Analysis of Precipitation Processes in Clouds Interacting with Complex Terrain during the OLYMPEX Field Campaign

Currently the Global Precipitation Measurement (GPM) Mission is measuring precipitation over a large fraction of the globe, including the mid- and high-latitudes, using both passive and active microwave sensors. Challenges are met in the mid- and high-latitudes with the accurate measurement of light rain and snowfall. Ground validation campaigns were launched in order to verify and improve GPM’s retrieved measurements, one of which focused on the mountainous Olympic Peninsula on the Pacific Coast of Washington State where orography poses great challenges to precipitation measurement. This study takes a radar-based approach to examining precipitation processes in the clouds interacting with the Olympic Mountains during the Olympic Mountain Experiment (OLYMPEX). Primarily using NASA’s S-band radar, NPOL, for its polarimetric variables, the evolution of the vertical hydrometeor profile is analyzed as clouds and precipitation approach the coast from the ocean and interact with the topography. Within these columns, ice water path (IWP) and liquid water path (LWP) are calculated and then compared to the detected ice water path by GPM’s Microwave Imager (GMI), a radiometer aboard GPM’s Core Observatory. Because ice scattering is the dominant radiometric signature used by the GMI for estimating precipitation over land masses, the purpose of this comparison is to examine the conservation of ice mass in relation to rainwater mass and how it might impact radiometer and radar-based estimations of precipitation from GPM. The overall goal is to use the collective data set obtained during OLYMPEX to relate the precipitation process occurring in a column to the actual precipitation measured at the ground and the measurements made from space during GPM Core Observatory overpasses to answer the following scientific questions: 1) How does the precipitation process impact uncertainties in space-based estimates of rain and snow? 2) Is there a conservation of ice mass and liquid water mass in a particular column in the cloud that can be used to track precipitation phase change during an interaction with orography? 3) Can the selected cases be categorized by their synoptic regime (i.e. extratropical cyclone warm sector, frontal, post-frontal) in order to systematically adjust GPM precipitation retrieval algorithms on a regime-specific basis?