The AMSR2 Surface Rain Rate (Day) layer indicates the rate at which rain is falling on the surface of the ocean, and is measured in millimeters per hour (mm/hr). Whereas precipitation includes both the liquid (rain) and solid (snow) forms, this layer only includes rain, the liquid form of precipitation. Precipitation is formed when water vapor condenses and becomes heavy enough to fall under the force of gravity. The characterization of precipitation over the ocean is important as, unlike on land, there are limited ground/surface stations that measure precipitation in the vast expanses of ocean. The surface precipitation layer is useful at different time scales. At a daily time scale, you are able to determine the diurnal cycle of precipitation. Precipitation usually falls in the early morning, local time in the ocean. At longer time scales, you are able to assess the change in regional patterns due to monsoon seasons and global patterns due to the El Niño-Southern Oscillation (ENSO).

The Advanced Microwave Scanning Radiometer 2 (AMSR2) instrument is a conically scanning passive microwave radiometer sensing microwave radiation at 12 channels and 6 frequencies ranging from 6.9 to 89 GHz on board the Global Change Observation Mission – Water 1 (GCOM-W1) satellite. This layer is part of the GCOM-W1 AMSR2 Level-2B rain and ocean (combined) product, which is calculated by the Goddard PROFiling algorithm (GPROF) 2010 version 2 using resampled brightness temperature (Level-1R) data provided by the Japanese Aerospace Exploration Agency (JAXA). The imagery resolution is 2 km and sensor resolution is 5 km. The temporal resolution is daily.

References: NRT AMSR2 L2B Global Swath GSFC Profiling Algorithm 2010; Advanced Microwave Scanning Radiometer for EOS (AMSR-E) and GCOM-W (AMSR2) Rainfall Products; AMSR-E/Aqua L2B Global Swath Surface Precipitation GSFC Profiling Algorithm, Version 3