1. Project Summary

Predicting the timing and location of convective cloud development is a fundamental challenge in the study of meteorology. Heterogeneities in the atmospheric boundary layer (ABL), particularly in ABL water vapor content and depth, lead to preferred locations for convective initiation. Land surface heterogeneity is an important cause of heterogeneity in the ABL. Hence, land-surface fluxes play an important role in ABL development and convective initiation. The minimum scale of land surface forcing that causes heterogeneity in ABL properties such as depth and water vapor content and consequently leads to preferred locations for cloud formation remains uncertain. Observations and models suggest scales ranging from a few km to nearly 100 km.

This project proposes an observational plan as part of the International H2O Project (IHOP) that will provide data concerning heterogeneity in land surface fluxes and the daytime ABL at scales from less than 1km to a few hundred km. The critical observations are maps of surface fluxes of latent and sensible heat over a region of at least 300 x 300km in Oklahoma and Kansas, and repeated airborne H2O DIfferential Absorption Lidar (DIAL) observations of ABL depth and lower tropospheric H2O vapor on east-west and north-south flight tracks approximately 300 km in length. The flux maps will be created from a network of surface flux towers, airborne measurements of surface fluxes over repeated flight tracks about 50km in length, satellite remote sensing of land surface temperature and vegetation cover, and a land surface model. One airborne DIAL will be coupled with an airborne Doppler lidar, yielding the ability to observe ABL flux profiles via remote eddy covariance. Flights will be focused on the midday hours of 10-15 relatively fair weather days, in order to capture the preconvective atmosphere with DIAL.

Observations will be analyzed to determine the degree of spatial heterogeneity in ABL water vapor, depth, and H2O budgets, and the causes of this heterogeneity, focusing especially on determining the spatial scales at which land surface heterogeneity is an important factor. Ten to fifteen days of data will be analyzed in an attempt to move beyond a case study approach.

Data assimilation will be used to ingest dense, mesoscale observations into a high-resolution mesoscale atmospheric model that includes a sophisticated land-surface scheme. The model will be used as an analysis tool to study mesoscale surface-ABL-cloud interactions captured in the observations. Further, the model will be used to assess the impacts of the DIAL observations, detailed land surface flux maps, and a new shallow cumulus parameterization on forecasts of ABL heterogeneity and convective initiation.

Products will include ABL depth maps, ABL H2O budget estimates, and model post-analysis fields incorporating all available IHOP observations for the 10-15 days of DIAL observations. Expected results include an improved understanding of the role of land surface heterogeneity in convective ABL development and convective initiation, the degree to which model prediction of ABL development and moist convection can be improved via dense observations of ABL H2O content and surface fluxes, and the impact of a shallow convection parameterization on model performance.

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3. Project Descriptiona. Results from Prior NSF Support.

Title: Regional forest-atmosphere coupling: Impacts on CO2 and climate. PI: Kenneth J. Davis, Penn State Univ. Co-I: A. Scott Denning, Colorado State Univ.Duration: 1 September, 1997 – 31 August, 2001.This project was funded by the Department of Energy via a joint DoE-NSF-NASA-EPA call for proposals (TECO) and received support from NCAR’s Atmospheric Technology Division via deployment of an Integrated Sounding System. The mean course of ABL growth was studied with a mixed layer model, radar observations, and eddy covariance observations of the surface buoyancy flux. Average convective ABL depth is predicted very well by a mixed layer model. Stable layer depth is a function of buoyancy and momentum flux (Yi et al, 2001a). ABL depths were used along with CO2 flux and mixing ratio measurements to estimate the gradient in CO2 mixing ratio between the ABL and the free troposphere. This estimate, performed on a daily basis for the entire growing season, shows the magnitude of the diurnal and seasonal CO2 rectifier effect (Denning et al, 1995). Our results suggest that simulations underestimated the diurnal rectifier effect and overestimated the seasonal rectifier (Yi et al, 2001b). Refereed publications:Yi, C., K.J. Davis, P.S. Bakwin, and B.W. Berger, 2001a. Long-term observations of the evolution of

the planetary boundary layer. J. Atmos. Sci., 58, 1288-1299.Yi, C., K.J. Davis, P.S. Bakwin, A.S. Denning and N. Zhang, 2001b. Is the simulated CO2

rectifier forcing too strong? Submitted to Nature.

b. Project Plan.

IntroductionPredicting the timing and location of mesoscale convective precipitation remains a fundamental challenge in the study of meteorology. Forecasting skill is limited by a lack of dense mesoscale atmospheric observations, imperfect atmospheric physics used in numerical models, and imperfect parameterizations for describing the full complexity of the atmospheric forcing provided by the earth’s surface. Heterogeneities in surface energy fluxes can lead to heterogeneities in ABL characteristics and the formation of mesoscale boundaries that influence the location and intensity of convection.

The earth’s soils, vegetation and topography are heterogeneous at far finer spatial scales than can be resolved by mesoscale atmospheric models. Strong ABL turbulence, typical of daytime when convective clouds and precipitation often form, tends to homogenize heterogeneous surface forcing. The spatial scales at which surface heterogeneity and the ABL must be resolved in order to capture the impact of ABL-surface coupling on convective initiation are not well known. One goal of this proposal, therefore, is to understand how land-atmosphere interactions create heterogeneous ABL structures, and whether or not that ABL structure leads to preferred locations for convective initiation.

Numerical simulations (e.g. Chen and Avissar, 1994), integrative studies (Shaw and Doran, 2001; Segal et al 1989), some observational campaigns (Sun et al, 1997; Kiemle et al, 1997; Desai et al 2001), and theoretical studies (Dalu et al, 1991; Mahrt, 2000) have explored this issue. Observations show that turbulence in the lower ABL is greatly influenced by land surface heterogeneity, including elevation, slope, vegetation and soil moisture, even at scales of 10km or smaller (e.g. Mahrt et al, 1994; Sun et al, 1997). Observations of mean ABL properties (e.g. depth, water vapor) over heterogeneous terrain however show relatively little correlation with heterogeneous surface fluxes at scales less than 50 km (Kiemle et al, 1997; Desai et al, 2001).

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This is supported by theoretical studies that suggest mesoscale flows are the strongest at scales of several tens of km (Chen and Avissar, 1994; Dalu et al, 1991). At larger scales (Desai et al, 2001) or extremely low wind speeds (Ehret et al, 1996) heterogeneous mean ABL structure driven by land surface heterogeneity emerges thoug these observations are limited to case studies. In the terminology of the ABL, this is the case where the atmospheric blending height (Mahrt, 2000) is greater than the ABL depth – that is, blending of surface heterogeneity does not occur. It is this situation, where the mean properties of the ABL can become very heterogeneous in space, that we suspect is most relevant to the creation of favored locations for convective initiation. Mahrt (2000) suggests that the blending height is a function of the spatial scale of surface heterogeneity, the magnitude of the flux heterogeneity, and the mean wind speed.

We hypothesize, therefore, that the origins of heterogeneity in mean ABL structure (e.g. ABL depth, water vapor content) must be understood in order to link land surface heterogeneity to convective initiation. Recent observations have focused on describing the response of ABL turbulence to land surface heterogeneity (e.g. Mahrt et al, 1994). Mean ABL properties are an expression of the integral of surface forcing, thus the spatial scales involved are larger, extending from the microscale well into the mesoscale. ABL observations that cover mesoscale domains and are coupled with highly resolved surface flux data are lacking. Thus we have insufficient data to confirm or refute existing theories and numerical studies of the coupling between land surface heterogeneity and mean ABL properties, and the resulting impact on convective initiation.

Regardless of the cause of mesoscale variability in the ABL, it is likely that improved descriptions of lower tropospheric structure, especially water vapor distributions and ABL depths, in combination with accurate surface energy fluxes, will improve the prediction of convective initiation. For example, in the limiting case of “homogeneous” surface forcing (i.e., blending height >> ABL depth) it has been suggested that the initiation of deep convection is sensitive to a combination of the surface energy balance and the thermodynamic structure of the ABL entrainment zone (Findell and Eltahir, 2001). ABLs with dry adiabatic entrainment zones are driven to convective initiation by high buoyancy flux while ABLs with moist entrainment zones are driven to instability more readily by high latent heat fluxes. Thus a second goal of this proposal is to evaluate the benefit of assimilating improved observations of ABL structure and surface fluxes on forecasts of ABL development and convective initiation.

Finally, ABL structure and lower tropospheric water vapor fields are not only influenced by surface fluxes, but by turbulence at the ABL top. In summer fair weather conditions, shallow cumulus convection has a significant impact on the development of the ABL and the subsequent initiation of deep convection. In addition to affecting the radiative flux profiles, shallow cumulus can substantially modify ABL budgets of energy and water vapor, and the moisture and temperature profiles of the troposphere above the convective ABL, therefore affecting the growth rate of the ABL and the properties of the air entrained at its top. These effects of shallow cumulus are well known (e.g., Stull 1985). However, shallow cloud effects are usually not included in operational numerical forecast models. Further, observations of the horizontal, cloud-scale distribution of water vapor, especially above the ABL, are rarely available. A third goal, therefore, is to evaluate the importance of improved model physics, namely the shallow cumulus parameterization, on ABL evolution and subsequent convective initiation. Unique International H2O Project (IHOP) data sets such as the airborne lidar water-vapor cross sections will allow us to quantify the effects of shallow cumulus in ways that have not been previously possible.

This investigation is based upon the enhanced data sets that will be available as part of IHOP, particularly surface flux measurements and airborne water vapor differential absorption lidar (DIAL) observations. We propose that these observations be focused on a spatial domain

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approximately 300km x 300km in the southern Great Plains (see Figure 1). This domain is large enough to encompass a broad spectrum of potential surface-ABL coupling and airmass modification, but small enough to allow aircraft to monitor the diurnal evolution of atmospheric conditions across the region. Observations would be repeated for 10-15 days in order to analyze surface-ABL coupling in a composite mode, rather than being limited to a few case studies.

Observational analyses will be conducted in cooperation with participating airborne lidar groups, flux measurement groups, and will incorporate high-resolution land use data and satellite remote sensing of the land surface. Surface energy balance will be observed using tower and aircraft-based eddy covariance, and mapped across the land surface using a combination of these direct observations and remote sensing. ABL depths as well as water vapor mixing ratios, fluxes and budgets will be resolved along airborne lidar flight tracks. The combination of highly resolved surface forcing and lower tropospheric structure, repeated over many days, will provide an unprecedented observational view of land-atmosphere coupling encompassing micro (1km or less) to mesoscale (up to 300 km) domains.

Our modeling effort will extend our work on the coupling between land-surface fluxes and ABL structure (Reen et al. 2001) and our work with shallow/deep cloud parameterizations (Deng et al. 2001a,b,c; Munoz et al.., 2001) by applying a coupled suite of advanced model physics within the Penn State / National Center for Atmospheric Research (PSU/NCAR) mesoscale model MM5, to the problem of convective initiation and warm-season quantitative precipitation forecasts (QPF) over the southern Great Plains. The model will be used both to support data analyses and to evaluate the impact of new data on forecasting skill. Unique IHOP data will be used for both four dimensional data assimilation (FDDA) and model verification. Nested grids and a multi-scale FDDA strategy will be used in the MM5 to represent and differentiate the many influences on the regional convection over the Southern Great Plains. In addition to local boundary-layer forcing, large-scale advective effects, such as elevated mixed layers and “lids” (e.g., Carlson et al. 1983), greatly influence the location and intensity of deep convection over the Great Plains. The differential advection of hot, dry air from elevated terrain to the southwest and the cooler, moist air ahead of a trough in the westerlies has a significant effect on severe weather forecasting in this region. The mesoscale modeling will complement observational analyses by identifying such processes that may be difficult to identify from observations alone. New data assimilation strategies within the ABL and a new shallow cumulus parameterization will take advantage of the unique combination of measurement systems available during IHOP.

Hypotheses

1. Surface flux heterogeneity at scales less than 10-20km does not create spatial heterogeneity in the mean properties (depth, H2O) of the midday convective ABL, except under extremely light-wind conditions. Turbulence in the ABL will reflect land-surface heterogeneity at much smaller spatial scales than mean ABL properties will.

2. Substantial spatial variability in the ABL depth, water vapor budget, lifting condensation level and level of free convection will be evident in the southern Great Plains at spatial scales of 50 km and greater. This heterogeneity will be caused in a significant number of cases by spatial heterogeneity in land surface energy fluxes. In a subset of cases, the ABL heterogeneity will provide a focus point for convective initiation.

3. Realistic surface sensible and latent heat fluxes resolved at scales of 5-10 km are necessary for a mesoscale model to accurately simulate spatial variability in ABL depth, ABL water vapor

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content, lifting condensation level and the level of free convection. Addition of such flux data improves the model’s ability to predict convective initiation.

4. Assimilation of airborne and ground-based lower tropospheric water vapor and ABL data including ABL depth and possibly vertical velocities will improve the ability of mesoscale models to forecast ABL development and convective initiation.

5. Incorporation of a shallow cumulus parameterization will improve a mesoscale model’s ability to simulate lower tropospheric water vapor distributions, ABL development, and convective initiation. Airborne DIAL data will verify that the parameterization is improving simulation of lower tropospheric H2O.

6. Doppler-DIAL, and DIAL-only data can be used to derive many of the terms of the ABL water vapor budget with a spatial resolution of about 50km.

Tools and Methods

More details of the array of instruments and observations proposed for IHOP can be found in the science overview document at http://www.atd.ucar.edu/dir_off/projects/2002/IHOP.html. This proposal will only provide brief descriptions of those instruments most central to this effort.

i. DIALDifferential absorption lidar (DIAL) is an active remote sensing method for profiling atmospheric constituents at high spatial and temporal resolution. Several airborne water vapor DIAL systems are currently in operation (Ehret et al, 1998; Ismail et al, 1998), and satellite deployment is being evaluated. Progress is also being made towards robust ground-based H2O DIAL systems that could parallel the operational wind profiler network. Airborne DIAL achieve an accuracy of 5% or better at spatial resolutions of 1 km or less in the horizontal and 200-300 m in the vertical, with ranges of 3-10 km depending on atmospheric conditions and the choice of water vapor absorption line. Higher resolution, down to order 100m x 100m in some cases, can be obtained at the cost of increased random noise in the derived water vapor field. Statistical techniques allow us to compute turbulent moments despite the increased instrument noise (Lenschow et al, 2000).

Three airborne DIALs currently proposed for participation in IHOP are the DLR system (Ehret et al, 1998), the the Service d’Aeronomie du Centre National de la Recherche Scientifique (CNRS) LEANDRE II system, and the NASA Langley Laser Atmospheric Sounding Experiment that participated in SGP97 (Ismail et al, 1998). These DIALs would be flown on board the DLR Falcon, the Naval Research Lab (NRL) P-3, and the NASA DC-8, respectively.

ii. Doppler lidarThe High-Resolution Doppler Lidar (HRDL) is a coherent, completely eye safe Doppler lidar operating at a wavelength of 2 μm operated by NCAR and NOAA/ETL. The range (vertical) resolution is 30 m. It has an along-track (horizontal) resolution of 75-150 m. Cross-sections are available in real time. HRDL is to be deployed on the DLR Falcon alongside the DLR H2O DIAL.

iii. Lidar data processingTwo products will be derived in addition to the backscatter intensity, water vapor mixing ratios, and vertical velocities obtained from the DIAL and Doppler lidars.

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ABL depth: Lidar backscatter data is retrieved at a spatial resolution of about 10m in the vertical and 1 to 10m in the horizontal (DLR - 100 Hz pulse rate; LEANDRE II or LASE - 10 Hz lidar pulse rates; on board an aircraft flying ~100 m s-1). ABL depths will be derived from lidar using the wavelet-based edge detection method of Davis et al (2000). ABL depth can be retrieved from the backscatter intensity from single unsmoothed lidar “shots,” thus providing a spatial resolution as small as 1m in the horizontal and 10m in the vertical for the duration of a flight.

H2O fluxes and budgets: The combination of HRDL with the DLR water vapor DIAL on the DRL Falcon will directly measure the latent heat flux profile using the eddy correlation technique. The airborne platform will considerably reduce the most important error source in latent heat flux profiling, the atmospheric sampling error. The error should be low enough to measure remotely the divergence of the flux profile, thus yielding a critical term in the convective ABL water vapor budget. Senff et al (1994), Giez et al (1999) and Wulfmeyer (1999) have all successfully demonstrated remote eddy covariance flux measurements using DIAL. When the Doppler lidar is not available fluxes can be computed using ABL similarity theory (Kiemle et al, 1997) where the turbulent flux is related to the mixing ratio variance, and variances are computed from the DIAL water vapor field. Both methods are most applicable to ABL entrainment fluxes for downward looking DIAL since the H2O signal-to-noise ratio is often highest at the ABL top. The advection terms in the ABL H2O budget will be computed using horizontal winds from the wind profiling radars in the study region and DIAL H2O fields (Senff et al, 1999).

Water vapor fluxes and budgets will be resolved at as small a scale as possible given the limits of atmospheric sampling error and instrument noise. The improved sampling of lidar over in situ aircraft (two dimensions vs. a line sample) is countered by the increase in instrument noise found in the differential remote measurements. The net impact is not yet clear. We anticipate that reasonable accuracy can be achieved for 50km segments along a single flight track (similar to ABL budgets from in situ flux aircraft – e.g. LeMone et al, 2001). Composites of multiple flights over the same terrain on a single day may yield more highly resolved ABL H2O budgets.

iv. In situ flux measurementsTower-based eddy-covariance flux measurements will provide point measurements of the surface energy balance that are continuous in time. Ten NCAR/ATD ISFF flux towers will be requested (see facilities request proposal by LeMone et al) to supplement exiting flux towers in the region. Existing flux towers include those that are part of DoE’s ARM-CART facility in Oklahoma, the Atmospheric Boundary Layer Experiment (ABLE) site in southern Kansas, and those that are part of the Oklahoma Atmospheric Surface-layer Instrumentation System (OASIS). Fractional coverage of vegetation and soil types in the region will be surveyed and compared to the distribution of existing flux towers in order to locate the NCAR ISFF towers, which will fill in undersampled soil/vegetation types and be located preferentially along flux aircraft and lidar aircraft flight tracks for intercomparison purposes.

The University of Wyoming King Air is being requested for airborne eddy covariance measurements during IHOP (see facilities request by LeMone et al). A major focus of the King Air ABL flights will be regionalization of the tower flux measurements. Flux legs of order 50km will be repeated many times in order to allow statistically significant fluxes to be computed over small (2-4km) subsets of the flight path using the methods of Mahrt et al (1994).

v. Flux mappingAdvances in flux measurement methodology and application of remotely sensed land surface data allows us to produce reasonably well-verified surface energy budget maps over entire mesoscale domains down to moderately fine spatial resolution, at least under clear-sky conditions

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(Mecikalski et al, 1999; Mahrt et al, 2001). Land surface fluxes of water and heat over the IHOP study region and experimental period will be computed with the Atmosphere-Land EXchange Inverse scheme (ALEXI, Mecikalski et al. 1999; Anderson et al. 1997) that is currently being adapted to run in real time over the continental United States. This system will provide the required spatial and temporal continuity necessary to construct maps of land-surface fluxes for the experimental region at a resolution of 5 km. Since the model is being developed to run in real time, it will be used to examine surface fluxes during IHOP for flight planning purposes.

ALEXI has been developed to diagnose surface fluxes, given thermal infrared remote sensing inputs, remotely-sensed vegetation data, plus a modicum of in-situ data and land-surface characteristics. This inversion uses the time rate of change in surface brightness temperature as an indicator of the land-surface energy balance. The morning surface temperature rise, as measured by ground-based infrared thermometry or from a satellite platform (e.g., GOES), guides the partitioning of surface fluxes into sensible and latent heating components. Given an estimate of vegetation cover derived from remote sensing spectral measurements (AVHRR-NDVI or MODIS; Sellers et al. 1996; Carlson and Ripley 1997), the surface brightness temperature measurements are decomposed into their soil and canopy contributions, allowing further partitioning of fluxes between the soil and canopy components of the scene. Energy closure is accomplished using an ABL height-energy relationship (McNaughton and Spriggs, 1986).

Many problems encountered in earlier thermal IR flux estimation methods are mitigated in ALEXI. A primary strength of this approach for remote-sensing applications is that it requires a limited number of ground-based input; in particular, the need for ancillary measurements of local air temperature is eliminated with the use of an energy closure based in the characteristics of the ABL. The errors associated with evaluation of fluxes using a surface-air temperature gradient across a short vertical distance (typically 2 m, as is characteristic of many other schemes), are reduced. Furthermore, because the model uses the time difference in brightness temperature rather than absolute temperature measures, it is relatively insensitive to time-independent biases inherent in infrared satellite observations of Earth's surface. This reduces the need for high accuracy in sensor calibration, in the estimation of atmospheric corrections to surface brightness temperature to obtain the surface radiometric temperature, and also in the specification of surface emissivity (Anderson et al, 1997).

ALEXI has been tested with data collected during three field experiments, and has been found to perform well in a variety of vegetative regimes (Sellers et al, 1998; Kustas et al, 1991; Mecikalski et al, 1999). Ongoing comparisons of ALEXI-derived fluxes with Oklahoma Mesonet flux measurements are being performed. Further comparisons will be made with the airborne and tower-based flux measurements obtained during IHOP.

ALEXI is essentially a clear-air technique, and while the mid-morning observation period maximizes chances of getting a clear-air view of the surface, there will obviously be times when regions will be cloudy and flux mapping will not be possible using ALEXI. During clear observing conditions we will use ALEXI to estimate a moisture index for the surface that will be carried in the Penn State MM5 model and maintained through cloudy periods. During these periods, the MM5 will be forced with accurate satellite-derived measurements of solar and longwave radiation (Diak et al. 1996; Diak et al. 2000) and through this merged monitoring/prediction system we will form a near-continuous record of fluxes of water and heat over the field experiment period.

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Higher spatial resolution fluxes can be obtained with accurate land use data. We will work with other IHOP investigators to obtain a high-resolution land use map based on 2002 ASTER imagery and ground-based verification.

Fei Chen of NCAR will also be computing land surface fluxes using the NCEP/OSU land-surface model (Chen and Dudhia 2001) in offline mode. This work will provide an alternative set of land surface fluxes, and this model will be adopted in MM5.

vi. MM5 modeling systemThe Penn State University / National Center for Atmospheric Research (PSU/NCAR) MM5 modeling system (Grell et al. 1994, Dudhia 1993) is a nonhydrostatic limited-area model with a terrain following sigma (normalized pressure) vertical coordinate. It will be configured here in a one-way nested-grid configuration with 36-, 12- and 4- km horizontal resolutions. The 36-km domain covers most of the continental United States and the 4-km domain is centered over the southern Great Plains ARM-CART site and will include the special IHOP data region in Oklahoma, Kansas and Texas. There will be 50-60 vertical layers between the surface and the model top at 50 hPa with two-thirds of these layers concentrated in the convective boundary layer. The model will use explicit grid-resolved predictive equations for cloud water, rain water and ice on all grids plus sub-grid moist convective parameterization (Kain and Fritsch 1990) for the 36-km and 12-km grids. A new shallow convective parameterization developed at PSU (Deng et al. 2001a,b) will be used to simulate the vertical distribution of moisture in shallow cumulus environments. The parameterization uses a unique hybrid mass-flux closure that is dependent on ABL TKE and CAPE. Updrafts in this scheme are calculated using a parcel buoyancy equation so that the scheme transitions to the Kain-Fritsch deep convective scheme during unstable conditions. Use of a shallow scheme within the model allows pre-conditioning of the convective environment by shallow clouds and sub-grid scale transport of moisture above the ABL (Munoz et al, 2001). Atmospheric and cloud radiative effects are also included.

Treatment of the planetary boundary layer physics will include the PSU 1.5-order turbulent kinetic energy (TKE) predicting scheme (Shafran et al. 2000, Stauffer et al. 1999) which uses variables conservative during phase changes. Land surface physics will include a simple force-restore model (Grell et al. 1994) and more sophisticated land-surface models: the NASA PLACE land-surface scheme (Wetzel and Boone 1995, Lynn et al. 2001) and the NCEP/OSU land-surface model (Chen and Dudhia 2001). These land-surface models include vegetative effects and allow soil moisture and temperature to change in time.

The MM5 includes the PSU four-dimensional data assimilation (FDDA) system based on Newtonian relaxation or nudging (Stauffer and Seaman 1990, 1994), and that can be used to assimilate either gridded analyses (“analysis nudging”) or individual observations distributed non-uniformly in space and time (“obs nudging”). The latter approach is especially attractive for assimilation of asynoptic field program data. A form of nudging has been recently adopted for assimilation of observed precipitation in a 12-h pre-forecast dynamic initialization of the NCEP Mesoeta model (Geoff DiMego, 2001, personal communication). A nudging FDDA system will be used here for both dynamic analysis and dynamic initialization. In a dynamic analysis, the model is used a “data interpolator” and data assimilation is applied throughout the model simulation to reduce error growth and obtain a better representation of the spatial and temporal relationships in the atmospheric features. In a dynamic initialization, FDDA is applied only during a model pre-forecast period to obtain improved initial conditions for a subsequent model forecast.

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Experiment Design

IHOP is scheduled for a six-week period from 13 May – 30 June, 2002. The observational area will be centered in Oklahoma at the DoE ARM-CART facility.

Airborne lidar flight plansFigure 1 shows proposed airborne lidar flight tracks. Summer winds in the southern Great Plains ABL are most often southerly, with turning towards westerlies above the ABL. The west-to-east flight legs will capture both the dry (west) to moist (east) gradient in surface conditions that is persistent in the central Plains, and the advection of air into the study area above the ABL. The north-to-south flight legs will sample the ABL parcels at different stages in their traverse across the plains along rough isolines of surface vegetation, describing surface-driven air mass modification as ABL air is advected into the study area. ABL and surface flux heteorogeneity will be examined both along and across the ABL wind, and along and across the dominant vegetation/precipitation gradient.

Ideally each flight track is to be covered by one aircraft, e.g. the DLR Falcon on the N-S leg and the NASA DC-8 on the E-W leg. The two tracks would be flown for approximately 4 hours, starting once convection is established (10-11 LST) and continuing into the mid afternoon (14-15 LST). The DC-8’s greater endurance might allow for 5-6 hours of sampling the diurnal cycle and a somewhat longer flight track if flight hours are available. At flight speeds of about 150 m s-1, each 300km leg would take about 35 minutes and the rectangular pattern could be repeated by the DLR Falcon about 3 times per flight with one flight per day.

The DIAL would be operated in a down-looking mode with the aircraft high enough above ground to capture the ABL and any elevated mixed layer found above the current ABL, or about

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Figure 1: Proposed instrumental array for IHOP. Ground-based instruments are from the IHOP science plan. Dotted rectangles are approximate locations of proposed airborne DIAL flight tracks. Suggested flight altitudes are approximately 5km above ground. Tracks are about 300km long. Shorter solid black lines are approximate locations of UWKA flux flight tracks. Flux aircraft would focus on repeated low level flux runs over fixed tracks with upper-ABL flights (about 0.7 zi) to characterize flux divergence.

4km above ground. Proposed 300km flight legs account for roughly 8 hours of atmospheric advection at 10 m s-1, so that upwind conditions will be well sampled for much of a diurnal cycle over a large portion of the 300x300 km domain. A 15 km leg separation serves both as a test of the degree of cross-track variability and as a source for estimates of cross-track advection of water vapor on times scales of about 1 hour. We propose to repeat this flight design 10-15 times over the course of the experiment in order to obtain a composite sample of the response of the ABL to land surface forcing. This is critical for isolating the importance of the land surface since there will be multiple forces creating mesoscale heterogeneity in the lower troposphere and limiting our ability to draw conclusive results from a small number of case studies.

If the Falcon alone were available only one of the flight tracks would be flown. In this case objectives could be met, but with less sampling of land surface variability for ABL-land coupling, and a loss of the benefit of upwind conditions from two directions for the convective initiation and data assimilation studies. It is possible that the P-3 with LEANDRE II or the DC-8 with LASE could cover both tracks if either were the only airborne DIAL available.

Flux aircraft flight plans:Figure 1 also shows the proposed flight tracks for the University of Wyoming King Air (UWKA) within the IHOP study area. The flight altitude for the UWKA would typically be as low as possible (order 30-50m) over 3-4 pre-selected 30-50km tracks intended to sample the range of land-surface heterogeneity found within the study region. These low-level legs would be repeated 10 or more times in a flight with perhaps half that number of legs at about 0.75 times the convective ABL depth. Low-level legs will be composited into ~2km resolution maps of surface fluxes (Mahrt et al, 1994) that can be compared to surface conditions (vegetation cover, elevation and, if available from the AQUA satellite, soil moisture). These flux measurements would then be compared to tower flux measurements in similar landscapes (some towers will be located along the UWKA tracks) to put the towers in the broader context of the landscape, and allow upscaling of the direct flux measurements to the landscape (Mahrt et al 2001; Desai et al, 2001), and to provide rigorous validation for our remotely sensed surface fluxes (ALEXI) and the land surface model run within MM5 (the NCEP/OSU model).

A second purpose of the UWKA flights is to provide validation of the DIAL-based ABL H2O budgets. The flight tracks are long enough to match a resolvable scale for remotely sensed fluxes. The multi-level UWKA flights will provide in situ measurements of the vertical flux divergence of H2O to compare to the remotely sensed fluxes. The horizontal gradients in H2O and time rate of change of H2O along the flight tracks will serve as validation of additional terms in the H2O budget that will also be measured via DIAL.

MM5 Experimental DesignThe goal of the numerical experiments will be to learn how to use the special data obtained from the IHOP field program to improve model predictions of the water vapor and boundary layer height in the pre-convective environment, and to improve the subsequent forecasts of convective precipitation. Model runs will include dynamic analyses where the model is used as a data interpolator constrained by the governing equations, and including all available observations from the IHOP domain. One set of experiments will involve removing various portions of the IHOP data, mainly degrading the resolution of the surface flux field and including or eliminating DIAL water vapor data, to assess the impact of these data on model simulation of ABL depth and convective initiation. A final experiment will test the impact of a shallow cumulus convection scheme on model forecasts using DIAL data as a uniquely appropriate method of evaluating this attempt to improve model physics.

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The following describes how the experiment design will address specific hypotheses.

Hypothesis 1:This hypothesis will be tested via the combination of airborne DIAL-based maps of ABL depth, ABL water vapor mixing ratios, lifting condensation level (LCL) and level of free convection (LFC). Note that variability in the horizontal and vertical distribution of temperature will be incorporated using the available observation network and the MM5 model as a data interpolator in order to derive LCL and LFC from the DIAL data. King Air data and balloon, aircraft and radiometer soundings will be used to test this approach within portions of the DIAL study area. These maps will be correlated in a composite sense with spatial patterns in surface fluxes and compared to the scaling arguments of Mahrt (2000), and the mesoscale patterns predicted by the studies of Chen and Avissar (1994) and Dalu et al (1991). The surface fluxes used for this analysis will be the tower and aircraft-validated ALEXI predictions, disaggregated using land use maps (Mecikalski et al, 1999) down to a scale of 1km. This analysis will follow the methods used in the case study of Desai et al (2001). The repetition of the flight pattern and analyses over many days will allow detection of coherence between ABL growth and surface forcing despite transient atmospheric structures that might confound the analyses of individual case studies. Wind measurements from the Doppler lidar, in situ winds from the UWKA, and perhaps clear-air winds from the UWKA’s Doppler radar may be used to extend this analysis to turbulent velocities and, if the data are sufficiently stable, mean vertical velocities.

Hypotheses 2 and 3:The key questions to be addressed here are to what degree ABL properties are influenced by surface forcing relative to other factors (e.g. spatial variability in the thermodynamic environment or mesoscale variability in atmospheric divergence), and the spatial scales where such surface-ABL coupling is expressed. Systematic observational studies at the mesoscale (tens to hundreds of km) are lacking.

Airborne lidar will document the heterogeneity of the lower troposphere across a wide spectrum of spatial scales (2 m for DLR DIAL backscatter data to 300 km for the whole flight leg). The scales at which land-surface heterogeneity and ABL properties do appear correlated will be determined via the observational analyses described for Hypothesis 1. This correlation will be examined in a more physically comprehensive fashion by numerical experiments with the MM5 following the work of Reen et al (2001). In these experiments data will not be assimilated at the 4km grid. All but ABL data will be assimilated within the coarser grids. The model with inner grid running at 4 km will attempt to simulate the properties of the ABL as observed via airborne lidar and other IHOP atmospheric profiling instruments. The sensitivity of ABL development to land-surface forcing will be assessed via degrading the surface fluxes from our most highly resolved and directly observed flux maps to nearly homogeneous conditions. These observational and numerical experiments will distinguish the scales at which the ABL integrates over surface heterogeneity and responds to areal average surface fluxes from the scales where the ABL develops mean properties that reflect the heterogeneity of the land surface.

Observations and modeling from the 1997 Southern Great Plains Experiment (SGP97) show significant land-ABL coupling at spatial scales much larger than the smallest scales of surface heterogeneity, but this work is limited to a case study. Desai et al (2001) found that the potential for ABL heterogeneity due to spatial variability in surface fluxes over the southern Great Plains in the summer (Figure 2) was large, with midday maximum ABL depths predicted by a one-dimensional thermodynamic growth model varying by more than a factor of two. The heterogeneity in surface fluxes existed over a wide spectrum of spatial scales, including field-to-field land use changes and larger scale patterns driven by recent precipitation. Observed

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variability in ABL depth (Figure 3), however, was most pronounced at scales of 50-100 km and was roughly correlated with recent precipitation patterns and associated differences in the surface energy balance. Variations due to the convective eddies are typical (Kiemle et al, 1997; Davis et al, 2000) and were also observed. The MM5 model, run in a mode similar to that we propose for this experiment, also showed substantially less structure in the ABL than was present in its surface flux fields (Figure 3). The model showed increased ABL heterogeneity at larger scales, similar to what was observed, when realistic spatial variations in surface buoyancy flux were included (Reen et al, 2001). The much deeper ABL in the southern portion of the study region led to substantially lower ABL mixing ratios (Figure 4), confirming the potential importance of these processes in convective initiation. This case study outlines the methodology we propose to evaluate Hypotheses 2 and 3.

Hypothesis 4:One series of experiments is designed to assimilate on the 12-km and 4-km model domains the DIAL water vapor and boundary layer heights in space and time along the flight paths of the IHOP aircraft(s). Other types of data will also be assimilated in this multi-scale FDDA approach. The emphasis will be placed on cases having DIAL data that later develop afternoon convection in the absence of major internal dynamic boundaries to organize where the convection begins (troughs, dry lines, left-over outflow boundaries from prior convection, etc.). Dynamic analyses and dynamic initializations based on these data, along with the improved surface, turbulence, and shallow cumulus cloud physics, will be used to better understand how the horizontal and vertical water vapor distributions develop leading up to the outbreak of deep convection. The 12-km domain, with its parameterized convection, is representative of the finest-scale current operational predictions. The 4-km domain, with explicitly resolved convection, is representative of the resolution anticipated for future non-hydrostatic operational models, such as the Weather Research and Forecast (WRF) model. The special IHOP data, as well as conventional data, satellite imagery, and the NCEP Stage IV hourly 4-km precipitation analyses, will be used to evaluate the model forecasts of convective initiation and QPF.

A major goal of IHOP is to assess the benefit of new data in improving model forecasts. Successful use of new and limited data requires innovative approaches. For example, in the data-sparse offshore regions of the northeast Pacific we performed a dynamic initialization using climatological heights of the marine-layer inversion base and prescribed temperature and moisture profiles built upwards every time step from the model’s surface-layer temperature and moisture fields within the marine ABL (Leidner et al. 2001). Use of this FDDA approach improved a short-term summertime forecast of ABL structure along the California coast.

We propose here an approach to assimilate the PBL and lower-atmospheric structures observed by DIAL and other instruments, and to determine their added value for prediction of ABL structure, and subsequent convective initiation and precipitation over the Great Plains. Stauffer et al. (1991) and others have demonstrated that assimilation of data within the ABL can improve model convective precipitation. The model will be adapted to modify its TKE based on the observed PBL height and surface fluxes. For example, observed surface sensible and latent heat fluxes can be used to correct low-level buoyant production in the model’s TKE budget. Improvement in the vertical mixing and PBL height will also improve moisture transport within the model. Multi-scale FDDA and a nested-grid strategy will be important for representing larger-scale advective effects, such as the “lid” and elevated mixed layer (e.g, Carlson et al. 1983), on the regional meteorology over Oklahoma and Kansas (e.g., Stauffer et al. 2000).

Hypothesis 5:The proposed work will also include testing and verification of the PSU shallow cumulus scheme

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at fine resolutions using the special IHOP moisture data. The role of shallow cumulus on the PBL and above-PBL structure (i.e. the pre-conditioning of the convective environment) is often discussed by operational forecasters but not usually included in the model forecast guidance. The data available during IHOP will allow better verification of the cloud and vapor fields than what is normally available, and will enable us to further modify the PSU shallow cloud scheme for the finer model resolutions important for prediction of convection and warm-season QPF. Current 3D work using this scheme is based on coarser 36-km model resolution (Munoz et al. 2001, Deng et al. 2001c).

Hypothesis 6:This hypothesis will be tested in the course of deriving spatially resolved ABL H2O budgets using airborne lidar. This is similar to the case study of Senff et al, (1999) (Davis et al, journal document in preparation) from the SGP97 experiment. The proposed IHOP data set spanning several days, including more direct in situ verification and extending to Doppler lidar for direct eddy covariance flux measurements will provide more extensive and definitive results than was possible from SGP97.

Workplan:Observations and data analyses:1. Work with NCAR/ATD and IHOP investigators to locate precise NCAR flux tower locations, UWKA flight tracks, and DIAL aircraft flight tracks. Participate in the field campaign. Jan. – June, 2002.

2. Work with DIAL and Doppler lidar groups on “added-value” data processing. This project will not conduct the basic lidar processing (e.g. mean H2O vapor fields computed from DIAL) but will, in cooperation with the lidar groups, take responsibility for producing: a) ABL depth maps for all flights, b) water vapor variance calculations in support of similarity-based flux estimates, c) similarity-based water vapor entrainment flux estimates, d) LCL and LFC maps based on DIAL H2O data. V. Wulfmeyer, G. Ehret and colleagues will conduct the eddy covariance flux calculations from the Doppler-DIAL lidar combination on the DLR Falcon. We will advise regarding computing fluxes over subsets of the flight legs and compositing these results vs. terrain for multiple flight legs. July, 2002 – June, 2003. ABL maps and flux estimates online by July, 2003.

3. Work with flux tower investigators to obtain a tower flux database. Work with UWKA staff and co-investigators (M. LeMone, NCAR, PI) to obtain surface flux and flux divergence data from the King Air with special focus on segregating low-level flux legs into ~2km segments and achieving statistically significant flux values via the compositing of multiple passes over the same terrain. July, 2002 – June, 2003.

4. Use ALEXI at 10km resolution to produce surface energy balance maps in nearly real time (~12 hour delay) for flight planning during the field campaign. Post-campaign estimates of surface energy budgets will be computed using ALEXI at finer spatial resolution (July – Dec., 2002) and compared to flux tower and aircraft surface flux measurements (Jan., – June, 2003). Flux maps made available by June, 2003. Needed adjustments made to ALEXI results based on comparison with direct flux measurements (July, 2003 – Dec., 2003). Assist with publications using these data, Jan. – Dec., 2004.

5. Assemble other components of the ABL H2O budget. Terms to be computed include advection and the time rate of change of H2O. Primary data will be DIAL data, but other sources (e.g. OK Mesonet, UWKA, rawindsondes and ground-based profiling) will be used where needed and to

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provide evaluation of DIAL budgets following our work in SGP97. Jan. – June, 2003. Journal publication preparation July – Dec., 2003.

6. Analyze the correlation between land-surface forcing and ABL properties using 10-15 days of DIAL ABL data and flux maps. Determine the degree to which observations show ABL mean properties (e.g. depth, H2O) integrating over land-surface heterogeneity. Determine the minimum scale at which surface heterogeneity influences ABL mean properties. July, 2002 – June, 2003. Merge with modeling analyses and work towards a joint journal publication, July – Dec., 2003.

Modeling1. Upgrade the MM5 physics to allow the NCEP/OSU surface scheme to be coupled with the Penn State TKE-predicting turbulence physics. Modify MM5 to allow assimilation of DIAL observations and ALEXI land surface fluxes. Perform preliminary tests and validations of the shallow cumulus parameterization in the 3-D MM5 using historical data sets from ARM program. Jan. – June, 2002.

2. Perform dynamic analysis for 10-15 days with DIAL data from ABL flight missions using the MM5 to provide 3-D fine-scale depictions of lower tropospheric water vapor and ABL height. Initialization of the land surface in MM5 will be based on offline land-surface modeling performed for IHOP by Dr. Fei Chen with funding from the USWRP. The ALEXI surface sensible and latent heat fluxes will be assimilated to correct the MM5 surface forcing. The output will be made available to IHOP. July – Dec., 2002 for first runs. Jan. – June, 2003 for “final” runs. “Data” available June, 2003.

3. Perform MM5 model-physics sensitivity experiments to determine the value added by using the shallow-convection parameterization and the sophisticated land-surface parameterization in the mesoscale model. Learn how these physics sub-models affect the development of the pre-convective and convective environments. July, 2002 – June, 2003. Prepare journal publication July – Dec., 2003.

4. Perform MM5 sensitivity experiments to determine how the spatial scale and intensity of the land-surface forcing affects the development of ABL and lower-tropospheric structures on 10-15 primary study days. July, 2003 – June, 2004. Prepare journal publication July – Dec, 2004.

5. Perform MM5 dynamic-initialization experiments ingesting all available data to investigate how improved resolution of surface forcing and ABL structure affects the initiation and development of shallow and deep convection. These experiments will be done with and without the assimilation of DIAL and surface flux data to determine the value added by these data. Model results will be evaluated against satellite cloud data and NCEP Stage-IV hourly precipitation analyses. The convective-initiation experiments will be performed on a subset of the IHOP cases selected for the presence of convection. Jan. – Dec., 2003. Prepare journal publication July – Dec., 2004.

Roles:Dr. Davis will serve as overall project coordinator. He will help to coordinate airborne observations, lead the observational analyses, and supervise one graduate student.Dr. Stauffer will lead the modeling work, including supervising a Ph.D. student.Dr. Seaman will consult regarding implementation of the shallow cumulus scheme.Drs. Mecikalski and Diak will provide surface flux mapping using ALEXI, as well as evalution of ALEXI fluxes using tower and aircraft flux data.

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.

Figure 4: DIAL-derived H2O mixing ratio (Ismail et al, 1998) from the same case. The plot shows a repeat over the same flight track where the aircraft flew from north to south to north again, with the break in the middle being the far southern portion of the domain from Figure 2, and the region of highest ABL depth from Figure 3.

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3 4.53 53 5.5363 6.53 73 7.5380

5 00

1 00 0

1 50 0

2 00 0

2 50 0

L atitud e (N )

Hei

ght (

m)

O b s erv ed 4 km M M 5 C lim a tolo gica l 4 km M M 5 O ff lin e P L A C E 4 km M M 5 O ff lin e P L A C E +E S TA R

Figure 3: Observed (Desai et al, 2001) and modeled ABL depth (Reen et al, 2001) over the same region as shown in Figure 2 for the same day. ABL depths are derived from the NASA Langley DIAL

Figure 2: Map of Bowen Ratio for 12 July, 1997 derived from a combination of flux towers and remote sensing data (Desai et al, 2001). Domain is similar to the N-S dotted rectangle (proposed DIAL flight track) in Figure 1

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