Improving Understanding and Prediction of Atlantic Tropical Cyclogenesis

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Intellectual merit: The goals/tasks of this research are to (a) perform 6-7 day cloud-resolving simulations of two tropical cyclone (TC) cases associated with African Easterly Waves (AEWs): one of which developed into a hurricane and the other did not; (b) explore the predictability of the two storms; and (c) examine the roles of different environmental conditions and internal dynamics in determining the different TC genesis and evolution scenarios. The following three research questions will be addressed: (i) To what extent are the two TCs predictable when the associated AEWs were still located over West Africa? (ii) What are the effects of the Saharan Air Layer (SAL), AEWs and convectively generated vortices (CGVs) on Atlantic TC genesis? (iii) What are the processes accounting for the transformation of low-level cold anomalies to a warm-cored vortex? Does TC genesis from AEWs occur from the bottom upward or the top downward? It is hypothesized that (a) AEWs provide the necessary quasi-balanced upward motion for the organization of deep convection and rotation via stretching; (b) TC genesis occurs from bottom up through vortical hot towers, inward potential vorticity flux convergence and upscale growth through CGV mergers; and (c) vertical shear, dry intrusion and convergence of the stable air mass associated with the SAL determine whether or not AEWs could develop to TCs with hurricane intensity. To validate/address the above hypotheses/questions, the Principal Investigator will simulate the TC systems using the Weather Research and Forecast (WRF) model with the finest grid resolution of 1 km. Then, ensemble integrations, initialized with random perturbations or with field observations over West Africa, will be carried out to examine the error growth characteristics and the predictability of TC genesis from AEWs. Diagnostic analyses and budget calculations will be performed to clarify the three-dimensional structures and evolution of the AEWs, and the SAL in relation to the TCs, and to study the upscale growth of CGVs and the thermodynamic transformation. The effects of the SAL, and AEWs on TC genesis will be investigated through sensitivity simulations by modifying their magnitudes and distributions in the model initial conditions with the piecewise potential vorticity inversion technique. Broad impacts: Successful completion of this project will improve understanding and prediction of Atlantic TC genesis from AEWs, particularly the interaction of the SAL with Atlantic storms. Results may benefit the operational 5-day prediction of Atlantic hurricanes, and improve evacuation planning and disaster reduction, thereby facilitating the protection of life and property in the coastal regions. In addition, this project will support graduate education and generate both peer-reviewed journal publications and conference presentations.