Gradient Imbalance and Upper Level Divergence in Tropical Cyclones from MAN12 Reanalysis

Tropical Cyclones (TCs) are known for their intense primary circulation which is sustained by their secondary circulation (i.e. in, up, out and down motion). This secondary circulation involves surface convergence and upper level divergence, and of these two, the mechanisms and the role of upper level divergence is poorly explored compared with all other aspects of TC dynamics. It is well known that near the surface the (quasi-steady) frictional convergence (which takes place due to the gradient imbalance) is crucial for the maintenance of the secondary circulation in TCs. At the same time, at the upper levels of TCs, a (quasi-steady) divergence of similar intensity is expected to be found. This upper level divergence can take place from two reasons (1) angular momentum fluxes of the rising air near the eyewall and (2) gradient imbalance as a result of strong curvature of the isobaric surfaces at upper levels. Theories of TCs intensity assume that the gradient balance hold everywhere above the boundary layer and thus include only the first of the two reasons for upper level divergence. In this study, an observational analysis is aimed to compare the contributions of both (1) and (2) to the divergence of the upper level wind field in several instance TCs. For this purpose the, high resolution, WRF based, North American Meso-Scale (NAM12) reanalysis data is used. The data are given at 12km-horizontal, 6h-temporal and 25mb-vertical resolutions. The gradient imbalance is calculated by the ratio of the geostrophic wind (Vg) to the curvature of the isobars (R) and the Coriolis parameter (f ), which must exceed a certain threshold for the gradient balance to hold (i.e. Vg/fR≤-¼), see Holton, JR. (2004) An Introduction to Dynamic Meteorology (4th edition). In those instance TCs, it is found indeed that the upper level pressure fields often breaks the Vg/fR threshold while in other parts of the upper levels (tropical or mid-latitude) atmosphere this threshold holds.