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David R. Ryglicki

Abstract

The interactions of the barotropic instability found at low levels in tropical cyclones and a shear forcing are presented. Previous works have indicated that at low levels of tropical cyclones, the inner edge of the core may be barotropically unstable and thereby able to support counterpropagating vortex Rossby wave interactions. It has also been demonstrated that hurricanes and other barotropic vortices possess innate, dry abilities to maintain themselves when under the duress of vertical wind shear. This work will address how these two separate processes interact with each other.

In this study, the barotropic ring is given additional vorticity in the outer regions to mimic observations more closely. This allows for the outward propagation of energy and simultaneous reduction of the radius of maximum wind. When this vortex is sheared, it is found that the shear forcing, which acts as a de facto wavenumber-1 forcing, does not noticeably alter the growth of the most unstable mode, wavenumber 3. The tilt precession of the vortex is altered greatly, as the tilt becomes both larger and slower. Palinstrophy and deformation analysis indicates that overall peak mixing is also reduced, owing to changes in the axisymmetrization process. Energetics analyses show that the radial component of the shear forcing acts to generate eddies while the tangential component of the shear tends to destroy eddies. The calculations are carried out a second time with another center-finding method, which shows the tilt to be much smaller and more variable while imparting a large wavenumber-1 signal in Fourier analyses.

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David R. Ryglicki, Daniel Hodyss, and Gregory Rainwater

Abstract

The interactions between the outflow of a tropical cyclone (TC) and its background flow are explored using a hierarchy of models of varying complexity. Previous studies have established that, for a select class of TCs that undergo rapid intensification in moderate values of vertical wind shear, the upper-level outflow of the TC can block and reroute the environmental winds, thus reducing the shear and permitting the TC to align and subsequently to intensify. We identify in satellite imagery and reanalysis datasets the presence of tilt nutations and evidence of upwind blocking by the divergent wind field, which are critical components of atypical rapid intensification. We then demonstrate how an analytical expression and a shallow water model can be used to explain some of the structure of upper-level outflow. The analytical expression shows that the dynamic high inside the outflow front is a superposition of two pressure anomalies caused by the outflow’s deceleration by the environment and by the environment’s deceleration by the outflow. The shallow water model illustrates that the blocking is almost entirely dependent upon the divergent component of the wind. Then, using a divergent kinetic energy budget analysis, we demonstrate that, in a full-physics TC, upper-level divergent flow generation occurs in two phases: pressure driven and then momentum driven. The change happens when the tilt precession reaches left of shear. When this change occurs, the outflow blocking extends upshear. We discuss these results with regard to prior severe weather studies.

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