Numerical Studies of Synoptic and Mesoscale Environments Conducive to Heavy Rainfall in Tropical and Extratropical Systems

Abstract

The purpose of this research was to examine the environments conducive to heavy rainfall production, specifically a landfalling hurricane, Hurricane Floyd (September 1999) and an Alpine event, MAP IOP-2B (September 1999). In addition to studying the two events independently, a third study examined the link between Floyd's extratropical transition and IOP-2B given that the two events occurred a few days apart. Analysis of observations of both events led to the formation of the hypothesis that the coupling of transverse ageostrophic circulations over a pre-existing low-level confluence zone was a key precursor to heavy rainfall production.

In both cases, a low-level confluence zone was found from the observations and simulations. For Floyd, the confluence zone developed as warm easterly winds ahead of the hurricane became juxtaposed with cooler northeast winds just inland over North Carolina and Virginia. In IOP-2B, the confluence zone developed as southerly winds from the Mediterranean became juxtaposed with easterly and southeasterly winds from eastern Italy. These easterlies and southeasterlies developed as southeast winds from the Adriatic Sea impinged upon the eastern Alps, and turned west in the form of a barrier jet. Also, in both cases, upper level diffluence, due to a split flow, became juxtaposed over the low-level confluence, enhancing the upward motion. MM5 simulations for both events revealed coupled thermally direct and thermally indirect circulations over the low-level confluence zone with their rising branches coupled over the zone, proving the hypothesis. Simulations of Floyd's extratropical transition showed a link existed between Floyd and IOP-2B. Parcels from Floyd's upper level circulation reached Italy around the time the heavy rainfall developed in IOP-2B. Simulations with and without latent heat release demonstrated the importance of latent heat release in maintaining the upper-level jets and split flow which in turn, aided in the maintenance of convection. Latent heat release was also found to be important in maintaining the strength of the transverse ageostrophic circulations