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COST 723 UTLS Summerschool
Cargese, Corsica, Oct. 3-15, 2005
Stefan A. Buehler
Institute of Environmental Physics
University of Bremen
www.sat.uni-bremen.de
OBS 13: Measuring Upper Tropospheric
Humidity with Operational
Microwave Satellite Sensors
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Overview
Water vapor in the Earths radiation balance
Operational meteorological microwave satellite instruments (AMSU-B)
AMSU-B measurements of upper tropospheric water vapor
Comparison with radiosonde measurements
Temperature uncertainty and supersaturation
The radiative transfer model ARTS
Summary
Cited papers can be found at
http://www.sat.uni-bremen.de
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Overview
Water vapor in the Earths radiation balance
Operational meteorological microwave satellite instruments (AMSU-B)
AMSU-B measurements of upper tropospheric water vapor
Comparison with radiosonde measurements
Temperature uncertainty and supersaturation
The radiative transfer model ARTS
Summary
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Earths Radiation Balance
Outgoing Longwave Radiation OLR
Incoming Shortwave
RadiationSun
Earth
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Earths Radiation Balance
Wavelength [μm]
λEλ
[nor
mal
ized
]
(Wallace und Hobbs, `Atmospheric Science', Academic Press, 1977.)
Radiative equilibrium temperature: -18°C
Global mean surface temperature: +15°C
34 K natural greenhouse effect
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Clear-Sky OLR Spectrum
A lot of the radiation comes from the UT
Water vapor and CO2 are the most important greenhouse gases
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Jacobians[10-14 W Hz-1 sr-1 m-2]
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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[10-14 W Hz-1 sr-1 m-2][10-14 W Hz-1 sr-1 m-2]
[10-14 W Hz-1 sr-1 m-2][10-14 W Hz-1 sr-1 m-2]
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Important Altitude Range
OLR is sensitive to changes of humidity in the upper troposphere, where it is difficult to measure.
Sensitivity peak below TTL.
[10-14 W Hz-1 sr-1 m-2]
MLS
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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15% change in humidity = double CO2 - (for a tropical atmosphere)
H2O is a stronger greenhouse gas than CO2
Higher surface temperature = more evaporation positive feedback.
(Buehler et al., JQSRT, submitted 2005)
Impact on Tropical OLR
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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The Water Vapor Feedback
Convection and cyclones transport moisture into the UT (see lectures of Heini Wernli and Andrew Gettelman)
Ascending air is dried by condensation processes
High spatial and temporal variability
Residence time of water substance ~10 days
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Variability of Clear-Sky OLR
Paradox: More humidity = more OLR!
Sim
ulat
ed O
LR [
W/m
2]
(Buehler et al., Q. J. R. Meteorol. Soc., submitted 2005)
Total water vapor [mm]
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Variability of Clear-Sky OLR
High temperature correlated with high humidity
Positive temperature signal outweighs negative humidity signal
Expected, otherwise runaway greenhouse effect
Water vapor signal strongest in the tropics
Simulated radiances agree with CERES OLR data
(Buehler et al., Q. J. R. Meteorol. Soc., submitted 2005)
Radiosondes
CERES Data
Sim
ulat
ed O
LR [
W/m
2]
Surface temperature [K]
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Variability of Clear-Sky OLR
(Buehler et al., Q. J. R. Meteorol. Soc., submitted 2005)
No strong temperature variations in the tropics
Temperature and Water Vapor variations are both important for clear-sky OLR
Radiosondes
CERES Data
Sim
ulat
ed O
LR [
W/m
2]
Del
ta O
LR [
W/m
2]
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Climate GCMs indicate that the feedback is positive.
A large part (about half) of the warming predicted by models for a CO2 rise is due to the water vapor feedback (Held and Soden, Annu. Rev. Energy Environ., 2000).
The UT is an important altitude region for this feedback, but humidity there is poorly known.
Radiosonde measurements:
Low spatial coverage
Poor data quality in the UT
Infrared satellite measurements:
Good global coverage, but affected by clouds
Clear sky bias
Microwave satellite measurements (today)
Radio occultation (Friday, OBS 16)
Ice clouds play also an important role in the UT radiation balance (Friday, OBS 15)
Comparison: Radiosondes ↔ Infrared Satellite Data
Big differences between the different data sets, for example:
+/-15 %RH difference between IR satellite and radiosonde
= 40% relative difference in humidity, as RH values are low in the UT.
(Soden and Lanzante, JGR 1996)
Problem: Large discrepancies, true climatology unknown (see e.g. SPARC UTLS H2O Assessment)
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Overview
Water vapor in the Earths radiation balance
Operational meteorological microwave satellite instruments (AMSU-B)
AMSU-B measurements of upper tropospheric water vapor
Comparison with radiosonde measurements
Temperature uncertainty and supersaturation
The radiative transfer model ARTS
Summary
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Microwave Satellite Data
SSM-T2 since 1995
AMSU-B since 1999
Passive microwave instruments (measuring thermal radiation from the atmosphere)
Less affected by cloud than infrared
Well calibrated
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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AMSU-B
Cross-track scanner
90 pixels per scan line
Outermost pixels 49° off-nadir
Swath with ≈ 2300 km
Global coverage twice daily
16 km horizontal resolution (at nadir)
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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AMSU-B Channels
(Details:John and Buehler,GRL, 31, L21108, doi:10.1029/2004GL021214)
Water vapor
Oxygen
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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AMSU-B Channels Water vapor
Oxygen(Figure by Viju O. John)
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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AMSU-B Jacobians
ARTS Simulation,
Atmosphere: Midlatitude-Summer
20 19 18 19 20
(Figure by Viju O. John)
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Jacobians depend on Atmospheric State
(Figures by Viju O. John)
Measurement not in TTL, but below
Altitude where OLR is very sensitive to H2O changes
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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AMSU-B Data (Channel 18)
Dry areas in the UT
(NOAA 16, Channel 18,
15.6.2004.
Figure: Oliver Lemke)
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Overview
Water vapor in the Earths radiation balance
Operational meteorological microwave satellite instruments (AMSU-B)
AMSU-B measurements of upper tropospheric water vapor
Comparison with radiosonde measurements
Temperature uncertainty and supersaturation
The radiative transfer model ARTS
Summary
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Retrieving humidity usually requires a priori, problematic for climate applications
Humidity Assimilation can destroy information on absolute value due to the bias corrections applied (compare lecture by Francois Bouttier)
Solution: Look for a humidity product that is related as closely as possible to the radiances
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Method originally invented by Brian Soden for IR data.
UTH = Jacobian-weighted relative humidity ≈ mean relative humidity between 500 and 200 hPa
Simple relation:
ln(UTH) = a + b Tb
Determine a and b by linear regression with training data set
Details: Buehler and John, JGR, 2004
Regression UTH Retrieval
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Coefficients independent of training data set
Basically another unit for radiance
Other humidity data must be processed in same way for comparison
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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AMSU UTH-Climatology
(AMSU-B, Channel 18, NOAA 15, Winter 1999-2000. Figure by Mashrab Kuvatov)
With deep apologies to Mark Baldwin for the weird color scale...
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Walker Circulation during La Nina
NOAA 15
AMSU-B UTH
DJF 99-00
500-200 hPa
HALOE, 82 hPa
Gettelman et al, 2001, J. Clim
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Comparison with an Infrared ClimatologyInfrared UTH (1981-1991), Soden and Bretherton, JGR, 101 (D5), 9333-9343, 1996
Microwave UTH (AMSU-B, NOAA 16, 2002),
Mashrab Kuvatov
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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(Figures by Mashrab Kuvatov)
UTH, AMSU-B, Channel 18, NOAA 16, 2002
Difference with and without cloud filter
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Overview
Water vapor in the Earths radiation balance
Operational meteorological microwave satellite instruments (AMSU-B)
AMSU-B measurements of upper tropospheric water vapor
Comparison with radiosonde measurements
Temperature uncertainty and supersaturation
The radiative transfer model ARTS
Summary
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Case Study for one selected Station
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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(Figure by Viju O. John)
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Finding Matches
Define target area (radius 50 km)
Compare mean satellite value to radiosonde
Take standard deviation σ50km as measure of sampling error
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Large variability in σ50km
Lowest values consistent with nominal radiometric noise
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Sources of error:
Radiometric noise of the AMSU measurement
Sampling error due to atmospheric inhomogeneity
Radiosonde measurement error in humidity and temperature
RT model error
AMSU calibration error
χ2 tests show that C0 can be taken as a global constant with a value of 0.5K.
Error Model
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Results
(Buehler et al., JGR, 109, D13103, doi:10.1029/2004JD004605, 2004)
Non-unity radiance slope
Possible reasons:
RT model
AMSU
Radiosonde
Increasing radiosonde dry bias under very dry conditions
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Comparison for a Different Sensor
Kem, Russia (64N, 34E)
Goldbeater’s skin type sondes
very large wet bias (expected from Soden and Lanzante, JGR, 1996 )
(Figure by Viju O. John)
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Comparing different Radiosonde Stations
40 European stations
Sonde data from BADC
(John and Buehler, ACP, 2005)
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Comparing different Radiosonde Stations
General dry bias (expected for Vaisala sensor)
Apparently erratic jumps can be understood by sensor and/or procedure changes for individual stations
Information about stations not readily available
Mystery: UK stations have less dry bias, although the are supposed to use similar sensors
See also poster by T. Suortti
(John and Buehler, ACP, 2005)
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Overview
Water vapor in the Earths radiation balance
Operational meteorological microwave satellite instruments (AMSU-B)
AMSU-B measurements of upper tropospheric water vapor
Comparison with radiosonde measurements
Temperature uncertainty and supersaturation
The radiative transfer model ARTS
Summary
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Supersaturation in UARS-MLS Data
exponential drop-off
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Gaussian T distr. Non-Gauss RH distr.
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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MLS Data Effect of 2K T uncertainty
Some of the observed supersaturation can be due to temperature uncertainties (Buehler and Courcoux, GRL, 2003).
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Overview
Water vapor in the Earths radiation balance
Operational meteorological microwave satellite instruments (AMSU-B)
AMSU-B measurements of upper tropospheric water vapor
Comparison with radiosonde measurements
Temperature uncertainty and supersaturation
The radiative transfer model ARTS
Summary
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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RTTOV
Fast RT model
Freely available from Eumetsat NWP SAF
Already configured for most meteorological sensors
Biases compared to more accurate ARTS model (see Poster by Nathalie Courcoux)
Not used for the calculations in this lecture
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Public Domain Program, developed together with Chalmers University, Göteborg and University of Edinburgh.
Two branches:
ARTS-1-0: Clear-sky
ARTS-1-1: with cloud scattering
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Radiative Transfer
RT Workshop 2004
RT Workshop 2005
Core Developers (2005)
Development and workshops since 1999.
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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ARTS Overview
Freely available: http://www.sat.uni-bremen.de/arts/
Clear-sky (arts-1-0):
Line spectra (HITRAN, JPL, GEISA)
Continua (H2O, N2, O2, CO2)
Trivial RT
Analytical Jacobians
Cloudy-sky (arts-1-1):
Two different algorithms for cloud scattering:Monte Carlo (MC) method
Discrete Ordinate Iterative (DOIT) method
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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ARTS Properties
All viewing geometries
Spherical
Polarized (up to 4 Stokes components)
Validated against various other physical RT models from microwave to infrared
Used as a reference to judge performance of RTTOV-8 (with scattering) within the NWP SAF
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Overview
Water vapor in the Earths radiation balance
Operational meteorological microwave satellite instruments (AMSU-B)
AMSU-B measurements of upper tropospheric water vapor
Comparison with radiosonde measurements
Temperature uncertainty and supersaturation
The radiative transfer model ARTS
Summary
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Summary
Upper tropospheric humidity (UTH) is an important parameter of the climate system.
Better absolute measurements of the global UTH distribution are needed.
Operational microwave sensors provide a new dataset that is independent of the IR satellite data.
Advantage: Less affected by clouds.
Disadvantage: So far short time series (since 1995 SSM T2, since 1999 AMSU-B).
RT model required for work with satellite measurements.
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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How to Compare Satellite Data to other Data
Find out which part of the satellite data is believed to be ok (check documentation and talk to others)
Need enough matches to get statistics (a single in-situ measurement is useless for satellite validation)
Set up error model, including sampling error (without error bars the comparison has no quantitative meaning)
Comparison in radiance space can avoid problems due to use of a priori information for satellite retrieval (you can scale back the radiance differences to uncertainties in geophysical parameters at the end)
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Outlook
Very promising UT humidity data is now becoming available from MLS on Aura.
Proposals for radio-occultation humidity measurements (my last lecture on Friday)
Clouds play also a crucial role for the radiation balance (my first lecture on Friday)
Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Thanks for your attention.Questions?
...