Precipitation dynamics in Ecuador and northern Peru during the 1991/92 El Nino: a remote sensing perspective

The formation, dynamics and spatial distribution of heavy precipitation dur ing the 1991/92 El Nino in Ecuador and northern Peru were examined by means of Meteosat-3 imagery, NOAA-AVHRR-based multichannel sea surface temperatu res (MCSST) and additional meteorological observations. The Convective and Stratiform Technique (CST) was used for rain retrieval by means of Meteosat IR data and a cross-correlation approach was applied to Meteosat image seq uences to derive cloud motion winds (CMW) which are essential for the analy sis of circulation patterns leading to severe precipitation. From an analys is of 45 days with severe precipitation it is proven that three mechanisms were responsible for the formation of heavy rains. Each mechanism reveals a specific localised impact. (1) The most frequent mechanism (frequency of s imilar to 61%) represents an extended land-sea breeze system. During such w eather conditions, predominantly locally confined precipitation patterns oc curred. Areas affected by the sea wind front during the day were the coasta l plains up to the 1000 m contour line on the western Andean slope. Local m axima in the frequency of cloudiness leading to precipitation could be foun d at isolated peaks of a lower coastal cordillera. At night the highest fre quency of precipitation was found over the warm water surface of the Gulf o f Guayaquil, mainly due to its coastal shape which significantly favours co nvergence of the nocturnal land breeze. (2) Convection, initiated in the co astal plain and on the western Andean slopes during the afternoon, was sign ificantly intensified by an entrainment of remainders of cirrus shields for m the Amazon basin. These cloud fragments spilled over the Andes with well developed trades in the mid/upper troposphere which blew in the opposite di rection to the daily sea/up-slope breeze. The spill over points were charac terized by areas of deep convection on the western Andean slopes and were f requently valley axes perpendicular to the mountain chain as well as the An dean depression in southern Ecuador. (3) During the main El Nino phase (Mar ch-April), heavy and persistent precipitation was extended over wide areas of the coastal plain showing neither a distinct diurnal cycle nor preferent ial areas. deep convection was frequently organized in mesoscale convective complexes (MCC) and was spatially correlated with MCSST > 27 degrees. The extensive instability of the troposphere during these weather conditions wa s marked by convective cloud streets and an intensification of the meridion al Hadley circulation off the coast of southern Ecuador and Peru.