Temperature, Relative

Humidity and Wind

Measurements in Clouds

Linnea Avallone

Phil Brown

Martina Krämer

Outline

For Winds, Temperature & Relative Humidity:

• Overview of needs/issues

• Review of existing instrumentation, both

operational and “research-quality”

• Detailed measurement requirements

Measurements of Winds

• Needs:

– Measurements of winds on small scales to

assess entrainment, vertical velocities; can be

critical to supersaturation, particle growth

• Issues:

– Accurate measurements on necessary scales

are difficult with existing sensors

– Wetting of sensor ports

Operational Instrumentation for Winds

• Typically a combination of INS/GPS and 5-

port pressure measurements

Operational Instrumentation for Winds

Platform Device Accuracy Response time

NASA ER-2 Litton 92 INS,

Litton 2001 GPS,

Rosemount p

Not reported Not reported

NSF C-130 IRS/GPS/radom

e pressure

± 0.2 m/s Not reported

NSF G-V Radome

pressure ports

(Mensor 6100)

± 0.1 m/s 0.05 s

FAAM BAe 146 Radome

pressure port &

Applanix GPS-

aided INS

± 0.05 m/s Not reported

Research Instrumentation for Winds

• INS coupled with

radome pressure

system (ParoScientific)

– P accuracy = 0.3 mb

– Angular measurement

is critical: wind

accuracy of ± 1 m/s at

TAS = 200 m/s requires

± 0.3°

– Vertical wind precision

is ± 0.003 m/s

NASA Ames Meteorological Measurement System (MMS)

Gust Probes

• Commonly used for flux and turbulence

measurements (20 – 100 Hz response)

– Typically 5-hole pressure port system

mounted on “sting” ahead of aircraft to avoid

flow distortion caused by nose and fuselage

Gust Probes

Supplier Product Accuracy Response Time

Airborne Research

Australia (ARA)

BAT probe (9-

port)

Precision of

0.04 m/s

50 Hz

Aventech, Inc. AIMMS-20 0.75 m/s 20 Hz

BAT probe

AIMMS-20

Issues for Wind Measurements

• Are there cloud effects on sensors?

• How well do we actually need to know

winds? Are the existing instruments

adequate in terms of accuracy and

precision?

• New developments?

Cloud Effects on Wind Measurements

These seem to be limited to wetting and/or

icing of pitot ports, rendering them

ineffective

How well do we need to know winds?

• Focus on vertical winds

– Can be very small (0.01 m/s) in quiescent

regions

– Lenschow et al. (1999) argue need to

measure to better than 0.03 m/s in most

cases to study entrainment

• Most current operational sensors do not achieve

this, even under ideal conditions

• Ames MMS has precision; accuracy not clear

• Turbulence/gust probes generally not accurate

enough

Wind Measurement Summary

• Current operational wind instrumentation

is generally not adequate for measuring

vertical velocities accurately. Precision

may be sufficient.

• Water-clearing technologies would help in

cloud (reverse air-flow)

• Gust probes work well on small scales, but

performance degrades with increased air

speed

Measurements of Temperature

• Needs:

– Accurate measurements for interpretation of microphysical processes, entrainment – few tenths ºC

• Issues:

– Icing/wetting of thermometric sensors causes underestimates of T

– Radiometric sensors have pathlength problems

Operational Instrumentation for Temperature

Thermometric (immersion) sensors are usually total

air temperature probes – need good TAS

measurements to obtain ambient air temperature

Operational Instrumentation for Temperature

Platform Device(s) Accuracy Response Time

NASA DC-8 Rosemount 102

AH2AG

± 1 °C Not reported

NSF C-130 Rosemount

102E2AL

De-iced Rosemount

102E

± 0.5 °C

± 1 °C

Not reported

NSF G-V Rosemount 102AL

De-iced HARCO

100990-1

± 0.5 °C

± 1 °C

0.02 s

WY King Air Reverse-flow inlet

with Minco element

± 0.5 °C Not reported

FAAM BAe-146 Iced/de-iced

Rosemount 102

± 0.3 °C 1 s

Research Instrumentation for Temperature

Radiometric sensors use the absorption

features of CO2 (e.g. 4.25 µm) to determine

air temperature

Issues for Temperature Measurements

• Thermometric sensors are affected by

wetting and/or icing

• Radiometric sensors have varying

pathlength and may be sensitive to aircraft

motions, presence of liquid water in path

• How well do we need to know T? Are

current instruments capable of providing

this?

Issues for Thermometric Sensors

Affected by wetting and/or icing– Negative biases, typically 1-2 °C, but instances as

large as 10 °C seen

– Even de-iced sensors and reverse-flow inlets can

accumulate water

– Wetting in Rosemount probes is not necessarily

complete, so cannot be easily corrected

Eastin et al. 2002

Issues for Radiometric Sensors

• At 4.25 µm, 90% of signal comes from within 10

m of aircraft; at 15 µm, within 200 m

– Geometry is problematic for looking at fine-scale

structure (entrainment) – signal from 20-100 m

• At 15 µm, there is absorption by liquid water

• Data can be difficult to interpret when plane

banks because signal can come from surface

Thermometric vs. Radiometric

Temperature

Clear Air In-cloud

Lawson & Cooper, 1990

Ophir

Rosem

ount

Reference = Reverse Flow

How well do we need to know T?

• For buoyancy and

convection studies,

~0.3 K or better

• For microphysical

studies, about 0.5 K

Wang & Sassen, 2002

ΔT =1 K

2-7%

1-4%

1-3%

Temperature Measurement Summary

• Operational temperature measurements are

accurate enough for most needs

• All immersion sensors are problematic in cloud –

wetting of sensors causes errors in T of ~ 1 °C.

There is no resolution to this problem.

• Radiometric sensors better, but have pathlength

issues that affect small-scale measurements

• Research-quality temperature measurements

are adequately calibrated for most needs

Measurements of Relative Humidity

• Needs:

– Accurate measurements of RH (or Td or H2Ov) to

understand extent of saturation

• Issues:

– Td measurements can be skewed by wetting of

sensors

– Direct measurements of H2Ov can be altered by

evaporation of cloud particles within instrument

– Calculations of RH affected by errors in T/Td/H2Ov

Operational Instrumentation for RH/Td

Standard instrumentation is chilled mirror

dewpoint/frostpoint hygrometer or humicap sensor

Operational Instrumentation for RH/Td

Platform Device(s) Accuracy Response Time

NASA DC-8 GE 1011C ± 0.1 °C 1 °C/s above -60°C

NSF C-130, G-V Buck Research

1011C

Lyman-α hygrometer

± 0.5 °C,Td > 0 °C

else ± 1 °C

5%

0.2 – 10 sec

WY King Air Licor 6262 NDIR ± 1% Not reported

FAAM BAe-146 GE 1011B ± 0.5 °C,Td > 0 °C

else ± 1 °C

2 °C/s

DLR Falcon GE 1011B w/reverse

flow inlet

Humicap (Vaisala

HMP230)

See above

± 1-3% RH 15 sec

Research Instrumentation for RH/Td

Technique Examples Accuracy Response Time

Lyman-alpha

photofragment

Harvard WV, FISH,

FLASH

6-10% 1 sec

Chilled mirror NOAA CFH 0.5 K (Td/f) [5%]

Varies with water

Tunable diode laser

spectroscopy

JLH, DLH, OJSTER 5 % 0.1 - 1 sec

JLH on DC-8 CFH

Lyman-α

Issues for RH/Td/H2Ov Measurements

• Chilled mirror sensors are common but not

necessarily accurate for all uses

• Many research instruments suffer from

artifacts related to particle evaporation

• How well do we need to know RH/Td? Are

current instruments accurate enough?

Issues with Chilled Mirror Sensors

• Errors of 1-2 °C above

saturation in wet environments

– Inlets that inertially separate

particles from gas help, but do not

completely eliminate problem

• Ambiguity of water phase

(frost/liquid) at temperatures

between 0 and -30 °C

• Slow response time at low Td/f

Issues with Research Hygrometers

Many water vapor

instruments have

internal sampling,

making them

susceptible to errors

from evaporation of

ingested particles Example: Harvard water vapor (HWV)

• Subisokinetic flow (150 – 200 m/s

decelerated to ~ 80 m/s) leads to

heating of air by at least 10 °C

Measuring Water Accurately

Blind intercomparison held at

AIDA chamber, Karlsruhe

Core instruments were full

participants

Referenced to AIDA TDL

Differences among

instruments for WV > 10 ppm

are ± 5-10%

Supersaturation and Measurement

Uncertainties

Figure from T. Peter

Uncertainties

from AquaVit

whitebook

Conversion of Td/f to RH

Formulations for vapor

pressure over liquid water

Formulations for vapor pressure

over ice

Figures from H. Voemel

Uncertainties in RH

RH = e/es = [f(Td) or f(H2Ov)]/f(T)

Errors in:

Td: > 1 °C

T ~ 0.5 - 1 °C

H2Ov ~ 10%

Conversion of T/Td to es/e ~ 2%

Error in RHi is a few percent at -40 °C,

growing larger at lower temperatures

RH/Td Measurement Summary

• Uncertainties in direct measurements of Td

result from wetting of sensor and phase

ambiguities at critical temperatures

• H2Ov measurements have reasonable level of

accuracy for determining RH at higher Td/[H2O],

but are not adequate at low T and low H2O

• Algorithms for conversion of Td to RH contribute

some error, but less than that of measurements