The Role of Cloud Size and Environmental Moisture in Shallow Cumulus Precipitation

Abstract

Cloud models show that precipitation is more likely to occur in larger shallow clouds and/or in an environment with more moisture, in part as a result of decreasing the impacts of entrainment mixing on the updrafts. However, the role of cloud size in shallow cloud precipitation onset from global satellite observations has mostly been examined with precipitation proxies from imagers and has not been systematically examined in active sensors, primarily because of sensitivity limitations of previous spaceborne active instruments. Here we use the more sensitive CloudSat/CALIPSO observations to identify and characterize the properties of individual contiguous shallow cumulus cloud objects. The objects are conditionally sampled by cloud-top height to determine the changes in precipitation likelihood with increasing cloud size and column water vapor. On average, raining shallow cumulus clouds are typically taller by a factor of 2 and have a greater horizontal extent than their nonraining counterparts. Results show that for a fixed cloud-top height the likelihood of precipitation increases with increasing cloud size and generally follows a double power-law distribution. This suggests that the smallest cloud objects are able to grow freely within the boundary layer but the largest cloud objects are limited by environmental moisture. This is supported by our results showing that, for a fixed cloud-top height and cloud size, the precipitation likelihood also increases as environmental moisture increases. These results are consistent with the hypothesis that larger clouds occurring in a wetter environment may be better able to protect their updrafts from entrainment effects, increasing their chances of raining.

Environmental effects on aerosol–cloud interaction in non-precipitating marine boundary layer (MBL) clouds over the eastern North Atlantic

“…A higher LTS magnitude represents a relatively stable environment and is favorable to the formation of marine stratocumulus (Medeiros and Stevens, 2011;Gryspeerdt et al, 2016). Note that the median LTS of 19.1 K in this study is close to the separation threshold of 18.55 K suggested by prior studies to distinguish the marine stratocumulus from a global assessment of marine shallow cumulus clouds (Smalley and Rapp, 2020). Therefore, leveraging the demarcation line at 19.1 K may allow us to investigate the aerosol-cloud relationships under contrasting thermodynamic regimes.…”

The relationship between precipitation and its spatial pattern in the trades observed during EUREC4A

“…That dry and moist scenes differ predominantly in the number of cells they contain, whereas the mean area of the cells only varies weakly with $$$ W $$$, was also found in radar observations (Louf et al ., 2019) and simulations (Brueck et al ., 2020) of deep convection. In a moist environment, clouds may be less affected by entrainment, which allows them to reach deeper and eventually start to precipitate (Smalley and Rapp, 2020). Also, clouds, and hence precipitating cells, may live longer in moister environments.…”

The relationship between precipitation and its spatial pattern in the trades observed during EUREC4A

“…That dry and moist scenes differ predominantly in the number of cells they contain, whereas the mean area of the cells only varies weakly with W, was also found in radar observations (Louf et al, 2019) and simulations (Brueck et al, 2020) of deep convection. In a moist environment, clouds may be less affected by entrainment, which allows them to reach deeper and eventually start to precipitate (Smalley and Rapp, 2020). Also, clouds, and hence precipitating cells, may live longer in moister environments.…”