Study of Tropical Cyclone with Method of Field Observations and Laboratory Simulation

摘要

The eye of a topical cyclone (TC) is investigated. The eye size, according to observational data, can change severalfold while the pressure in the TC center remains unchanged; on average, the eye size changes more slowly during TC development than during its attenuation. On the basis of the results obtained with a laboratory simulation of an intensive convective vortex (ICV) of a moist type, it is proposed, as a version, to explain the cause of the change in TC eye diameter. Conditions required for the concentrated vortex are provided by an experimental unit representing a modified Fitzgerald camera. The model vortex structure is studied from basic similarity criteria. It corresponds to the central part of a developed tropical cyclone in the lower troposphere. The thermal Froud number is a governing criterion of similarity for vortices of such a class. The results of investigation, derived with the physical ICV model under conditions of a distinct eye in the vortex center, are considered. Measurements of relative moisture in the vortex allow us to treat the condensation heat release at a water phase transition as a local heat source. The available heat core in the central vortex leads to an increased density instability in the region. When the density instability reaches some critical value, it is resolved in the form of a thermal breakdown of the boundary layer, which determines the explosive nature of the vertical heat and moisture transfer in moist ICVs. The process is repeated with some quasiperiodic frequency. It is noted that the density instability increases in the vortex center more slowly than its relaxation. It is shown that the vortex eye diameter is also subject to quasiperiodic variations. An explanation for the pulsation of the vortex eye diameter is suggested: the stage of decrease in the eye diameter corresponds to the accumulated density instability in the vortex core, while its decrease is a result of a warm air outbreak during an explosive mechanism of heat and moisture removal. It is presumed that variations in TC eye diameter are also connected with heat breakdowns.