New Investigation of a Tropical Cyclone: Observational and Turbulence Analysis for the Faraji Hurricane

Abstract

In the general framework of the atmosphere, the in-depth study of extreme events of complex nature such as tropical cyclones remains an open problem. Nowadays, there are different useful studies aiming to enlarge the current knowledge of these events. Under this perspective, the search for mechanisms and geographical areas of formation and dynamics of tropical cyclones at different latitudes needs better constraints on their preliminary results.This work focuses on a diagnostic analysis of a tropical cyclone, with the aim of identifying key points that characterize its evolution, from a dynamic and observational point of view. The study is applied to Hurricane Faraji, the most powerful tropical cyclone of the 2021 Indian Ocean season, classified as fifth-category on the Saffir–Simpson intensity scale. The study develops in three main sections, all related to each other. The starting point is a large set of satellite products from both polar and geostationary platforms. From the data acquired by the polar instruments, an accurate study of the evolution of the hurricane is carried out, focused on the extraction of physical information related to the system (temperature and altitude of the associated cloudy system, temperature gradient, pressure in the different regions) at moderate resolutions. From the data acquired by the geostationary instruments, it is possible to obtain very high temporal resolution pictures of the temperature field of the cyclone, from which a study of the turbulent dynamics is carried out. In particular, to investigate the maximum energy content in the different regions of the cyclone, the Proper Orthogonal Decomposition (POD) technique is used to extract the associated spectra of both the spatial and temporal components, studied separately on three different ranges of scales.

A Satellite Analysis: Comparing Two Medicanes

“…As in the analysis for the case study of a tropical cyclone [24], the high-quality scanning technique operated by VIIRS, combined with daily nighttime acquisition at 00:00 UTC, enhances accuracy in the results by minimizing reflected radiance effects.…”

“…This study highlights a new perspective on diagnosing extreme events related to atmospheric mesoscale over the Mediterranean basin using a satellite remote sensing approach. As previously ac-knowledged for studying the diagnostics of a tropical cyclone at its maximum intensity category [23,24], a similar methodology is employed for comparing two Medicanes, aiming to seek atmospheric characterization from the medium-high resolution satellite data provided by two important LEO sensors. Their technical specifications for imagery acquisition are summarized in Table 1.…”

“…These constants allow deriving a real temperature value, providing T amb , given by the daily averaged output value of air temperature at 2 m above mean sea level (AMSL), acquired from the BOLAM forecasting model database. The overall calculation of H and T e f f must be completed by a generally known criterion used for assuming that the satellite-recorded T b values, under suitable instability conditions, can coincide with the dew, T D , at the altitudes of the condensation level [24]. With this procedure, by imposing T b = T D , system (2) is solved, and the quantities H and T e f f are obtained (Equation ( 3)).…”

“…Starting from the radiation amount taken from the sensor, brightness temperature is used to extract other meaningful atmospheric fields, providing an improved assessment of the TLC cloud systems. Satellite observations are analyzed by deriving cloud top altitude and temperature, vertical temperature gradient, and atmospheric pressure field [23,24]. Altitude and temperature are obtained by considering drops in air temperature and the dew point in standard atmospheric conditions, while sea surface temperature is taken into account in the evaluation of the atmospheric instability through the vertical temperature gradient.…”