Imerg V07: Sharpen

One key change in the upcoming V07 algorithm of the Integrated Multi-satellitE Retrievals for GPM (IMERG), the U.S. GPM Science Teams gridded precipitation product, is the introduction of the Scheme for Histogram Adjustment of Ranked Precipitation Estimates in the Neighborhood (SHARPEN). SHARPEN is designed to reverse the distortion to the distribution of precipitation rates caused by averaging multiple precipitation fields in the Kalman filter step of IMERG. The distortion manifests as a shift in the distribution towards the lower intensities, resulting in an increased precipitation area and reduced occurrence of high intensities. This study will review SHARPEN as implemented in IMERG V07 with more details and examples of its behavior, with particular focus on the improvement in representing intense precipitation. More importantly, we will clarify that SHARPEN is not a downscaling or feature sharpening scheme, but rather sharpens a precipitation system by approximately reversing the blurring effects of averaging different input precipitation fields at the original 0.1° resolution. We will also discuss the implications of an increased challenge in interpreting the source of the estimates through the IMERG processing chain. With IMERG V07 scheduled for production at the start of 2022, this study will serve to emphasize the impending changes associated with SHARPEN and alert users to its potential improvements.