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Susanne Crewell1 & MICAM Team2
1Meteorologisches InstitutUniversität Bonn
2 Laurent Chardenal (CETP), Gunnar Elgered (Chalmers),
Catherine Gaffard (Metoffice), Jürgen Güldner (DWD),
Boris Kutuza (IRE Moskva),
Lorenz Martin (IAP Bern) etc..
First Results of Microwave Radiometer Intercomparison
Campaign (MICAM)
2
Setup eight microwave radiometer with very different design August 1-14, 2001 in Cabauw, The Netherlands time series observation at different observation angles 34 radio soundings (Metoffice)
Objectives estimate accuracy of brightness temperature measurements assess quality of CLIWA-NET CNN measurements assess quality of derived liquid water path (LWP) and integrated
water vapor (IWV) and influence of instrument specifications set constraints on gas absorption at microwave frequencies optimze low-cost microwave radiometer design
Leipzig, May 14 2002
Setup and Objectives of MICAM
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U. Bern, Switzerland
Chalmers U., Sweden
CETP Velizy, France
UK Metoffice
U. Bonn, Germany
German Weather Service
German Weather Service
Inst. Radioeng., Russia
Leipzig, May 14 2002
Overview of MICAM Frequencies
4 Leipzig, May 14 2002
Radiometer Specifications
Instrument Integration Beam Elevation Time /s Width /Angle /
Conrad 3 2.2 - 3.1 0-180
DRAKKAR 1 11 - 13.3 fixed at 90
MARSS 0.11 10 every 10 deg
MICCY 1 0.4 - 0.9 0-90
STPE 6 ~10 fixed at 40
THPR 1; t=420 2.2 - 6.1 0-90
TROWARA 30 4.6 - 4.7 0-90
WVRA 60 4.6 - 5.5 0-90
Azimuth orientation: West
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MICAM WWW Site
Leipzig, May 14 2002
http:/cliwaftp.meteo.uni-bonn.de/CLIWANET/MICAM/
time series of brightness temperatures (TB) in original resolution
- similiar frequency channels are shown together- each observation period is shown separately
time series of TB averaged to 10 min mean values
differences between radiometers averaged over observation periods
calculated and measured TB for each radiosounding for all frequencies
- temperature, humidity and calculated liquid water content
Six hourly plots of derived IWV and LWP from all radiometer
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Rain
Rate
mm/h
wet radiometer gives questionable measurements
short integration time and high beam resolution give highest LWP values
Time Series of Brightness Temperatures
Leipzig, May 14 2002
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Comparison of 10 Minute Means
Leipzig, May 14 2002
Comparison has to be limited to cloud free periods
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Direct comparison of Brightness Temperatures
Leipzig, May 14 2002
zenith observation
closest match in time (<36 s)
homogeneous atmosphere (<1 K)
Bias = 1.1 K
RMS = 0.5 K
Correlation = 0.99
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center of H2O line
cloud sensitive frequency
Leipzig, May 14 2002
Comparison with Radiative Transfer
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all cloudfree casesN=16
10 min means past launch
partly cloudyScencesremoved
Comparison with Radiative Transfer
Leipzig, May 14 2002
slope of regression line is significantly < 1 for many channels
description of water vapor line absorption and continuum might have opposite bias
14 Leipzig, May 14 2002
Conclusions and Implication on Retrieval
Conclusions relative accuraccy is much higher than absolute! DRAKKAR needs recalibration (at 23.8 GHz) calibration of Russian radiometer shouldn‘t be trusted discrepancies between radiometer are as high as uncertainties in
radiative transfer modelling
gas absorption (water vapor/continuum) needs clarification
Implications on Retrieval agreement between microwave profilers and radiative transfer is
good along oxygen absorption complex (temperature profiling) discrepancies at typical LWP/IWV frequencies are about 1-2 K
(as assumed in the retrieval algorithm development) bias in water vapor profiles due to uncertainty at line center
15 Leipzig, May 14 2002
Future work
Implications on low-cost microwave radiometer
appropriate rain detection and protection neccesary reference absolute calibration load
Future work investigate differences at 90 GHz (cloud sensitive) analyse raw data and skydip calibration procedure perform radiative transfer calculations with MONORTM investigate spectral behaviour of TB differences
analyse influence of instrument specifications on time series
characteristics
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Retrieval Accuracy
EGS, Nice, April 25, 2002
LWP is derived from perfect brightness temperature (TB) measurements ill-determined problem
Retrievals rely on accuracy of radiative transfer; Uncertainties: - gas absorption (e.g. water vapor)- refractive index of water (e.g. <0C [Westwater et al., 2001])
Error characteristics of TB are difficult to defineabsolute uncertainty (1 K) is lower than relative (~0.2 K)
Microwave Intercomparison Campaign (MICAM)
Influence of statistical assumptions in algorithm (e.g. LWC)
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Conclusions / Outlook
uncertainty of gas absorption at microwave frequencies about as high as differences between different radiometers
LWP=20-40 gm-2
laboratory measurements needed for further improvement
evaluation of LWP in cloud free conditions (Ceilometer/IR)
synergetic algorithms (TBs, ceilometer, IR radiometer) to improve estimates of zero and low LWP cases
Brightness temperature simulations from atmospheric model output
low-cost microwave radiometer with improved precipitation detection/protection system and reference calibration
EGS, Nice, April 25, 2002