Rain height variability in the Tropics

Most prediction methods relating to propagation in rain and other hydrometeors take into consideration the height of the melting layer in order to separate the effects of rain from ice. Here, we present the annual variability of this height, from five years´ of data from the precipitation radar on board the Tropical Rainfall Measurement Mission (TRMM) satellite. The variability is derived for latitudes ranging from -30° to +30°, and compared with the corresponding freezing heights derived from the ECMWF reanalysis data. The two sets of heights show a strong variation with latitude, with the tropics showing the narrowest distribution. Their differences also show a latitude dependence, and indicate that the melting layer thickness is more variable for the subtropics. The implications are examined with reference to hydrometeor scatter interference. The recommended global standard prediction procedure for this mode of interference is evaluated using the variable distribution of the melting layer heights and their effects are examined in terms of the overall interference signal levels at 11 GHz and 30 GHz. When compared with the results using the fixed distribution specified in the current recommendation, small but significant differences at the 0.01 time percentages are observed, particularly for the 30 GHz case. The evaluation of an enhanced version of the procedure shows that significant differences can be observed even at 11 GHz, thus demonstrating the need to use a latitude-dependent rain height variability for the full evaluation of rain scatter interference levels.