JP3.5
Investigating barotropic instability of the African Easterly Jet and its potential for development in the NASA fvGCM
Marangelly Fuentes, Howard University Program in Atmospheric Sciences, Washington, DC; and O. Reale and V. Morris
General Circulation Models (GCMs) have generally not been considered suitable to properly represent tropical cyclones (TC) because of the inadequate resolution necessary to fully resolve the cyclone's structure [e.g. Hou et al., 2004]. The resolution needed to start resolving the structure of the TC circulation would be at least of the order of 20-30 km or higher, whereas global models used for operational weather forecasting until one or three years ago operated at a resolution of 50km or lower. As a consequence, the vortices represented in GCMs tend to be too weak, their scale too large, and their fine structure is not properly represented [Atlas et al., 2005b]. NASA Goddard has developed a finite-volume General Circulation Model (hereafter GEOS), based on a finite-volume dynamical core with terrain-following Lagrangian control and volume dynamical discretization. This model has been showing the capability of spontaneously producing tropical cyclones and resolving part of their structure [Atlas et al., 2005] without the aid of bogusing technique. Its dynamical core [described by Lin 2004], has allowed real time simulations at a quarter of degree resolution or higher [Shen, 2006a, 2006b]. The focus of this work is to understand how the quarter degree resolution GEOS model spontaneously produces tropical cyclogenesis. To study these processes we investigate the role of barotropic instability and kinetic energy transfer in the GEOS cyclogenetic process. This work shows results from ten numerical model simulations of Atlantic Tropical Storms performed between August and September 2004. The primary reasons for investigating this temporal window was the opportunity to test the ability of the model to reproduce a unique case in which there were two systems simultaneously present, but only one ultimately developed into a hurricane. We observe several features associated with the barotropic instability in the developing system (Hurricane Frances) that are not present in the non-developing system. These findings are suggestive that barotropic instability plays an important role in the early tropical development as represented in the numerical results from the NASA GEOS model. As a second part of this work we analyze the loss of kinetic energy from the African Easterly Jet, compared with the gain of kinetic energy of the vortex associated with Hurricane Frances as it appears in the model. The purpose of this second part is to further understand the dynamics and transfer of energy from the jet during the early cyclogenetic stage in the model.
Joint Poster Session 3, Tropical Cyclones and Climate Change Poster Session
Monday, 21 January 2008, 2:30 PM-4:00 PM, Exhibit Hall B
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