Interactions between Simulated Tropical Cyclones and an Environment with a Variable Coriolis Parameter

Abstract

Numerical simulations of tropical cyclones are performed to examine the effects of a variable Coriolis parameter on the structure and intensity of hurricanes. The simulations are performed using the nonhydrostatic fifth-generation Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model using a 5-km fine mesh and fully explicit representation of moist processes. When a variable Conolis parameter ( f ) environment is applied to a mature tropical cyclone, a persistent north-northwesterly shear develops over the storm center as a result of an interaction between the primary circulation of the storm and the gradient in absolute vorticity. As a result, the variable-f storm quickly develops a persistent wavenumber-1 asymmetry in its inner-core structure with upward motion and rainfall concentrated on the left side of the shear looking downshear, in agreement with earlier studies. In comparison, the constant-f storm develops weak transient asymmetries in structure that are only partially related to a weak vertical wind shear. As a result, it is found that the tropical cyclone with variable f intensifies slightly more slowly than that with constant f, and reaches a final intensity that is about 5 mb weaker. It is argued that this "beta shear" is not adequately represented in large-scale analyses and so does not figure into calculations of environmental shear. Although the effect of the beta shear on the tropical cyclone intensity seems small by itself, when combined with the environmental shear it can produce a large net shear or it can reduce an environmental shear below the apparent threshold to impact storm intensity. If this result proves to be generally true, then the presence of an additional overlooked beta shear may well explain differences in the response of tropical cyclone intensification to westerly versus easterly shear regimes.

Effects of Vertical Wind Shear on Tropical Cyclone Precipitation

“…16d is the least asymmetric within the inner core (100 km) with values ranging from 9.5 to 3.8 mm h 21 , which may perhaps be due to its small sample size (63 overpasses). The differences are consistent with an enhancement by the beta gyre, as in numerical studies by Tuleya and Kurihara (1981), Ritchie and Frank (2007), and others. 16a) is more than double its southeasterly counterpart and has substantially larger sample size (i.e., 496).…”

“…This does not account for precipitation being carried cyclonically as it falls, which would further contribute to a left-of-shear precipitation enhancement. Ritchie and Frank (2007) examined how planetary vorticity affects vertical wind shear in simulated tropical cyclones in the absence of environmental shear. The strong shear (15 m s 21 ) case began to weaken within 3 h while the weak shear case (5 m s 21 ) did not weaken until ;36 h. A clear wavenumber-1 asymmetry (with maxima of rainfall and convection on the downshear-left side) did develop within a few hours for both the weak and strong shear cases, although this asymmetry did occur more rapidly for the 15 m s 21 simulation.…”

Tropical‐cyclone intensification and predictability in three dimensions

“…In fact, the large-scale β-gyres are virtually the same among all ensemble members. Our conclusions in this section differ profoundly from those of a recent paper by Ritchie and Frank (2007). They compared two deterministic experiments of a similar type to ours, using a similar version of MM5 with 5 km horizontal resolution.…”

Objective Identification of Annular Hurricanes

“…The greater asymmetry in tropical cyclone (TC) structure in these TC simulations is in a large part due to the vertical wind shear variations that result from the inclusion of the planetary vorticity advection. Simulations of tropical cyclones using environmental conditions similar to those documented in K03 have also been shown to result in a more axisymmetric tropical cyclone (Ritchie 2004). One can interpret these results as implying that beta shear in these simulations produces greater TC asymmetries, and if the environmental wind shear opposes the beta shear, these asymmetries are reduced.…”