MMM SEMINAR NCAR
Equatorial Wave
Confluence Zones and Tropical Cyclone Genesis
Greg Holland
Aerosonde North America
Abstract
The manner in which Rossby-mode waves in the equatorial duct can accumulate energy in regions where there the zonal winds are increasingly easterly towards the east has been described in a series of seminal papers by Webster and Chang (1988, 1997) and Chang and Webster (1990, 1994). Their major interest lay in the manner in which tropical energy accumulated in specific regions and then emanated to higher latitudes. Holland (1995) examined the wave interactions that seemed to be supporting tropical cyclone formation in the western North Pacific and suggested that the same accumulation mechanism could be contributing to the genesis process.
This talk provides an extension of theses studies in joint work with Peter Webster and Hai-Ru Chang to examine the manner in which wave accumulation can focus energy down to small scales, where it can provide the trigger for tropical cyclone genesis. I will start by describing the basic process using a simple WKB analysis and 2-D modeling. A climatological study will show that almost all tropical cyclone form within wave accumulation regions around the globe. In the more marginal areas active and inactive cyclone phases are associated with wave accumulation and stretching, respectively. A major step is to understand the manner in which the accumulation occurs in a fully non-linear environment with latent heating, ocean processes and vertical shear. I will present the some thoughts on the effects of vertical shear and the results from a preliminary analysis of the North Atlantic region using 2D model studies, which reproduces the manner in which easterly waves intensify, weaken and intensify as they move away from Africa.
This research has highlighted data limitations that are restricting further understanding. I will conclude by briefly describing how we plan to work with field experiments in the eastern Pacific and eastern Atlantic to further our understanding of these important processes. These experiments will be based on UAV platforms, which will have the endurance to remain on station and observe the full scale interaction process in detail.
Tuesday, June 1, 2004, 1:45
PM
NCAR-Foothills
Laboratory
3450 Mitchell Lane
Bldg 2, Rm 1022,
Auditorium