Mysteries of the last minutes of rain drops: slopes of maximum radar reflectivity profiles below freezing level in precipitation systems

The maximum reflectivity profiles of precipitation systems are generated using 12 years of TRMM precipitation radar (PR) observations. The slopes of the maximum reflectivity profiles below the freezing level are found more frequently decreasing toward the ground in the precipitation systems over land than in those over ocean. This phenomenon is also found from ground radar observations over Tucson and Darwin. This study first demonstrates the geographical distributions and seasonal variations of the mean slopes of the maximum reflectivity profiles below 4 km using 12 years of TRMM data. Then the relations between the characteristics of the precipitation systems and the slopes are examined. The possible explanations to the decreasing radar reflectivity toward the ground from below the freezing level, including evaporation and radar attenuation, are discussed. A simple evaporation model is developed to help interpret the impact of the evaporation on the radar reflectivity value variations. The observations and model simulations show some discrepancies that are difficult to interpret without detailed microphysical information.