A synoptic-scale investigation of entropy fluxes during Tropical Cyclone Gonu

Document Type : Research Article

Author

Assistant Professor, Atmospheric Science Center, Iranian National Institute for Oceanography and Atmospheric Science, Tehran, Iran

Abstract

Tropical cyclones (TC) have been investigated from different points of view. Development of forecast of TC intensity and its track is often the shared purpose of all previous researches. To this aim, various empirical indices and different frameworks, based on various parameters, have been defined to provide deep knowledge of TC dynamics and thermodynamics. In this research, using the thermodynamic parameter of entropy, entropy fluxes (including surface, lateral and vertical fluxes) have been calculated. A theoretical framework based on hypothesized mechanism, introduced by Tang and Emanuel (2010), has been used to calculate the vertical flux of entropy. This ideal framework used a set of rigid assumptions including steadiness, axisymmetry and slantwise neutrality to assess the effects of vertical entropy flux on TC intensity via the possible pathway of downdrafts outside the eyewall. The lateral entropy flux has been computed based on radial component of surface wind. Azimuthal average of lateral entropy flux has been calculated to analyze vertical extension and strength of inflow (in the lower part of boundary layer) and also outflow (in the upper part of troposphere). Here, Tropical Cyclone Gonu (TCG) has been focusedon. TCG, formed at 18:00 UTC 1 June 2007 and decayed on 7 June, passed intensity of Saffir-Simpson Category-5 and affected southern coast (Makran) of Iran. All above parameters have been computed and analyzed during TCG lifetime using (1) Era-Interim reanalysis data (from European Center for Medium Range Weather Forecast) with 0.125 degree horizontal resolution, 12 vertical levels from 1000 to 200 hPa and 6-hour time intervals, and (2) data produced by India Meteorological Department. The variables were used both at the surface and also at pressure levels, the surface values were temperature and humidity (both at 2 m height), wind vector (at 10 m height), mixing ratio and sea level pressure. Synoptic–scale analysis has been done using data in a circular region centered by TCG center with a radius of 500 km. Results of horizontal patterns and time series of radial and tangential components of wind vector indicated that the value of radial component was maximized simultaneously with maximum activity of TCG. At TCG peak activity time, the tangential component had a comparatively minimum value embedded between two relative maximum values. Time series analysis showed that the integrated values of all three parameters of surface, vertical and lateral entropy fluxes experienced their extremum values before TCG reached its maximum intensity. It is worthwhile to be noted that their lead time varied from 6 hours (for surface entropy flux), 18 hours (for lateral entropy flux) to 30 hours (for vertical entropy flux). A comparative analysis between the values of entropy fluxes during TCG and those for Haiyan Tropical Cyclone (TCH, the strongest TC formed over the Pacific Ocean), reported by Pegahfar and Gharaylou (2019), indicated that entropy surface flux and lateral entropy flux during TCG were respectively two and one order of magnitude larger than the related values during TCH. In contrast, TCG experienced entropy vertical flux with two orders of magnitude smaller than that during TCH. Hence it can be concluded that the accumulation of energy helped TCG to travel to the higher latitudes. Moreover, the strongest inflow and outflow occurred before and after TCG maximum intensity, respectively. In a period that TCG reaches category-5 intensity and more, vertical extension of inflow layer was minimized while outflow layer started from the lower levels, comparing with results from the beginning of TCG life cycle. Conclusively, findings of this research showed that surface, vertical and lateral entropy fluxes, even in synoptic scale, have the ability to be served as empirical indices and also need to be focused in theoretical, computational and practical frameworks, for all prognostic purposes of TC intensity.

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References

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Journal of the Earth and Space Physics
Volume 45, Issue 2
July 2019
Pages 459-472

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