Xiaoping Cheng

Effects of the Cold Core Eddy on Tropical Cyclone Intensity and Structure under Idealized Air–Sea Interaction Conditions

AbstractThe 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...

Observational Occurrence of Tropical Cyclone Ducts from GPS Dropsonde Data

AbstractOn 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, esp...

Sensitivity of tropical cyclone intensity and structure to vertical resolution in WRF

In this paper the impacts of vertical resolution on the simulations of Typhoon Talim (2005) are examined using the Weather Research and Forecasting (WRF) model, with cumulus parameterization scheme representing the cumulus convection implicitly. It is shown that the tropical cyclone (TC) track has little sensitivity to vertical resolution, whereas the TC intensity and structure are highly sensitive to vertical resolution. It is partly determined by the sensitivity of the planetary boundary layer (and the surface layer) and the cumulus convection processes to vertical resolution. Increasing vertical resolution in the lower layer could strengthen the TC effectively. Increasing vertical resolution in the upper layer is also beneficial for the storm intensification, but to a lesser degree. In contrast, improving the midlevel resolution may cause the convergence of environmental air, which inhibits the TC intensification. The results also show that the impacts of vertical resolution on features of the TC structure, such as the tangential winds, secondary circulations and the evolution of the warm-core structure, are consistent with the impacts on the TC intensity. It is suggested that in the simulations of TCs, the vertical levels should be distributed properly rather than the more the better, with higher vertical resolution being expected both in the lower and upper layer, while the middle layer should not hold too many levels.

A Numerical Study on the Combined Effect of Midlatitude and Low-Latitude Systems on the Abrupt Track Deflection of Typhoon Megi (2010)

AbstractIn 2010, Supertyphoon Megi experienced an abrupt track deflection in the South China Sea (SCS) after traversing Luzon Island. To reveal the physical processes responsible for the timing and location of the sudden track deflection, the potential vorticity (PV) diagnosis and numerical simulations with initial strength perturbations are applied to investigate the individual and combined effects of environmental systems on Megi’s motion based on the steering flow theory. Results indicate that Megi’s northward track deflection was mainly determined by the effect of the midlatitude circulation, or rather, the break of the high pressure belt joined by the continental high (CH) and the Pacific subtropical high (SH). The retraction of CH played a particularly critical role in the break of the high pressure belt, making it the most important feature of the midlatitude circulation to determine Megi’s deflection. In addition, a small low-latitude anticyclone (SA) southeast of Megi was crucial in affecting the...

Characteristics of Mesoscale Convective Systems over China and Its Vicinity Using Geostationary Satellite FY2

AbstractThis 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 p...

A Numerical Study of the Interaction between Two Simultaneous Storms: Goni and Morakot in September 2009

Significant anomalous tracks were observed when the severe tropical storm Goni (0907) and typhoon Morakot (0908) in September 2009 were evaluated in short distances. The relationship between the two is regarded as a case of binary interaction. Based on an MM5 model (fifth-generation mesoscale model of Pennsylvania State University-National Center for Atmospheric Research), in this study a series of sensitivity experiments were designed to determine the binary interaction between them. The sensitivity of the storm characteristics to the binary interaction was demonstrated through modeling experiments with different TC intensities and sizes based on the bogus vortices initialization. Furthermore, the contributions of large-scale environmental flow and the effects of interaction between the motions of the cyclones were distinguished by numerical experiments using only one of the TC vortices.Results from these experiments show that Morakot (0908) had a greater impact on the motion of Goni (0907), whereas Goni (0907) had a relatively limited impact on Morakot (0908). At the upper level, the northeasterly jet flow in the third quadrant of Morakot (0908) enhanced the upper-level divergence of Goni (0907) and had an important role in maintaining and increasing Goni’s (0907) intensity. And at the lower level, Morakot (0908), with strong convergence and ascending airflow, made a stable transport channel of southwesterly warm and wet flow, thus supporting the lower-level water vapor convergence of Goni (0907). Goni (0907), which was located upriver of the southwesterly flow, intercepted part of the water vapor transportation in the southwesterly flow, causing the water vapor convergence to strengthen while the water vapor convergence of Morakot (0908) weakened.

Effects of sea spray evaporation and dissipative heating on intensity and structure of tropical cyclone

To examine effects of sea spray evaporation and dissipative heating on structure and intensity of a real tropical cyclone, the sea spray flux parameterization scheme was incorporated into the fifth-generation Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model (MM5). Sensitivity tests were performed with varying the spray source function intensities and with and without dissipation heating. The numerical results indicate that sea spray evaporation increases the interfacial sensible heat flux, which is increased by 16% for the moderate spray and 47% for the heavy spray, but has little effect on the interfacial latent heat flux. The net effect of sea spray evaporation is to decrease the total sensible heat flux and to increase the total latent heat flux. The total enthalpy flux is increased by 1% and 12% with moderate and strong spray amounts, respectively. Consistent with these results, the intensity of the tropical cyclone is increased by 5% and 16% in maximum 10-m wind speed, respectively, due to sea spray evaporation. Sea spray evaporation and dissipative heating modify the tropical cyclone structure in important but complex ways. The effect of sea spray on the near-surface temperature and moisture depends on the spray amounts and its location within the tropical cyclone. Within the high-wind region of a tropical cyclone, the lower atmosphere becomes cooler and moister due to the evaporation of sea spray. However, the dissipative heating offsets the cooling due to sea spray evaporation, which makes the lower atmosphere warmer.

Effect of wind-current interaction on ocean response during Typhoon KAEMI (2006)

The Weather Research and Forecasting (WRF) model, the Princeton Ocean Model (POM), and the wave model (WAVEWATCH III) are used to develop a coupled atmosphere-wave-ocean model, which involves different physical processes including air-forcing, ocean feedback, wave-induced mixing and wave-current interaction. In this paper, typhoon KAEMI (2006) has been examined to investigate the effect of wind-current interaction on ocean response based on the coupled atmosphere-ocean-wave model, i.e., considering the sea surface currents in the calculation of wind stress. The results show that the wind-current interaction has a noticeable impact on the simulation of 10 m-winds. The model involving the effect of the wind-current interaction can dramatically improve the typhoon prediction. The wind-current interaction prevents excessive momentum fluxes from being transferred into the upper ocean, which contributes to a much smaller turbulence kinetic energy (TKE), vertical diffusivity, and horizontal advection and diffusion. The Sea Surface Temperature (SST) cooling induced by the wind-current interaction during the initial stage of typhoon development is so minor that the typhoon intensity is not very sensitive to it. When the typhoon reaches its peak, its winds can disturb thermocline, and the cold water under the thermocline is pumped up. However, this cooling process is weakened by the wind-current interaction, as ocean feedback delays the decay of the typhoon. Meanwhile, the temperature below the depth of 30 m shows an inertial oscillation with a period about 40 hours (∼17°N) when sudden strong winds beat on the ocean. Due to faster currents, the significant wave height decreases as ignoring the wind-current interaction, while this process has a very small effect on the dominant wave length.

Asymmetric distribution of convection in tropical cyclones over the western North Pacific Ocean

Forecasts of the intensity and quantitative precipitation of tropical cyclones (TCs) are generally inaccurate, because the strength and structure of a TC show a complicated spatiotemporal pattern and are affected by various factors. Among these, asymmetric convection plays an important role. This study investigates the asymmetric distribution of convection in TCs over the western North Pacific during the period 2005–2012, based on data obtained from the Feng Yun 2 (FY2) geostationary satellite. The asymmetric distributions of the incidence, intensity and morphology of convections are analyzed. Results show that the PDFs of the convection occurrence curve to the azimuth are sinusoidal. The rear-left quadrant relative to TC motion shows the highest occurrence rate of convection, while the front-right quadrant has the lowest. In terms of intensity, weak convections are favored in the front-left of a TC at large distances, whereas strong convections are more likely to appear to the rear-right of a TC within a 300 km range. More than 70% of all MCSs examined here are elongated systems, and meso-β enlongated convective systems (MβECSs) are the most dominant type observed in the outer region of a TC. Smaller MCSs tend to be more concentrated near the center of a TC. While semi-circular MCSs [MβCCSs, MCCs (mesoscale convective complexes)] show a high incidence rate to the rear of a TC, elongated MCSs [MβECSs, PECSs (persistent elongated convective systems)] are more likely to appear in the rear-right quadrant of a TC within a range of 400 km.

Development and validation of an evaporation duct model. Part I: Model establishment and sensitivity experiments

Based on the Coupled Ocean-Atmospheric Response Experiment (COARE) bulk algorithm and the Naval Postgraduate School (NPS) model, a universal evaporation duct (UED) model that can flexibly accommodate the latest improvements in component (such as stability function, velocity roughness, and scalar roughness) schemes for different stratification and wind conditions, is proposed in this paper. With the UED model, the sensitivity of the model-derived evaporation duct height (EDH) to stability function (ψ), ocean wave effect under moderate to high wind speeds, and scalar roughness length parameterization, is investigated, and relative contributions of these factors are compared. The results show that the stability function is a key factor influencing the simulated EDH values. Under unstable conditions, the EDH values from stability functions of Fairall et al. (1996) and Hu and Zhang (1992) are generally higher than those from others; while under stable conditions, unreasonably high EDHs can be avoided by use of the stability functions of Hu and Zhang (1992) and Grachev et al. (2007). Under moderate to high wind speeds, the increase in velocity roughness length z0 due to consideration of the true ocean wave effect acts to reduce modeled EDH values; this trend is more pronounced under stable conditions. Although the scalar roughness length parameterization has a minor effect on the model-derived EDH, a positive correlation is found between the scalar roughness length z0q and the model-derived EDH.