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Summer convection in urban environments and the flash floods


Augusto J. Pereira Filho

Departamento de Ciências Atmosféricas, Instituto Astronômico e Geofísico, Universidade de São Paulo.

Presented by IGOR G. PACCA

The Metropolitan Area of São Paulo (MASP) is one of the largest urban environments of the planet with about sixteen million inhabitants. Flash floods in the MASP are more often in the summer (Pereira Filho et al. 1991). Anomalous atmospheric circulation patterns developed under the influence of the El Niño in South America during February 1998. A large-scale high-pressure system anomaly over the State of São Paulo inhibited organized rainfall. Rainfall accumulations were above average along the coastline and below average elsewhere (Climanálise 1998). Furthermore, heavy showers and flash floods were above average in the MASP within this period. These events were monitored with the São Paulo weather radar (Braga Junior 1988). Radar measurements were used to estimate the rainfall spatial frequency and distribution. The results indicate a nucleous of rainfall accumulation over the MASP higher than four times the spatial average within 150km from the São Paulo weather radar. Similarly, there is a nucleous of high rainfall probability over the MASP between 1400 UTC and 2000 UTC. GOES-8 IR data reveals above average temperatures within the MASP caused by the local heat island effect (Lombardo 1984). Furthermore, surface data from IAGUSP indicate the increase of moisture content due to inflow of the sea breeze (De Oliveira & Silva Dias 1982). These data suggest there might be a strong interaction between the heat island and the sea breeze to destabilize the atmosphere. On the other hand, infiltration has been largely reduced within the MASP with a proportional increase in surface runoff. Since the summer rainfall tend to be higher within the MASP, it can be inferred that there is higher risk of flash floods in this area. Moreover, the urban environment tends to have a positive feedback on flash floods. Higher temperatures combined with the sea breeze moisture inflow generate stronger updrafts and higher rainfall accumulations that results in large amounts of runoff and associated flash floods. Therefore, as the MASP increases, the flash floods tend to be more destructive.

— ( November 19, 1999 ) .

Publication Dates

  • Publication in this collection
    07 Aug 2000
  • Date of issue
    June 2000
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