| Diurnal, decoupling-related variations in the summertime marine boundary layer cloud structure over the eastern Atlantic Ocean are investigated using data collected during the Atlantic Stratocumulus Transition Experiment (ASTEX). The data used in this study, which was collected from the island of Santa Maria in the Azores, consists of measurements from radiosondes, a laser ceilometer, a surface micrometeorology system, and a prototype 94 GHz radar.; Radiosonde data collected at 3-hour intervals show the nocturnal MBL on 15 June to be well-mixed and daytime MBL to be intermittently decoupled. Concomitant with this diurnal evolution in thermodynamic structure is an important change in the cloud structure.; The coupled nocturnal MBL on 15 June is found to contain mesoscale cellular convection of a horizontal scale of approximately 8 km that is characterized by warmer, moister updrafts and cooler, drier downdrafts. Higher cloud tops, thicker clouds, and more subcloud drizzle is seen in the ascending branches of these convective cells.; In contrast to this coupled nocturnal cloud structure, daytime decoupling is shown to increase the mixing ratio in the surface mixed layer. This increase has two important effects: it generates latent instability in the surface mixed layer and it lowers the surface lifting condensation level. The potential energy generated by this latent instability is shown to be sufficient to support updrafts between 3 and 5 ms{dollar}sp{lcub}-1{rcub}{dollar}. Furthermore, it is shown that the instability is realized when the lifting condensation lowers into the surface mixed layer from its initial location in the decoupled mixed layer above. The daytime MBL is found to contain transient cumulus elements that occasionally recouple the MBL. These cumulus elements are observed to rise from the top of surface mixed layer into a sheet of stratocumulus located beneath the inversion and to produce considerable drizzle that modifies the subcloud thermodynamic structure. During the majority of these drizzle events, subcloud drizzle evaporation is found to cool and moisten the layer, but produce no change in {dollar}thetasb{lcub}rm e{rcub}{dollar}. Some of these drizzle events, however, are shown to contain penetrating downdrafts. |
