Annual Variability in the Force Balance at the Outflow Layer of Tropical Cyclones from NAM12-Reanalysis

The gradient imbalance is examined in the outflow level of large number of Tropical Cyclones (TCs) in a high-resolution reanalysis data. The gradient imbalance exists only around High-pressure centers when Vg/fR<-¼ (see Holton, 2004: An Introduction to Dynamic Meteorology, 4th edition) here Vg is the geostrophic wind R is the curvature of the isobars and f is the Coriolis parameter. The formation of a High-pressure center is possible only at the top of storms with a clear equivalent barotropic structure (isobars are parallel to isotherms on the planner view) and therefore an index is defined which quantifies the ability of a storm to develop such a High at its top and potentially have gradient imbalance there. We examine the correlation between this equivalent barotropic index, the gradient imbalance and the storms angular intensity in more than 50 storms (altogether having more than 700, 6-hourly, pressure maps) in the years 2004-2015 in the East Pacific and West Atlantic basins near the American continent. We use the, WRF based, North American Meso-Scale (NAM12) reanalysis in which data are given at 12km-horizontal, 6h-temporal and 25mb-vertical resolutions. The correlation between equivalent bartropic structure, gradient imbalance, upper level wind divergence and intensity is found to have clear annual variance. Moreover, in those years in which the gradient imbalance is dominant and correlates to intensity, the storms tend to be smaller more organized (and axi-symmetric) while in years where gradient imbalance is not dominant, the storms tend to be larger and less organized (and less axi-symmetric). The strong correlation between gradient imbalance and storm intensity in those cases shows that axi-symmetric storms require such imbalance as a divergence mechanism and motivates the study of axi-symmetric TCs in idealized models in which the assumption of gradient wind balance (i.e. the balanced vortex hypothesis as given by the Sawyer Eliassen equation) is relaxed.