Effects of surface exchange coefficients for high wind speeds on typhoon structure

Recent studies have revealed the behavior of surface exchange coefficients, especially, for momentum under high wind speed conditions such as hurricane conditions; the coefficients tend to decrease in strong winds.

Based on the previous studies, we examine the sensitivity of typhoon structure as well as intensity to the surface drag by using a coupled atmosphere--wave model that incorporates the surface exchange coefficients behaviors in strong winds.

Specifically, the coefficients both for heat/moisture and momentum level off as wind speed increases and decrease under conditions where the wind speed further increases and the ratio of the coefficients is constant.

By using the models, we conducted numerical simulations for Typhoon IOKE (2006).

It is shown that the reduced exchange coefficients for momentum and heat in strong winds produce a remarkable increase (decrease) of surface wind speed (surface fluxes) but little difference of minimum surface pressure.

In addition, the total water content and vertical velocity, as a proxy for convective intensity, decrease only in the eyewall region, due to the reduced surface fluxes.

As a result, the precipitation within the eyewall also decreases.

Although the modified profile of drag coefficient levels off at high wind speed, surface convergence does not change, rather increase slightly outside the radius of maximum wind speed.

It is suggested that the structure of typhoon is critically determined by the surface heat fluxes.

In order to focus on the dynamical and thermodynamical influences on typhoon structure, we also conducted numerical experiments under idealized conditions.

In these experiments, we will specifically examine the sensitivity of typhoon structure to the ratio of surface exchange coefficients for enthalpy and momentum, for instance, how the ratio affects the convection activities not only in the eyewall region but also in the spiral rainband region.