Tracking parcels that are entrained across cloud tops

In order to study the processes affecting entrained parcels, a Lagrangian parcel tracking model has been implemented in a large-eddy simulation model. A night-time stratocumulus cloud was simulated with horizontal grid spacing of 5 m and vertical grid spacing of 2.5 m. Over 40 million parcels were tracked, and entrained parcels were identified based on each parcel's history. The temporal and spatial changes of entrained parcels across the top of marine stratocumulus clouds have been analyzed. Buoyancy changes prior to and during entrainment by longwave radiation, evaporation, and mixing have been diagnosed and compared. Analysis of the entrained parcels shows that entrainment occurs in dry regions of sinking air. A mixing fraction analysis has been used to partition cooling due to various effects. Radiative cooling is obtained using direct time integration, then, the mixing fraction and evaporative cooling are diagnosed. Results show that radiative and evaporative cooling are comparable, but the largest contribution to buoyancy reduction is due to mixing. The radiative cooling field is strongly inhomogeneous, and only weak cooling and warming generally exist in regions of entrainment, so that the entrained parcels do not experience strong radiative cooling. Parcels descend slowly through the inversion layer, mainly cooled by mixing with wisps of air detrained from the cloud top. When these parcels descend enough to feel the flow of the cloud top, they are carried into the downdraft area by the convergent flow at the cloud top. Cooling due to radiation and evaporation accelerate the parcel's descent.