How do Cyclones Spin up? Hurricane Dennis as a Case Study.

Application of the flux form of the vorticity equation shows that a cyclone will develop when the tendency of convergence to enhance the low-level circulation of a system is greater than the tendency of surface friction to spin the system down. In the present work we study the developing stages of Hurricane Dennis by applying this form of the vorticity equation. We use dropsonde data released from the two NOAA P-3 aircraft in July of 2005 as part of the IFEX (Intensity Forecasting EXperiment) campaign. We find the convergence of absolute vorticity into the system directly from the dropsonde data. Then we find the spin-down tendency due to frictional forces by making the assumption that the surface stress is distributed uniformly through the depth of the inflow layer, taken to be the 100mb layer closest to the surface. These calculations enable us to evaluate the spin-up rate of the system. Our results show a negative tendency in the development of the cyclone which implies that it should not have spun up. Nevertheless, we know the system evolved into a hurricane. This supports earlier results obtained in TEXMEX (Tropical EXperiment in MEXico). We will discuss the possible causes of this discrepancy. The most likely cause is that the eddy transport of momentum is more complex than we have envisioned here.