The helical feature calculation for the velocity field of a developing tropical cyclone

The paper presents a part of the results obtained within the framework of the research program on numerical analysis of helical features of the developing tropical cyclone, by using the regional ETA model of the atmosphere. A set of integral characteristics and of gridded diagnostics is selected and substantiated, allowing revealing and tracing evolution of large-scale helical-vortex instability associated with specific properties of helical convective turbulence under condition of tropical cyclogenesis. The helicity vertical flux (helicity index) is calculated following the technique by Kurganskii for two tropical cyclones: Man-Yi (2007) and Wilma (2005). It is shown that the helicity index changes during the tropical cyclone evolution reflect adequately the tendencies of pressure and velocity changes and thus the index can be used as an indicative characteristic of intensity of the developing tropical cyclone.