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Zhanhong Ma, Jianfang Fei, Xiaogang Huang, Xiaoping Cheng, and Lei Liu

Abstract

In Part II of this study, the influence of an oceanic cold-core eddy on the atmospheric boundary layer structures of Typhoon Francisco (2013) is investigated, as well as a comparison with the cold wake effect. Results show that the eddy induces shallower mixed-layer depth and forms stable boundary layer above and near it. The changes of these features shift from northwest to southeast across the storm eye, following the translation of Francisco over the eddy. Nonetheless, the decrease in mixed-layer depth and formation of stable boundary layer caused by the cold wake are located at right rear of the storm. The sensible heat fluxes at the lowest atmospheric model level are mostly downward over the sea surface cooling region. Due to their different relative locations with Francisco, the diabatic heating in the northwest quadrant of the storm can be more effectively inhibited by the cold-core eddy than by the cold wake. The asymmetric characteristics of surface tangential wind are less sensitive to sea surface cooling than those of surface radial wind, implying a change in surface inflow angle. Different from previous studies, the surface inflow angle is found to be reduced especially above the cold-core eddy and cold wake region. An analysis of radial wind tendency budget indicates that the decrease in radial pressure gradient is dominant in changing the acceleration rate of surface radial wind, rather than the decrease in the Coriolis and centrifugal forces, and therefore more outward surface flow is induced by both the cold-core eddy and cold wake.

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Zhanhong Ma, Jianfang Fei, Xiaoping Cheng, Yuqing Wang, and Xiaogang Huang

Abstract

In Part II of this study, the roles of surface sensible heat fluxes (SHX) in tropical cyclones (TCs) are further investigated in the context of sea spray processes. Results show that the sea spray evaporation is favorable for the TC intensification through enhancing the surface latent heat fluxes (LHX).

Unlike the results in Part I, the removal of SHX has led to a somewhat weaker TC by inclusion of sea spray. This is because the spray-mediated latent heat fluxes are simultaneously diminished after cutting down the SHX. Without the warming of SHX from the ocean, the surface air becomes cooler and thereby closer to saturation, which substantially hinders the evaporation of sea spray droplets. Therefore, the SHX are instrumental for sustaining the release of latent heat fluxes by sea spray evaporation. In the experiments of Part I and this study, the reduced total surface enthalpy fluxes as a result of the removal of SHX do not necessarily result in weakened TCs, while the larger LHX basically correspond to stronger TCs. This suggests that the TC intensity is largely dependent on the LHX rather than the total surface enthalpy fluxes, although the latter is generally dominated by the former. Relative roles of thermal and moisture effects in radially elevating the surface equivalent potential temperature θ e are also compared. The contributions of thermal effects account for 30%–35% of the total changes in θ e for mature TCs, no matter whether SHX from the ocean are included. This further implies that the SHX contribute insignificantly to the spinup of a TC.

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Xudong Peng, Jianfang Fei, Xiaogang Huang, and Xiaoping Cheng

Abstract

Official forecasts of tropical cyclone (TC) tracks issued by the China Meteorological Administration (CMA); the Regional Specialized Meteorological Centre in Tokyo, Japan; and the Joint Typhoon Warning Center (JTWC) were used to evaluate the accuracies, biases, and trends of TC track forecasts during 2005–14 over the western North Pacific. Overall, the JTWC demonstrated the best forecasting performance. However, the CMA showed the most significant rate of improvement. Two main zones were discovered in the regional distribution of forecast errors: a low-latitude zone that comprises the South China Sea and the sea region east of the Philippines, and a midlatitude zone comprising the southern Sea of Japan and the sea region east of Japan. When TCs moved into the former zone, there were both translational speed and direction biases in the forecast tracks, whereas slow biases were predominated in the latter zone. Twelve synoptic flow patterns of TCs with the largest error have been identified based on the steering flow theory. Among them, the most two common pattern are the pattern with the combination of cyclonic circulations, subtropical ridges, and midlatitude troughs (CRT, 26 TCs), and the pattern of the TCs’ track that cannot be explained by steering flow (NSF, 6 TCs). In the CRT pattern, TCs move northwestward forced by the cyclonic circulations and the subtropical ridges and then turn poleward and eastward under the influence of the midlatitude troughs. In the NSF pattern, storms embedded in the southwest flow by the cyclonic circulation and the steering flow suggest TCs should turn to the right and move northeastward but instead TCs persisted in moving northwestward.

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Zhanhong Ma, Jianfang Fei, Xiaogang Huang, and Xiaoping Cheng

Abstract

The contributions of surface sensible heat fluxes (SHX) to the evolution of tropical cyclone (TC) intensity and structure are examined in this study by conducting cloud-resolving simulations. Results suggest that although the peak values of SHX could account for nearly 30% of those of the total surface latent and sensible heat fluxes, the impact of SHX on TC intensification is nonetheless not distinct. However, the TC size shows great sensitivity to the SHX that the storm is shrunk by over 20% after removing the SHX.

A potential temperature budget analysis indicates that the adiabatic cooling accompanying the radial inflow is largely balanced by the transfer of sensible heat fluxes rather than the entrainment of subsiding air from aloft. If there is upward transfer of SHX from underlying ocean so that the near-surface potential temperature decreases upward, the SHX will play a vital role; instead, if the upward SHX are absent so that the potential temperature increases upward near the surface, the downward sensible heat fluxes become the dominant contributor to warm the inflow air. The changes in TC size are found to be primarily caused by the rainband activities. The SHX help maintain high convective available potential energy as well as the cold pool feature outside the eyewall, thus being crucial for the growth of outer rainbands. If without upward transport of SHX, the outer-rainband activities could be largely suppressed, thereby leading to a decrease of the TC size.

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Juli Ding, Jianfang Fei, Xiaogang Huang, Xiaoping Cheng, and Xiaohua Hu

Abstract

On the basis of global positioning system dropsonde data, Japan Meteorology Agency Regional Spectral Model analysis data, National Centers for Environmental Prediction reanalysis data, satellite products from the Naval Research Laboratory, and best-track tropical-cyclone (TC) datasets from the Shanghai Typhoon Institute, the statistical characteristics of the ducts induced by TCs (TC ducts) over the western North Pacific Ocean were analyzed for the period from September 2003 to September 2006, and two typical strong-duct cases with remarkable differences in formation cause were analyzed and compared. Of the total of 357 dropsondes, there are 212 cases that show ducting conditions, with an occurrence percentage of ~59%. Of the 212 TC-duct events, profiles with multiple ducting layers make up nearly one-half, with the main type of ducts being elevated ducts; in contrast, weak ducts make up over one-half, resulting in a weak median duct strength and thickness. Ducts formed in the transition zone, especially on the left side of TC tracks, tend to be much stronger and thicker than those formed inside TCs. The former are induced by the interaction between TCs and their surrounding systems, such as the inrush of dry and cold air from the north on the left side of TC tracks. The latter are associated with successive subsidence in the gaps between spiral cloud bands. With increasing TC intensities, the associated ducts inside TCs tend to be much stronger and thicker and to appear at higher altitudes.

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Zhanhong Ma, Jianfang Fei, Xiaogang Huang, and Xiaoping Cheng

Abstract

The moist static energy (MSE) is derived from the first law of thermodynamics and has been widely used in tropical cyclone (TC) studies because of its energetic and conventionally recognized conservation properties. This study investigates the validation of the MSE application in TC systems based on cloud-resolving numerical simulations. By examining the approximations made in deriving the MSE, neglecting the horizontal advection of pressure (namely, the generation of kinetic energy) relative to the vertical advection of pressure is found to be in error in the boundary layer of TCs with the horizontal advection of pressure even being several times larger than the vertical advection of pressure near the surface. Such a problematic approximation has broken down the conservation property of MSE in adiabatic conditions. An investigation of the energetic characteristics based on an MSE budget equation demonstrates that the MSE has created significant bias in evaluating the energy transport in the inner region of the TC boundary layer. Neglecting the kinetic energy conversion term in the boundary layer leads to a more strengthened cool-pool feature of MSE relative to the equivalent potential temperature; therefore, the interchangeable relationship between these two terms may also be inaccurate in the boundary layer. It is concluded that, although the MSE is an instrumental term for TC studies, caution should be taken when it is used in the boundary layer of TCs.

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Zhanhong Ma, Jianfang Fei, Lei Liu, Xiaogang Huang, and Xiaoping Cheng

Abstract

The impacts of ocean feedback on tropical cyclones (TCs) are investigated using a coupled atmosphere–ocean model under idealized TC and cold core eddy (CCE) conditions. Results reveal negative impacts of the ocean coupling on TC development. The cold wake induced by a TC not only weakens the TC intensity but also limits the expansion of the storm circulation. The presence of CCE has boosted the TC-induced sea surface temperature cooling, which conversely inhibits the TC development. The TC appears to be weakened as it encounters the CCE edge. The intensity reduction attains a maximum shortly after the TC passes over the CCE center, and simultaneously the CCE-induced asymmetry of the storm structure is most significant as well. The TC undergoes a period of recovery after departure from the CCE, lasting about 36–48 h. During this time the residual asymmetry caused by the CCE is smoothed gradually by storm axisymmetrization. The CCE has induced smaller TC size throughout the simulation even after the TC intensity has completely recovered, an indication of longer recovery time for the TC size. Notably cooler and moister eye air in the lower troposphere, just under the warm-core height, is found in the experiment with CCE. The water vapor mixing ratio budget analysis indicates that it is primarily attributed to changes in vertical advection that occurred in the eye, that is, the undermined eye subsidence associated with the suppressed eyewall convection. The horizontal patterns of vertical motion in the boundary layer are also distinctly changed by the CCE.

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Xiangcheng Li, Xiaoping Cheng, Jianfang Fei, Xiaogang Huang, and Juli Ding

Abstract

The duration of the eyewall replacement cycle (ERC) in typhoons is determined by the rate of dissipation of the inner eyewall and intensification of the outer eyewall, which is an important indicator for predicting changes in the intensity and structure of typhoons. Previous studies on ERCs have focused on the internal storm dynamics associated with the interactions between the concentric eyewalls (CEs), but the impacts of the sea surface cooling (SSC) on ERCs remain not adequately investigated. The slow movement of Typhoon Trami results in remarkable SSC. Using a coupled atmosphere–ocean model, the simulation for Trami generates an ERC that matches observations, whereas an unrealistic long-lived ERC is produced in the uncoupled simulation. Numerical simulations suggest that the typhoon-induced nonuniform SSC cannot only weaken the typhoon, but can also modulate the duration of the ERCs. The SSC acts like a catalyst for triggering the negative feedback between the surface heat exchange and the circulations of Trami to reduce the energy supply to the inner eyewall more severely where the sea surface temperature (SST) dropped more sharply. The SSC works in concert with the interactions between the CEs to weaken the inner eyewall faster, thus terminating the ERC of Trami rapidly. The results indicate that a better understanding of the modulation effect of SSC is required for the accurate forecast of ERCs.

Significance Statement

The duration of the eyewall replacement cycle in typhoons is determined by the rate of dissipation of the inner eyewall and intensification of the outer eyewall. While much is known about the cutoff effects of the outer eyewall on the dissipation of the inner eyewall, few studies have examined the dissipation induced by sea surface cooling. Using the coupled atmosphere–ocean model, the simulation for Trami generates an eyewall replacement cycle that matches observations, whereas an unrealistic long-lived one is produced in the uncoupled simulation. The results suggest that the typhoon-induced nonuniform sea surface cooling cannot only weaken the typhoon, but can also modulate the duration of the eyewall replacement cycle, which is essential for the accurate forecasting of eyewall replacement cycles.

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Yahua Wang, Xiaoping Cheng, Jianfang Fei, and Bowen Zhou

Abstract

This study investigates simulated fair-weather shallow cumulus-topped boundary layer (SCTBL) on kilometer- and subkilometer-scale horizontal resolutions, also known as the numerical gray zone of boundary layer turbulence. Based on a priori analysis of a simulated classic SCTBL with large-eddy simulation, its gray zone scale is determined. The dominant length scale of the cloud layer (CL) is found to be the effective cloud diameter, while that of the underlying mixed layer (ML) is the size of organized convection. The two scales are linked by a simple geometric argument based on vertically coherent updrafts, and are quantified through spectral analysis. Comparison to a simulated dry convective boundary layer (CBL) further reveals that the ML gray zone scale does not differentiate between clear and cloudy conditions with the same bulk stability. A posteriori simulations are then performed over a range of resolutions to evaluate the performance of a recently developed scale-adaptive planetary boundary layer (PBL) scheme. Simulation results suggest indifferences to the scale-adaptive capability. Detailed analyses of flux partition reveal that, in the absence of a shallow cumulus scheme, overly energetic resolved fluxes develop in the CL at gray zone and coarse resolutions, and are responsible for overpredicted resolved convection in the ML. These results suggest that modifications are needed for scale-adaptive PBL schemes under shallow cumulus-topped conditions.

Significance Statement

Shallow cumulus (ShCu) clouds play an important role in the dynamical and radiative processes of the atmospheric boundary layer. As the grid resolution of modern numerical weather prediction models approach kilometer and subkilometer scales, also known as the gray zone, accurate modeling of ShCu clouds becomes challenging due to difficulties in their parameterization. This study identifies the spatial scale that sets the gray zone of ShCu clouds, providing the key to building better parameterizations. Performance of existing parameterizations developed for clear-sky conditions is evaluated for cloudy conditions, exposing deficiencies and motivating further development.

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Xiangrong Yang, Jianfang Fei, Xiaogang Huang, Xiaoping Cheng, Leila M. V. Carvalho, and Hongrang He

Abstract

This study investigates mesoscale convective systems (MCSs) over China and its vicinity during the boreal warm season (May–August) from 2005 to 2012 based on data from the geostationary satellite Fengyun 2 (FY2) series. The authors classified and analyzed the quasi-circular and elongated MCSs on both large and small scales, including mesoscale convective complexes (MCCs), persistent elongated convective systems (PECSs), meso-β circular convective systems (MβCCSs), meso-β elongated convective system (MβECSs), and two additional types named small meso-β circular convective systems (SMβCCSs) and small meso-β elongated convective systems (SMβECSs). Results show that nearly 80% of the 8696 MCSs identified in this study fall into the elongated categories. Overall, MCSs occur mainly at three zonal bands with average latitudes around 20°, 30°, and 50°N. The frequency of MCSs occurrences is maximized at the zonal band around 20°N and decreases with increase in latitude. During the eight warm seasons, the period of peak systems occurrences is in July, followed decreasingly by June, August, and May. Meanwhile, from May to August three kinds of monthly variations are observed, which are clear northward migration, rapid increase, and persistent high frequency of MCS occurrences. Compared to MCSs in the United States, the four types of MCSs (MCCs, PECSs, MβCCSs, and MβECSs) are relatively smaller both in size and eccentricity but exhibit nearly equal life spans. Moreover, MCSs in both countries share similar positive correlations between their duration and maximum extent. Additionally, the diurnal cycles of MCSs in both countries are similar (local time) regarding the three stages of initiation, maturation, and termination.

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