Tropical Storm Debby: Genesis dynamics and the relevance of the Saharan air layer

This study uses a mesoscale ensemble to study the dynamics and predictability of tropical storm Debby, which formed off the African coast in August 2006. Debby slowly intensified to a minimal tropical storm from 21 to 23 August, and it remained at that intensity until dissipating on 26-27 August. Storm strength in the ensemble after 84 h of integration, valid when Debby was a tropical storm, ranges from that of a tropical depression to a strong Category 1 hurricane.

Differences between ensemble members are used to compute statistical correlation, which elucidates why storms in some ensemble members strengthen and others do not. Storms in all ensemble members originate from a mesoscale, low-level vorticity anomaly present over Africa in the initial conditions. Though the initial strength of this anomaly is only weakly related to final storm intensity, after 24 h of integration the relation becomes very strong. Approximately half of final intensity spread is determined by variance in low-level vorticity at this time.

Thus, factors favoring intensification of the initial vortex are examined. Of particular interest is potential interaction with the Saharan air layer (SAL), which is thought to have impeded Debby's development. The role of CAPE and mid-level moisture (independent of the SAL), elevated levels of which have previously been shown to favor tropical cyclone intensification, are also examined. Finally, the relevance of jet dynamics early in the genesis period and deep-layer shear over the entire forecast are explored.