Exploring precipitation processes in stratocumulus clouds from satellite-derived cloud properties
Date
2021
Authors
Murakami, Yasutaka, author
Kummerow, Christian D., advisor
van den Heever, Susan C., advisor
Chiu, Christine, committee member
Venkatachalam, Chandrasekaran, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
Marine stratocumulus clouds are low-level convective clouds that develop within the marine atmospheric boundary layer and have a large impact on the global radiation budget and hydrological cycle. Drizzle plays an important but complicated role in their longevity and microphysical properties. Many studies have examined the response of cloud base rain rate to varying cloud droplet number concentrations and cloud thickness, as well as liquid water path (LWP), and found that cloud base rain rates are enhanced with lower cloud droplet number concentrations and greater cloud thickness or LWP. In warm stratocumulus clouds, cloud base rain rate is a combination of raindrop embryo production through collision coalescence (i.e. autoconversion) and raindrop embryo growth by collecting cloud droplets (i.e. accretion). Previous studies have shown that cloud base rain rate depends on LWP or cloud thickness and the geographical location of stratocumulus clouds, but the dependence of the autoconversion process on these variables is not well known because cloud base rain rate represents the effects of both autoconversion and accretion. This two-part dissertation explores the dependence of stratocumulus cloud precipitation processes on cloud thickness and geographical location by examining the cloud properties retrieved by A-Train satellite observations from CloudSat's Cloud Profiling Radar (CPR), CALIPSO's Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) and Aqua's Moderate Resolution Imaging Spectroradiometer (MODIS). In the first part, the relations between cloud top properties (radar reflectivity, LWC and cloud droplet number concentration) and cloud geometrical thickness are investigated for subtropical stratocumulus clouds. Satellite-observations show that cloud top LWC and effective radius increase as clouds become thicker. The data also suggest that autoconversion may be more efficient in thicker clouds. These findings are consistent with previous studies that have shown that thicker clouds have larger cloud droplets and thus produce more rain embryos. However, it is also found that clouds separate into two sub-groups as they transition from thick (i.e. geometrical thickness of 384-480m) to very thick clouds (i.e. geometrical thickness of 624-720m). Drizzling clouds have higher LWC and their drops have larger effective radii, whereas non-drizzling clouds have lower cloud top LWC and smaller effective radii. In the second part, the climatology of satellite-derived cloud top properties (radar reflectivity, LWC and cloud droplet number concentration) for 8 stratocumulus cloud regions are presented. While LWP tends to be larger for midlatitude clouds, cloud top LWC tends to be larger at subtropical stratocumulus clouds. Since midlatit0ude stratocumulus clouds are thicker, these results suggests that effective condensation rates are larger for subtropical stratocumulus clouds. Both cloud top and cloud base radar reflectivity also tend to be larger for subtropical stratocumulus clouds. Based on these findings, the sensitivity of cloud top radar reflectivity on LWC and cloud droplet number concentration are examined. Cloud top radar reflectivity is more (less) sensitive to changes in LWC and cloud droplet number concentration for clouds with stronger (weaker) cloud top radar reflectivity. This is consistent with previous findings that collision-coalescence efficiency between liquid water droplets (i.e. approximately 20 μm in diameter) increases non-linearly with droplet size. The overall results presented in this dissertation indicate that the autoconversion process can be represented with a globally applicable function of cloud top LWC and cloud droplet number concentration for all stratocumulus clouds regardless of their geolocation and geometrical thickness. It is also demonstrated that cloud top raindrop embryo generation rate is an important factor for determining the precipitation generation rate for stratocumulus clouds as a whole. In general, accretional growth is controlled by both the total cross-sectional area of rain drops and LWP. By comparing spatial patterns of cloud top radar reflectivity (i.e. total cross-sectional area of rain drops) and radar reflectivity increase from cloud top to bottom (i.e. accretional growth), it is found that accretional growth depends more on total cross-sectional area of rain drops and less on LWP in stratocumulus clouds. These conclusions can explain the findings of previous studies that cloud base rain rate depends on LWP (or cloud thickness) and geographical location of stratocumulus clouds. Cloud base rain rate is dependent on geometrical thickness because cloud top LWC increases as cloud become thicker. Subtropical stratocumulus clouds tend to have stronger precipitation at a given LWP compared to midlatitude stratocumulus clouds because the effective condensation rate of subtropical stratocumulus clouds is greater and so is the cloud top LWC. In this study, the effect of Cloud Condensation Nuclei on warm rain processes is represented by varying cloud droplet number concentration. The results presented in this dissertation represent more than one hundred thousand independent pixels and provide a statistically robust benchmark that numerical models should reproduce.
Description
Rights Access
Subject
cloud base rain rate
droplets
clouds
raindrop embryo
cloud thickness