Susanne Crewell 1 & MICAM Team 2 1 Meteorologisches Institut Universität Bonn

1. First Results of Microwave Radiometer Intercomparison Campaign (MICAM) • 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..

2. Setup and Objectives of MICAM 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

3. Overview of MICAM Frequencies U. Bern, SwitzerlandChalmers U., SwedenCETP Velizy, FranceUK MetofficeU. Bonn, GermanyGerman Weather ServiceGerman Weather ServiceInst. Radioeng., Russia Leipzig, May 14 2002

4. 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 Leipzig, May 14 2002

5. Impressions from MICAM IRE WVR MICCY TROWARA 20 m MARSS Conrad Drakkar

6. Observation Schedule Zenith observation during day Leipzig, May 14 2002

7. MICAM WWW Site • 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 Leipzig, May 14 2002

8. Time Series of Brightness Temperatures wet radiometer gives questionable measurements short integration time and high beam resolution give highest LWP values Rain Rate mm/h Leipzig, May 14 2002

9. Comparison of 10 Minute Means Comparison has to be limited to cloud free periods Leipzig, May 14 2002

10. Comparison of 10 Minute Means Leipzig, May 14 2002

11. Direct comparison of Brightness Temperatures • zenith observation • closest match in time (<36 s) • homogeneous atmosphere (<1 K) • Bias = 1.1 K • RMS = 0.5 K • Correlation = 0.99 Leipzig, May 14 2002

12. Comparison with Radiative Transfer center of H2O line cloud sensitive frequency Leipzig, May 14 2002

13. Comparison with Radiative Transfer all cloudfree casesN=16 10 min means past launch partly cloudy Scences removed slope of regression line is significantly < 1 for many channels  description of water vapor line absorption and continuum might have opposite bias Leipzig, May 14 2002

14. 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 Leipzig, May 14 2002

15. 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 Leipzig, May 14 2002

16. Retrieval Accuracy • LWP is derived from perfect brightness temperature (TB) measurements  ill-determined problem • Error characteristics of TB are difficult to defineabsolute uncertainty (1 K) is lower than relative (~0.2 K) • Retrievals rely on accuracy of radiative transfer; Uncertainties: - gas absorption (e.g. water vapor)- refractive index of water (e.g. <0C [Westwater et al., 2001]) • Influence of statistical assumptions in algorithm (e.g. LWC) Microwave Intercomparison Campaign (MICAM) EGS, Nice, April 25, 2002

17. 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