Synoptic and spatial variability of the rainfall along the northern Peruvian coast during the 1997–8 El Niño event

The 1997-8 El Niño event was the strongest in recent history. Because the event was anticipated some months in advance it was possible to plan additional meteorological observations to examine the regional aspects of the anticipated heavy rains along the coast of northern Peru. A temporary network of seven pilot balloon stations was installed just prior to the onset of the rains in December 1997, and several hundred raingauges were established to more accurately quantify the rainfall and its temporal variability. These special observations have been used to describe the synoptic variability of the rainfall. During the period January-April 1998 the daily rainfall from approximately 100 stations was averaged to obtain the daily rainfall per station. A time series of this value showed large variations of roughly one week periodicity. Composites of wet and dry days were prepared using these values; both pilot balloon and NCEP reanalysis windfelds showed a clear difference between wet and dry days with westerly wind anomalies of about 1-2 m/s (with respect of mean conditions) in the lower troposphere.

The importance of a flexible and fast response observing system, the need for cooperative observers for measuring rainfall, and the importance of understanding regional climatology and meteorology before designing a regional weather forecasting system will be discussed.