Formation and Quasi-Periodic Behavior of Outer Spiral Rainbands in a Numerically Simulated Tropical Cyclone*

Abstract

The formation and quasi-periodic behavior of outer spiral rainbands in a tropical cyclone simulated in the cloud-resolving tropical cyclone model version 4 (TCM4) are analyzed. The outer spiral rainbands in the simulation are preferably initiated near the 60-km radius, or roughly about 3 times the radius of maximum wind (RMW). After initiation, they generally propagate radially outward with a mean speed of about 5 m s 21 . They are reinitiated quasi-periodically with a period between 22 and 26 h in the simulation. The inner spiral rainbands, which form within a radius of about 3 times the RMW, are characterized by the convectively coupled vortex Rossby waves (VRWs), but the formation of outer spiral rainbands (i.e., rainbands formed outside a radius of about 3 times the RMW) is much more complicated. It is shown that outer spiral rainbands are triggered by the inner-rainband remnants immediately outside the rapid filamentation zone and inertial instability in the upper troposphere. The preferred radial location of initiation of outer spiral rainbands is understood as a balance between the suppression of deep convection by rapid filamentation and the favorable dynamical and thermodynamic conditions for initiation of deep convection.The quasi-periodic occurrence of outer spiral rainbands is found to be associated with the boundary layer recovery from the effect of convective downdrafts and the consumption of convective available potential energy (CAPE) by convection in the previous outer spiral rainbands. Specifically, once convection is initiated and organized in the form of outer spiral rainbands, it will produce strong downdrafts and consume CAPE. These effects weaken convection near its initiation location. As the rainband propagates outward farther, the boundary layer air near the original location of convection initiation takes about 10 h to recover by extracting energy from the underlying ocean. Convection and thus new outer spiral rainbands will be initiated near a radius of about 3 times the RMW. This will be followed by a similar outward propagation and the subsequent boundary layer recovery, leading to a quasi-periodic occurrence of outer spiral rainbands. In response to the quasi-periodic appearance of outer spiral rainbands, the storm intensity experiences a similar quasi-periodic oscillation with its intensity or intensification rate starting to decrease after about 4 h of the initiation of an outer spiral rainband. The results provide an alternative explanation or one of the mechanisms that are responsible for the quasi-periodic (quasidiurnal) variation in the intensity and in the area of outflow-layer cloud canopy of observed tropical cyclones.

Contributions of Surface Sensible Heat Fluxes to Tropical Cyclone. Part I: Evolution of Tropical Cyclone Intensity and Structure

“…For both runs, the surface-based CAPE ranges from 600 to 1200 J kg 21 in the outer region during the storm's lifetime. In the inner core, the CAPE decreases inward monotonously and possesses quite small values in the vicinity of the eyewall, which is consistent with the observational and modeling results (e.g., Bogner et al 2000;Li and Wang 2012;Molinari et al 2012). At the initial time (3 h), the storm intensity is slightly weakened by removing SHX in NOSH (Fig.…”

“…In the remainder of the simulation the eyewall in CTRL tends to be more compact. The inner and outer rainbands remain active but with fluctuations present, which should be related to the quasiperiodic behavior of rainbands (Li and Wang 2012). For NOSH the eyewall convection and inner rainbands are also active, but the characteristics of outer rainbands exhibit great discrepancy relative to those in CTRL.…”

“…negatively correlated (Fang and Zhang 2011;Li and Wang 2012). On the other hand, the CAPE could accumulate as a result of upward SHX (Fang and Zhang 2011).…”

A Comparison of Inner and Outer Spiral Rainbands in a Numerically Simulated Tropical Cyclone

“…Although rainbands are distinct features of a TC, they may interact with the eyewall in one way or another, modulating the TC structure and intensity (May and Holland 1999;Franklin et al 2006;Wang 2009;Wang and Xu 2010;Xu and Wang 2010a,b). Their activity, on one hand, is controlled by the TC internal dynamics (Li and Wang 2012) and, on the other hand, may be affected largely by different large-scale environmental forcings (Willoughby et al 1984;Guinn and Schubert 1993).…”

A Spatial Filter Approach to Evaluating the Role of Convection on the Evolution of a Mesoscale Vortex

“…Horizontal and vertical shear can force wavenumber-1, -2, and higher-order structure as part of the storm (Elsberry 1995). There are often spiral bands (Moon and Nolan 2010;Li and Wang 2012) that traverse less than half the storm azimuthally, vortex Rossby waves that exist on the strong vorticity gradient around the storm core (Montgomery and Kallenbach 1997), and the core itself can be highly asymmetric (Schubert et al 1999). All of these structures contain significant asymmetric structure, and thus when conventional separation approaches are used, are partially or totally removed from the vortex field and incorrectly distributed into the "nonvortex" field.…”