Three Examples of Fair-Weather Mesoscale Boundary-Layer Convection in the Tropics

Abstract

Evidence indicates that fair-weather to towering cumulus clouds over the East Atlantic Ocean during GATE were frequently organized into mesoscale structures. Three examples of such structures are examined, using gust-probe aircraft data collected in parallel straight-and-level flight tracks at 150 m, and covering an area greater than 30?30 km. The aircraft (two cases) or rawinsonde (one case) data provide vertical profiles of mean wind, temperature and mixing ratio. Cloud patterns are revealed from an upward-looking infrared sensor on the aircraft and radar and satellite pictures. The data show that the cumulus were organized into bands with horizontal wavelengths of 15?25 km. The circulations appear to extend through the subcloud layer, with all the fields at 150 m well related to the cloudiness overhead. Since the circulations are aligned with the subcloud-layer shear and travel in a direction parallel to the subcloud-layer wind (in the two cases for which band movement is documented), it is believed that they are primarily subcloud-layer phenomena. The subcloud-layer depth is about 600 m, giving aspect ratios of the bands from 25 to 50, in the range of mesoscale cellular convection observed in midlatitudes. Several physical mechanisms which might explain the bands are discussed.