*2.3. P-IN-SEVIRI*

P-IN-SEVIRI is a precipitation product developed in the Satellite Application Facility on Support to Operational Hydrology and Water Management (H SAF) project [78], providing instantaneous precipitation rate at spatial and temporal SEVIRI resolution. It is provided by EUMETSAT, and it is based on an underlying collection of time and space overlapping overpasses from SEVIRI IR imagers and surface rain rate estimates (through the use of algorithms based on Low Earth Orbit-Passive Microwave (LEO PMW) radiometers), which constitutes a look up table of geo-located relationships between rain rate and TB at 10.8 μm, updated as soon as new overlapping SEVIRI IR and LEO PMW overpasses are available. The processing method is called "Rapid Update" (RU) blending technique [79].

As new input datasets (MW and IR) are available in the processing chain, the MW-derived rain rat (RR) pixels are paired with their time and space-coincident geostationary 10.8 μm IR TB data, using a 10-min maximum allowed time offset between the pixel acquisition times and a maximum space offset of 10 km between the pixel coordinates. Each co-located data increments the histograms of TB and RR within a latitude-longitude box 2.5◦ wide (i.e., a 2.5◦ × 2.5◦ box), as well as the eight surrounding boxes (this overlap ensures a fairly smooth transition in the histogram shape between neighboring boxes). The rationale behind these threshold values for time collocation and box size is discussed by [80].

In order to set-up a meaningful statistical ensemble, the method can look at older MW-IR slot intersections (no older than 24 h), until a certain (75%) box coverage is reached and a minimum number of coincident observations are gathered for a 2.5◦ × 2.5◦ region (at present 400 points, this is a tunable parameter in the procedure). Thus, the RU technique requires an initial start-up time period (~24 h), to allow for establishing meaningful, initial relationships all over the considered area.

As soon as a box is refreshed with new data, a probabilistic histogram matching relationship is updated using the MW RR and IR TB probability distribution functions (PDF), and an updated lookup table (histogram file) is created.
