Abstract:
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 (TB). An error in flagging during an active or recent precipitation event can either produce an overestimation or underestimation of soil moisture due to short-term surface wetting of vegetation and/or surface ponding. 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 analysis is conducted globally using IMERG precipitationCal (a multi-satellite estimate with gauge calibration) product. The elements of the contingency table and skill scores (hit rate, false alarm ratio, threat score, gilbert skill score) are evaluated for every 3-hr time step. The performance statistics (ubRMSE, RMSE, B, R) are computed for this scenario. This analysis is conducted for ascending and descending SMAP overpasses over 4 years (2015-2018) of data. Key results include: i) IMERG precipitation generally shows higher spatial variability than GEOS-5 precipitation, ii) the percentage of detecting very low rain intensities is slightly larger in IMERG, which is very likely related to the constellation of GPM satellite observations used in estimating the IMERG product, iii) the effect of incorporating IMERG rainfall measurements in lieu of GEOS-5 precipitation forecasts are minimal on the SMAP soil moisture retrieval accuracy.