Laboratory for
Climatology and Remote Sensing

Abstract:

This Letter describes the adjustment of the Convective-Stratiform Technique (CST) to estimate 1991± 93 El NinÄ o rainfall in Ecuador and Peru by means of IR radiation data recorded by the Meteosat-3 VISSR sensor. The adjustment includes (1) a regionalization of a one-dimensional cloud model using local radiosonde data, (2) a ® eld-of-view correction of the VISSR-derived cloud top temperature to model requirements and (3) an adapted discrimination scheme between raining and non-raining clouds. Validation reveals a strong correlation between calculated and observed rainfall for all climatic regimes and rainfall intensities within the study area.

Abstract:

The formation, dynamics and spatial distribution of heavy precipitation during the 1991/92 El Nin˜o in Ecuador and northern Peru were examined by means of Meteosat-3 imagery, NOAA-AVHRR-based multichannel sea surface temperatures (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 sequences to derive cloud motion winds (CMW) which are essential for the analysis of circulation patterns leading to severe precipitation. From an analysis of 45 days with severe precipitation it is proven that three mechanisms were responsible for the formation of heavy rains. Each mechanism reveals a speci? c localized impact. (1) The most frequent mechanism (frequency of ~61%) represents an extended land–sea breeze system. During such weather conditions, predominantly locally con? ned precipitation patterns occured. Areas a? ected by the sea wind front during the day were the coastal plains up to the 1000m contour line on the western Andean slope. Local maxima in the frequency of cloudiness leading to precipitation could be found at isolated peaks of a lower coastal cordillera. At night the highest frequency of precipitation was found over the warm water surface of the Gulf of Guayaquil, mainly due to its coastal shape which signi? cantly favours convergence of the nocturnal land breeze. (2) Convection, initiated in the coastal plain and on the western Andean slopes during the afternoon, was signi? cantly intensi? ed by an entrainment of remainders of cirrus shields from the Amazon basin. These cloud fragments spilled over the Andes with well-developed trades in the mid/upper troposphere which blew in the opposite direction to the daily sea/up-slope breeze. The spill over points were characterized by areas of deep convection on the western Andean slopes and were frequently valley axes perpendicular to the mountain chain as well as the Andean depression in southern Ecuador. (3) During the main El Nin˜o phase (March– April ), heavy and persistent precipitation was extended over wide areas of the coastal plain showing neither a distinct diurnal cycle nor preferential areas. Deep convection was frequently organized in mesoscale convective complexes (MCC) and was spatially correlated with MCSST > 27ß . The extensive instability of the troposphere during these weather conditions was marked by convective cloud streets and an intensi? cation of the meridional Hadley circulation o? the coast of southern Ecuador and Peru.

Abstract:

An analysis of heavy precipitation dynamics during the strongest Super El Niño of the last century (1997/98) for Ecuador and northern Peru is presented. Additionally, a unique event of severe flooding in the Rio Ica catchment (central Peruvian Andes) during this event is examined. All investigations are based on the exploitation of satellite data. Precipitation maps are calculated by means of the Enhanced Convective Stratiform Technique (ECST) and Cloud Motion Winds (CMW) are extracted from time series of GOES-8 data. AVHRR-based sea surface temperatures are also used as data of the Precipitation Radar (PR) onboard the TRMM platform. Spatio-temporal dynamics of heavy precipitation during EN 1997/98 has been proven to reveal the same mechanisms as during a normal El Niño (1991/92). However, rainfall amount is significantly increased. Evidence for clear spill-over effects of cirrus remainders from the Amazon on rainfall formation in the coastal lowlands could be found by means of TRMM PR data and CMWs. The study reveals that the severe flood event in the Rio Ica catchment was caused by anomalous strong zonal trades which initiated the heavy precipitation event from the Amazon lowlands.