A validation network prototype matches-up TRMM Precipitation Radar (PR, at left) and quality-controlled NEXRAD reflectivity data (right) for 19 ground radar sites in the Southeastern United States. Match-up data sets are available starting in August 2006. Gridded data in netCDF format are available via the GPM ground validation password-protected ftp site in the /PR_GV_netCDF subdirectory. Check with the GPM GV Contacts if you need ftp access or additional information. The animation above illustrates radar reflectivity in 9 of the 13 vertical levels available in the 300 by 300 km PR and NEXRAD match-ups. In this case, the data range from 0 to 13.5 km in elevation in 1.5 km steps over the Miami, Florida site (KAMX) on August 30, 2006. A GPM Ground Validation prototype is currently underway to compare TRMM satellite data to similar measurements from the national network of operational weather radars. The goal of the Validation Network is to identify and resolve significant discrepancies between the US national network of ground radar observations and satellite observations. Conceptually, the Validation Network also extends to include targeted hydrometeorological assets external to continental US (e.g., Kwajalein and other national networks). 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 GPM Validation Network prototype is being implemented as a match-up dataset consisting of WSR-88D (a.k.a. NEXRAD) radar reflectivity and TRMM/ Precipitation Radar (PR) reflectivity. These comparisons build on research results published by Anagnostou et al. (2000), Bolen and Chandrasekar (2000) and by Liao et al. (2001). Validation Network data are available at the GPM password-protected ftp site. The username and password for this site are available on request from the GPM Ground Validation points-of-contact. A User's Guide is also available that describes the format and content TRMM Precipitation Radar and NEXRAD match-up dataset.

The Validation Network prototype is intended to exercise many of the functional and performance requirements defined in the draft Ground Validation System Level 3 Operations Concept and the Ground Validation System Level-3 Requirements. Future versions of the National MapValidation Network prototype will extend its capabilities to generate automated comparisons of NEXRAD and TRMM PR data, including checking the long-term calibration stability of the NEXRAD sites. Additional information on the Validation Network can be found in the preprint articles by Schwaller et al. (2007) and Morris et al. (2007). A User's Guide to the VN is also available. Other Direct Validation Frameworks The Coordination Group for Meteorological Satellites (CGMS) supports the International Precipitation Working Group (IPWG) as forum for operational and research users of satellite precipitation measurements. The goal of the IPWG is to exchange information on methods for measuring precipitation and the impact of space borne precipitation measurements in numerical weather and hydrometeorological prediction and climate studies. The IPWG web site has links to data, products, and tools of interest to precipitation ground validation. Location of the NEXRAD sites in the Validation Network prototype

The specific objectives of the prototype include:

• collection of NEXRAD data and associated TRMM PR overpass data for a 20-site subset of the 59 NEXRAD radars that are within view of the PR instrument. This 20-site subset falls within a bounding area of the continental US with latitudes <33^oN and longitudes >98^oW.
• transformation of the NEXRAD and TRMM PR data to a common Cartesian grid
• harvesting metadata from the NEXRAD and TRMM PR data, and storage of the metadata into a relational database
• archive of the raw and transformed NEXRAD and TRMM PR overpass data
• deployment of a simple browser interface for access to the archive data and match-up code
• completion of prototype lessons learned, particularly with regard to the manual quality checking of the data and other aspects related to scaling the National Map to the full suite of 169 NEXRAD radars during the GPM time-frame.

NEXRAD Data

The NEXRAD data used in the Validation Network prototype consists of TSDIS product 2A-55: gridded 3D NEXRAD reflectivity and rain type. This product is generated from NEXRAD Level II archive data which have been acquired, quality controlled and postprocessed by the TRMM GV project. The gridded 2A-55 and raw Level II NEXRAD data and related products are made available to the GPM Science Team for the Validation Network prototype by special arrangement with the TRMM GV team. The native 2A-55 product is gridded so that each grid element occupies a 2km x 2km horizontal and 1.5 km vertical volume. The product is interpolated to 4km horizontal resolution to correspond exactly to the PR re-sampled product so that they can be compared directly to one another. Each NEXRAD product contains data for the volume scan whose start time is nearest in time (within a threshold time range) and prior to the PR overpass of the site, stored in a single netCDF file. More details on the quality controlled NEXRAD products can be found in Wolff et al. (2005).

The National Weather Service/NOAA produces a national reflectivity mosaic image from NEXRAD at 10-minute intervals. These images are saved for the times of TRMM overpasses of the Validation Network area. These mosaics are simply for viewing purposes should a researcher desire to see the weather situation for a particular overpass case.

1. TRMM Precipitation Radar (PR) re-sampled product.

This product consists of PR satellite data and derived rainfall characteristics that have been re-sampled to a uniform grid. The grid is centered on the location of a NEXRAD radar, and the grid has a horizontal and vertical dimension that is matched to the useful data range of the corresponding NEXRAD. Data are saved in netCDF format. There is one PR re-sampled product generated each time the PR passes within a 100km radius of a NEXRAD radar.

The PR re-sampled product consists of:

• Raw PR radar reflectivity (Zr) from TSDIS product 1C-21
• Attenuation-Corrected PR radar reflectivity (Zc) from TSDIS product 2A-25
• Rain rate at the same levels as Zr and Zc.
• Rain type and surface rain rate and vertical level of the bright band from TSDIS product 2A-25
• PR rain characteristics (rain/no-rain) from TSDIS product 2A-25

Zr, Zc and rain rate are re-sampled to a uniform volume with the following characteristics:

• data are re-sampled to a uniform cube using a bilinear horizontal interpolation approach, with parallax correction and vertical averaging to 1.5km layers, with each 3D grid element occupying a 4km x 4km x 1.5 km high volume
• map projection for re-sampling is a simple Cartesian grid centered on the ground radar location, with the +y axis pointing north along the local meridian. It is a half-resolution (horizontal) subset of the 2km 2A-55 grid
• x-y-z dimension of the grid is 75x75km horizontal (300x300km), and 13 vertical (1.5-19.5km)

Rain type and rain/no-rain characteristics are re-sampled via a nearest-neighbor interpolation to the same x-y grid as the Zr and Zc data, but the rain type and rain/no-rain characteristics grids are each only 1 level deep in the vertical. All corresponding PR grids for one overpass of a given site are saved in the same netCDF file.

Examples of expected results from the Validation Network (from Liao et al., 2001)

References:

E.N. Anagnostou, C.A. Morales, T. Dinku. 2000. The Use of TRMM Precipitation radar observations in determining ground radar calibration biases. Journal of Atmospheric and Oceanic Technology 18:616-628.

S.M. Bolen and V. Chandrasekar. 2000. Quantitative cross validation of space-based and ground-based radar observations. Journal of Applied Meteorology 39:2071-2079.

L. Liao, R. Meneghini, T. Iguchi. 2001. Comparisons of rain rate and reflectivity factor derived from TRMM Precipitation Radar and the WSR-88D over the Melbourne, Florida, site. Journal of Atmospheric and Oceanic Technology 18:1959-1974.

Wolff D. B., D. A. Marks, E. Amitai, D. S. Silberstein, B. L. Fisher, A. Tokay, J. Wang, and J. L. Pippitt, 2005: Ground validation for the Tropical Rainfall Measuring Mission (TRMM). J. Atmos. Ocean. Technol. 22, 365-380.