Mesoscale variability and drizzle in southeast Pacific stratocumulus
Large regions of subtropical stratocumulus clouds are responsible for significant net cooling in the earth's radiative budget. Stratocumulus are maintained by a complex interaction of processes on small scales (turbulent eddies) and mesoscales (10-100 km) that are not currently well represented in climate models. In northern-hemisphere stratocumulus, drizzle has been found to affect the boundary layer dynamics and cloud properties, but until recently, few observations had been obtained in the large, persistent southeast Pacific stratocumulus region.This study uses observations from the Eastern Pacific Investigation of Climate (EPIC) 2001 stratocumulus cruise in the southeast Pacific, including data from scanning and vertically-pointing radars. Relationships are derived between rain rates and radar reflectivity, and areal-averaged rain-rates are obtained from scanning-radar data. While substantial drizzle falls from the clouds in this region (0.2-2.3 mm day-1), about 70% of it evaporates before reaching the surface.Stratocumulus have significant diurnal modulations, with boundary layer mixing and cloud thickness decreasing during the afternoon and increasing at night. Drizzle is most frequently observed in the early morning when clouds are typically thickest. Sizable drizzle cells could last about two hours, suggesting that moisture from sub-cloud evaporated drizzle recirculates back into the cloud layer.Southeast Pacific stratocumulus typically exhibit closed-cellular patterns in cloud structure, although open-cellular patterns are also common. Closed cells are mesoscale cloudy regions surrounded by thin breaks. They are characterized by a well-mixed boundary layer and little or no precipitation (rain rates <∼1 mm day-1). During EPIC, open-cellular cloud structures, with large clear regions surrounded by smaller cloud patches, were associated with a greater frequency of high rain rates (> 1 mm day-1).Regardless of the overlying cloud patterns, drizzle cells have consistent vertical structures. They can be parameterized as overturning cells on 2-30 km horizontal scales, with convergence below cloud and divergence in the cloud layer.Regions of open-cellular cloud structure can appear within otherwise closed-cellular stratocumulus. Open-cellular regions frequently appear before sunrise, so drizzle is likely contributing to their formation. Because the cloud structure patterns impact the radiative properties of the stratocumulus sheet, future observations and modeling studies should further investigate their formation mechanisms.
- Atmospheric sciences