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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)

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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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Impressions from MICAM

MICCY

Conrad

MARSS

WVR

IRE

TROWARA

Drakkar

20 m

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Observation Schedule

Leipzig, May 14 2002

Zenith observation during day

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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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Comparison of 10 Minute Means

Leipzig, May 14 2002

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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