Gradient Non-Balance: A New Approach for the Outflow at the Top of Tropical Cyclones in Models and Observations

This work address a new flow region that may develop at the top of TCs called gradient non balance and examines its occurrence and importance in both Weather, Research and Forecasting (WRF) simulations and observations.

The gradient non-balance is a flow-regime in which the gradient wind balance has no solutions for the pressure map around a high. This regime occurs when the sum of the outwards pointing centrifugal force (v²/r) and pressure gradient (∂φ/∂r=fv_g) forces cannot be balanced by the inward pointing Coriolis force (fv). The transition to this flow regime can be diagnosed by the point in which a non-dimensional number, the geostrophic Rossby number Ro_g=v_g/fr is smaller than -1/4. Due to its strong dependence on the radius, compact high pressure centers, such as those that may develop above a warm core of Tropical Cyclone (TCs hereafter), are likely to violated the Ro_g>-1/4 threshold for gradient balance. In a non-balance case some part of the radial pressure gradient acts on the radial velocity making the self-induced dynamics of the outflow at the top of TCs important.

Here we present an analysis Ro_g values in both model and observations at the top (15km height) of TCs. Observed pressure maps are given from the recent, high altitude, dropsoude observations taken during the Tropical Cyclone Intensity (TCI) field campaign. The simulations were done using the WRF model in different setups. The Ro_g values found at the top of TCs, both in observed and in simulated storms, strongly violate the gradient wind balance (Ro_g<-1/4), which contradict one of the key assumptions of the balance vortex model.

In simulated storms it is found that non-balance is accompanied by changes in both maximum wind and radius of maximum wind. While idealized storms return to balance within several days, simulations of real-world tropical cyclones retain a considerable degree of non-balance throughout the model integration. Comparing mean and maximum values of different simulated storms shows that peak non-balance correlates with either peak intensity or intensification, implying the possible importance of non-balance at upper levels for the near surface winds.