Modeling Studies of Transitions from Slow to Fast Tropical Cyclone Intensification

Northwest Research Associates, Incorporated, Seattle WA

Investigators

Abstract

This research project is part of a broader effort to advance current understanding of how underdeveloped tropical cyclones intensify into potentially devastating hurricanes. Computer modeling studies will be conducted to address deficiencies in our knowledge of the nature and effectiveness of the intensification process that operates when there exists markedly asymmetric convection linked to substantial misalignment of the mid-level and low-level cyclonic circulations (tilt). Changes to the asymmetric state that are required for a fast intensification mechanism to supersede a slow mechanism, and the time scale for such changes to occur under a variety of environmental conditions will be elucidated. Knowledge gained from this project will assist complementary efforts to improve the accuracy of operational intensification forecasts that determine the preparations needed to adequately mitigate damage to coastal or island communities that lie in the predicted path of a tropical cyclone. In addition to offering new insights relevant to forecasting, this project will support student internships that will contribute to the development of the next generation of atmospheric scientists. The research strategy will entail a combination of reduced and full-physics modeling studies. The reduced modeling studies will add parameterized diabatic forcing to the dry primitive equations primarily to represent the heating associated with convection concentrated downtilt of the lower tropospheric vortex center. A variety of intensification mechanisms are expected to exist over the expansive multidimensional parameter space of the diabatic forcing, the state of the vortex, and the environmental vertical wind shear. The spectrum of possibilities ranges from very slow asymmetric modes of spinup to a fast mode associated with core reformation. The domain of applicability for each distinct intensification mechanism will be determined through extensive numerical experiments. Theoretical formulas will be sought for the hypersurfaces that separate regions of parameter space in which different intensification mechanisms operate. The full-physics simulations will be used to elucidate the moist thermo-fluid dynamics governing the convection that drives each intensification mechanism, and to elucidate the processes that lead a tropical cyclone to transition from slow to fast spinup. The sea surface temperature and vertical wind shear will be varied to uncover an assortment of transition-types and the environments in which they occur. Many of the transitions are expected to transpire through contraction or shear-relative reorientation of the tilt vector that characterizes the misalignment of the tropical cyclone. Thus, to better understand what controls the timing of a transition to fast spinup, a sizeable part of this project will involve analyzing how diabatic processes regulate the decay and precession rates of the tilts of the simulated tropical cyclones. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.