Investigation of the Rapid Intensity Changes of Landfalling Hurricanes Harvey and Irma (2017) through the Formalism of Scale Interactions

In this study, the energetics leading to the rapid intensity changes experienced by hurricanes Harvey and Irma (2017) are addressed using a scale-interaction approach. In this approach, a storm-centric cylindrical coordinate system is taken and the energies of the azimuthal harmonics are investigated in the wavenumber domain. The main questions asked here relate to the role of eddies organized at the scales of the hurricane (i.e wavenumbers 0,1,2) and at disorganized cloud scales (higher-order wavenumbers) in maintaining and disrupting the organization of deep convection in these hurricanes. Energy transfers in the wavenumber domain can either be (i) in-scale, such as the generation of eddy potential energy (covariance of convective heating and temperature) and the conversion of eddy potential energy to eddy kinetic energy due to vertical overturning (covariance of vertical velocity and temperature); or (ii) out-of-scale/across scales via nonlinear triad interactions. Examination of the energy transfers using the above approach along with the power spectral analysis of kinetic energy and rainwater ratio, highlight the importance of such eddies in the maintenance and disruption of the organization of convection, thereby leading to changes in hurricane intensity.

High-resolution, cloud permitting, Hurricane Weather Research Forecast model (HWRF) outputs are used for the diagnostics in this study. The examination is performed at the upper, middle and lower troposphere, for the inner, middle and outer radii as the hurricanes are at different proximities from the land (pre, during and post-landfall) when there is a major change in the organization of convection (E.g. rapid intensity changes, interactions with upper-level PV anomalies and landfall). This is primarily to understand the dominant scales in play and how they communicate within the hurricane vortex, in the hurricane environment, and between the environment and vortex. In addition, the potential impact of the locking and unlocking of the phases of the hurricane asymmetries on the hurricane intensity changes is noted.

Finally, to demonstrate that this approach has a universal message for the qualitative prediction and understanding of rapid intensity changes in hurricanes, the energetics and behaviors of Harvey and Irma are compared with two previous tropical cyclones - Phailin and Lehar (2013) that experienced contrasting intensity changes over the Bay of Bengal.