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Danyang Wang and Yanluan Lin

Abstract

The size and structure of tropical cyclones (TCs) are investigated using idealized numerical simulations. Three simulations are conducted: a pure dry TC (DRY), a moist reversible TC (REV) with fallout of hydrometeors in the atmosphere disallowed, and a typical TC (CTL). It was found that the width of the eyewall ascent region and the radius of maximum wind r m are much larger in DRY and REV than those in CTL. This is closely related to the deep inflow layer (~4 km) in DRY and REV associated with a different entropy restoration mechanism under the subsidence region. With the wide ascents, the close link between r m and the outer radius in DRY and REV can be well predicted by the Emanuel and Rotunno (ER11) model. The magnitude of subsidence, mainly controlled by the vertical gradient of entropy in the mid- and upper troposphere, is nearly one order greater in DRY and REV than that in CTL. This study demonstrates that the falling nature of hydrometeors poses a strong constraint on the size and structure of real world TCs via the entropy distribution in the subsidence region. The wide ascent, self-stratification in the outflow, and decently reproduced wind profile in DRY and REV suggest that DRY and REV behave like a prototype of the ER11 model with CTL being an extreme type.

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Danyang Wang and Yanluan Lin

Abstract

Tropical cyclone (TC) wind structure is important for its intensity change and induced damage, but its modulating factors remain to be explored. A heat-engine-based surface wind structure parameter α, reflecting TC’s relative compactness, is introduced and derived based on an entropy budget framework. We found that α is modulated by three key parameters: the thermodynamic efficiency ϵ PI in potential intensity theory, the Carnot efficiency ϵ C of the system, and the degree of irreversibility α irr of the system. A higher α irr contributes to a larger α and a lower heat engine efficiency. An expression linking TC intensity and compactness also emerges under this framework. Idealized simulations of a typical moist TC (CTL), a dry (DRY) TC, and a moist reversible TC (REV; in which hydrometeors do not fall out) evinced that the significantly higher α irr in CTL, due to irreversible entropy productions from precipitation dissipation, water vapor diffusion, and irreversible phase changes, contributes to its much larger compactness compared to DRY and REV. The study illustrates the importance of irreversible entropy production processes in modulating TC surface wind field. Simple estimate suggests that α will increase due to a hypothesized increased α irr with warming because of increased water content. This indicates that TCs will become more compact in a warmer climate.

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Danyang Wang, Jun-Ichi Yano, and Yanluan Lin

Abstract

The vorticity variability associated with the Madden–Julian oscillation (MJO) is examined. The analysis is focused on the 150-hPa pressure level, because a clear dipolar-vortex signal, reminiscent of the theoretically proposed strongly nonlinear solitary Rossby wave solution (albeit with the opposite sign), is seen in raw data at that level. A local empirical orthogonal function (EOF) analysis over the equatorial region of the Eastern Hemisphere (0°–180°E) identifies the two principal components representing an eastward propagation of a dipolar vortex trapped to the equator. Association of this propagation structure with the moist convective variability of the MJO is demonstrated by regressing the outgoing longwave radiation (OLR) against this EOF pair. The obtained evolution of the OLR field is similar to the one obtained by a direct application of the EOF to the OLR. A link of the local vorticity variability associated with the MJO to the global dynamics is further investigated by regressing the global vorticity field against the time series of the identified local EOF pair. The Rossby wave trains tend to propagate toward the Indian Ocean from higher latitudes, just prior to an initiation of the MJO, and in turn, they propagate back toward the higher latitudes from the MJO active region over the Indian Ocean. A three-dimensional regression reveals an equivalent barotropic structure of the MJO vortex pair with the signs opposite to those at 150 hPa underneath. A vertical normal mode analysis finds that this vertical structure is dominated by the equivalent height of about 10 km.

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