On The Genesis and Predictability of Hurricane Julia (2010)

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Tropical cyclogenesis (TCG) continues to be one of the least understood processes in tropical meteorology today. While a robust theoretical frame- work for TCG within African Easterly Waves (AEWs) has recently been developed, little work explores the mesoscale processes and interactions with the AEW during TCG. This study investigates the TCG of Hurricane Julia from the 2010 north Atlantic hurricane season using a series of high-resolution model simulation with the finest grid size of 1 km. In addition to a control simulation used to study the mesoscale processes during TCG, 20 ensemble simulations are conducted to identify key dynamical and thermodynamical processes taking place during TCG. These ensembles also serve to quantify the predictability of TCG while determining the processes responsible for ensemble solution disagreements.

It is found that the TCG of Hurricane Julia is triggered by the pronounced upper-tropospheric warming associated with organized deep convection. The upper-level warming is able to intensify and become a meso-α-scale feature due to a storm-scale outflow beyond the Rossby radius of deformation. The simulation confirms previous ideas by demonstrating that the intersection of the AEW's trough axis and critical latitude is a preferred location for TCG, while supplementing such work by illustrating the importance of upper-tropospheric warming and meso-α-scale surface pressure falls during TCG. Ensemble simulations further elaborate on the mechanisms by depicting substantial parametric differences between the stronger and weaker members. The dominant pattern of mean sea-level pressure ensemble differences is associated with the intensity of the pre-tropical depression (pre-TD), explaining nearly half of the total variance at the time of TCG. Similar patterns of differences are found for the low-level absolute vorticity and upper-tropospheric temperature anomalies.

An additional sensitivity simulation removing the latent heat of fusion associated with deposition results in significant changes to the TCG process. It is shown that the fusion heating occurring during deposition is important for the upper-tropospheric thermodynamic changes occurring during TCG and thus, yields fundamental changes to structure and intensity of deep convection. Overall, removing fusion heating from deposition results in a weaker MSLP disturbance and one that is not self-sustaining.