Shou-Jun Chen

Development of a next-generation regional weather research and forecast model.

The Weather Research and Forecast (WRF) project is a multi-institutional effort to develop an advanced mesoscale forecast and data assimilation system that is accurate, efficient, and scalable across a range of scales and over a host of computer platforms. The first release, WRF 1.0, was November 30, 2000, with operational deployment targeted for the 2004-05 time frame. This paper provides an overview of the project and current status of the WRF development effort in the areas of numerics and physics, software and data architecture, and single-source parallelism and performance portability.

A Modeling Case Study of Heavy Rainstorms along the Mei-Yu Front

On 12‐13 June 1991, a series of convective rainstorms (defined as mesoscale precipitation systems with rainfall rates exceeding 10 mm h21) 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 21. 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.

Climatology of Explosive Cyclones off the East Asian Coast

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 Nino episodes during 1958–87. The physical relationship between these two phenomena, however, is not well understood.

A Numerical Study of a Mesoscale Convective System over the Taiwan Strait

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-b- and meso-g-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 ue 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.

The Effect of Dust Radiative Heating on Low-Level Frontogenesis

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 tha...

A GPS/MET Sounding through an Intense Upper-Level Front

A Global Positioning System Meteorology (GPS/MET) proof-of-concept experiment became a reality on 3 April 1995. A small satellite carrying a modified GPS receiver was launched into earth orbit to demonstrate the feasibility of active limb sounding of the earths neutral atmosphere and ionosphere using the radio occultation method. On 22 October 1995, a GPS/MET occultation took place over northeastern China where a dense network of radiosonde observations was available within an hour of the occultation. The GPS/MET refractivity profile shows an inflection, and the corresponding temperature retrieval displays a sharp temperature inversion around 310 mb. Subjective analyses based on radiosonde observations indicate that the GPS/MET occultation went through a strong upper-level front. In this paper, the GPS/MET sounding is compared with nearby radiosonde observations to assess its accuracy and ability to resolve a strong mesoscale feature. The inflection in the refractivity profile and the sharp frontal inversion seen in the GPS/MET sounding were verified closely by a radiosonde located about 150 km to the east of the GPS/MET occultation site. A similar frontal structure was also found in other nearby radiosonde observations. These results showed that high-quality GPS/MET radio occultation data can be obtained even when the occultation goes through a sharp temperature gradient associated with an upper-level front.

Numerical Study of a Typhoon with a Large Eye: Model Simulation and Verification

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 out...

Climatology of deep cyclones over Asia and the Northwest Pacific

SummaryA frequency analysis of deep cyclones with central pressure less than or equal to 990 hPa over Asia and the Northwest Pacific in the period 1958–1989 is presented. The most active areas of deep cyclones are: 1) Western Siberia, east of the Ural Mountains; 2) Northeastern China, east of the Mongolia Plateau and, 3) South-west of the Kamchatka Peninsula. The first most active area is related to European cyclones (Schinke, 1993) and starts in the lee of the Ural Mountains; the second is related to cyclones in the lee of Altai-Sayan and the third to East Asian coastal cyclones. After zonal averaging, two frequency maxima of deep cyclones emerged, one between 62.5–67.5° N and the other between 47.5–52.55° N. This is different from the European and North Atlantic regions where only one maximum occurs. The seasonal frequency deep cyclones in Northeastern China reaches maximum in spring and summer while in western Siberia and the Northwest Pacific deep cyclones are more frequent in winter. The annual trend of deep cyclones over the Northwest Pacific shows an increase from the sixties to the eighties while deep cyclones over East Asia decreased during this period. In the 1980s, more deep cyclones occurred over the Northwest Pacific and less deep cyclones over main land Asia which may be associated with the northern hemisphere warming. The monthly number of oceanic deep cyclones in December and January appeared to be positively correlated with the August and September sea surface temperatures over the East Pacific (El Nino regions 1 + 2).

Altai-Sayan lee cyclogenesis: Numerical simulations

SummaryIn this paper we compare and analyse two cases of lee cyclogenesis observed near the Altai-Sayan massif (14–16 April 1988; 3–5 March 1986). The interpretation of the events is based on numerical simulations with an isentropic primitive equation model, where runs without orography are compared to those with realistic orography. The runs with mountains are successful in depicting the observed developments, so that the no-mountain experiments can be assumed to provide further information on the dynamics of both cases.The case of 14–16 April 1988 is dominated by rapid cyclogenesis and cut-off low formation aloft linked to the southward motion of a potential vorticity maximum towards the Altai-Sayan massif. Cyclone formation would have occurred without mountains as well. However, cyclogenesis at low levels is enhanced by the presence of the mountains through blocking of the cold air and corresponding deformation of the surface cold front. So this is a case of orographically modified cyclogenesis whereas the cyclogenesis of 3–5 March 1986 is induced by the mountain. In that case a broad and almost steady trough moves eastward over the Altai-Sayan region. Cyclone formation is absent in the no-mountain run. However, both cases exhibit some similarities with respect to the low-level developments induced by the mountains. A conceptual model is presented in order to further elucidate the key features of both cases.

Numerical simulation on mesoscale convective system along Mei-Yu front in Southern China

A mesoscale convective system (MCS) was generated over the South China Sea at around 115° E, 21°N on 0000 UTC 7 June 1998. Riding along the Mei-Yu front, the system moved through the Taiwan Strait and finally hit the southern part of the Taiwan Island, produced over 300 mm of rainfall over parts of Taiwan in the next 24 hours. It was found that the Penn State-NCAR Mesoscale Model Version 5 (MM5) did quite well in simulating the evolution of the MCS. Diagnostic studies on the mesoscale structure of MCS, moisture concentration process and the momentum budget were based on the model output. The concept model of the MCS in the mature phase can be concluded as the following: At the 850 hPa level, the mesoscale low level jet (mLLJ) was found to the southwest of the MCS, which was also associated with a mesoscale low generated by convection, the mesoscale upper level jet (mULJ) was found due east of the MCS (and an upper-level mesoscale high) at the 300 hPa level. Horizontal advection of momentum acted to accelerate the exit of the mLLJ but decelerate the entrance of the mLLJ where the pressure gradient force acted to compensate this lost and maintain its strength. For maintenance of the mULJ, vertical advection of momentum and the pressure gradient force were both found to be important.