On the statistical relationship between cloud optical and microphysical characteristics and rainfall intensity for convective storms over the Mediterranean

Abstract. The relationship between the multi-spectral cloud field characterization from the Advanced Very High Resolution Radiometer (AVHRR) and the rainfall intensities from the Advanced Microwave Sounding Unit-module B (AMSU-B) data were studied for a convective storm event, which occurred during the first 15 days of June 2007 over the Mediterranean. The cloud products exploited in this analysis, cloud mask, type, optical thickness, and effective radius, are obtained from the NOAA-NESDIS operational processing system Clouds from the AVHRR-Extended algorithm (CLAVR-x), whereas the rain intensity values are retrieved from the AMSU-B brightness temperatures via a fast algorithm, using opaque frequencies (centred at 183 GHz) to correct for the presence of water vapour affecting the retrieval results. The algorithm is conceived to discriminate between convective and stratiform rain using a suitable set of thresholds; the retrieval is subsequently carried out separately for the two types.

A test for the discrimination of precipitating from non-precipitating areas was based on the comparison between the precipitation information and the retrieved cloud parameters. The test produced a cloud optical thickness threshold value, beyond which the precipitation initiates, and an effective radius range for the identification of the precipitating clouds. The results stemming from the application of the test to the June 2007 case study are very encouraging, although still preliminary and restricted to the analyzed Mediterranean storms. In particular, the test shows high potential for delineating non-precipitating areas (more than 90% of successful cases for every considered cloud type) and to identify precipitating ice clouds related to convective rain (confirmed by 82% of hits). On the other hand, the relative inability to address the stratiform cloud systems is proved by the fact that the majority of the missed cases, for each cloud types, is characterized by light rain intensities (≤3 mm h⁻¹).