An Evaluation of IMERG Orographic Precipitation during the 2017-18 Cool Season

Wintertime orographic precipitation in the western United States is a vital component for the region’s annual water resources. Providing accurate precipitation measurements in elevated terrain is crucial when making political and societal decisions regarding water use in the semi-arid west. Remote sensing techniques, like the Integrated Multi-satellite Retrievals for GPM (IMERG) product, offer advantages for precipitation monitoring that ground-based radar and observations systems cannot provide in complex terrain. The purpose of this study was to begin evaluating geographic and seasonal consistency between IMERG, the High Resolution Rapid Refresh (HRRR) numerical weather model, and the Parameter-elevation Relationships on Independent Slopes Model (PRISM) orographic precipitation datasets during the cool season (Oct - April). IMERG was found to struggle in the northern Rocky Mountains and with light precipitation events characterized by low snow rates. Seasonally, a decreased correlation between the HRRR and IMERG datasets was noticed as the cool season progressed into the deep winter months (JFM). This is most likely due to IMERG’s precipitation algorithm having difficulty when distinguishing between surface snowpack in the mountains from new, light snowfall events falling on top of the current snowpack. A case study was then used to identify to what degree brightness temperatures from the GMI Microwave Imager could be utilized in greater detail to outline potential precipitation areas during the deep winter months for IMERG orographic precipitation improvement.