A rain rate product for AltiKa

Bruno PICARD (CLS, France)


Marie-Laure Fréry (CLS, FRANCE); Mathilde Siméon (CLS, France); Estelle Obligis (CLS, France); Nathalie Steunou (CNES, France); Nicolas Picot (CNES, France)

Event: 2014 SARAL/AltiKa workshop

Session: Instrument processing

Presentation type: Type Poster

The AltiKa/SARAL mission is a complement of the Jason altimeter series. A two-channels (23.8 GHz and 37 GHz) microwave radiometer (MWR) is combined to the altimeter in order to correct the altimeter range for the excess path delay (dh) resulting from the presence of water vapour in the troposphere.

The impact of the rain on altimeter measurements is of importance for SARAL/AltiKa since the 37 GHz Ka band used by the altimeter is more sensitive to rain events that the 13.5 GHz Ku band used by previous missions. In order to help for the study of a given rain rate on the waveform parameters, a rain rate product is estimated from the two brightness temperatures (TB) and the altimeter backscattering coefficient (sigma0).

First, a "combined" rain rate product is built from observations provided by instruments dedicated to the atmosphere monitoring. Three different sources have been used: SSMIS (Special Sensor Microwave Imager/Sounder) F16 and F17 under the Defense Meteorological Satellite Program (DMSP) and Windsat radiometer on Coriolis. For each AltiKa measurements, the closest observation in time and space amongst the three external sources is recorded, together with the time lag (Delta-t_RR) between the altimeter and the external observation date.

In order to take into account potential collocation issues, a "false alarm" flag is raised, based on AltiKa radiometer cloud liquid water content (CLWC), when the combined rain rate is positive but CLWC is null. A cleaned (w.r.t. this flag) learning dataset can be built, relating the two TBs and altimeter sigma0 to the "combined" rain rate.

A sensitivity study is performed using datasets built with all the "combined" rain rate records available or with a limit for the acceptable Delta-t_RR (1h, 30min, 10min, 5min).
Comparisons to SSMI monthly rain rates show the very good agreement with the AltiKa dedicated rain rate with a better correlation than the "combined" product without any collocation issues.