Rapid development from a depression into a tropical storm was documented on the second of these missions. During this period spinup due to vorticity convergence far exceeded the spindown tendency of surface friction. This convergence was aided by the presence of ambient vorticity approximately 10 times the Coriolis parameter. The vorticity pattern at low levels had a great deal of fine-scale structure, much of which appeared to be associated with deep convective towers. Spinup associated with tilting was moderately strong, but variable in sign and less important than the convergence of vorticity.
The extreme imbalance between the spinup tendency due to convergence and the spindown tendency due to friction indicates that the Ekman pumping mechanism used in many conceptual and simplified numerical models to represent the low-level convergence in tropical cyclones can underestimate this convergence by a factor of 10 in developing systems.
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