SURFACE-BASED OBSERVATIONS OF MESOSCALE CUMULUS-STRATOCUMULUS INTERACTION DURING ASTEX

Data obtained from the island of Santa Maria in the Azores, during the Atlantic Stratocumulus Transition Experiment (ASTEX) are used to desc ribe cloud and boundary-layer structure for a 24-h period on 15 June 1 992 over the east-central Atlantic (37 degrees N, 25 degrees 10'W). Th e evolution of the mesoscale cloud structure during a 24-h period in t he vertical column above the surface site was characterized using a 94 -GHz radar, a laser ceilometer, 3-h radiosonde ascents, and surface mi crometeorological 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 per iod, a single layer of stratocumulus that averaged similar to 200 m in thickness topped a well-mixed marine boundary layer. Mesoscale cellul ar convection (MCC), which had ascending regions with horizontal dimen sions of similar to 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. T hese variations included mesoscale patches of cumulus that rose from t he surface-lifting condensation level into the overlying stratocumulus (cumulus-stratocumulus interaction), coexisting with patches of decou pled stratocumulus that occasionally had small, shallow cumulus beneat h. The average horizontal scale of cumulus-stratocumulus interaction r egions was found to be on the order of similar to 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 obs erved at the surface were shown to correspond with the deepening of th e cumulus layer. Evidence was presented that mesoscale cumulus-stratoc umulus interaction regions were affecting the surrounding decoupled re gions 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 precipitat ion events to nearby decoupled regions. It was suggested that the enha nced precipitation due to cumulus convection may be an important feedb ack mechanism between the cloud and subcloud layers in the transition region.