Temperature, Relative Humidity (Water vapor) and Wind
Transcript of Temperature, Relative Humidity (Water vapor) and Wind
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Temperature, Relative
Humidity and Wind
Measurements in Clouds
Linnea Avallone
Phil Brown
Martina Krämer
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Outline
For Winds, Temperature & Relative Humidity:
• Overview of needs/issues
• Review of existing instrumentation, both
operational and “research-quality”
• Detailed measurement requirements
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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
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Operational Instrumentation for Winds
• Typically a combination of INS/GPS and 5-
port pressure measurements
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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
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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)
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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
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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
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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?
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Cloud Effects on Wind Measurements
These seem to be limited to wetting and/or
icing of pitot ports, rendering them
ineffective
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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
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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
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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
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Operational Instrumentation for Temperature
Thermometric (immersion) sensors are usually total
air temperature probes – need good TAS
measurements to obtain ambient air temperature
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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
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Research Instrumentation for Temperature
Radiometric sensors use the absorption
features of CO2 (e.g. 4.25 µm) to determine
air temperature
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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?
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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
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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
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Thermometric vs. Radiometric
Temperature
Clear Air In-cloud
Lawson & Cooper, 1990
Ophir
Rosem
ount
Reference = Reverse Flow
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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%
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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
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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
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Operational Instrumentation for RH/Td
Standard instrumentation is chilled mirror
dewpoint/frostpoint hygrometer or humicap sensor
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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
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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
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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?
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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
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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
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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%
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Supersaturation and Measurement
Uncertainties
Figure from T. Peter
Uncertainties
from AquaVit
whitebook
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Conversion of Td/f to RH
Formulations for vapor
pressure over liquid water
Formulations for vapor pressure
over ice
Figures from H. Voemel
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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
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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