Mechanism Governing the Size Change of Tropical Cyclone-Like Vortices

Abstract

To understand the basic mechanism governing the size evolution of tropical cyclones (TCs), we systematically perform numerical experiments using the primitive equation system on an f-plane. A simplified, TC-like vortex is initially given and an external forcing mimicking cumulus heating is applied to an annular region at a prescribed distance from the vortex center. Moist process and surface friction are excluded for simplification. We focus on the sensitivity of size evolution to the location of the forcing. The vortex size is defined as the radius of 15 m s -1 lowest-level wind speed (R15). The evolution of R15 depends on the forcing location, and its dependence can be understood by considering radial transport of the absolute angular momentum (AAM) at R15 due to the heat-induced secondary circulation (SC), whose structure is governed by the distribution of inertial stability. When the forcing is applied to the outer part of a vortex but still inside R15, where inertial stability is weak, the SC extends to the outside of R15 and carries AAM inward. Thus, R15 increases. Conversely, when the forcing is applied near the center of the vortex, where inertial stability is strong, the SC closes inside R15 and R15 hardly increases. These results indicate that extension of the heat-induced SC to the outside of R15 is important for the evolution of the vortex size. Moreover, the further beyond R15 the SC extends, the more the vortex size increases. This relationship is consistent with the result of the parcel trajectory analysis; the larger the extent of SC, the longer distances the parcels cover, conserving larger AAM. Finally, when the forcing is applied to the outside of R15, smaller AAM is carried outward by the SC on the inward side of the heating location, resulting in the decrease of R15.

The effects of initial vortex size on tropical cyclogenesis and intensification

“…Clearly, the effectiveness of convection in producing inflow depends not only on its radial location, but also on its intensity and areal (including azimuthal) extent. It is therefore not surprising that cyclones in which deep convective rain bands extend relatively far from the circulation centre tend to have larger outer circulations compared with storms where deep convection is more confined, a feature demonstrated, for example, in the numerical simulations by Fudeyasu and Wang (2011) and in highly idealized simulations by Tsuji et al (2016). Nevertheless, the question still remains as to why there are areal differences in the distribution of convection.…”

“…There have been numerous theoretical attempts to account for the size differences between storms, including the size changes that may occur during the lifetime of an individual storm (DeMaria and Pickle, 1988; Xu and Wang, 2010; Smith et al , 2011; Chan and Chan, 2014; Frisius, 2015; Kilroy et al , 2016; Tsuji et al , 2016). Some of these studies have pointed to a monotonic relationship between the final size of storms and the size of the initial disturbance from which they form, a finding that motivates the present study in part.…”

“…The further deep convection extends from the vortex centre, the more capable it will be of producing inflow at large radii, where the weaker inertial stability will be less of an impediment to radial motion. Thus, the existence of outer convection must be conducive to increasing the storm outer size, as demonstrated by Xu and Wang (2010) and Tsuji et al (2016). The foregoing angular momentum principle does not apply in the boundary layer, because the frictional torque reduces the value of M, i.e.…”

The Outer-Core Wind Structure of Tropical Cyclones

“…These include: a reduction of evaporative cooling in the outer rainbands (Sawada and Iwasaki 2010); cloud-radiative forcing (Bu et al 2014); changing the atmospheric temperature profile (Stovern and Ritchie 2016); and prescribing heating elements within certain rings around a vortex to mimic cumulus heating (Tsuji et al 2016). The main conclusions from these studies are essentially similar.…”

Effect of a Low-Frequency Vortex on the Size of Typhoon Lan (2017)

“…Previous studies suggested that numerous factors such as latitude, ocean basin, environmental pressure, relative humidity, the distribution of diabatic heating and sea surface temperature might affect TC size (Atkinson 1971;Merrill 1984;Frank and Gray 1980;Weatherford and Gray 1988;Cocks and Gray 2002;Liu and Chan 2002;Kimball and Mulekar 2004;Hill and Lackmann 2009;Chavas and Emanuel 2010;Chan and Chan 2012;Knaff et al 2014;Frisius 2015;Tsuji et al 2016;Sun et al 2017). For instance, Holland and Merrill (1984) found that the size of a TC was affected by its interaction with the environmental flow.…”