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P. Bauer
,
L. Schanz
, and
L. Roberti

Abstract

This paper presents a simple approach to adjust microwave brightness temperature distributions obtained from slant-path measurements for projection effects. Horizontal displacement in the direction of sight is caused by signal contributions from other than near-surface layers that are projected to the footpoint of observation. In particular at frequencies sensitive to ice particle scattering the horizontal projection effect can amount to values as big as the vertical cloud extent. Based on cloud model–generating, three-dimensional hydrometeor distributions at subsequent model time steps and a modified one-dimensional radiative transfer model, the high correlation of effective radiance contribution altitudes and brightness temperatures at 37.0 and 85.5 GHz is demonstrated. For these altitudes, described by the centers of gravity of the spectral weighting functions, regression equations are derived with standard errors below 0.61 km at 85.5 GHz and 0.22 km at 37.0 GHz for both the Special Sensor Microwave/Imager (SSM/I) and Tropical Rainfall Measurement Mission Microwave Imager. Once the centers of gravity are retrieved a simple geometry correction can be applied to the measurements.

Application to model cloud fields at various time steps and different oberservation geometries shows a significantly improved correspondence of brightness temperature and hydrometeor distributions. This method is also applied to SSM/I observations during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment in the equatorial Pacific. Considerable improvements of single-channel rain retrievals based on 85.5-GHz measurements compared to shipborne radar data were achieved, which suggests that a major uncertainty of so-called scattering algorithms can be explained by geometry effects that can be easily corrected. Multichannel algorithms, however, require a more elaborate integration scheme to allow for both frequency and scene-dependent adjustments.

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P. Bauer
,
L. Schanz
,
R. Bennartz
, and
P. Schlüssel

Abstract

The status of current rainfall-retrieval techniques by satellite radiometry has been evaluated by recent international algorithm intercomparison projects. As a general result, passive microwave techniques perform superiorly for instantaneous applications over oceans, while infrared or combined infrared–microwave techniques show improved monthly rainfall accumulations, mainly due to the high temporal sampling by geosynchronous observations. Merging microwave, visible, and infrared imagery data available on the same satellite such as the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) and the Visible Infrared Scanner (VIRS) provides further potential for the improvement of instantaneous retrievals. A case study is shown that demonstrates the stepwise degradation of information contained in the microwave signals when three-dimensional cloud effects and realistic antenna patterns are simulated for a convective cloud obtained from Doppler polarization radar soundings. Simultaneous visible and infrared data may contribute mainly to better rain-regime classification, in particular when sophisticated cloud identification techniques and cloud parameter retrievals are incorporated. Although the beam-filling problem is not solved by the TMI–VIRS combination alone, some other progress, for example, concerning better coastline treatment, is shown.

With respect to monthly products and the climatologically important observation of diurnal rainfall variations, the TRMM sensor combination will provide a calibration standard to be applied to geosynchronous sensors.

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