Satellite Radar Measurements of Precipitation and Freezing Level in Atmospheric Rivers

Atmospheric rivers (AR) account for more than 90% of the total meridional water vapor flux in mid latitudes and are the primary driver of precipitation in the western United States, among other locations. Here, reflectivity profiles from the Global Precipitation Measurement Mission’s Dual-Frequency Precipitation Radar (GPM DPR) are used to evaluate precipitation and temperature characteristics of ARs over the West Coast and Eastern Pacific Ocean. Initial validation of GPM DPR freezing level heights, based on comparisons of satellite-derived bright band elevation with a network of ground-based vertically pointing radars along the West Coast, demonstrated significant agreement. However, precipitation comparison with gridded observations in the western US identified deficiencies in GPM DPR’s ability to reproduce the spatial distribution of winter precipitation accumulation, related to sampling frequency. Despite challenges in directly comparing GPM DPR precipitation estimates with in-situ observations, it is shown that bulk precipitation statistics and freezing level elevation acquired from GPM DPR overpasses of AR events yield unique and useful information for process-based studies and model validation over the ocean. Here, nearly 70% of GPM DPR-measured precipitation in the eastern North Pacific Ocean was associated with AR features during the study period, and these observations further illuminate the proportions of convective (26%), warm-type stratiform (23%) and cold-type stratiform (51%) precipitation within the study domain. Additionally, brightband retrievals from GPM DPR are compared with freezing level height from reanalyses. These analyses demonstrate exceptional agreement between satellite and reanalysis freezing level height, despite considerable event-to-event variability, and their results suggest that GPM DPR brightband elevation may be used as independent data for evaluating freezing level variability and change in regional and global climate models over the ocean.