Evaluating the vertical structure of tangential winds in tropical cyclone simulations: comparison to observations and theory

We have previously examined the vertical structure of tangential winds in tropical cyclones from both an observational and theoretical perspective (Stern and Nolan 2009). In particular, we investigated what determines the outward slope with height of the Radius of Maximum Winds (RMW), and whether or not the RMW is a surface of constant absolute angular momentum (M). Here, we compare our prior results to those of a set of idealized numerical simulations.

In the context of the axisymmetric theory of Emanuel (1986), we have shown that the RMW is very nearly an M surface, because of the constraints of thermal wind balance and slantwise moist neutrality. Observations confirm this prediction to a certain extent, although there is a small but systematic tendency for M to decrease upwards along the RMW of real storms. In contrast, simulated hurricanes evolve towards a state where M increases substantially upwards along the RMW.

Theoretically, the outward slope of M surfaces and the RMW should increase approximately linearly with radius (at constant height), and should be essentially independent of intensity. Observations agree quite well with this prediction, while the simulations yield mixed results.

The degree to which slantwise moist neutrality is a valid approximation for tropical cyclones is investigated in the simulations. Within the eyewall, the vortex is actually conditionally unstable below about 4.5 km, while it is stable above. In spite of this, the rate at which the maximum winds decay with height is often well predicted by theory.