Effect of Beta Shear on Simulated Tropical Cyclones

Abstract

Through cloud-resolving simulations, this study examines the effect of ß on the evolution of tropical cyclones (TCs). It is found that the TC simulated on a /3 plane with variable Coriolis parameter/is weaker in intensity but larger in size and strength than the TC simulated on an/plane with constant/. Such differences result mainly from the effect of the ß shear rather than from the variation of/due to the latitudinal change of the TC position, as illustrated in a three-stage conceptual model developed herein. The first stage begins with the establishment of the ß shear and the emergence of asymmetries as the TC intensifies. The ß shear peaks in value during the second stage that subsequently leads to the formation of an extensive stratiform region outside of the primary eyewall. The evaporative cooling associated with the stratiform precipitation acts to sharpen the low-level equivalent potential temperature gradient into a frontlike zone outside of the eyewall region, which leads to the burst of convection outside of the primary eyewall. The third stage is characterized by a weakening ß shear and the corresponding TC vortex axisymmetrization and expansion. The convection on the inner edge of the stratiform region becomes more organized in the azimuthal direction and eventually causes the TC structure to evolve in a manner similar to the secondary eyewall formation and eyewall replacement usually observed in TCs. It is the active convection outside of the primary eyewall that contributes to a relatively weaker but larger TC on the ß plane than that on the /plane.

Simulated Sensitivity of Tropical Cyclone Size and Structure to the Atmospheric Temperature Profile

“…The model has been adapted for use with idealized initial conditions to permit careful control of the initial-value experiments that enables us to examine the physical mechanisms associated with TC structural changes. WRF has been used by many previous sensitivity studies that explore mechanisms concerning TC size and intensity changes (e.g., Frank and Ritchie 2001;HL09;Fang and Zhang 2012;Mallard et al 2013). …”

Mechanism Governing the Size Change of Tropical Cyclone-Like Vortices

“…Because many external environmental factors affect the outer region of TCs, TC size is affected more by the number of external factors than TC intensity. In fact, many previous studies have indicated that various external factors affect TC size; examples are ocean basins and seasons (e.g., Merrill 1984;Chan and Chan 2012;Knaff et al 2014), synoptic environments (e.g., Liu and Chan 2002;Yuan et al 2007), β-effect (Fang and Zhang 2012), and latitude (e.g., Weatherford and Gray 1988a, b;Kimball and Mulekar 2004). The involvement of so many external environmental factors complicates the quantitative evaluation of the relationship between TC size and those factors.…”

The Roles of Asymmetric Inflow Forcing Induced by Outer Rainbands in Tropical Cyclone Secondary Eyewall Formation

“…This inflow is likely to enhance the boundary layer convergence and trigger convection in the inner-core region of TCs. Fang and Zhang (2012) suggested that the development of new convection preceding SEF might be facilitated with a frontlike feature in the low-level equivalent potential temperature field. The sharpening of equivalent potential temperature gradient was due to the evaporative cooling of rainwater under a wavenumber-1 stratiform cloud deck induced by b shear.…”