud droplets werelarge (> 25 µm diameter, also typically some drizzledrops present) and in concentrations of at least a fewper cubic centimeter. Overall cloud droplet concentra-tions were low (< 100 cm−3) in these cases as well.Conversely, colder boundary layer clouds (down toaround −12°C) with smaller cloud droplets generallydid not contain ice. Also, overall cloud droplet con-centrations were higher (> 100 cm−3) in these cases.These observations support the picture presented byHobbs and Rangno (1998).

b. AerosolsDuring the FIRE Arctic Clouds Experiment, the

atmosphere was frequently pristine near the surface.However, haze layers several hundreds to thousandsof meters thick were common aloft, indicating long-range transport. Figure 7 shows an example of a ver-tical profile flown by the UW Convair-580 througha thick haze layer that probably originated in Asia.Tenuous cirrostratus cloud was present above 5 km;at 100-m altitude there was a very thin (16 m) stratuslayer. The profiles shown in Fig. 7 show relatively highvalues of extinction and scattering at altitudes exceed-ing 4000 m, and the haze layer shows considerablestructure. The extinction coefficient has at least eightdistinct maxima over the depth of the profile.Furthermore, within each maximum there are varyingdegrees of absorption, suggesting differing sourcesand/or aging of the aerosol.

Within 100 m of the surface, Fig. 7 shows that ex-tinction and scattering were very low. Accumulation-mode particle concentrations were 10 times greateraloft than at the surface. Precipitation scavenging ofthese particles by boundary layer stratus may havecreated these unusually clean conditions near the sur-face. During May, the NCAR C-130Q generally ob-served very low concentrations of CN, CCN (cloudcondensation nuclei), and IFN in the boundary layer,indicating a very clean background and fairly efficientor long-duration scavenging mechanisms. The CN val-ues of ∼ 10 cm−3 for extended periods are among thecleanest in the world. Occasionally during May, theC-130Q observed small-scale regions near the surfacewith high concentrations of aerosol particles, particu-larly IFN, possibly associated with open water in leads.It has been hypothesized that ocean bacteria may be

active as ice nuclei, but it is not clear how these couldenter into the atmosphere from the small areas of openwater in the Arctic Ocean.

During July, very high concentrations of smallaerosol particles were observed from the C-130Q inthe boundary layer. Volatility tests suggest that theparticles were sulfuric acid, which is consistent withlocal production of dimethyl sulfide in the ArcticOcean during the melt season (Ferek et al. 1995).Local production of aerosols was also observed in thehumidity inversion above boundary layer clouds andalso in dissipating cloud layers. In about 30% of thecloudy boundary layer cases sampled by the UWConvair-580, the total particle concentrations in alayer just above the top of the cloud layer were ∼ 1000cm−3 (in some cases 8000 cm−3) greater than those im-mediately above or below the layer. The cases withenhanced particle concentrations above cloud top wereassociated with a cloud-top temperature and a humid-ity inversion (e.g., see Fig. 3). Nucleation-mode par-ticles may have been responsible for the increases intotal particle concentration, possibly formed by gas-to-particle conversion in the layer of enhanced humid-ity. A similar phenomenon was observed from the

FIG. 5. Data obtained from a transect flown at 300-m altitudeby the Canadian Convair-580 aircraft across Cape BathurstPolynya in the Beaufort Sea (70°12′N, 134°45′W) on 25 April1998. The polynya is shown in (c) between points A and B: (a)air temperature (thin) and its vertical turbulent flux (bold),(b) specific humidity (thin) and its vertical turbulent flux (bold),(c) reflected solar radiation flux in the wavelength interval 0.45–0.52 µm.

20 Vol. 81, No. 1, January 2000

C-130Q. In some cloud layers, even after the cloudshad dissipated, enhanced CN (but not IFN) were ob-served, presumably associated with particle produc-tion by gas-to-particle conversion in the humid air.

On 14 June, between 1950 and 2130 UTC, the UWConvair-580 aircraft flew a series of three horizontaltransects through the lower, middle, and upper por-tions of a uniform stratus cloud about 200 m thick. Thetransects were flown from 43 km downwind of Bar-row (point A) to 129 km downwind of Barrow (pointB). Transects flown below the cloud and in the cloudshowed a clear gradient in interstitial CN concentra-tions, with concentrations highest closest to Barrow.These measurements point to the advection and dis-persion of an anthropogenic plume downwind of Bar-row. The effects of particle emissions from Barrowon the microstructure of the stratus cloud was readilyapparent. Mean droplet concentrations in the vicinityof A and B were 68 and 25 cm−3, respectively. Themean values of the cloud droplet effective radius atA and B were 9.5 and 11.1 µm, the mean drizzle fluxesat A and B were 0 and 1.7 mm day−1, and the mean

liquid water contents at A and B were 0.18 and0.15 g m−3, respectively. Thus, the Barrow plume in-creased droplet concentrations in the stratus, decreasedcloud droplet effective radius, and effectively shut offdrizzle. These observations highlight the sensitivityof the arctic stratus clouds to modification by anthro-pogenic aerosol.

c. RadiationNumerous radiometers on the aircraft and at the

surface provide a detailed picture of the radiation en-vironment in the vicinity of the SHEBA ship. Here wedescribe some elements of the surface and cloud ra-diation characteristics.

The evolution of area-average surface albedo (ob-tained from the ratio of the upwelling to downwellingbroadband solar flux) in the vicinity of the SHEBAship, measured by the UW Convair-580 and theNCAR C-130Q, is shown in Fig. 8. During May, theaverage surface albedo values for the six NCAR C-130Q flights where the surface could be observedranged from 0.85 to 0.67. No change was obvious in

FIG. 6. Observations from the downward-pointing lidar aboard the Canadian Convair-580 on 27 April 1998, across the Cape BathurstPolynya in the Beaufort Sea (near 70°N, 134.5°W). The upwind edge of the polynya is near the left edge of the figure. The scale barrepresents backscatter ratio (no units) from 0 to 10, indicating the amount of scatter relative to “clear” air. The cloud plume is shownby the highest backscatter values. The growth of the internal boundary layer over the polynya is indicated by the solid line.

21Bulletin of the American Meteorological Society

FIG. 7. Vertical profiles of the aerosol light-scattering coefficient, σscat

(dotted line), andthe aerosol light-extinction coefficient, σ

ext (solid line), at a relative humidity of 30% and a

wavelength of 550 nm. The measurements were obtained aboard the UW’s Convair-580aircraft over the Barrow ARM site between 2247 and 2442 UTC on 19 June 1998.

amounts of open water or newice. The highest albedo (0.85)was measured on 18 May aftera fresh snowfall beneath an op-tically thick liquid cloud. Thelowest value of 0.67 was mea-sured under a clear sky on 20May. On 29 May, the first rain-fall of the season caused meltmetamorphism to begin and thefirst surface melt ponds beganforming during the first week ofJune. The largest gradient in sur-face albedo occurred during theperiod 7–18 June, when most ofthe snowmelt occurred. By 18June, most of the surface snowhad disappeared. Area-averagedsurface albedos in the vicinity ofthe ship ranged from 0.42 to0.56 during July, depending onthe evolution of the surface meltand the cloud characteristics.Surface albedo was strongly in-fluenced by cloud optical depth.On 18 July, we determined a to-tally diffuse surface albedo of0.56 in the vicinity of the shipunder heavy overcast. Just eastof the ship under clear-sky con-ditions, with no obvious differ-ences in surface features, the surface albedo was 0.40.

During April and the first half of May, consider-able inhomogeneities in upwelling radiation wereseen, associated with ice of different thickness and alsoopen water. Figure 9 shows a time series of hemi-spheric upwelling shortwave and longwave radiationobtained on 7 May from the C-130 at an altitude of30 m. Surface temperatures ranged from −15°C overmultiyear ice to −1.7°C over open water. The impactof leads on the upwelling fluxes is seen in Fig. 9 fromthe coincident low values of upwelling shortwave ra-diation and the high values of the upwelling longwaveradiation. During July, when the sea ice was melting,the surface was very complex (see cover), with openwater in leads and the surface meltponds clearly in-fluencing the surface albedo (the hemispheric short-wave radiometers were not capable of resolving theindividual small features).

The NASA Ames Solar Spectral Flux Radiometerwas deployed on the NASA ER-2, on the UW

Convair-580, and on the SHEBA ship. Figure 10shows 22 consecutive hours of spectral downwellingradiance at the SHEBA ship on 15–16 May. Localnoon is around 2300 UTC and coincides with maxi-

FIG. 8. Time series plot from 4 May through 29 June 1998 ofaverage broadband (300–3000 nm) surface albedos obtained fromthe C-130Q (May, July) and the UW Convair-580 (June) in thevicinity of the SHEBA ship.

22 Vol. 81, No. 1, January 2000

mum signal in clear sky on 15 May; the minimumsolar elevation is 6° at approximately 1100 UTC.Before 0300 UTC, water vapor and oxygen featuresdominate the spectra. By 0400 UTC the effects ofcloud are clearly seen by the saturated oxygen band at762 nm and water vapor band at 940 nm.

The diffuse radiation re-flected by arctic stratus clouds andsea ice was measured aboard theUW Convair-580 aircraft whileflying a clockwise circular orbitand scanning the scene belowthe aircraft with the NASAGoddard scanning radiometer(CAR). This radiometer scans ata rate of 100 rpm from zenith tonadir on the starboard side of theaircraft and can map the entirereflection pattern of the surfaceduring a complete circular orbitof the UW Convair-580. With a1° field of view of the radiom-eter, a complete bidirectionalreflectance distribution functioncan be obtained for eight wave-lengths of the radiometer simul-taneously (King 1992). Figure11 shows the reflectance func-tion of 0.68 and 1.64 µm for a

homogeneous stratus cloud and for the sea ice surface.Of particular interest in Fig. 11a is the enhanced back-scattering maximum for the stratus cloud that occursin opposition to the sun. Surrounding this glory fea-ture is a pronounced rainbow (apparent on the left-hand side of Fig. 11a), which is characteristic of water

droplet clouds. Figure 11b illus-trates the reflection function ofsea ice. In contrast to Fig. 11a,sea ice shows no rainbow orglory pattern, and is consider-ably darker than stratus waterclouds at 1.64 µm due to themuch larger absorption of solarradiation by ice than by water atthis wavelength.

Measurements of cloud opti-cal properties were obtained froma cloud integrating nephelom-eter, a new instrument flown forthe first time on an aircraft (theUW Convair-580) during theFIRE Arctic Clouds Experi-ment. This instrument is de-signed for measurements of theasymmetry (g) parameter andoptical scattering and extinctioncoefficients at 0.635 µm in liq-

FIG. 9. Time series plot of upwelling longwave (red) and shortwave (blue) radiation ob-tained by the NCAR C-130 aircraft at an altitude of 30 m on 7 May 1998. The location ofleads is indicated in green.

FIG. 10. Observations of spectral downwelling solar irradiance obtained from the SolarSpectral Flux Radiometer at the SHEBA ship on 15–16 May 1998.

23Bulletin of the American Meteorological Society

uid and ice clouds. An especially interesting case wasfound on 1 June when the aircraft passed through anice cloud containing predominantly rosette ice crys-tals. The optical extinction coefficient and the asym-metry parameter are shown in Fig. 12 for this case. Theaverage-measured asymmetry parameter for this case(g = 0.715) is significantly smaller than that found inliquid clouds (with typical values g = 0.835).

d. Remote sensingRemote-sensing data from the NASA ER-2 aircraft

are illustrated in Fig. 13 for a case on 20 May near thecoast of Barrow. The swath includes tundra coveredby snow (and cloud), open water near the coast, andsea ice floes offshore. The ER-2 was flying down theimage from top to bottom, encountering single-layerstratus clouds with a cloud-topaltitude of around 600 m (CLS).Clouds are readily observed bythe MAS at 1.62 µm since waterclouds are quite reflective at thiswavelength, in contrast to openwater and sea ice, which are bothquite dark at this wavelength(cf. Fig. 11). The MAS visibleimage (0.66 µm) suggests thatthe clouds were optically thickover the tundra, thinning and be-coming semitransparent over theopen water. This is also con-firmed by examining the verticalcross section of the CLS. TheAMPR 37-GHz image clearlyshows the surface features, sinceatmospheric emission at thiswavelength is minimal. Openwater has low emissivity andhence low brightness tempera-ture at the AMPR frequency of37 GHz, while snow and icehave high emissivity at 37 GHzand hence appear bright. Com-parison of the images at 220 and37 GHz (MIR) shows that overopen water the brightness tem-perature at 220 GHz is signifi-cantly higher because of thehigher surface emissivity andsignificant atmospheric emis-sion at this frequency. Over seaice, the low brightness tempera-

ture at 220 GHz (but not at 37 GHz) indicates that thereis some snow cover on the sea ice, which is detectedat 220 GHz because of its shallower penetration depth.This case illustrates the complexity of the polar sur-faces and the utility of the combination of visible, near-infrared, submillimeter, and microwave wavelengthsin separating out the characteristics of the surface andclouds.

An example of a retrieval of cloud properties fromthe NOAA-14 AVHRR is shown in Fig. 14 for2300 UTC 4 May 1998 (corresponding to the case de-scribed in Figs. 2–4). The satellite data consists of1-km AVHRR data taken over the SHEBA Ice Station(76.0°N 165.4°W). Figure 14a shows the channel 3(3.75 µm) image that depicts a relatively complexcloud system over the area. The data taken in the box

FIG. 11. Bidirectional reflectance function obtained from the Cloud Absorption Radiom-eter (CAR) on the UW Convair-580 at 0.68 and 1.64 µm for (a) homogeneous stratus cloudon 29 May 1998 at 72°48′N, 158°44′W when the solar zenith angle was 53.65°, and (b) thesea ice surface on 23 June 1998 at 77°44′N, 16°36′W when the solar zenith angle was 54.45°.

24 Vol. 81, No. 1, January 2000

centered over the SHEBA ship (outlined in the satelliteimage) were analyzed with the solar-infrared, infrared,split-window technique that matches calculations fromradiative transfer parameterizations of reflectance andemittance at 3.75, 10.8, and 11.9 µm to determinecloud phase, particle size, and optical depth for each

pixel (Minnis et al. 1998). Of the 870pixels, 18% were classified as liquidwater and 82% as ice. The mean effec-tive radius for the water droplets was6.3 µm, while the mean effective diam-eter for the ice crystals was 10.2 µm.Images of the retrieved particle sizes andoptical depths are shown in Fig. 14e. TheC-130 in situ instruments observed highconcentrations of small, nearly sphericalparticles near cloud top. Further into thecloud, some liquid water was observedwith indications of icing and somedrizzle. These observations provide ten-tative confirmation of the satellite re-trievals of cloud particle size and phase.The radar and flight logs, however, showthat the retrieved cloud-top altitude isapparently responding to the low concen-tration of ice crystals between 1 and3 km.

During July 1998 the NOAA-15AVHRR became available. This instru-

ment differs from previous AVHRRs because chan-nel 3 measures 1.6-µm radiances during the daylightand 3.75-µm radiances at night. NOAA-15 is the firstoperational meteorological satellite with the 1.6-µmchannel. This spectral band provides extremely gooddiscrimination of snow, clouds, ocean, and land as

shown in the multispectral image inFig. 15. It is also useful for cloud phaseand particle size determination. Snowcrystals and water bodies are strong ab-sorbers, so they appear black in the1.6-µm imagery. Land surfaces are typi-cally more reflective at 1.6 µm than at0.63 µm while ice clouds are less reflec-tive than liquid clouds. Thus, the combi-nation of the visible, infrared, and 1.6-µmimages shows the cold ice pack as pink,clouds as white, and clear water as deepblue. Thin clouds over the ice indiscern-ible in either visible or infrared imagesare quite evident in the 1.6-µm image andmultichannel overlay. The new AVHRRdata should provide much improved sat-ellite retrievals of cloud properties overthe Arctic. The aircraft flights during July1998 should be quite valuable for vali-dating the interpretation of this new sat-ellite resource.

FIG. 12. Time series of the volume extinction coefficient and asymmetry pa-rameter measured with the cloud integrating nephelometer during horizontal flightof the UW’s Convair-580 through an ice cloud with T = −40.5°C. The red dashedline indicates a typical value of g for a liquid water cloud.

FIG. 13. Composite of observations obtained from the NASA ER-2 aircraft on20 May 1998 in the vicinity of the Barrow coast. MAS: MODIS Airborne Simu-lator; AMPR: Advanced Microwave Precipitation Radiometer; MIR: Millimeter-wave Imaging Radiometer; CLS: Cloud Lidar System.

25Bulletin of the American Meteorological Society

e. Single-column modelingData obtained during the FIRE

Arctic Clouds Experiment havebeen used to assess the perfor-mance of several large-scale mod-els during May: the EuropeanCentre for Medium-Range WeatherForecasts (ECMWF) numericalweather prediction model as it wasoperational at the time of the ex-periment, and single-columnversions of the Colorado State Uni-versity (CSU) GCM (Fowler et al.1996) and the Arctic Regional Cli-mate System Model (ARCSyM)(Pinto et al. 1999). The single-column models are forced withtime series of large-scale advec-tivetendencies and divergencesobtained from the ECMWF initial-ized analyses. The surface turbu-lent fluxes were specified usingECMWF data in the CSU SCM,while turbulent fluxes were mod-eled in the ARCSyM SCM. TheECMWF column is not allowed todevelop its own short-term climatesince results are from successiveglobal 12–36-h forecasts; therefore,the simulation is not susceptible tomodel drift as are the two SCMsdescribed above.

Table 6 shows a preliminary com-parison of the monthly averaged valuesfor May of selected cloud and radiationparameters for the ECMWF, CSU, andARCSyM models against the field ob-servations. The surface radiation fluxeswere obtained from a 2-m stand locatednear the SHEBA ship. Cloud fraction,and base and top heights were deter-mined from the cloud radar and lidar;and cloud liquid water path (LWP) wasobtained from the surface-based micro-wave radiometer. Comparisons of themicrowave-derived LWP with that ob-tained from profiles flown through thecloud layer by the NCAR C-130Q air-craft reveal that the microwave valuesmay be biased toward larger values.Further calibration of the aircraft and mi-

FIG. 14. NOAA-14 AVHRR image and derived cloud radiation products over SHEBA(denoted by “S” in each panel) and the surrounding ice pack at 2252 UTC 4 May 1998.(a) Visible, 1-km resolution image with box outlining area of cloud analysis; (b) cloudmask with coincident C-130Q flight track; (c) broadband shortwave albedo; (d) outgo-ing longwave radiation in W m−2; (e) 3.7-µm image (left panel) and r

e in µm refers to

effective radius of cloud liquid water droplets, De denotes effective diameter of ice cloud

particles; and gray areas indicate that particle sizes could not be retrieved.

Downwelling longwave 244 231 227 191radiation (W m−2)

Downwelling shortwave 248 275 291 250radiation (W m−2)

Total cloud fraction 0.82 0.69 0.51 0.89

Liquid water path (g m−2) 43 9 22 13

Cloud-top height 3.3 3.8 3.2 6.9(top layer) (km)

Cloud-base height 1.2 0.8 0.5 1.9(bottom layer) (km)

TABLE 6. Preliminary comparison of observations (OBS) with model output(ECMWF, ARCSyM, CSU) for a single grid cell centered on the SHEBA ship forthe month of May 1998.

Parameter OBS ECMWF ARCSyM CSU

26 Vol. 81, No. 1, January 2000

crowave data should bring these observations closertogether.

The ECMWF and ARCSyM models significantlyunderpredict the total cloud fraction. Cloud fractionestimates obtained individually from the lidar and ra-dar differ by a few percentage points, but this discrep-ancy is small relative to the discrepancies with the modelsimulations. The ECMWF model is fairly successfulat reproducing the observed cloud heights. Althoughthe total cloud fraction modeled by CSU agrees wellwith the observations, the model appears to producemore high clouds and fewer low clouds than observed.All three models substantially underpredict the LWP.

The errors in the modeled cloud properties contrib-ute to the biases in the surface radiative fluxes.ECMWF and ARCSyM models predict surface down-welling shortwave radiation fluxes that are too largeand surface downwelling longwave radiation fluxesthat are too small, which is consistent with modelunderprediction of cloud fraction and liquid water

path. The CSU CSM reproduces very well the down-welling shortwave radiation flux but the downwellinglongwave radiation flux is substantially underesti-mated, which is consistent with the predominance ofhigh clouds simulated by this model.

Further details of the intercomparison of the obser-vations with the SCMs are shown in Fig. 16 for thetime series of cloud LWP and surface radiation fluxes.It is seen that cloud events in both models are eithermissed (16–19 May) or the LWP of the modeledclouds is underpredicted. The underprediction of LWPis partly associated with inaccurately representingcloud layers as crystalline. As seen on 12 May, theunderpredicted LWP may result in a large error in themodeled net shortwave flux but small errors in the down-ward longwave flux. Missed cloud events result ingross errors in both the downwelling longwave and netshortwave radiation, as seen on 29 May. The first 10days of both simulations are characterized by low liq-uid clouds that occur in a layer with a significant warm

FIG. 15. NOAA-15 AVHRR 1-km imagery at 2021 UTC 26 July 1998 over the Arctic Ocean. The location of the SHEBA ship isdenoted with an “S.” (a) 0.63-µm image; (b) 11-µm image; (c) 1.6-µm image; and (d) pseudo–color image with red, green, and blueintensities determined by the 0.63-µm reflectance, 1.6-µm reflectance, and the reversed 11-µm temperature (snow: pink, open water:dark blue, thick water cloud: white, ice clouds: light blue-gray).

27Bulletin of the American Meteorological Society

bias that is associated with the large-scale forcing sup-plied by ECMWF; hence, when clouds with signifi-cant LWP are simulated, the downwelling longwaveradiation is substantially overestimated. The “clear-sky” radiative transfer in the ARCSyM SCM ishandled fairly well while larger clear-sky biases areevident in the CSU GCM (as indicated by 22–25 Mayin Fig. 16). The modeled clear-sky downwelling so-lar flux is too large in both models around solar mid-night. This could be the result of problems with theplane-parallel approximation at high zenith angles.The radiative impact of aerosols, which has not beenincluded in these models, may also contribute to thebias seen in the net solar flux at the surface.

6. Conclusions

The FIRE Arctic Clouds Experiment successfullymet its operational objectives, namely to conduct amultiaircraft study of clouds and radiation over theArctic Ocean in the vicinity ofthe SHEBA ice station and theARM Barrow site. The overalldesign of the experiment was tocombine measurements at thesurface, from research aircraft,and from space to address prob-lems of arctic clouds, radiation,and aerosols, including their mod-eling and remote sensing. Thecomplex experimental designwas judged to be very success-ful in terms of its efficiency,economy, and completeness.

Preliminary analysis of thedata indicates that the datasetcontains a wealth of informationon clouds, radiation, and aerosolto address the scientific ques-tions, remote sensing issues,and modeling objectives of theproject. Preliminary scientifichighlights that illuminate thescience questions presented insection 2 include the following:

• A persistent humidity inver-sion above boundary layercloud tops, associated withthe static stability of the arc-

tic environment, contributes to the homogeneityand persistence of the cloud by inhibiting evapo-rative cooling associated with entrainment mixingat cloud top.

• Evidence was found of direct association of low-level clouds with open water in polynyas.

• Large variations in the relationship between cloudtemperature and phase were found, influenced bythe presence of ice nuclei, seeding of the cloud byice particles falling from above, and the size of theliquid drops.

• Aerosol composition is commonly highest aloft, es-pecially during spring, associated with long-rangetransport and scavenging in the lower atmosphereby boundary layer clouds. Local production ofaerosols was observed in dissipating cloud layers,the humidity inversion, and was associated withlocal production of dimethyl sulfide.

• There is some evidence that the open water in leadsmay provide a source of ice forming nuclei.

• Direct evidence was given for the susceptibility of

FIG. 16. Comparison of simulations of the ARCSyM and CSU single-column modelswith SHEBA observations during the month of May: (top) liquid water path; (middle) down-welling surface longwave radiation; (lower) net surface shortwave radiation.

28 Vol. 81, No. 1, January 2000

arctic cloud microphysical and optical propertiesto modification by anthropogenic aerosol.

• The spatial inhomogeneity of the surface physicaland optical characteristics is very large, especiallyduring the summer melt season. Temporal varia-tions in surface albedo were documented to occurin response to atmospheric forcing: the first sea-sonal rainfall markedly decreased the snow surfacealbedo and heralded the onset of snowmelt; sub-stantial variations (up to 20%) in surface albedoarose from variations in cloud optical depth thatwere not associated with any variations in surfacecharacteristics; and late summer surface albedochanges occurred in response to freezing and thaw-ing of the surface in response to storms.

• Measurements of the asymmetry parameter showthe substantial effect that habit and size of the icecrystals have on the radiative transfer.

• The combination of visible, near-infrared, sub-millimeter, and microwave wavelengths showsconsiderable promise in discriminating the charac-teristics of the surface and clouds under many con-ditions that have hitherto been difficult to identifyusing only a single sensor.

• The ECMWF, CSU, and ARCSyM models tend tounderpredict low cloud amount and the column liq-uid water path, resulting in simulations of surfacenet shortwave radiative fluxes that are too large,and downwelling longwave radiative fluxes thatare too small.

Our initial assessment of the dataset indicates thatit will provide the basis for definitive answers to someof the science questions articulated for the project, andprogress toward addressing the others. Preliminarysingle-column modeling studies indicate that consid-erable work needs to be done to assess and minimizethe impact of uncertainties in the boundary advection,but significant progress is already being made in de-veloping and testing new parameterizations using thisdataset. The dataset is already being used to evaluatesatellite cloud and sea ice retrievals and is forming thebasis for improved satellite remote sensing algorithmsusing multiple sensors.

Ultimate achievement of the full objectives of theFIRE Arctic Clouds Experiment will require exten-sive analysis of the data and comparison with modelsand satellite retrievals by many scientists, both withinand beyond the FIRE Science Team. To facilitate col-laboration, the datasets are being archived in a formthat will make them readily accessible worldwide.

Further information on the FIRE Arctic Clouds Ex-periment and data archival can be found at http://eosweb.larc.nasa.gov/ACEDOCS/ace_intro.html andhttp://ltpwww.gsfc.nasa.gov/MAS/FIREIII.pdf.

Acknowledgments. The FIRE Arctic Clouds Experiment wasfunded by the National Aeronautics and Space Administration,National Science Foundation, Department of Energy, Environ-ment Canada, the Canadian National Research Council, and theCanadian Panel on Energy Research and Development. Wewould like to thank the following individuals and groups whohelped make this experiment a success: D. McDougal and theFIRE Project Office, K. Laursen and the staff of the NCAR Re-search Aviation Facility, the flight crews of the University ofWashington and Canadian Convair-580s, the U.S. Army at FortWainwright, the Geophysical Institute at the University ofAlaska, the NASA Langley Research Center DAAC and Com-munications and Computer Systems Branch, scientists and tech-nical personnel on the Des Groseilliers, and all of the scientists,students, and technical personnel who participated in the fieldexperiment. We are grateful to the ECMWF for providing a spe-cial assimilated dataset for the single-column model experiments.We would like to also thank R. Hood, J. Wang, J. Spinhirne, theARM program, and the SHEBA Project Office for contributingdata to this paper. Comments from the anonymous reviewers sig-nificantly improved the presentation of the paper. Members ofthe FIRE Arctic Science Team are C. Bretherton, W. Cotton,J. Curry, A. Del Genio, W. Eberhard, C. Fairall, H. Gerber,J. Hallett, C. Jakob, P. Hobbs, J. Hudson, G. Isaac, M. King,Y. Kogan, S. Kreidenweis, S. Krueger, P. Lawson, D. Lenschow,M. Miller, P. Minnis, P. Pilewskie, D. Randall, W. Rossow,J. Spinhirne, S. Tsay, T. Uttal, F. Valero, S. Wang, Q. Wang,B. Wielicki, and D. Wylie.

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