Why does maximum radar reflectivity tend to increase downward toward the ocean surface, but tend to decrease downward toward the land surface?

The maximum reflectivity profiles of precipitation systems are generated using 13 years of TRMM precipitation radar (PR) observations. The maximum reflectivity below the freezing level usually decreases toward the ground in precipitation features over land, but increases toward the surface over the ocean. Because the TRMM PR algorithms are Z-R-based, we are motivated to understand this observational fact. We review evidence from surface-based C-band and S-band radar profiles, which are not subject to the attenuation correction issues as are the TRMM PR profiles, and we are able to conclude that these differences between land and ocean are genuine and not artifacts. This study first demonstrates the geographical distributions and seasonal variations of the mean slopes of the maximum reflectivity profiles below 4 km using 13 years of TRMM data. Then the relations between the characteristics of the precipitation systems and the slopes are examined. A simple evaporation model is developed to help interpret the profiles, and initial indications are that it fails to explain the observations adequately over land, and does nothing to explain the observations over water. Some alternative hypotheses are presented that involve the relative importance of warm rain vs. rain from melted graupel, and the relative importance of weak vs. strong updrafts in the low troposphere. The observations and model simulations show some discrepancies that are difficult to interpret without detailed microphysical information.