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Qingqing Li
,
Yuqing Wang
, and
Yihong Duan

Abstract

The impact of evaporation of rainwater on tropical cyclone (TC) intensity and structure is revisited in this study. Evaporative cooling can result in strong downdrafts and produce low–equivalent potential temperature air in the inflow boundary layer, particularly in the region outside the eyewall, significantly suppressing eyewall convection and reducing the final intensity of a TC. Different from earlier findings, results from this study show that outer rainbands still form but are short lived in the absence of evaporation. Evaporation of rainwater is shown to facilitate the formation of outer rainbands indirectly by reducing the cooling due to melting of ice particles outside the inner core, not by the cold-pool dynamics, as previously believed. Only exclusion of evaporation in the eyewall region or the rapid filamentation zone has a very weak effect on the inner-core size change of a TC, whereas how evaporation in the outer core affects the inner-core size depends on how active the inner rainbands are. More (less) active inner rainbands may lead to an increase (a decrease) in the inner-core size.

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Qingqing Li
,
Yuqing Wang
, and
Yihong Duan

Abstract

The effects of diabatic heating and cooling in the rapid filamentation zone (RFZ), within which inner rainbands are often active, on tropical cyclone (TC) structure and intensity are investigated based on idealized numerical experiments using a cloud-resolving TC model (TCM4). The results show that removal of heating (cooling) in the RFZ would reduce (increase) the TC intensity. Diabatic heating in the RFZ plays an important role in increasing the inner-core size whereas diabatic cooling tends to limit the inner-core size increase or even reduce the inner-core size of a TC. Removal of both diabatic heating and cooling in the RFZ greatly suppresses the activity of inner rainbands but leads to the quasi-periodic development of a convective ring immediately outside of the inner core. A similar convective ring also develops in an experiment with the removal of diabatic heating only in the RFZ. With diabatic cooling removed only in the RFZ, an annular-hurricane-like structure arises with the outer rainbands largely suppressed.

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Qingqing Li
,
Yuqing Wang
, and
Yihong Duan

Abstract

The dynamical process of outer rainband formation in a sheared tropical cyclone (TC) is examined in this study using the fully compressible, nonhydrostatic TC model. After the easterly vertical wind shear of 10 m s−1 was imposed upon an intensifying strong TC, an outer rainband characterized by a wavenumber-1 structure formed as a typical principal rainband downshear. Further analysis indicates that the outer rainband formation was closely connected to the activity of the inner rainband previously formed downshear. Moving radially outward, the inner rainband tended to be filamented owing to the strong radial gradient of angular velocity. As the inner rainband approached the outer boundary of the inner core, convection in its middle and upwind segments reinvigorated and nascent convective cells formed upwind of the rainband, caused mainly by the decreased filamentation and stabilization. Subsequently, the rainband reorganized into a typical outer rainband. Three different scenarios are found to be responsible for the outer rainband formation from downshear inner rainbands. The first is the outer rainband forming from an inner rainband downshear as a sheared vortex Rossby wave. The second is the outer rainband forming directly from a single deformation-induced inner rainband. The third is the outer rainband developing from an inner rainband downshear organized from a blend–merger of inner rainbands that were initiated from locally deformed convection upshear right.

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Qingqing Li
,
Yihong Duan
,
Hui Yu
, and
Gang Fu

Abstract

In this study, the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) is used to simulate Typhoon Rananim (2004) at high resolution (2-km grid size). The simulation agrees well with a variety of observations, especially for intensification, maintenance, landfall, and inner-core structures, including the echo-free eye, the asymmetry in eyewall convection, and the slope of the eyewall during landfall. The asymmetric feature of surface winds is also captured reasonably well by the model, as well as changes in surface winds and pressure near the storm center.

The shear-induced vortex tilt and storm-relative asymmetric winds are examined to investigate how vertical shear affects the asymmetric convection in the inner-core region. The inner-core vertical shear is found to be nonunidirectional, and to induce a nonunidirectional vortex tilt. The distribution of asymmetric convection is, however, inconsistent with the typical downshear-left pattern for a deep-layer shear. Qualitative agreement is found between the divergence pattern and the storm-relative flow, with convergence (divergence) generally associated with asymmetric inflow (outflow) in the eyewall. The collocation of the inflow-induced lower-level convergence in the boundary layer and the lower troposphere and the midlevel divergence causes shallow updrafts in the western and southern parts of the eyewall, while the deep and strong upward motion in the southeastern portion of the eyewall is due to the collocation of the net convergence associated with the strong asymmetric flow in the midtroposphere and the inflow near 400 hPa and its associated divergence in the outflow layer above 400 hPa.

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Jinjie Song
,
Philip J. Klotzbach
, and
Yihong Duan

Abstract

The impacts of El Niño on tropical cyclone (TC) activity over the western North Pacific (WNP) are examined through investigation of three types of tropical Pacific warming episodes according to where the maximum sea surface temperature (SST) anomalies occur in the equatorial Pacific: the eastern Pacific El Niño (EPE), the central Pacific El Niño (CPE), and the mixed El Niño (ME). More TCs form over the eastern part of the WNP in all three El Niño types, whereas the frequency of TCs over the western part of the WNP increases as the peak SST anomalies migrate from east to west. Although TCs more frequently recurve at higher latitudes during EPE and CPE, the most frequent region for recurving is much closer to the East Asian continent in CPE years than in EPE years. In contrast, more TCs track westward and threaten the Philippines in ME years. The increased TC genesis over the western part of the WNP can be explained by enhanced low-level relative vorticity, reduced vertical wind shear, and increased maximum potential intensity during CPE and increased midlevel moisture during EPE and ME. This increase is further related to updraft anomalies near the date line driven by an anomalous Walker circulation and an anomalous low-level cyclonic circulation over the WNP. The TC track differences among the different El Niño types are linked to the east–west shift of the western Pacific subtropical high, possibly caused by an anomalous Hadley circulation from 120° to 130°E that is strongly coupled with the anomalous Walker circulation.

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Hao Hu
,
Yihong Duan
,
Yuqing Wang
, and
Xinghai Zhang

Abstract

The diurnal variation of rainfall over China associated with landfalling tropical cyclones (TCs) is investigated using hourly rain gauge observations obtained from 2425 conventional meteorological stations in China. Records between 12 h prior to landfall and 12 h after landfall of 450 landfalling TCs in China from 1957 to 2014 are selected as samples. The harmonic analysis shows an obvious diurnal signal in TC rainfall with a rain-rate peak in the early morning and a minimum in the afternoon. The diurnal cycle in the outer region (between 400- and 900-km radii from the storm center) is found to be larger than in the core region (within 400 km of the storm center). This could be attributed to the effect of land on the inner core of the storms as the diurnal cycle is distinct in the core region well before landfall. As the result of this diurnal cycle, TCs making landfall at night tend to have cumulative precipitation, defined as the precipitation cumulated from the time at landfall to 12 h after landfall, about 30% larger than those making landfall around noon or afternoon. Moreover, the radial propagation of the diurnal cycle in TC rain rate, which has been a controversial phenomenon in some previous studies with remote sensing observations, was not present in this study that is based on rain gauge observations. Results also show that the diurnal signal has little dependence on the storm intensity 12 h prior to landfall.

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Hui Wang
,
Yuqing Wang
,
Jing Xu
, and
Yihong Duan

Abstract

This study examines the evolution of the warm-core structure during the secondary eyewall formation (SEF) and the subsequent eyewall replacement cycle (ERC) in a numerically simulated tropical cyclone (TC) under idealized conditions. Results show that prior to the SEF, the TC exhibited a double warm-core structure centered in the middle and upper troposphere in the eye region, and as the storm intensified with a rapid outward expansion of tangential winds, the warm core strengthened and a secondary off-center warm ring developed between 8- and 16-km heights near the outer edge of the eye. During the SEF, both the upper-level warm core and the secondary off-center warm ring rapidly strengthened. As the secondary eyewall intensified and contracted and the primary eyewall weakened and dissipated, the off-center warm ring extended inward and merged with the inner warm core to form a warm core typical of a single-eyewall TC. Results from the azimuthal-mean potential temperature budget indicate that the warming in the eye is due to subsidence and the warming above 14-km height outside the eye is largely contributed by radial warm advection in the outflow. The development of the off-center warm ring is largely due to the subsidence warming near the inner edge of the primary eyewall and in the moat area and the warming by diabatic heating in the upper part of the inner eyewall below 14-km height. Further analysis indicates that the eddy advection also played some role in the warming above 12-km height in the upper troposphere.

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Hui Wang
,
Yuqing Wang
,
Jing Xu
, and
Yihong Duan

Abstract

The axisymmetric and asymmetric aspects of the secondary eyewall formation (SEF) in a numerically simulated tropical cyclone (TC) under idealized conditions were analyzed. Consistent with previous findings, prior to the SEF, the tangential wind of the TC experienced an outward expansion both above and within the boundary layer near and outside the region of the SEF later. This outward expansion was found to be closely related to the top-down development and inward propagation of a strong outer rainband, which was characterized by deeper and more intense convection upwind and shallower and weaker convection downwind. In response to diabatic heating in the outer rainband was inflow in the mid- to lower troposphere, which brought the absolute angular momentum inward and spun up tangential wind in the inflow region and also in the convective region because of vertical advection. As a result, as the outer rainband intensified and spiraled cyclonically inward, perturbation tangential and radial winds also spiraled cyclonically inward and downward along the rainband. As it approached the outer edge of the rapid filamentation zone outside the primary eyewall, the downwind sector of the rainband in the boundary layer was rapidly axisymmetrized. Continuous inward propagation and axisymmetrization and secondarily the merging with inner rainbands led to the spinup of tangential wind in the boundary layer, enhancing surface enthalpy flux and convection and eventually leading to the simulated SEF. Our results demonstrate that the simulated SEF was a top-down process and was mainly triggered by asymmetric dynamics.

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Jianing Feng
,
Yihong Duan
,
Qilin Wan
,
Hao Hu
, and
Zhaoxia Pu

Abstract

This work explores the impact of assimilating radial winds from the Chinese coastal Doppler radar on track, intensity, and quantitative precipitation forecasts (QPF) of landfalling tropical cyclones (TCs) in a numerical weather prediction model, focusing mainly on two aspects: 1) developing a new coastal radar super-observation (SO) processing method, namely, an evenly spaced thinning method (ESTM) that is fit for landfalling TCs, and 2) evaluating the performance of the radar radial wind data assimilation in QPFs of landfalling TCs with multiple TC cases. Compared to a previous method of generating SOs (i.e., the radially spaced thinning method), in which the density of SOs is equal within the radial space of a radar scanning volume, the SOs created by ESTM are almost evenly distributed in the horizontal grids of the model background, resulting in more observations located in the TC inner-core region being involved in SOs. The use of SOs from ESTM leads to more cyclonic wind innovation, and larger analysis increments of height and horizontal wind in the lower level in an ensemble Kalman filter data assimilation experiment with TC Mujigae (2015). Overall, forecasts of a TC’s landfalling position, intensity, and QPF are improved by radar data assimilation for all cases, including Mujigae and the other eight TCs that made landfall on the Chinese mainland in 2017. Specifically, through assimilation, TC landing position error and intensity error are reduced by 33% and 25%, respectively. The mean equitable threat score of extreme rainfall [>80 mm (3 h)−1] forecasts is doubled on average over all cases.

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Johnny C. L. Chan
,
Yihong Duan
, and
Lynn K. Shay

Abstract

The interaction between a tropical cyclone (TC) and the underlying ocean is investigated using an atmosphere–ocean coupled model. The atmospheric model is developed from the Pennsylvania State University (Penn State)–National Center for Atmospheric Research (NCAR) mesoscale model version 4 MM4 and the ocean model consists of a mixed layer and an inactive stagnant layer beneath. Coupling between the atmosphere and the ocean models is achieved through wind stress and surface heat and moisture fluxes that depend on the sea surface temperature (SST). In the absence of a background flow, the atmospheric component consists of only a predefined vortex with an initial central pressure and the radius of the 15 m s−1 wind. The basic control experiments demonstrate that the coupled model can simulate the development of a TC and its interaction with the ocean.

Changes in TC intensity are sensitive to those of SST and the response is almost instantaneous. An SST of ∼27°C is found to be the threshold for TC development. In addition, the initial depth of the ocean mixed layer has an appreciable effect on TC intensity, which also depends on the movement of the TC. Furthermore, the vertical structure of ocean (vertical temperature gradient in the stagnant layer and temperature differential between the two layers) plays a significant role in modulating TC intensity.

In the presence of a warm core eddy (WCE), a TC intensifies before its center reaches the edge of the WCE. Although the TC attains maximum intensity at the center of the WCE, it does not weaken to its original intensity after leaving the WCE. During the entire passage of the TC, the SST at the center of the WCE decreases by about only 1°C, and the WCE generally maintains its original characteristics. However, two cold pools are observed around its periphery. A similar intensification process occurs when a TC moves over a sharp SST gradient and a locally deep ocean mixed layer. These results are explained by the interaction between the ocean and the TC circulation as well as the change in the total surface heat flux.

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