Meso-β Scale Vortex Coalescence and its Consequences on Tropical Cyclone Development in Idealized Numerical Experiments

Previous observational studies and numerical simulations suggest that developing tropical weather systems commonly contain a variety of mesoscale vortices. Such vortices may undergo complicated interactions that culminate in coalescence. This presentation will summarize recent findings of an ongoing investigation that aims to elucidate the diverse mechanisms and consequences of meso-β (20-200 km) scale vortex coalescence in a cloud resolving numerical model. The discussion will focus on computational experiments that simulate the merger of relatively weak cyclonic vortices on a tropical oceanic f-plane with a sea-surface temperature between 26 and 30 ^oC. Experiments having vortices initialized with maximal intensity in the middle troposphere have produced a number of unexpected results. To begin with, they have revealed that binary vortex interactions leading to middle-tropospheric merger can be detrimental to tropical cyclone development over a relatively cool ocean, if the initial vortex separation distance (D) falls within a special interval. The hindering of development found for such values of D distinctly coincides with considerable misalignment of surface and midlevel relative vorticity cores as merger progresses. The mechanics of the pertinent merger process has been examined in detail in the limiting case of dry adiabatic dynamics. In short, middle tropospheric merger appears to result primarily from the rotational tendencies of core misalignments that gradually amplify in each vortex as a consequence of their mutual shearing. Complementary theoretical work based on the preceding conceptual model has correctly predicted the conditions and time scale for merger to occur in a subset of simulations where diabatic effects are minimal during the vortex interaction period. Theory and simulations have also revealed that (in the relevant parameter regime) divergent winds have an important role in the adiabatic dynamics responsible for the rotation of vortex misalignments and middle-tropospheric merger. Although earlier studies have shown that diabatic processes often accelerate the coalescence of mesoscale vortices, moist convection and a simplified parameterization of atmospheric radiation are here found to impede and decelerate the merger process of interest under a variety of conditions. Such slowdown coincides with the diabatic hindering of the vortex misalignments that are deemed necessary for the adiabatic merger mechanism to operate. Comparisons of diabatic slowdown scenarios to diabatic acceleration scenarios will be presented, along with a discussion of the relevance of ideal two dimensional merger dynamics amid moist convection in developing systems. This work has been supported by NSF grant AGS-1250533.

References:

Schecter, D.A., 2017: A computational study on the nature of meso-β scale vortex coalescence in a tropical atmosphere. J. Adv. Model. Earth Syst., 9, 1366-1398.
Schecter, D.A., 2016: Development and nondevelopment of binary mesoscale vortices into tropical cyclones in idealized numerical experiments. J. Atmos. Sci., 73, 1223-1254.