Simulations and observations of extreme low-level updrafts in Hurricane Isabel

Very high-resolution (444 m) simulations of Hurricane Isabel are examined and are found to exhibit extreme (15-30 m/s) low-level updrafts which are substantially stronger than those simulated at lower resolution (1.33 km), even when interpolated to the same lower resolution. The height at which the maximum updrafts occur is also lower in the very-high resolution simulations, with a sharp peak in maximum vertical velocities around 2 km height, followed by a slower decay with height above. This low-level maximum is not seen in the lower resolution simulations.

The origins of these simulated extreme updrafts are investigated. Preliminarily, it appears likely that the extreme updrafts are dynamically forced, in association with large near-surface intensification of small scale low-level vortices. The simulated updrafts are also compared with dropsonde-observed extreme updrafts in terms of structure, magnitude, and location. The sensitivity of low-level updrafts to boundary layer parameters are explored through multiple simulations with varying drag coefficient (Cd) and boundary layer depth. When the boundary layer depth is forced to be reduced to 80% of that of the control simulation, the maximum updrafts are found 500-750 m lower, and are stronger. In contrast, when Cd is reduced to 80% of that of the control, the maximum updrafts are weakened by several m/s, while the height of the maximum is unchanged.