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Shou-Jun Chen and Lorenzo Dell'osso

Abstract

The relative importance of latent beat release and surface sensible heat flux in a case of East Asian coastal cyclogenesis were investigated by performing numerical experiments with the ECMWF limited-area model. In the control experiment that included all physical processes, the cyclone developed rapidly in a way similar to that observed. In the experiment without latent beat feedback, only a shallow low appeared when the upper short-wave trough approached the inverted surface trough situated on the coast, but no further development took place. This suggests that the baroclinic forcing was enhanced by the feedback of physical process.

An increasingly unbalanced subtropical jet streak, an ageostrophic low-level jet and the associated vertical indirect circulation prior to the major development were well simulated in the Control but they were not simulated in the “dry” experiment (without latent heating). The latent heating had a profound impact on the amplifying jet streak circulation and the vertical coupling within the system which appeared to prime the rapid cyclogenesis along the coast.

The sensible beating contributed newly 18% to the surface development. It helped to build a potential temperature contrast along the coast below 900 mb. Without sensible heating, the model latent heat release was reduced. The results from the experiment without sensible and latent beating indicated that, the impact of sensible heating was partly through the moist process rather than direct beating.

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Shou-Jun Chen, Le-sheng Bai, and Stanley L. Barnes

Abstract

A real-time quasi-geostrophic omega diagnostic scheme, based on Hoskins' Q-vector analysis and developed by Barnes, was applied to a cold mesoscale vortex with severe convection over northeast China in summer. The limited area model used at the Beijing Weather Center did not predict this event because the baroclinic forcing was rather weak, but the Q-vector analysis clearly indicated the forcing 12 h before. In addition to Barnes' diagnostics, we estimate divergence tendency in low levels through computation of the rotational component of the Q-vector. Combined with the diagnosed stability tendency, moisture analysis, and low-level wind convergence zone, the convective area can be identified. This microcomputer diagnostic-graphics scheme, when coupled with intelligent use of conventional data, has potential as an aid for local short-range weather forecasting.

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Shou-Jun Chen, Ying-Hwa Kuo, Wei Ming, and Hong Ying

Abstract

Severe dust storms frequently occur over northwestern China during spring. They are often associated with strong fronts. In this paper, numerical simulations are performed to examine the effect of dust radiative heating on surface frontogenesis.

The absorption and multiple scattering of the dust are included in an atmospheric radiation scheme. A two-dimensional primitive equation model with 20 levels in the vertical is used for idealized simulations. After a 12-h integration a strong narrow front zone is created below 650 mb. The horizontal potential temperature gradient reaches 6 K (100 km)−1, which is three times as large as that in the initial data. A direct vertical transverse circulation is established along the frontal zone. which is qualitatively similar to the observations.

The results show positive interaction between low-level frontogenesis and dust radiative heating. The adiabatic frontogenesis forcing is enhanced by the feedback of the dust radiative heating. These results suggest that the dust heating can significantly affect mesoscale weather systems in arid and desert regions.

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Shou-Jun Chen, Le-Sheng Bai, and Ernest C. Kung

Abstract

To explicitly describe the energy exchange between meso and synoptic-scale motions, a diagnostic scheme of kinetic energy has been developed. By using a horizontal filtering technique, meteorological variables are separated into synoptic and mesoscale components. A set of budget equations are derived for the kinetic energy of synoptic scale motion V̂, the kinetic energy K′ of mesoscale motion V′, and the scalar product V̂·V′.

The scheme is applied to diagnose a severe rainstorm case over northern China during summer. The results show that the scale interactions between wind and height fields produce V̂·V′, which transfers kinetic energy to and K′. The term V̂·V′ thus acts as a medium in scale interactions conveying the energy between meso- and synoptic-scale motions and the potential energy source residing in the mass field.

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Shou-Jun Chen, Ying-Hwa Kuo, Wei Wang, Zu-Yu Tao, and Bo Cui

Abstract

On 12–13 June 1991, a series of convective rainstorms (defined as mesoscale precipitation systems with rainfall rates exceeding 10 mm h−1) developed successively along the Mei-Yu front. During this event, new rainstorms formed to the east of preceding storms at an interval of approximately 300–400 km. The successive development and eastward propagation of these rainstorms produced heavy rainfall over the Jiang-Huai Basin in eastern China, with a maximum 24-h accumulation of 234 mm. This study presents the results of a numerical simulation of this heavy rainfall event using the Penn State–NCAR Mesoscale Model Version 5 (MM5) with a horizontal resolution of 54 km.

Despite the relatively coarse horizontal resolution, the MM5, using a moist physics package comprising an explicit scheme and the Grell cumulus parameterization, simulated the successive development of the rainstorms. The simulated rainstorms compared favorably with the observed systems in terms of size and intensity. An additional sensitivity experiment showed that latent heat release is crucial for the development of the rainstorms, the mesoscale low-level jet, the mesolow, the rapid spinup of vorticity, and the Mei-Yu frontogenesis. Without latent heat release, the maximum vertical motion associated with the rainstorm is reduced from 70 to 6 cm s−1.

Additional model sensitivity experiments using the Kain–Fritsch cumulus parameterization with grid sizes of 54 and 18 km produced results very similar to the 54-km control experiment with the Grell scheme. This suggests that the simulation of Mei-Yu rainstorms, the mesoscale low-level jet, and the mesolow is not highly sensitive to convective parameterization and grid resolution. In all the full-physics experiments, the model rainfall was dominated by the resolvable-scale precipitation. This is attributed to the high relative humidity and low convective available potential energy environment in the vicinity of the Mei-Yu front.

The modeling results suggest that there is strong interaction and positive feedback between the convective rainstorms embedded within the Mei-Yu front and the Mei-Yu front itself. The front provides a favorable environment for such rainstorms to develop, and the rainstorms intensify the Mei-Yu front.

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Qing-Hong Zhang, Shou-Jun Chen, Ying-Hwa Kuo, Kai-Hon Lau, and Richard A. Anthes

Abstract

Typhoon Winnie (1997) was the fourth supertyphoon in the western North Pacific in 1997. In its mature stage, an outer eyewall, consisting of deep convection with a diameter of 370 km, was observed by satellite and radar. Within this unusually large outer eyewall existed an inner eyewall, which consisted of a ring of shallow clouds with a diameter of ∼50 km. In this study, Typhoon Winnie is simulated using a nested-grid version of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) with an inner grid length of 9 km. The model reproduces an outer cloud eyewall with a diameter of ∼350 km. The simulated radar reflectivity and hourly precipitation are verified with satellite microwave, infrared, and cloud brightness temperature images.

Analysis of the model results indicates that the large outer eyewall in many ways possesses the structure of a typical hurricane eyewall. This includes strong tangential winds and radial inflow outside the eyewall as well as an extremely large horizontal wind shear right at the eyewall. The outer eyewall is characterized with a ring of high vorticity (RHV). This RHV is closely related to a ring of high convergence (RHC). This RHC is caused by organized convective systems along the eyewall. The eye simulated by Winnie is characterized by a broad region of warm, dry slowly sinking air.

The factors determining the diameter of eyes in tropical cyclones are discussed by considering the scale of the environmental angular momentum and the maximum kinetic energy achieved by parcels of air originating in the environment and reaching the radius of maximum wind. It is hypothesized that the formation of a large eye is favored by large circulations in which parcels of air are drawn in toward the center of the storm from great distances, and trajectories of air in Winnie that support this hypothesis are shown.

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Shou-Jun Chen, Ying-Hwa Kuo, Pei-Zhong Zhang, and Qi-Feng Bai

Abstract

A climatological analysis of cyclogenesis over east Asia and the adjacent northwest Pacific for the period 1958–87 based on the Beijing Meteorological Center' historical surface maps is presented. The most active cyclogenetic areas were: 1) the lee sides of the Altai-Sayan, Stanovoi, and Great Xinganling mountains, and 2) the East China Sea and the Sea of Japan. The former was related to lee cyclogenesis and the latter to coastal cyclogenesis.

After zonal average, the primary zone of cyclogenesis emerged between 45° and 50°N, constituted mainly by the Altai-Sayan lee cyclogenesis. The Altai-Sayan lee cyclogenesis occurred in all seasons, with peak frequencies from April to May and from August to September. The secondary zone of cyclogenesis, located at 30°–35°N with half the frequency of the primary zone, was a result of East China Sea cyclogenesis. The coastal cyclogenesis occurred only in the cold season and disappeared in summer and early autumn. The total cyclogenetic events over east Asia reached a minimum in January. By contrast, cyclogenesis is still quite active over North America in January.

The trend of east Asia cyclogenesis showed a decline from 1958–77, which was coincident with the finding of Whittaker and Horn in North America. After 1977, no such decline was found.

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Shou-Jun Chen, Ying-Hwa Kuo, Pai-Zhong Zhang, and Qi-Feng Bai

Abstract

The climatology of explosive cyclogenesis off the east Asian coast was studied, based on 30 years (1958–87) of surface analyses. There were two favorable areas for explosive deepening, one over the eastern Sea of Japan, and the other over the northwestern Pacific, east and southeast of Japan. The latter was located close to the warm Kuroshio Current. The frequency of explosive cyclogenesis reached a local minimum over Japan. The geographic distribution of explosive-cyclone frequency suggests that the explosive cyclogenesis is influenced by the Japanese islands. In addition, a positive correlation is found between explosive-cyclogenesis frequency and the El Niño episodes during 1958–87. The physical relationship between these two phenomena, however, is not well understood.

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Qing-Hong Zhang, Kai-Hon Lau, Ying-Hwa Kuo, and Shou-Jun Chen

Abstract

On 7 June 1998, a mesoscale convective system (MCS), associated with a mesoscale cyclone, was initiated on the south side of a mei-yu front near Hong Kong and developed over the Taiwan Strait. In this study, numerical simulations for this event are performed using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). The model captures the evolution of the MCS, including the shapes of clouds and the rainfall rate.

In the mature phase of the simulated MCS, the MCS is composed of several meso-β- and meso-γ-scale convective clusters possessing commalike shapes similar to that of a midlatitude occluded cyclone. The cluster at the head of the “comma” consists of convective clouds that are decaying, while the tail of the comma is made up of a leading active convective line. A mesoscale cyclone, associated with a mesolow, at the tailing region of the leading convective line is well developed below 500 hPa. At 850 hPa, a mesoscale low-level jet (mLLJ) is located on the south side of the mesolow, and is directed toward the comma-shaped convective clusters. At 300 hPa, a mesohigh develops over the leading cluster. A mesoscale upper-level jet (mULJ) is located on the east side of this mesohigh. Relative streamline and trajectory analyses show that the mLLJ, associated with low θ e and sinking air motion, is a rear-inflow jet, while the mULJ is the outflow jet of the MCS. Monsoon air from the boundary layer in front of the MCS feeds deep convection within the MCS.

Momentum budget calculations are performed in the regions of the mLLJ and mULJ, at the developing and mature stage of the MCS. The pressure gradient force and the horizontal advection are the main contributors to the development of mLLJ in the developing stage. Although the effects of the pressure gradient force are weakened considerably when the MCS reaches maturity, the horizontal advection continues to accelerate the mLLJ. Vertical advection tends to decelerate the mLLJ both in its developing and mature stages. The pressure gradient force and vertical advection are responsible for generating the mULJ in the early stages, and maintaining the mULJ in its mature stage. Strong convective upward motion, which carries the horizontal momentum upward, from the exit of the mLLJ to the entrance of the mULJ, is crucial in the vertical coupling of the mLLJ and mULJ.

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