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.
