Recovery Processes and Factors Limiting Cloud-Top Height following the Arrival of a Dry Intrusion Observed during TOGA COARE

Abstract

This study investigates the recovery of the tropical atmosphere to moist conditions following the arrival of a dry intrusion observed during the Tropical Ocean and Global Atmosphere Program Coupled Ocean?Atmosphere Response Experiment (TOGA COARE). A cloud-resolving model was used to quantify the processes leading to the moistening of the lower and middle troposphere. The model replicates the general recovery of the tropical atmosphere. The moisture field in the lower and middle troposphere recovered in large part from clouds repeatedly penetrating into the dry air mass. The moistening of the dry air mass in the simulation was due to lateral mixing on the edges of cloudy regions rather than mixing at cloud top. While the large-scale advection of moisture played a role in controlling the general evolution of moisture field, the large-scale thermal advection and radiation tend to directly control the evolution of the temperature field. The diurnal variations in these two terms were largely responsible for temperature variations above the boundary layer. Thermal inversions aloft were often found at the base of dry layers. The study also investigates which factors control cloud-top height for convective clouds. In both the observations and simulation, the most common mode of convection was clouds extending to ?4?6 km in height (often termed cumulus congestus clouds), although the period also exhibited a relatively wide range of cloud tops. The study found that cloud-top height often corresponded to the height of the thermal inversions. An examination of the buoyancy in the simulation suggested that entrainment of dry air decreased the parcel buoyancy making these inversions more efficient at controlling cloud top. Water loading effects in the simulation were generally secondary. Thus, there is a strong coupling between the dry air and thermal inversions as clear-air radiative processes associated with the vertical gradient of water vapor produce these inversions, while inversions and entrainment together limit the vertical extent of convection. One positive impact of dry air on convection occurred early in the simulation when clouds first penetrate the extremely dry air mass just above the boundary layer. At this time in the simulation, water vapor excesses within the rising parcels strongly contributed to the positive buoyancy of the clouds. In general, however, the impacts of dry air are to limit the vertical extent of convection and weaken the vertical updrafts.