Abstract
Data obtained from the island of Santa Maria in the Azores, during the Atlantic Stratocumulus Transition Experiment (ASTEX) are used to describe cloud and boundary-layer structure for a 24-h period on 15 June 1992 over the east-central Atlantic (37°N, 25°10′W). The evolution of the mesoscale cloud structure during a 24-h period in the vertical column above the surface site was characterized using a 94-GHz radar, a laser ceilometer, 3-h radiosonde ascents, and surface micrometeorological instrumentation. Mesoscale circulations and drizzle were found to be key elements of the boundary-layer clouds observed in this region.
During the late-night and predawn hours of the study period, a single layer of stratocumulus that averaged ∼200 m in thickness topped a well-mixed marine boundary layer. Mesoscale cellular convection (MCC), which had ascending regions with horizontal dimensions of ∼7 km, was observed during this period. At sunrise, decoupling was imposed on this MCC, and extreme mesoscale variations in the cloud thickness and surface precipitation rate were observed. These variations included mesoscale patches of cumulus that rose from the surface-lifting condensation level into the overlying stratocumulus (cumulus–stratocumulus interaction), co-existing with patches of decoupled stratocumulus that occasionally had small, shallow cumulus beneath. The average horizontal scale of cumulus–stratocumulus interaction regions was found to be on the order of ∼12 km, and mesoscale variations in the cloud thickness of as much as 400 m were indicated in the remote sensor data during the daytime.
Major drizzle events observed at the surface were shown to correspond with the deepening of the cumulus layer. Evidence was presented that mesoscale cumulus-stratocumulus interaction regions were affecting the surrounding decoupled regions through (i) the vertical transport of properties of the surface moist layer to decoupled stratocumulus by nearby cumulus and (ii) the extended influence of subcloud-layer wakes induced by major precipitation events to nearby decoupled regions. It was suggested that the enhanced precipitation due to cumulus convection may be an important feedback mechanism between the cloud and subcloud layers in the transition region.
