Quantitative Intercomparison of TRMM PR and KPOL Radar Observations

S. Bolen and V. Chandrasekar Department of Electrical Engineering, Colorado State University

Simultaneous space and ground-based measurements were collected from the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) and the WSR-93S radar located at Kwajalein Island - also referred to as the KPOL radar. Data was collected from five different time periods over nine different spatial areas.

Direct inter-comparisons between space and ground based samples can be a daunting task. Differences in viewing aspects between space and earth point observations, propagation frequencies, resolution volume size and time synchronization mismatch between space and ground-based measurements can contribute, in a significant way, to direct point-by-point inter-comparison errors. This problem is further exasperated by geometric distortions induced upon the space-based observations caused by the movements and attitude perturbations of the spacecraft itself.

To minimize errors, ground and PR data were collected during simultaneous intervals not exceeding 2-3 minutes in time difference. Both sets of data were re-mapped to a PR-centered Cartesian coordinate system, using a non spherical earth model (WGS-84 model), with the origin located at the intersection of the TRMM ground track and scan that passes through the center of the storm area located within a 50 x 50 km window - as depicted in the example shown in Figure 1.

The horizontal images of KPOL data for each of the study cases are shown in Figures 2-6. The radar images are taken at the 2 km horizontal altitude and show the TRMM track overlay, PR swath path, scan line and 50 x 50 km study regions with KPOL location at the center of the image.

Ground and PR data were interpolated to 0.5 km grids in the horizontal and 0.25 km in the vertical at altitudes ranging from 0.5 to 15.0 km. The (x,y) coordinates of each PR beam are then found and each beam is linearly averaged at the specified (x,y) coordinate in the vertical and horizontal directions. Vertical limits are taken as the maximum vertical resolution between either the PR or ground radar vertical resolution at that point calculated according to the resolution geometry. Thus, a three dimensional volume, with dimensions in the horizontal plane equal to the PR horizontal resolution and height equal to the resolution of the PR or ground radar at that point, is constructed at each PR (x,y) beam location. Similarly, ground radar reflectivity is also linearly averaged in the same three-dimensional volume. This ensures that observations are taken from similar co-located volumes. The geometry is illustrated in Figure 7.

After volume matching, the average reflectivity (in linear scale) vs. altitude was determined for each of the nine different regions and the plots are shown in Figures 8-16. The difference between PR and KPOL was determined from the plots at altitude and reflectivity levels were it was assumed that there was insignificant attenuation in PR measurements. (PR reflectivity version 4 data was used in the comparison). A summary of the difference between PR and KPOL observations is shown in Figure 17.