The Front Range Pilot Project for GPM: An instrument and concept test

In the summer of 2004, demonstration of the supersite ground validation (GV) concept for the Global Precipitation Measurement (GPM) mission was carried out in the form of a Front Range Pilot Project (FRPP), involving scientists from CSU and NOAA's Environmental Technology Laboratory and Aeronomy Laboratory. The project's specific aims included evaluation of dual-wavelength polarimetric radar observations for estimating rainfall rates in moderate to heavy rainfall, as well as developing errors associated with these techniques by comparison to ground-based measurements from gauges and disdrometers. To accomplish this component, the project used the S-Band CSU-CHILL radar at Greeley, CO and NOAA-ETL's X-Band radar at Erie, CO. The X-Band radar's improved phase sensitivity in light rain was also investigated to determine the application of polarimetric-based techniques for mapping light rain (1-5 mm hr-1). The pilot project also focused on selection of UHF profiler frequencies that would best complement S-Band profiler measurements to allow for accurate retrieval of drop size distribution characteristics. A further goal of the pilot project was to perform quantitative comparisons of drop size distribution (DSD) characteristics between the profilers and scanning radars in order to evaluate assumptions in the scanning radar retrieval technique (e.g., equilibrium drop shape relationship) as well as spatial variability of the DSD. We also looked to demonstrate the complimentary role played by rain gauges and surface disdrometers (both 2-D video and J-W types) in determining the error characteristics of multi-frequency profiler DSD estimates and dual-frequency radar DSD and rain estimates. We will present selected case studies that; compare rain rates from the S- and X-band scanning radars (over a broad range of rain rates, including some cases where hail was present); demonstrate the utility of a 449 MHz profiler combined with an S-band profiler for retrieving DSD's; and compare the retrieval of Dm (mass-weighted mean diameter) and Do (median volume diameter) from the scanning radars to surface based disdrometer measurements. Based on these results, we also make recommendations for the experimental design at the GPM ground validation supersites.