Determination of the Calibration Factor for the Rain Rate Derived From TRMM Satellite Using MRR Data over Akure, Nigeria

The ground based radar calibration of a space borne radar is an important means of testing the reliability of Tropical Rain Measuring Mission- Precipitation Radar (TRMM- PR) derived radar reflectivity factor (Z) and rain rate (R) values over a location. For this purpose, a well-calibrated ground based Micro Rain Radar (MRR) located at Department of Physics, The Federal University of Technology, Akure (Lat 5.3⁰E, Long 7.3⁰N) in south-western Nigeria was used to validate data from TRMM satellite data. The measurements of the vertical profile of reflectivity factor and rain rate were carried out using TRMM-PR satellite radar and a MRR simultaneously in 2008 and 2010 during the raining season. The data collected from 49 different range gates covering heights from 250 m -1225 m in 250 m steps from TRMM-PR and 30 different range gates covering height from 0 – 4800 m in 160 m steps for MRR were compared. From the two years data used, 28 days (overpasses) of significant rainfall at two different heights 1.5 km and 4.0 km for the two days of most significant rainfall were selected for analysis. The data analysis at the  1.5 km and 4.0 km height and for 28 days of overpasses show good correlation ranging from the maximum 0.99 to  the least 0.53 at 4 km and 0.99  to the least 0.59 at 1.5 km. The  ‘b’  parameter value of the power law  Z = aR^b  empirically derived  from MRR over Akure were used to calibrate TRMM-PR values for rain rate estimate, the mean difference of  radar reflectivity factor at heights 4.0 km and 1.5 km were 2.4 dB and 2.5dB respectively. The results also show that the radar reflectivity (dBZ) derived from the PR data after attenuation correction agrees to within 2.0 dB when compared with that of the MRR with a relative variation of ±0.9 dB at different heights.