Saturation Point Structure of Marine Stratocumulus Clouds

Abstract

The microstructure of a Pacific stratocumulus capped boundary layer was examined. The boundary layer flow was over an area of sea-surface temperature gradient, so that no steady state was present. Conserved variable diagrams showed a complex structure with one branch of the mixing line extending, from the mixed layer saturation point to the ocean surface saturation point, a second branch in the cloud layer and a third upward into the inversion. They correspond well with a conceptual model for the unstable, radiatively cooled cloud topped boundary layer. Saturation point pairs for ascending and descending branch of the internal boundary layer circulation were isolated by a simple conditional sampling technique. Pair separation decreased downward towards the surface, as did the variance around the mean ascent and descent values. Also, the variance was smaller for the ascending branch than for the descending branch. Spectral analysis of the saturation pressure showed that the primary circulation scale was 5 km, the same scale that was observed by lidar observations of cloud tops. A budget diagram for the time-dependent boundary layer is used to derive the time evolution of the layer and the entrainment rate using radiative flux estimates from a model incorporating cloud top heights from the lidar, and cloud liquid water and temperature from a mixing line model. A reasonable cloud top entrainment rate of 1 cm s⁻¹ was obtained. The internal convective circulation velocity was found to be an order of magnitude higher corresponding to an internal circulation time scale of about 1 hour. The external adjustment time of the layer was 10 hours and the lag of the layer mean from equilibrium agreed with the advection over a 3°C warmer sea surface in 10 hours.