A Global Precipitation Mission (GPM) validation network prototype

A Validation Network (VN) prototype is currently underway that compares data from the Tropical Rainfall Measuring Mission (TRMM) satellite Precipitation Radar (PR) to similar measurements from the U.S. national network of operational weather radars. This prototype is being conducted as part of the ground validation activities of the Global Precipitation Measurement (GPM) mission. The purpose of the VN is to provide a means for scientists in the precipitation community to identify and resolve significant discrepancies between the U.S. national network of ground radar observations and satellite observations. The ultimate goal of such comparisons is to understand and resolve the first order variability and bias of precipitation retrievals in different meteorological/hydrological regimes at large scales. The VN dataset consists of matched TRMM PR and quality-controlled, ground-based WSR-88D (Weather Surveillance Radar — 1988 Doppler, or NEXRAD) radar reflectivity, for 21 sites in the Southeastern U.S. located within a bounding rectangle extending from 24° to 35° N latitude and 80° to 98° W longitude. The current period of record for the VN data starts on August 8, 2006 and runs to the present.

A new event record is added to the VN database any time the TRMM PR ground track passes within 250 km of one of the VN sites. For each overpass event, a WSR-88D volume scan (Level II archive product) for the closest volume scan beginning at or prior to the satellite overpass is acquired. WSR-88D Level II data are processed to a standard TRMM 2A-55 gridded dataset, including automated and manual quality control. Coincident PR data are extracted from the standard TRMM 1C-21 and 2A-25 datasets, which include radar reflectivity (both raw and attenuation corrected, respectively) as well as other parameters. Because the input WSR-88D datasets for the VN match-ups are quality controlled by a human analyst there is a time lag of up to several weeks from observation to VN product generation. Both the original Level II and final quality-controlled WSR-88D products are retained in the VN dataset, along with all PR data for the portion of the orbit within the bounding rectangle.

VN grid data matchup products are generated from the VN database for cases where there is ≥25% confirmed rain in the overlap area and ≥25% percent areal overlap between coincident PR and WSR-88D grids, although these criteria are adjustable. Each match-up dataset contains coincident TRMM PR data and WSR-88D data, with both data types re-sampled to a common 3-dimensional Cartesian grid centered locally on the overpassed radar site. Each grid has a 4 km horizontal and 1.5 km vertical spacing, and extends 300 km by 300 km in the horizontal and from 1.5 to 19.5 km in the vertical. On average, about 515 coincident events with complete data are collected each month. Of these coincident events with data, about 22 per month meet the overlap and rain criteria. Data for events that meet these criteria are stored in netCDF files, one pair per event.

Reflectivity comparison statistics are routinely generated for each match-up dataset on a site-by-site basis. Tables of mean error, standard deviation, and bias are computed for each event for a selected vertical level. Graphics are also generated including histograms and scatter plots of reflectivity. Results are shown for pooled data. Results are also stratified between convective and stratiform rain, and are further stratified by whether the data lie over land, coast or ocean. Additional routine products such as probability density function and vertical reflectivity profiles are being developed; methods for addressing the temporal and spatial offsets between the satellite and ground radars are being evaluated. As the number of coincident precipitation events grows, it is expected that the statistical products will provide information on the reflectivity calibration accuracy and stability of the WSR-88D radars relative to the well-calibrated and stable TRMM PR.

The VN is being used as a tool for evaluating the current TRMM PR attenuation correction algorithm. It is expected that these investigations will be used as a prototype for validating the attenuation correction and precipitation retrieval algorithms of the Dual-frequency Precipitation Radar (DPR) in the GPM era. Although the VN currently supports 21 match-up sites, the network was designed to be scalable. There are plans underway to add more U.S. domestic and international validation sites in the near-term, and additional participants are welcome to join the network. Information about the match-up grid data files and statistical products, including documentation and how to gain access to them on a password-protected ftp site, can be found on the GPM ground validation web site located at this URL: http://gpm.gsfc.nasa.gov/groundvalidation.html.