Genesis of Hurricane Julia (2010) from an African Easterly Wave: Wave Structure and Possible Triggers

Tropical cyclogenesis (TCG) is one of the least understood processes in tropical meteorology today. The formation of tropical depressions (TDs), which under the right conditions grow into tropical storms (TSs), has a plethora of interacting processes. Furthermore, fewer than 10% of African Easterly Waves (AEWs) spawn named tropical cyclones (TCs). In this study, a 78 h cloud-resolving simulation of an AEW and its transition into TC Julia during the north Atlantic 2010 hurricane season is obtained using the Weather Research and Forecasting (WRF) model. Our study focuses on understanding the evolution of the AEW, the synoptic environment and the multi-scale processes leading to the TCG of Hurricane Julia (2010). The genesis of Julia from an AEW is traceable back five days before, based on satellite and in situ observational data. TCG occurred rapidly, taking only 18 h from the AEW's costal transition to TD status. The WRF model reasonably reproduces the track and intensity of the storm as it goes through genesis. The AEW provides a region of preferred development for convection with enhanced low-level convergence in a region of moderate vertical wind shear and sufficient low tropospheric moisture. Cyclonic vorticity generation is a bottom-up process with multi-scale interaction and the conglomeration of mesoscale cyclonic vorticity maxima associated with vortical hot towers (VHTs). Furthermore, the African Easterly Jet (AEJ) provides shear-generated cyclonic vorticity on its southern side, which aids in the development of mid-level cyclonic vorticity within the AEW. The merging of two near-surface vortices, each with different air mass characteristics, triggered TCG. The northern surface vortex was created by strong thermal heating below 850 hPa in conjunction with weak forced adiabatic descent in association with the movement of the AEW over higher terrain. This vortex had characteristics of high Convective Available Potential Energy (CAPE) and Equivalent Potential Temperature (theta-e) values near the surface but low column moisture content between 925 and 700 hPa (RH < 70%). The southern surface circulation remained in tact as the surface circulation associated with the AEW. This vortex had low CAPE and theta-e values but high low-level moisture content (RH > 70%). The vertical thermodynamic structure of the southern low consisted of a warm anomaly in the upper troposphere and weak cooling below 700 hPa. As these two vortexes merged off the west African coast, the high CAPE/theta-e air interacted with the moist low-tropospheric column associated with the AEW. Genesis took place just 30 h after the surface vortex merger with low and mid-level cyclonic vorticity elements quickly merging and becoming more coherent.