Rainfall intensity estimation by ground-based dual-frequency microwave radiometers

Many investigators have used satellite data to derive rainfall intensity an d to compare them with rain gauge data. However, there has always been a pr oblem: what is the optimal time period for the two different types of data? A set of well-controlled data collected by ground-based dual-frequency mic rowave radiometers at the National Central University (24.9 degreesN, 121.1 degreesE) in Taiwan between January of 1996 and December of 1997 was used to find the answer. The results show that a 1-h interval would be the optim al time period and that hourly data will provide a better accuracy than oth er options (5, 10, or 30 min or 2 h). Two algorithms, the differential and the brightness temperature, were established to estimate rainfall intensity using ground-based dual-frequency microwave brightness temperature and rai n gauge data. The results show that the root-mean-square error and the corr elation coefficient are 0.63 mm h(-1) and 0.88, respectively, for the diffe rential method, and 0.91 mm h(-1) and 0.71 for the brightness temperature m ethod. The analysis also shows that because the atmospheric background and environmental influence in the continuous observations are identical, the c hanges in brightness temperature are only caused from the changes in liquid water content in the air. That probably made the differential method a bet ter choice for rainfall intensity estimation than the brightness temperatur e method. Moreover, ground-based radiometers measure downwelling radiation from bottom up, and little ice-particle scattering or horizontal inhomogene ity is involved. The results can be compared with retrievals from satellite microwave radiometers for a better understanding of the physics of microwa ve emission and scattering due to raindrops or ice particles.