A Case Comparison Between the TRMM Data and Airborne Radar Data in Rainfall Retrieval

Precipitation radar (PR) on the Tropical Rainfall Measuring Mission (TRMM) is the first spaceborne instrument capable of producing three dimensional structure of rainfall. Information about each individual tropical cyclone (TC) can also be provided by airborne radar. Both airborne and spaceborne radars have a lot in common and their observing data are comparable. But it's difficult to get proper cases with good coincidence to perform significant comparison. Only hurricane Bonnie is selected as a study case. On August 26, 1998, a flight is made by NOAA/P3 plane at 11:30 along the east coast of America, and a close observation on hurricane Bonnie is made by the dual beam airborne radar placed on it. Almost at the same time the TC core is fully observed by TRMM orbits.Before comparing with TRMM, P3 radar data need to be processed. The measured reflectivity factor (Z-field) of P3 radar is corrected for path attenuation using the hybrid dual beam/stereoradar algorithm. The absolute error in the radar calibration is also corrected. The typical sampling time of a hurricane by the airborne P3 radar is about one hour, while it is one or two minutes for the TRMM PR. Therefore, the nearly instantaneous rain pattern observed by the PR may evolve and move during the airborne radar sampling. Such an effect is reduced by restricting the region for comparisons to a domain where the maximum time lag between the two samplings do not exceeding 600 s. Also, the rain pattern is corrected for advection using a model of horizontal tangential speed (V_t) of individual rain cell versus the radial distance to the eye center. Considering different cross range resolution, TRMM is interpolated on the P3 grid.Then a serial of comparisons is made between P3 and TRMM data. Comparisons of vertical cross section of TRMM and P3 corrected reflectivities are given, and the data agree with each other well within the comparison domain. Outside this area, a large part of the hurricane where the time lag between the two samplings exceeds 600 s can't be used for significant comparisons. Point to point comparis ons of all rain, stratiform rain and convective rain show that most of the data points are along the diagonal and have a standard deviation of 3.9 dBz. The associated histogram of differences ΔZ is sharply peaked, the mean difference is 0.7 dBz with a standard deviation of 6.1 dBz. The mean vertical rainfall density profiles retrieved from the P3 and TRMM show they agree with each other very well above 5 km and have a little difference within 2—5 km. All the comparison results show that these data are in good agreement.By comparing the data of P3 and TRMM in hurricane Bonnie, the TRMM results a re validated, so is the hybrid algorithm and also the understanding of TC precipitation field is improved.