Genesis of tropical cyclone Agni: Physical Mechanisms Amit Kesarkar*, Sudipta Banerjee, J Venkata Ratnam Computational Atmospheric Sciences, Centre for Development of Advanced Computing, Pune Unversity Campus, Ganesh Khind, Pune 411007, India * Corresponding Author: Dr. Amit P Kesarkar, e-mail: akesarkar@cdac.in , Fax:91-020-25694004 1. Introduction Mesoscale convective complex plays an important role in the development of tropical disturbances. The pathway by which these mesoscale cumulus complexes organize to form a large-scale tropical cyclone vortex is one of the great unsolved problems in dynamical and tropical meteorology. The factors interacting on various spatial scales, and playing an important role in the genesis of tropical deep convections are described by Gray, 1968; Cheung, 2004. Besides SST and vertical wind shear (850 – 200 hPa), the other atmospheric parameters which can contribute to the genesis of the tropical cyclones include conditional instability and cyclonic absolute vorticity in lower troposphere; Relative Humidity (RH) in middle troposphere (500 – 700 hPa); meridional wind shear; anti-cyclonic relative vorticity in upper troposphere; 200 hPa divergence and sensible heat. The conceptual model of cooperative intensification of organized moist convection and cyclone vortex through destabilization of tropical atmosphere resulting from radiative heating, surface forcing etc. was well known for last few decades (Ooyama, 1964; Charney and Eliassen, 1964). In this mechanism the convection is described as the selective release of CAPE in regions where the large scale circulation happens to converge replacing it elsewhere. This model describes well the spin-up of the vortex in response to the convective heating and mass flux occurs however the control of convection by vortex scale flow/ cumulus parametrization is not well understood. In atmosphere, the observed rate of change of CAPE in the maritime tropical atmosphere is much smaller than what would occur if de-stabilization of the atmosphere by large scale process were unopposed by convection (Arakawa and Schubert, 1974). Thus the temperature (mass distribution) of tropical atmosphere is determined by conditions that convection is in statistical equilibrium with large scale and not by selective release or stored CAPE. The required thermodynamic dis- equilibrium has to be provided by large reservoir of energy with the mechanism of releasing energy through the wind dependent rate of transfer of enthalpy. This mechanism termed as wind- evaporation feedback mechanism (Neelin et al., 1987) and wind- induced surface heat exchange (WISH) by Yano and Emanuel, 1991. In this mechanism the amount of relizable energy in marine tropics can be represented by thermodynamic disequilibrium between tropical ocean and atmosphere (Emanuel, 1987). This mechanism can be explained based on exchange of sensible heat and latent heat. In this mechanism total increase in heat contains is proportional to lag increase in water vapor contain and decrease in pressure ans is known as heat input by isothermal expansion. However the major unsolved problem of tropical cyclogenesis is now to understand how a weak amplitude tropical disturbance transformed into a surface vortex of sufficient strength that can amplified by WISHE process. Recent studies by Bister and Emanuel, 1997; Ritchie & Holland, 1997 suggested that mesoscale convective vortices which form in stratiform precipitation region at mid-levels of the troposphere in disturbed weather regions are precursore to tropical cyclo-genesis. Both these studies suggested that mid- level mesoscale convective vortices are necessary for building a surface vortex. Bister and Emanuel hypothesized downward advection of vertical velocity via mesoscale subsidence in association with stratiform precipitation. Ritchie & Holland hypothesized downward development by succession of mid-level merger between mesoscale vortices that form in stratiform precipitation region following deep convective episode. Montgomery and Farrell, 1993; Montegomery & Enagonio, 1998 and Hendricks et al., 2004, suggested all together different mechanism for development of surface vortices. Hot towers are in general termed as a positive influence to genesis via subsidence warming around them and the attaindant surface pressure fall (Simpson et al., 1998). The low- level vortex merger and anti-symmetrization of small scale diadiabatically generated cyclonic potential vorticity anomalies (convective bursts) could intensify a large scale vortex on realistic time scale Tangential momentum spin-up can be explained through the organizational mechanism process and potential merger or diadiabatic vortex merger pre-conditioned atmosphere. In this work we have simulated the pregeneis period of the cyclone using a mesoscale model Advanced Research WRF version 2.1.1 (ARW) to understand the role of mesoscale features and to identify associated physical processes in the genesis of tropical cyclone Agni. 2. Synoptic History of Agni Two deep convective mesoscale disturbances were observed in the Equatorial Indian ocean on 19 November 2004 and at 18UTC 26 November 2004. The first disturbance was located at about 800 km southeast of Colombo, Sri-Lanka and the second at about 160 Km north of equator. The second disturbance originated from the remnants of the first disturbance as low-level circulation, with the maximum wind speed of around 11 m s -1 (20 Knots). From 00 UTC 27 November 2004 to 03 UTC 28 November 2004 the second disturbance organized itself to form Tropical Cyclone Agni. The deep convection associated with it organized with maximum surface wind speed of around 15 m s -1 (30 knots) surrounding the center. During the organization, it was observed that the centers of the intensification moved about half degree south of the equator without losing its counter clockwise rotation. This erratic behavior questioned the necessary condition of required large Coriolis parameter either side of equator for the genesis of tropical cyclone. After surviving its excursion south of equator, at 06 UTC 28 November 2004 this tropical disturbance strengthened to tropical storm and was located around 75 Km 1