Stratocumulus cloud top height estimates and their climatic implications

A depth-dependent boundary layer lapse rate was empirically deduced from 156 radiosondes released during sixmonth-long research cruises to the southeast Pacific sampling a variety of stratocumulus conditions. The lapse-rate dependence on boundary layer height is weak, decreasing from a best fit of 7.6 to 7.2 K km21 as the boundary layer deepens from 800m to 2 km. Ship-based cloud-base heights up to 800m correspondwell to lifting condensation levels, indicating well-mixed conditions, with cloud bases.800m often 200–600m higher than the lifting condensation levels. The lapse rates were combined with Moderate Resolution Imaging Spectrometer 11-mm-derived cloud-top temperatures and satellite microwave-derived sea surface tempera-tures to estimate stratocumulus cloud-top heights. The October-mean cloud-top height structure of the southeast Pacific was then spatially and diurnally characterized. Coastal shoaling is apparent, but so is a significant along-coast cloud-top height gradient, with a pronounced elevation of the cloud-top heights above theArica Bight at;208S. Diurnal cloud-top height variations (inferred from irregular 4-times-daily sampling) can locally reach 250 m in amplitude, and they can help to visualize offshore propagation of free-tropospheric vertical motions. A shallow boundary layer associated with the Chilean coastal jet expands to its north and west in the afternoon. Cloud-top heights above the Arica Bight region are depressed in the afternoon, whic