Evaluation of Precipitation Flag for SMAP Soil Moisture Accuracy using IMERG and GOES-5

The precipitation flag in the Soil Moisture Active Passive (SMAP) Level 2 passive soil moisture retrieval algorithm is used to indicate the presence or absence of precipitation at the time of the SMAP overpass. The quality of the precipitation flag can adversely bias soil moisture retrieval due to its large impact on SMAP brightness temperatures (T_B). An error in flagging during an active or recent precipitation event can either produce an overestimation or underestimation of soil moisture as the radiometer's sensing depth is likely to be different because of varying wetness conditions. Currently, the SMAP precipitation flag is set based on forecasts of precipitation from NASA's Global Modeling and Assimilation Office's (GMAO) GEOS-5 model. However, the precipitation estimates from numerical weather prediction (NWP) models are only as good as the physical models and assimilated data inputs.

NASA's Global Precipitation Measurement (GPM) mission provides an opportunity to evaluate the GEOS-5-based precipitation flags used in SMAP soil moisture retrievals. The Integrated Multi-satellite Retrievals for GPM (IMERG) version 5 "early" product (latency of 4 hours) are compared to SMAP precipitation flags derived from the GMAO GEOS-5 model. The IMERG "early" product provides passive microwave (PMW) and infrared (IR) satellite precipitation estimates, which are remapped to match the SMAP EASE-2 grid. The evaluation is conducted using merged PMW and IR, PMW only, and IR only precipitation estimates. The differences between the precipitation estimates from IMERG and GOES-5 products are quantified and used to evaluate their effect on the accuracy of the SMAP retrieved soil moisture.