Previous work has involved the development of analytical and numerical models of the tropical cyclone boundary layer. Key predictions have been that the wind maximum in the upper part of the boundary layer is up to about 25% super gradient in a stationary storm, depending on the radial wind profile, and that the ratio of the surface to gradient wind speeds increases towards the centre of the storm. In a moving storm, the jet is strongest and lowest to the left front in the Northern Hemisphere, while the wind speed ratio is highest on the left of the storm and can approach unity at the radius of maximum winds on the left hand side.

Here, a variety of observations are used to test these predictions. These include observations from aircraft, surface-based platforms, profilers, and dropsondes.