ANALYSIS OF HELICAL FEATURES OF THE VELOCITY FIELD IN MODELING OF TROPICAL CYCLONES Galina Levina 1,2 and Michael T. Montgomery 3,4 Russian Academy of Sciences, 1 Institute of Continuous Media Mechanics, Perm, 2 Space Research Institute, Moscow levina@icmm.ru 3 Naval Postgraduate School, Monterey, CA, USA, 4 NOAA/Hurricane Research Division, Miami, FL, USA mtmontgo@nps.edu Introduction. There exists a fundamental theoretical hypothesis about a small-scale helical turbulence that may result in a large-scale instability governing the structure formation. The specific properties of small-scale helical turbulence were first discovered in magnetohydrodynamics by Steenbeck, Krause and Rädler [1]. This phenomenon is known as the alpha-effect. The first theoretical example in general (non-MHD) hydrodynamics was proposed by Moiseev and co-authors [2], and on its basis a mechanism for intensification of large-scale vortex disturbances in the atmosphere – the so-called turbulent vortex dynamo [3]. In both, MHD and non-MHD cases the theory gives thresholds for the large-scale helical instability. A numerical approach was developed and applied by Levina et al. [4] to simulate helical-vortex effects under developed thermal convection. Resulting findings also showed the threshold for larger scale structure generation as well as a way of development of large- scale helical instability by merging of cells and consequent intensification of newly forming helical vortices. At the same time a paper by Montgomery and co-authors [5] appeared which proposed a new scenario of tropical cyclogenesis and showed by near cloud resolving simulations how a mesoscale tropical depression vortex could develop from cumulonimbus convection as a result of upscale vorticity cascade. In the simulations the growth of flow scales occurred by multiple mergers of small-scale convective structures. The simulations first generated from convective updrafts (each up to 2-5 km horizontal scale) a number of vortical hot towers, each of 10-30 km horizontal scale, which eventually resulted in an intense mesoscale helical vortex of tropical depression of 80-100 km horizontal scale. Results of works [4,5] were brought in together for a common discussion in seminars of Montgomery Research Group (2006). That gave a start for our collaboration and paved the way to introducing the analysis of helical characteristics of the velocity field in numerical investigations of tropical cyclones by use of atmospheric modeling systems. Our current common work is focused on the analysis of the helicity field during this vortical hot tower (VHT) route to tropical cyclogenesis. Some first results will be discussed. Numerical simulation of tropical cyclogenesis by use of atmospheric modeling systems. For calculations of helical characteristics of the velocity field we take the output of the Regional Atmospheric Modeling System (RAMS) numerical simulations performed within a series of experiments on the VHT route to tropical cyclogenesis by Montgomery et al. [5]. The RAMS is a three-dimensional nonhydrostatic numerical modeling system developed at Colorado State University and comprising time-dependent equations for velocity, nondimensional pressure perturbation, ice-liquid water potential temperature, total water mixing ratio, and cloud microphysics (see [5] and references therein for details). RAMS utilizes an interactive multiple nested grid scheme which allows explicit representation of cloud-scale features within the finest grid while enabling a large domain size to be used, thereby minimizing the impact of lateral boundary conditions. A standard