Formation of Convective Clouds at the Foothills of the Tropical Eastern Andes (South Ecuador)

Abstract

This study examines the seasonal and diurnal dynamics of convective cloud entities?small cells and a mesoscale convective complex?like pattern?in the foothills of the tropical eastern Andes. The investigation is based on Geostationary Operational Environmental Satellite-East (GOES-E) satellite imagery (2005?07), images of a scanning X-band rain radar, and data from regular meteorological stations. The work was conducted in the framework of a major ecological research program, the Research Unit 816, in which meteorological instruments are installed in the Rio San Francisco valley, breaching the eastern Andes of south Ecuador. GOES image segmentation to discriminate convective cells and other clouds is performed for a 600 ? 600 km2 target area, using the concept of connected component labeling by applying the 8-connectivity scheme as well as thresholds for minimum blackbody temperature, spatial extent, and eccentricity of the extracted components. The results show that the formation of convective clouds in the lowland part of the target area mainly occurs in austral summer during late afternoon. Nocturnal enhancement of cell formation could be observed from October to April (particularly February?April) between 0100 and 0400 LST (LST = UTC ? 5 h) in the Andean foothill region of the target area, which is the relatively dry season of the adjacent eastern Andean slopes. Nocturnal cell formation is especially marked southeast of the Rio San Francisco valley in the southeast Andes of Ecuador, where a confluence area of major katabatic outflow systems coincide with a quasi-concave shape of the Andean terrain line. The confluent cold-air drainage flow leads to low-level instability and cellular convection in the warm, moist Amazon air mass. The novel result of the current study is to provide statistical evidence that, under these special topographic situations, katabatic outflow is strong enough to generate mainly mesoscale convective complexes (MCCs) with a great spatial extent. The MCC-like systems often increase in expanse during their mature phase and propagate toward the Andes because of the prevailing upper-air easterlies, causing early morning peaks of rainfall in the valley of the Rio San Francisco. It is striking that MCC formation in the foothill area is clearly reduced during the main rainy season [June?August (JJA)] of the higher eastern Andean slopes. At a first glance, this contradiction can be explained by rainfall persistence in the Rio San Francisco valley, which is clearly lower during the time of convective activity (December?April) in comparison with JJA, during which low-intensity rainfall is released by predominantly advective clouds with greater temporal endurance.