Vorticity budget in developing typhoon Nuri

The TPARC/TCS-08 project carried out in the Western Pacific basin in August and September of 2008 provides an outstanding data set on the genesis and development of typhoons in this region. Tools used in this project include dropsondes from multiple aircraft and airborne Doppler radar data from the ELDORA radar mounted on the NRL P-3. In this paper we use dropsonde and radar observations of developing typhoon Nuri to document all terms in the vorticity equation from pre-depression to typhoon. This development occurred over 4 days, with excellent Doppler radar and dropsonde coverage available for the first 3 days. The 4th day had dropsonde coverage only.

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.