Unexpected peak near -15°C in CloudSat echo top climatology

We examine a radar-derived climatology of Earth's cloudiness, focusing on the vertical distribution echo tops. Between the two main peaks at ∼2 km and ∼14 km associated with low and high clouds, respectively, lie two distinct midlevel peaks in the tropics. One is in the 5-6 km layer, the other between 7 and 8 km. Both are seen day and night, over land and sea, over each tropical ocean basin, and in published ground radar climatology. In the extratropics, the vertical echo top distribution by temperature reveals a strikingly sharp peak in the -15 to -20°C layer (corresponding to the 7-8 km peak in the tropics). Again the signal is robust (night-day, land-sea, summer-winter). We consider interpretations of the -15°C peak ranging from data artifacts (unlikely to mere radar brightening, to possible true microphysical or dynamical cloudiness enhancement mechanisms.