SYNOPTIC ANALYSIS OF AN EXTREME AND PERSISTENT RAINFALL EVENT OVER STATE OF RIO GRANDE DO SUL, BRAZIL, IN MAY 2004

Raidel Báez Prieto, Mateus da Silva Teixeira

Abstract


Between 04 to 09 May 2004, part of the state of Rio Grande do Sul (RS), Brazil; have recorded rainfall above 300mm. The east region of this state, mainly on the coast, had the highest rainfall accumulation occurred. A synoptic analysis of this period has shown a persistent trough west of RS in high and middle levels of troposphere, in the first three days of this event. This trough started a cyclonic vortice in the mid-low-tropospheric levels in the following days. The upward movement associated with this trough stayed semi-stationary over the state of RS and acted in almost entire tropospheric layer during great part of this period. In addition, it was observed that this tropospheric layer had high values of relative humidity from surface up to 300 hPa, over the state of RS – greater than 70%. In 09 May 2004, the cyclonic vortice has lost their configuration and has distanced from RS. The state of RS was dominated by an anticyclonic circulation and by a drier tropospheric layer, what disfavors rainfall occurrence in this region. During all analyzed period, it could be observed that the configuration in the middle troposphere has extended up to higher levels, but the same does not occurred to the surface. Also, a cyclonic circulation in 850 hPa was observed over the state of RS, but any cyclogenesis has occurred at surface. A surface cyclogenesis was observed over the coast of the state of Sao Paulo (SP), in 05 May. This cyclone has moved to south, approaching the coast of the states of RS e Santa Catarina (SC), in 08 May. A comparison of the atmospheric behavior with the rainfall recorded in the state of RS between 03 and 09 May suggest that this low-pressure center has no direct influence in the cumulative rainfall observed in this period. In addition, in 08 and 09 May, this low-pressure center weakened and move away from the coast of the states of RS and SC. Therefore, although the weak low-level temperature advection and low-level moisture flux convergence observed over this region, a persistent mid-low-tropospheric trough, what favor persistent upward movement, together with a deep and very humid layer, seems to be the main responsible to the great amounts of rainfall registered in the state of RS.

Keywords


Extreme rainfall. Persistent rainfall.

References


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DOI: https://doi.org/10.5902/2179460X21265

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