Gradient Balance in Tropical Cyclones

Analysis of a large inventory of in situ observations from research aircraft shows that the gradient wind approximates the axisymmetric swirling flow in the free atmosphere within 150 km of the centers of Atlantic hurricanes and tropical storms. In the middle and lower troposphere, the rms difference between the azimuthal cream swirling and gradient winds is typically < 1.5 m s−1 with zero bias. This balance prevails only for the azimuthal mean, not locally, nor is balance to be expected in either the surface friction layer or the upper tropospheric outflow layer where the radial flow is comparable with the swirling flow. It is theoretically possible that axisymmetric supergradient flow may occur in response to rapid radial acceleration where the radial flow slows in the friction layer beneath the eyewall or where it converges into intense diabatically forced updrafts. Nevertheless, the observations in the free lower and midtroposphere show that systematic departures of the azimuthal mean vortex from balance are too small to measure.