Search Results

You are looking at 1 - 10 of 33 items for

  • Author or Editor: Zhe-Min Tan x
  • Refine by Access: All Content x
Clear All Modify Search
Xin Qiu and Zhe-Min Tan

Abstract

This study analyzes the secondary eyewall formation (SEF) process in an idealized cloud-resolving simulation of a tropical cyclone. In particular, the unbalanced boundary layer response to asymmetric inflow forcing induced by outer rainbands (ORBs) is examined in order to understand the mechanisms driving the sustained convection outside the primary eyewall during the early phase of SEF.

The enhancement of convection in the SEF region follows the formation and inward contraction of an ORB. The azimuthal distribution of the enhanced convection is highly asymmetric but regular, generally along a half circle starting from the downwind portion of the ORB. It turns out that the descending radial inflow in the middle and downwind portions of the ORB initiates/maintains a strong inflow in the boundary layer. The latter is able to penetrate into the inner-core region, sharpens the gradient of radial velocity, and reinforces convergence. Consequently, warm and moist air is continuously lifted up at the leading edge of the strong inflow to support deep convection. Moreover, the inflow from the ORB creates strong supergradient winds that are ejected outward downwind, thereby enhancing convergence and convection on the other side of the storm. The results provide new insight into the key processes responsible for convection enhancement during the early phase of SEF in three dimensions and suggest the limitations of axisymmetric studies. There are also implications regarding the impact of the asymmetric boundary layer flow under a translating storm on SEF.

Full access
Kekuan Chu and Zhe-Min Tan

Abstract

Annular hurricanes, characterized by annular structure, are a subset of mature-stage intense tropical cyclones, and they tend to be stronger and persist longer than average tropical cyclones. The characteristics of annular hurricanes in the North Atlantic and eastern-central North Pacific Oceans are well documented by Knaff et al. However, little is known about the annular typhoons in the western North Pacific (WNP). This study investigates the general features of annular typhoons in the WNP based on a 20-yr analysis (1990–2009) of global storm-centered infrared brightness temperature and passive microwave satellite datasets. Similar to annular hurricanes, annular typhoons also only form under a specific combination of environmental conditions, resulting in a quite low occurrence rate (~4%), and only 12 annular typhoons occur during this period. The concentric eyewall replacement is one effective pathway to annular typhoon formation. Three annular typhoons experienced the concentric eyewall replacement within 24 h prior to their annular phases during this period. There are two seedbeds, located east of Taiwan and in the central WNP, for annular typhoon formation within a narrow zonal belt (20°–30°N). The former is conducive to the landfall of annular typhoons, in particular six of the nine annular typhoons that formed in this region eventually made landfall. Because the average time interval between landfall of the annular typhoons and the end of their annular phase is relatively short, about 30 h, they can maintain near-peak intensities and hit the landfalling areas with record intensities. They present a unique threat to eastern Asia but have received little attention from the scientific community so far.

Full access
Bolei Yang and Zhe-Min Tan

Abstract

Self-aggregation of convection can be considered as the simultaneous occurrence of dry patch initiation/amplification and wet patch contraction/intensification from initially uniform moisture and temperature fields. As the twin of wet patches, dry patches play an important role in moisture and energy balance during convective self-aggregation. In this study, the WRF Model is used to study the initiation of dry patches in convective self-aggregation, especially the continuous drying in their boundary layer (BL). In the dry patch BL, increased air density leads to an enhanced high pressure anomaly, which drives an amplifying BL divergent flow and induces an amplifying BL subsidence. The virtual effect of drying by subsidence counteracts warming by subsidence and the BL process, further increasing BL air density. Our analysis indicates the existence of a dry-subsidence feedback, which leads to the initiation of dry patches in convective self-aggregation. This feedback is shown to be important even in very large-scale (3000 km × 9000 km) cloud-resolving convective self-aggregation simulations.

Free access
Bolei Yang and Zhe-Min Tan

Abstract

Interactive radiation helps accelerate tropical cyclogenesis, but the mechanism is still unclear. Using idealized numerical modeling in the radiative–convective equilibrium framework, it is revealed that interactive radiation can bring forward tropical cyclogenesis by accelerating the development of the midlevel vortex. A strong horizontal longwave radiative warming anomaly in the layer between 6 and 11 km altitudes in the vortex region, caused by large concentration of ice-phased particles at high levels, is critical to the development of the midlevel vortex. This longwave radiative warming anomaly induces more upward water vapor flux (mainly in the nonconvective region) and then results in more latent heating at upper levels and more sublimation and melting cooling at lower levels. This leads to an increase of the vertical diabatic heating gradient, and then the intensification of the midlevel vortex. A stronger upward water vapor flux also produces more condensates at upper levels and further enhances the horizontal longwave radiative warming anomaly in the upper troposphere, constituting a positive feedback, and then accelerates tropical cyclogenesis.

Free access
Yi-Fan Wang and Zhe-Min Tan

Abstract

Secondary eyewall formation (SEF) could be considered as the aggregation of a convective-ring coupling with a tangential wind maximum outside the primary eyewall of a tropical cyclone (TC). The dynamics of SEF are investigated using idealized simulations based on a set of triplet experiments, whose differences are only in the initial outer-core wind speed. The triplet experiments indicate that the unbalanced boundary layer (BL) process driven by outer rainbands (ORBs) is essential for the canonical SEF. The developments of a secondary tangential wind maximum and a secondary convective ring are governed by two different pathways, which are well coupled in the canonical SEF. Compared with inner/suppressed rainbands, the downwind stratiform sectors of ORBs drive significant stronger BL convergence at its radially inward side, which fastens up the SEF region and links the two pathways. In the wind-maximum formation pathway, the positive feedback among the BL convergence, supergradient force, and relative vorticity within the BL dominates the spinup of a secondary tangential wind maximum. In the convective-ring formation pathway, the BL convergence contributes to the ascending motion through the frictional-forced updraft and accelerated outflow associated with the supergradient force above the BL. Driven only by inner rainbands, the simulated vortex develops a fake SEF with only the secondary convective ring since the rainband-driven BL convergence is less enhanced and thus fails to maintain the BL positive feedback in the wind-maximum pathway. Therefore, only ORBs can promote the canonical SEF. It also infers that any environmental/physical conditions favorable for the development of ORBs will ultimately contribute to SEF.

Open access
Jing Xu, Yuqing Wang, and Zhe-Min Tan

Abstract

An empirical relationship between sea surface temperature (SST) and the maximum potential intensification rate (MPIR) of tropical cyclones (TCs) over the North Atlantic has been developed based on the best-track TC data and the observed SST during 1988–2014. Similar to the empirical relationship between SST and the maximum potential intensity of TCs previously documented, results from this study show a nonlinear increasing trend of the MPIR with increasing SST, with a more rapid increasing trend when SST is higher than 27°C. Further analyses indicate that about 28% of intensifying TCs over the North Atlantic reached 50% of their MPIR and only 7% reached 80% of their MPIR at the time when they were at their lifetime maximum intensification rates. Moreover, a TC tended to have a larger intensification rate when it was located in regions with higher SST and lower vertical wind shear (VWS). This indicates that although the MPIR–SST relationship is much stronger than that for the IR rate versus SST for most TCs, the actual intensification rate of a TC is determined by not only the SST but also other environmental effects, such as VWS. Additional results from a simplified dynamical system previously developed for TC intensity prediction suggest an SST-dependent TC MPIR, similar to that fitted from observations. However, the MPIR obtained from the observational fitting seems to underestimate the MPIR in regions with low SST at higher latitudes where VWS is often large. Nevertheless, this study provides the observational evidence for the existence of the MPIR for TCs.

Full access
Yan Liu, Zhe-Min Tan, and Zhaohua Wu

Abstract

The interaction between tropical convective heating and thermally forced circulation is investigated using a global dry primitive-equation model with the parameterization of wave-conditional instability of the second kind (CISK). It is demonstrated that deep convective heating can hardly sustain itself through the moisture convergence at low levels regardless of the fraction of immediate consumption of converged moisture. In contrast, when the fraction is large, shallow convective heating and its forced circulation exhibit preferred growth of small scales. As the “CISK catastrophe” mainly comes from the instantaneous characters of moisture–convection feedback in the conventional wave-CISK, a noninstantaneous wave-CISK is proposed, which highlights the accumulation–consumption (AC) time scale for the convective heating accumulation and/or the converged moisture consumption. In the new wave-CISK, once moisture is converged, the release of latent heat takes place gradually within an AC time scale. In this sense, convective heating is not only related to the instantaneous moisture convergence at the current time, but also to that which occurred in the past period of the AC time scale. The noninstantaneous wave-CISK could guarantee the occurrence of convective heating and/or moisture convergence at larger scales, and then favor the growth of long waves, and thus solve the problem of CISK catastrophe. With the new wave-CISK and AC time scale of 2 days, the simulated convective heating-driven system bears a large similarity to that of the observed convectively coupled Kelvin wave.

Open access
Xin Qiu, Zhe-Min Tan, and Qingnong Xiao

Abstract

A high-resolution, full-physics model initiated with an idealized tropical cyclone–like vortex is used to simulate and investigate the secondary eyewall formation. The beta skirt axisymmetrization (BSA) hypothesis previously proposed is examined and the roles of axisymmetrizing vortex Rossby waves (VRWs) in the secondary eyewall formation are further investigated. During the formation period, convection outside the inner-core region is organized into an outer spiral rainband. The PV dipoles that are persistently generated by convective updrafts through tilting effect move along the rainband and inward toward inner-core region and are finally axisymmetrized in the preexisting beta skirt region. The formation of the secondary eyewall is preceded by a rapid intensification period, during which vortical hot towers, discrete VRWs, and sheared VRWs dominate the inner-core asymmetric structures. Sheared VRWs are repeatedly emanated from the outer edge of the eyewall and become more concentric when propagating outward, leading to the formation of a weak but nonnegligible secondary circulation near the VRWs’ stagnant radius. The mean tangential flow is accelerated by the low-level convergence associated with the secondary circulation and also by the wave–mean flow interaction mechanism, both of which are elucidated by absolute angular momentum budget calculation. The mean radial gradient of relative vorticity is enhanced across the stagnant radius, causing the extension of beta skirt to outer radii in the lower-tropospheric levels. Results from this study suggest that the stagnant radius mechanism and the BSA mechanism may work cooperatively in the sense that the former helps to establish an extensive beta skirt and the latter takes charge from then on.

Full access
Yi-Peng Guo and Zhe-Min Tan

Abstract

This study investigated the impacts of the interannual variability in the boreal spring regional Hadley circulation over the Indo-Pacific warm pool (IPWP) on the tropical cyclone (TC) activity over the western North Pacific (WNP). The principal modes of the interannual variability in the IPWP Hadley circulation were calculated using empirical orthogonal function (EOF) analysis. The leading mode (EOF-1) features cross-equatorial southerly wind anomalies over the Indian Ocean and Maritime Continent and has an evident impact on WNP TC activity during summer. In the summer following a positive phase of the EOF-1, a cyclonic circulation anomaly, with upward motion, positive relative vorticity anomalies, and weak sea level pressure, dominates the WNP, and this favors increased TC genesis. However, large positive vertical wind shear anomalies over the South China Sea and Philippine Sea inhibit the TC intensification. A positive wind–sea surface temperature (SST)–precipitation feedback was found to facilitate the ability of the signal of the EOF-1 to persist until the summer. The westerly wind anomalies converge around 10°N over the WNP, thus increasing precipitation, and this increased precipitation enhances the westerly wind anomalies via a Gill-type response. The strengthened westerly wind anomalies increase total wind speeds, which in turn cool the SST in the Bay of Bengal and the South China Sea, and warm the SST in the eastern WNP, increasing the zonal SST gradient. Consequently, this increased zonal SST gradient further enhances the westerly wind anomalies, strengthens the monsoon trough, and increases the WNP precipitation further. Therefore, the WNP precipitation anomalies are sustained into the summer.

Open access
Yan Liu, Zhe-Min Tan, and Zhaohua Wu

Abstract

Recent study indicates that the non-instantaneous interaction of convection and circulation is essential for evolution of large-scale convective systems. It is incorporated into cumulus parameterization (CP) by relating cloud-base mass flux of shallow convection to a composite of subcloud moisture convergence in the past 6 h. Three pairs of 19-yr simulations with original and modified CP schemes are conducted in a tropical channel model to verify their ability to reproduce the Madden–Julian oscillation (MJO). More coherent tropical precipitation and improved eastward propagation signal are observed in the simulations with the modified CP schemes based on the non-instantaneous interaction. It is found that enhanced feedback between shallow convection and low-level moisture convergence results in amplified shallow convective heating, and then generates reinforced moisture convergence, which transports more moisture upward. The improved simulations of eastward propagation of the MJO are largely attributed to higher specific humidity below 600 hPa in the free troposphere to the east of maximum rainfall center, which is related to stronger boundary layer moisture convergence forced by shallow convection. Large-scale horizontal advection causes asymmetric moisture tendencies relative to rainfall center (positive to the east and negative to the west) and also gives rise to eastward propagation. The zonal advection, especially the advection of anomalous specific humidity by mean zonal wind, is found to dominate the difference of horizontal advection between each pair of simulations. The results indicate the vital importance of non-instantaneous feedback between shallow convection and moisture convergence for convection organization and the eastward MJO propagation.

Restricted access