26th Conference on Hurricanes and Tropical Meteorology

Wednesday, 5 May 2004
Relationships between windspeed and precipitation in the Pacific ITCZ
Richelieu Room (Deauville Beach Resort)
Larissa E. Back, MIT, Cambridge, MA
Poster PDF (83.4 kB)
The relationship between daily windspeed and precipitation in the Pacific ITCZ is analyzed using four years of daily gridded SSM/I and TMI satellite passive microwave retrievals averaged over 2.5 degree boxes. Faster winds are associated with substantially more precipitation throughout the ITCZ. The slope of this relationship varies geographically and rapidly increases as the atmosphere becomes moister. Surface wind speed explains a small but highly statistically significant fraction of the daily rainfall variability, and provides insight into how surface forcing modulates tropical convection.

Physically, we anticipate higher windspeeds promote more evaporation, which destabilizes the boundary layer and can trigger deep convection. The observed increases in precipitation are much greater than evaporation changes associated with the increased windspeed. This implies a convergence feedback by which small increases in evaporation induce moisture convergence that feeds larger increases in precipitation. Horizontal moisture advection is a complicating factor; in some regions, high windspeed correlates with strong lower tropospheric dry advection that counteracts the increased surface evaporation.

Other data sources are analyzed to show the result is robust. Area-averaged precipitation estimates derived from a radar at Kwajalein Island are compared with the microwave precipitation estimates. 2.5 degree vector mean winds computed from Quickscat are compared with the SSM/I and TMI-derived windspeeds.

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