On the relation between satellite observed liquid water path, cloud droplet number concentration and cloud base rain rate and its implication for the auto-conversion rate in stratocumulus clouds
Date
2020
Authors
Murakami, Yasutaka, author
Kummerow, Christian D., advisor
van den Heever, Susan C., advisor
Venkatachalam, Chandrasekaran, committee member
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Abstract
Stratocumulus clouds are low-level convective clouds that develop within the atmospheric boundary layer. Their persistence and broad coverage of the earth's surface produces important impacts on the global radiation energy budget and hydrological cycle. Precipitation processes of these stratocumulus clouds play a large role in their longevity and spatial distribution through their interaction with the vertical profiles of humidity and temperature within the atmospheric boundary layer. This has led to a number of field campaigns to understand the precipitation processes of stratocumulus clouds. However, because of the limited field campaign domains and limited amount of these observations, it is difficult to draw statistically significant conclusions on the precipitation processes of global stratocumulus clouds from these data. In this study, space-borne observations from A-Train satellites are utilized to obtain robust relations among the liquid water path, cloud droplet number concentration and cloud base rain rate for three geographical regions with similar large-scale environments, namely the north east Pacific off the coast of California, the south east Pacific off the coast of Peru and the south east Atlantic off the coast of Namibia, where strong subsidence flow from the subtropical-high is observed. Radar reflectivity from CloudSat's Cloud Profiling Radar (CPR) is employed to estimate the cloud base rain rate (Rcb). Liquid water path (LWP) and cloud droplet number concentration (Nd) are estimated from Moderate Resolution Imaging Spectroradiometer (MODIS) cloud optical thickness and effective radius. The relation between cloud base rain rate (Rcb) and the ratio of liquid water path to cloud droplet number concentrations (LWP/Nd) are obtained from a large number of A-train observations that show similar probability density distribution for all three target areas in this study. Rcb has a positive correlation with LWP/Nd and the increase in Rcb becomes larger as LWP/Nd increases, which is consistent with the results from previous ground-based observations. The research presented here also shows that the increase of Rcb in respect to LWP/Nd become more gradual in larger Nd regions, which suggests that the relation between Rcb and LWP/Nd changes with different cloud droplet number concentrations. These findings are consistent with our theoretical understanding of cloud physics processes in that 1) auto-conversion and accretion growth of rain embryos becomes more effective as cloud droplet number concentrations near cloud top decrease, and 2) auto-conversion is suppressed when the cloud droplet radius is small enough. The sensitivity of the auto-conversion rate to cloud droplet number concentration is investigated by examining pixels with small LWP in which the accretion process is assumed to have little influence on Rcb. The upper limit of the dependency of auto-conversion on the cloud droplet number concentration is assessed from the relation between cloud base rain rate and cloud top droplet number concentration since the sensitivity is exaggerated by the accretion process. The upper limit of the sensitivity of auto-conversion found in this study was found to be a cloud droplet number concentration to the power of -1.44±0.12. This study demonstrates that satellite observations are capable of detecting the average manner in which precipitation processes are modulated by the liquid water path and drop number concentrations.
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Subject
cloudphysics
stratocumulus
satellite observation
auto-conversion