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Sandra Yuter, North Carolina State University
In marine environments with low concentrations of CCN, the cloud lifetime hypothesis postulates in part that more rain causes decreased cloud fraction (all other factors being equal). Previous work has highlighted examples of the co-occurrence of open-cellular broken cloud with heavy drizzle. Several key examples in the literature of the observed co-occurrence of heavy drizzle and cloud breakup coincidentally occur near dawn. These previous case studies and LES modeling suggest that precipitation causes cloud breakup at night. The large shipboard radar data set collected during VOCALS-Rex, as well as 8 years of satellite-derived drizzle frequency and cloudiness transitions for the southeast Pacific indicate that scenarios where heavier or more widespread precipitation is associated with cloud break up at night are an exception rather than the rule.
While examples can be found for a wide variety of cloud fraction and precipitation conditions, the majority of nocturnal overcast conditions with precipitation present did not result in cloud break up. Cloud can also break up overnight without any strong drizzle in the vicinity. Precipitation is not a necessary condition for overnight cloud breakup in marine stratocumulus. During the night, the probability of cloud breakup is not clearly related to precipitation area or intensity. During the day, clouds with larger precipitation areas are less likely to change between overcast and broken conditions (less than full cloud coverage) than clouds with smaller precipitation areas.
Based on typical liquid water path and rain rates of the stronger drizzle cells, rain out should occur within ~20-30 min. However, the results of cell tracking for drizzle cells that persist at least 15 min indicate that stronger drizzle cells within marine stratocumulus are more likely to have larger areas and longer durations. The top 15% of tracked cells last longer than 42 min. A few cells persisted for over 90 min. Drizzle cells preferentially occur in relatively close proximities to each other (2-4 km), forming clusters of cells that may foster the recycling of water vapor from evaporated drizzle among cells.
Our large data set allowed us to subdivide the data by time of day and cloud condition. Contrary to precipitation in clean marine conditions reducing cloud fraction, we found that increased precipitation tends to maintain a given cloud fraction. We postulate that the diurnal cycle of solar radiation is the neglected common cause of co-varying cloud fraction and drizzle variations in marine stratocumulus.
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