HR: 1400h
AN: A23B-04    [Abstracts]
TI: Vorticity balance in East Pacific tropical cyclones
AU: * Marin, J
EM: juliocma@atmosfera.unam.mx
AF: Centro de Ciencias de la Atmosfera, Universidad Nacional Autonoma de Mexico, Mexico City, 04510, Mexico
AU: Raymond, D J
EM: raymond@kestrel.nmt.edu
AF: Physics Department, New Mexico Institute of Mining and Technology, Socorro, NM 87801, United States
AU: Raga, G B
EM: raga@servidor.unam.mx
AF: Centro de Ciencias de la Atmosfera, Universidad Nacional Autonoma de Mexico, Mexico City, 04510, Mexico
AB: There are still questions about how a tropical cyclone develops out of an unorganized region of convection over the tropical oceans. In this study we investigate the evolution of a few cyclones that developed in the East Pacific during the EPIC2001 and IFEX2005 projects. We have analyzed in those cyclones the relative importance of the terms derived from the vorticity equation: the time rate of change of its absolute circulation, the convergence of absolute vorticity into the system at each level and the spin-down tendency (friction term). We use FNL analyses for September-October 2001 and GFS model runs for June-July 2005 to calculate the two first terms, while the friction term was calculated as a residual from the other two terms and also using a simple bulk formulation from the FNL analyses. The output of the GFS model runs (which were especially for this study run by Jongil Han from NOAA/NCEP) includes the time evolution of the friction term, which can also then be compared with the residual calculation. Results from the FNL data show that the agreement between the residual calculation and the bulk formula is better during the early stages of cyclone development, when a not too deep (up to 2 km) layer is considered. As the tropical cyclones go from the tropical depression (TD) stage to the hurricane stage, the agreement is better for a shallower layer (0.5-1 km). Results from the GFS runs show that the balance is observed in a shallow layer with almost the same frequency as in a very deep layer. The results indicate that as the tropical cyclones start to intensify, the convergence of absolute vorticity dominates in the vorticity balance producing the consequent increment circulation in the system. As storm evolution proceeds the friction term becomes more important in the balance, resulting in a decrease in the circulation and leading to the final decay of the cyclone. Furthermore, the convergence of mass seems to dominate in the convergence of absolute vorticity term. However, in the hurricane stage, the absolute vorticity seems to have a stronger role on this term that causes the intensification of tropical storms.
DE: 3314 Convective processes
DE: 3329 Mesoscale meteorology
DE: 3339 Ocean/atmosphere interactions (0312, 4504)
DE: 3355 Regional modeling
DE: 3374 Tropical meteorology
SC: Atmospheric Sciences [A]
MN: 2007 Joint Assembly