A new technique for detecting precipitation at mid-latitudes during daytime using Meteosat Second Generation SEVIRI

A new method for the detection of precipitation during daytime using MSG SEVIRI data is proposed. The approach is not only applicable for the detection of mainly convectively induced precipitation by means of the commonly used relation between infrared cloud top temperature and rainfall intensity but enables also the detection of stratiform precipitation (e.g. in connection with mid-latitude frontal systems). The new technique is based on the conceptual model that precipitating cloud areas are characterized by a combination of particles large enough to fall, an adequate vertical extension (both represented by the cloud water path (cwp)), and the existence of ice particles in the upper part of the cloud. The technique considers the VIS0.6 and the NIR1.6 channel to gain information about the cloud water path. Additionally, the channel differences ΔT8.7-10.8 and ΔT10.8-12.1 are considered to supply information about the cloud phase. The information about the cloud water path and the cloud phase of the four variables is merged and incorporated into the new developed precipitation detection algorithm. The detection of the precipitating cloud areas is realized by using a minimum threshold for the pixel based rainfall confidence as a function of the respective value combination of the four variables VIS0.6, NIR1.6, ΔT8.7-10.8 and ΔT10.8-12.1. The calculation of the rainfall confidence is based on a comparison of the value combinations of these four variables with ground based radar data. The results of the algorithm were compared with corresponding ground based radar data not used for deriving the transfer function. The proposed technique performs better than existing optical retrieval techniques using only IR thresholds for cloud top temperature. The new developed algorithm shows encouraging performance concerning the detection of precipitation during daytime in the mid-latitudes using MSG SEVIRI data. Together with the existing precipitation detection algorithm during nighttime for MSG SEVIRI (Thies et al., 2008) the new algorithm offers the great potential for a 24 h technique for precipitation detection with a high spatial and temporal resolution.