Despite the truism that precipitation comes from clouds, modern observation systems and global models have traditionally separated these two parameters. Recently, however, data from NASA’s A-Train satellite constellation have allowed scientists to begin utilizing global observations to systematically study the characteristics of clouds producing precipitation.
Now Stephens et al. have harnessed this technology to examine the nature of weather systems occurring between 30°N and 30°S latitude. The team combined CloudSat precipitation and cloud observations from 2007 to 2010 with Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) lidar and Global Precipitation Measurement data to analyze the relationship between precipitation and the heights of the tops of the cloud columns, which they term “cloud top height,” as well as the elevations of what the authors interpret to be the tops of the rain-bearing clouds (“rain top height”).
The results indicate that about 6% of the observed tropical weather systems produce rain and that most of the rain that falls from these systems comes from relatively “shallow,” or low-elevation, rain clouds, about 40% of which are located beneath higher cloud layers. The team also discovered that shallow precipitation, despite being widespread above the tropical oceans, is almost completely absent above land at these latitudes.
According to the authors, distinguishing between cloud top and rain top heights in their analysis has significant scientific implications: Since observation systems can’t peer below upper cloud layers, these tools significantly overestimate the height of the rain-producing clouds throughout the tropics. Jointly capturing the characteristics of clouds and precipitation in this region is also crucial for estimating their effects on diabatic heating, an exchange of energy that is fundamental to numerous atmospheric processes, including air circulation, the organization of storms, and alterations in the hydrologic cycle. (Journal Geophysical Research: Atmospheres, https://doi.org/10.1029/2018JD029394, 2018)
—Terri Cook, Freelance Writer