East Asia and Western Pacific Meteorology and Climate

The following sections are included:

• FOREWORD

• PREFACE

• CONTENTS

The global monsoon systems consist of regions with large seasonality (Zeng et al. (1994), Zeng and Zhang (1998), Xue and Zeng (1999)). It is discovered that in the lower troposphere there are one tropical monsoon region (coincident with the classical monsoon region), two subtropical monsoon regions (one each in Northern and Southern Hemisphere and occupied by the seasonal migration of subtropical high) and two middle-high latitudinal monsoon regions (associated with the storm tracks of westerlies). The division of seasons is determined by the normalized similarity (Zeng et al. (1994), Zeng and Zhang (1992), Zhang and Zeng (1998)). It is revealed that in the monsoon regions, the transitions from winter to summer and from summer to the next are abrupt.

Observations from the SCSMEX/GAME sounding domains have been used to document several features of the onset of the 1998 East Asian summer monsoon. Marked transitions of the low-level and upper-level flow occurred in the vicinity of the South China Sea (SCS) in mid-May as the summer monsoon became established over this region. Over the northern SCS, the onset of westerly monsoon flow occurred around 15 May, whereas over the southern SCS the onset was on 20 May. Deep tropospheric moistening accompanied the monsoon onset in the northern SCS as precipitation began in that region. Changes in the moisture profile in the southern SCS were more complex, although there appeared to be a transition from a tradewind-like structure (moist trade cumulus layer capped by dry air aloft) to a moistening of the mid-to-upper troposphere accompanying the onset.

Preliminary results from SCSMEX indicate that the monsoon onset over the South China Sea is rather complex and far from homogeneous over this oceanic region. A prominent mixed Rossby-gravity wave was detected at the time of onset over the southern SCS. The onset of the 1998 East Asian summer monsoon was distinctly different from the onset of South Asian or Indian monsoon.

This paper attempts to relate the intensity of the South Asia summer monsoon, (SASM) to large-scale circulation anomalies associated with the East Asia summer monsoon (EASM). By partitioning the monthly mean horizontal winds at various tropospheric levels into the vertical mean and vertical shear components, the kinetic energy of the shear flow (K_s) averaged between June and September within the box (40° − 100°E, 0 − 20 °N) is found to be strongly related to the SASM, and is therefore defined as the south Asian summer monsoon index (SASMI) to represent the intensity of the SASM. A good correlation is found between the SASMI and the other intensity indices of the SASM.

Besides the conditions in south Asia, the value of SASMI is also closely related to the large-scale circulation anomalies associated with the EASM. When the value of the SASMI is large, positive (negative) temperature anomalies are found over the Asian landmass (Indian Ocean). This gives rise to an above-normal north-south land-sea temperature gradient. As a result, low-level southwesterlies extend all the way from the Indian sub-continent to the Philippines, which results in a monsoon trough over south China. The southerlies to the north of this trough converge with the northerlies in the midlatitudes over northern China to form a monsoon frontal trough. For small values of the SASMI, the entire atmospheric pattern apparently shifts southward.

The impacts of space-based observations of ocean surface wind vectors in our understanding of the environmental and climatic changes in East Asia and the Western Pacific are demonstrated. The daily global coverage and the high spatial resolution of the data open up new studies in typhoons, monsoons, and their subsequent flooding over land. Information on data access and future satellite missions is also discussed.

The following sections are included:

• INTRODUCTION

• EARLY VS. LATE ONSET YEARS

• POSSIBLE MECHANISMS

• CONCLUDING REMARKS

• REFERENCES

Observational evidence is presented to show an active ENSO-East Asian monsoon interaction during the mature and decay phases of ENSO. During the mature phase of major El Niño episodes, an anomalous low-level anticyclone establishes over the Philippine Sea. This anticyclone develops rapidly in late fall of the ENSO development year and persists until the following spring or early summer, causing anomalous above normal rainfall along the East Asian monsoon front stretching from southern China northeastwards to the Kuroshio extension.

Why do mature phases of ENSO have a delayed impact on the East Asian summer monsoon? We found that the anomalous anticyclone over the Philippine Sea is accompanied by the enhancement of the in situ sea surface cooling in front of the anticyclone. The persistence of the Philippine Sea anticyclone is attributed to a positive feedback between the atmospheric Rossby wave response and associated sea surface cooling through surface evaporation. This positive feedback provides a mechanism to maintain Philippine Sea anomalies until next summer against dissipations.

On the other hand, the Philippine Sea anticyclonic anomalies, which occur prior to the peaks of major warm events, induce anomalous easterlies in the equatorial western Pacific. By forcing equatorial upwelling Kelvin waves and elevating the thermocline, the easterly anomalies provide a continuing negative feedback to the eastern Pacific warming. The proposed mechanism is different from, but complementary to the delayed oscillator mechanism. The latter relies on oceanic wave adjustment process. Since the formation and maintenance of the Philippine anticyclone depend on the existence of mean circulation during the boreal cold season, the proposed negative feedback favors ENSO turning around in boreal winter. This may partially explain why the reversal of warming or cooling trends of ENSO cycles occurs most frequently toward the end of the calendar year.

Comparison experiments based upon a climate model are conducted to study the impacts of land surface processes on the mean July distributions of precipitation and subtropical anticyclone and on the seasonal variation of the Asian circulation. Results show that through changing the recycling precipitation ratio, the surface latent heat flux over land surface affects the global precipitation apparently. The latent heating of the Asian monsoon contributes to the distribution of the subtropical anticyclone over the region; and the strong surface sensible heat flux over North Africa in summer is very important for the configuration of the subtropical anticyclone over the North Atlantic Ocean. It is also shown that the surface sensible heat flux over the Tibetan Plateau is the basic factor mat causes the abrupt seasonal transition from winter to summer of the circulation in the surrounding area.

The following sections are included:

• INTRODUCTION

• EXPERIMENTAL DESIGN

• Precipitation anomalies during ENSO events

• Circulation anomalies during ENSO events

• Ocean-atmosphere coupling processes

• Discussion

• References

In this study, the feasibility of applying NCEP/RSM to predict the regional climate and further with GWLF hydrological model to forecast the streamflow in Taiwan are investigated. At first, 1992 monthly rainfall in Taiwan simulated by applying NCEP/GSM/RSM were compared with the observations. It is shown that when using 25 km × 25 km grid resolution in horizontal, monthly rainfall in winter can be simulated well but tends to be over-estimated. Sensitivity runs indicated that although higher resolution models can simulate finer structure of the rainfall distribution, rainfall amount is severely over-estimated. The skills of the rainfall prediction for the other seasons are rather poor.

Monthly streamflow in Taiwan can be simulated well by applying a daily weather generating model and GWLF hydrological model. A model system consist of CCM3/ RSM climate model, daily weather generating model, and GWLF hydrological model is applied to do the assessment of climate change impacts on the regional climate and streamflow in Taiwan. Two different resolution RSM runs have been done. One is 50 km × 50 km (RSM1). The other one is 15 km × 15 km (RSM2). The CCM3/RSM model system has been run on current CO₂ scenario to reproduce the present regional climate and on doubled CO₂ scenario to predict the regional climate change in the future. All the climate models (CCM3, RSM1 and RSM2) predict a consistent warming trend of 1.5 to 2.3 C. However different resolution models predict rainfall and probability of rainy day vary in locations and seasons.

CCM3/GWLF predict streamflow will increase in all area while RMM1/GWLF predict the streamflow will increase in northern but decrease in central, southern, and eastern Taiwan year around. Furthermore, RSM2/GWLF predict the streamflow will increase in the southern and eastern but decrease in the northern and central Taiwan year around. Uncertainties could be caused by any links. Therefore, much more research is needed before any solid conclusion can be reached.

Based on the success and scientific results of the GPS/MET (Global Positioning System/Meteorology, Ware et al. 1996) project, COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate), a follow-on project for weather and climate, space weather, and geodetic science is being developed jointly by many partners. COSMIC plans to launch eight satellites in 2004. Each satellite will retrieve about 500 daily profiles of key ionospheric and atmospheric properties from the tracked GPS radio signals as they are occulted behind the Earth limb.

This paper discusses some of the applications of COSMIC data for meteorology and climate. COSMIC will provide high vertical resolution temperature, pressure and water vapour information for a variety of research studies. The COSMIC data set will allow mapping of the global atmospheric flow of water vapour that is crucial for understanding and predicting weather and climate. The data set will provide accurate geopotential heights, enable the detection of gravity waves from the upper troposphere to the stratosphere, reveal the height and shape of the tropopause globally with unprecedented accuracy, support the investigation of fronts and other baroclinic structures, and improve our understanding of tropopause-stratosphere exchange processes.

COSMIC data will complement other observing systems and improve global weather analyses, particularly over the oceans and polar regions, and numerical weather forecasts made from these analyses. Through assimilation in numerical models, COSMIC data will improve the resolution and accuracy of the global temperature, pressure and water vapour fields, and through the model's dynamical and physical adjustment mechanisms, the wind fields as well.

For climate research and monitoring COSMIC will provide an accurate global thermometer that will monitor Earth's atmosphere in all weather with unprecedented long-term stability, resolution, coverage, and accuracy. COSMIC will provide a data set for the detection of climate variability and change, the separation of natural and anthropogenic causes, the calibration of other satellite observing systems and the verification and improvement of climate models.

Based on the data analyses and numerical simulations with a tropical Pacific coupled model, the important relationship between occurrences of El Nino/La Nina events and anomalies of sub-surface ocean temperature (SOT) in the Pacific warm pool region are studied in this paper.

The data analyses showed that most of ENSOs since 1950s are similar to the situation of 1997-98 ENSO. There have been positive anomalies of SOT in the warm pool region before the occurrence of El Nino event, but negative anomalies of SOT in the warm pool region prior to the occurrence of La Nina event. As positive (negative) anomalies of SOT propagated from warm pool region towards the equatorial eastern Pacific, the El Nino (La Nina) event would occur soon.

Eastward propagation of the anomalous SOT is closely associated with zonal wind anomaly over the equatorial western Pacific. If there is continued westerly (easterly) wind anomaly over the equatorial western Pacific, the positive (negative) anomaly of SOT in the warm pool region will propagate eastwards, and zonal wind anomaly appears a little earlier than the eastward propagation of the anomalous SOT.

Numerical simulations with a tropical Pacific atmosphere-ocean coupled model (IAP-CGCM) also showed that the anomalies of SOT in warm pool region played an important role in the occurrences of El Nino and La Nina events. These are similar to observations.

1997/98 was one of the strongest El Niño period on record, the estimated global mean surface temperature for land and marine areas averaged are 0.44°C above the 1961-1990 normal period. From the mid-1998 till now, it began La Niña year, both of the above episodes affect the weather all around the world.

In this paper, we based on our observation data, find out some relationship between Anomaly and weather (precipitation and air temperature).

The relationship between the SSTA in the western Pacific and the June precipitation in the mid and the lower Changjiang valleys(MLCJ) is discussed. A key ocean area is determined over the western Pacific (WP) in 134.5°E∼147.5°E, 20.5°N∼30.5°N. It is found that the period from the previous October to January in the same year is the WP SSTA leading period with the best positive correlation. The years with more precipitation (MPY) and the years with less precipitation (LPY) in June in the MLCJ are selected. Also, the years with higher SST (HTY) and the years with lower SST (LTY) in the WP area in the Oct. to Jan. period are chosen, too. The composed June anomaly patterns of precipitation in China are obtained for both the HTY and the LTY. It is seen that in HTY and LTY the distributions of June precipitation in China are almost in totally opposite phase. The composed June climatic anomaly patterns of meteorological fields are finally discussed in order to analyze the mechanism. It is found that the June climatic anomaly patterns of MPY and HTY are similar, and so are the patterns of LPY and LTY. Therefore, the SSTA in the WP area does have influence on the June precipitation anomalies in the MLCJ, and even in whole China.

Characteristics of climatological distributions of annual and monthly precipitations as well as the pattern of drought/flood variation in Taiwan over a 48-year period of 1951 - 1998 were statistically analyzed. It was found that the northeastern part had for many years a maximum in mean annual precipitation, while the mid-western and southwestern parts had a minimum. The mean annual precipitation over the southeastern coast of mainland was as much as that for southwestern Taiwan. Flood season over most parts of Taiwan occurred between April and June; but in the northeastern part, intense precipitation occurred between September and December. For the southeastern coast of mainland, intense precipitation usually occurred between April and June. Correlation of monthly precipitation and comparison of drought/flood years between Taiwan and the southeastern coast of mainland were made. They revealed a significant relationship regulating the drought/flood variation in the flood season over both sides of the Taiwan Strait and the corresponding changes in monthly precipitation. Causes of the observed phenomena were discussed.

The Pacific Climate Information and Prediction System (PCIPS) is a U.S. Government Internet facility that provides climate data as well as climate forecasts for the tropical Pacific region. The Pacific region is strongly influenced by both the beneficial and detrimental rainfall impacts of the El Nino/Southern Oscillation (ENSO). This report highlights some of the prominent features of this information facility, and illustrates both the level of skill of the climate forecasts and the implied utility of information to the inhabitants of Pacific island nations.

The tropospheric biennial oscillation (TBO) and El Nino-Southern Oscillation (ENSO) are interannual variations that affect the Asian-Australian monsoon region and the tropical Pacific. Some investigators view the two oscillations are actually one system, with TBO a “weaker sister” of ENSO. Others view the two as separate modes, with TBO primarily driven by the monsoon and ENSO primarily driven by atmosphere-coupling of the tropical eastern Pacific. In this paper the Chang-Li theory, which belongs to the latter category, is used as the basis to study the interactions of the two oscillations. It is shown that the nonlinear interaction of the two modes may produce a variety of solutions that may explain the complexity of the observed monsoon-ENSO relationship.

The following sections are included:

• INTRODUCTION

• GOVERNING EQUATIONS FOR NONLINEAR KELVIN WAVES

• Small amplitude approximation

• References

The horizontal PGF in σ-coordinates can be separated into its irrotational and rotational components and expressed by , where φ_e is referred to as an equivalent geopotential, and η is called a geo-streamfunction. The equivalent geopotential φe(x, y, σ, t) can be used in σ-coordinates in the same way as φ(x, y, p, t) is used in p-coordinates. The small difference between two large terms over steep slopes on the right hand of (1.2) is eliminated automatically by using expression (1.3). The quantity − ∇²η is referred to as a surface pressure drag on vorticity, and it acts as a pressure torque. In the region where −∇²η < 0, the vorticity decreases in the atmosphere and it transfers to the surface of mountains through the surface pressure stress, and vice versa. The surface pressure stress torque can be computed exactly at each time step in a numerical model by using the geo-streamfunction. A computed example shows that a negative center of the geostreamfunction develops in the lee of the Rockies during the growth of a lee cyclone, and the vorticity transfer from the atmosphere to the surface of the Rockies also grows in the region where the lee cyclone develops. The prediction of the strength of the lee cyclone is improved by using the equivalent geopotential and geo-streamfunction.

The typhoon-track data over the Western North Pacific (WNP) were collected since 1897 when routine hourly observations were first available from the weather stations around Taiwan area. Among the 666 cases recorded in 1897-1996, 374 either approached or made landfalls on Taiwan. These storms are thus referred to as the “invading” typhoons in this study, and are also further categorized into 9 sub-groups mainly according to their moving paths and partly their origins. The averaged yearly total invading typhoons are 3.7, but the number can be as high as 10 or none at all for individual years. The yearly total of the invading typhoons seemed to exhibit a significant 30-year oscillation according to a power spectral analysis. We have also ranked the intensity of storms in terms of locally observed wind and precipitation data. From the statistics of the first 10 such most intensive invading typhoons in each local area, the occurrence of local maximum wind and rainfall are often strongly related to local topography.

The orographic effects in changing the storm motion when passing over the Central Mountain Range (CMR) were characterized into 15 conceptual models from the historical data set, which can then be readily applied to operational use. In analyzing the occurring frequency of the storms that locally reached the hazardous intensity for office-closing, we also found the great importance to local wind forecasts of the blocking effect by the CMR and the channel effect associated with the Taiwan Strait. As to the distribution of precipitation, it is again noted to be mostly phase-locked with the topography of Taiwan.

In this study, the vertical momentum budget is examined by inverting three-dimensional (3D) buoyancy and dynamic perturbation pressures using a high-resolution, explicit simulation of Hurricane Andrew (1992), in order to gain insight into the processes leading to the subsidence warming in the eye and the vertical lifting in the eye-wall.

It is found that a large portion of surface perturbation pressures is caused by the moist-adiabatic warming in the eye-wall and the subsidence warming in the eye. However, the associated buoyancy-induced pressure gradient force (PGF) is mostly offset by the buoyancy force, and their net effect is similar in magnitude but opposite in sign to the dynamically-induced PGF. Of importance is that the dynamically-induced PGF points downward in the eye to account for the maintenance of the general descent. But it points upward in the outer half portion of the eye-wall, particularly in the northern sector, to facilitate the lifting of high-θe air in the lower troposphere. Furthermore, this dynamic force is dominated by the radial shear of tangential winds. Based on this finding, a new theoretical explanation, different from previously reported, is advanced for the relationship among the subsidence warming in the eye, and the rotation and vertical shear in the eye-wall.

The following sections are included:

• Introduction

• Summary of results

• A preliminary test of large-error determination

• REFERENCES

The performance of U.S. Naval Operational Global Atmosphere Prediction System (NOGAPS) on the predictions of tropical cyclones in August of 1998 using two different convective parameterization schemes, the Arakawa-Schubert scheme and the Emanuel scheme, is evaluated. The Emanuel scheme provided superior track forecasts than the Arakawa-Schubert scheme did and the structures and precipitation were also better simulated by the Emanuel scheme.

The typhoon rainfall distributions over Taiwan area are investigated through empirical orthogonal function analysis. The data set includes the hourly rainfall observed at 23 stations when a typhoon was inside the region between 18°N and 28°N, 116z°E and 126°E from 1970 to 1996. The rainfall pattern of the leading EOF modes shows that the Taiwan topography is a dominant factor to the distribution of typhoon rainfalls. The rainfalls are enhanced on the up-slope side of the mountain range and the rainfalls are suppressed on the down-slope side of the mountain. By investigating the relationship between the amplitude of each EOF mode with the typhoon intensity, Julian day, typhoon moving speed and direction, some favourable conditions for enhancing or suppressing the rainfalls with the selected mode are found.

High-resolution numerical simulations and detailed analyses are carried out to examine the ability of the model in simulating the track, intensity change, and the detailed mesoscale wind and precipitation distributions associated with Typhoon Herb (1996). The 2.2 km MM5 successfully simulates the mesoscale rainfall distribution associated with Herb, although the predicted maximum 24-h rainfall of 1290 mm was about 75% of the observed amount. It is shown that the ability of the model in successfully simulating the observed rainfall hinges on the model's horizontal grid resolution in describing the Taiwan terrain. The presence of the Central Mountain Range plays a key role in substantially increasing the total rainfall amounts associated with Typhoon Herb by lifting the moisture-laden air over the upslope regions of the mountains. Overall, Herb is a case in point to indicate the intimate relation between the rainfall distribution and the Taiwan topography.

During the formation of typhoon Mike (1990), the trade wind to the north of the system was very strong over a very large domain. However, there were evident cross-equatorial flows associated with the formation of typhoon Betty (1987). The intensities of both systems increased greatly after they reached tropical storm intensity. To help understand the influences of the environmental momentum field on the formation of typhoon, we used the PSU/NCAR MM5 to simulate the formation process of Betty and Mike. The ECMWF grid point data were used as the model initial field. Results showed that the strong trade wind associated with Mike's formation and the cross-equatorial flows associated with Betty's formation were simulated reasonably well. The intensities of model typhoons were also close to the observations for Betty. However, the model vortex developed faster than the observation for Mike.

Both the tangential momentum and the vorticity budgets were analyzed using model-simulated data. Results showed that the low-level eddy horizontal angular momentum transport caused by the strong trade winds played a significant role to the spin-up of the model vortex, thus was favorable for the formation of Mike. The horizontal vorticity advection caused by the cross-equatorial flow associated with Betty's formation did not contribute significantly to the vorticity generation of model vortex. However, the low-level convergence enhanced by the cross-equatorial flow might have caused the vorticity generation near system centre and increased heating efficiency. Such effect might have played an important role on the organization of convection and helped the formation of tropical cyclone.

The Meiyu fronts affecting northern Taiwan during the period of May-June 1991-1996 were categorized into the moving, the stationary, and the retreating types. Radar echoes observed at the CKS International Airport and surface observations from the Central Weather Bureau for the same period were analyzed to reveal the relationship between daytime land convection and local circulations under the influence of different types of fronts over northern Taiwan.

Results show that the sea breeze and localized vortices prevailed over northern Taiwan. A convergent line was located over the northwestern slope of the Snow Mountain Range with localized cyclonic vortices along the line. Convective activities were modulated strongly by the sea breeze and the convergent line for different types of fronts. More widespread convection was triggered on the day when front entered northern Taiwan for the stationary type as compared to the moving and the retreating types. Convection was enhanced on the day when front entered northern Taiwan and suppressed on the following day for the moving type and the reverse was true for the stationary type. On the other hand, it was enhanced and organized consecutively from the day when front entered northern Taiwan to two days after for the retreating type.

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 Meiyu front, the system moved through the Taiwan Strait and brought over 300 mm of rain to parts of Taiwan in the 24-hour period after hitting the southern part of the island. Torrential rain resulted in severe flooding. We found that the Penn State-NCAR Mesoscale Model Version 5 (MM5) did quite well in simulating the evolution of the MCS, including the shape of clouds and the amount of precipitation over Taiwan island.

In the mature phase of the MCS, a mesoscale low, a mesoscale low-level jet (mLLJ) and a mesoscale upper-level jet (mULJ) could be clearly identified in die numerical simulations. At the 850 hPa level, the mLLJ was located to the southwest of the MCS, which was also associated with a mesoscale low generated by convection. In the 300 hPa level, the mULJ was found due east of the MCS (and a upper-level mesoscale high). Budget studies were used to investigate the physical processes associated with the maintenance of the mLLJ and mULJ. Horizontal advection of momentum acted to accelerate the jet at the exit of the mLLJ but decelerate the jet at the entrance of the mLLJ where the pressure gradient force acted to compensate this loss and maintain the strength of the jet in its mature phases. For maintenance of the mULJ, vertical advection of momentum and the pressure gradient force were both found to be important.

Doppler radar observations of a mesoscale vortex associated with a quasi-stationary mesoscale convective system in the Taiwan Mei-yu season is presented. By using GBVTD technique, the mean tangential winds are retrieved. The results showed the vortex had a dipole structure in perturbed Doppler radial winds embedded within the prevailing southwesterly winds. Heavy rainfall occurred at downstream of the enhanced southwesterly flows induced by the mesoscale vortex.

Based on the wind observations of the wind profiler located in Hong Kong, the mesoscale phenomena in the lower troposphere for the severe rainstorm case of 7-9 June 1998 were analyzed. The wind profiler revealed an easterly jet stream in the boundary layer bringing a cold air mass from the East China Sea prior to the occurrence of the rainstorm. The development of the South China Sea monsoon was also shown in details by the wind profile. The mesoscale phenomena of the jet stream were found in the low levels and the boundary layer during the period of torrential rain.

Atmospheric water vapor data were obtained by two Global Positioning System (GPS) receivers at Shantou and Yangjiang during the Huanan (southern China) Torrential Rainfall Experiment from 5 May to 26 June 1998. The ground-based GPS technique was tested to assess its accuracy for meteorological applications. Integrated water vapor was estimated through zenith wet delay and the results were compared with routine radiosonde data (at mandatory and significant levels). The trends shown by the two methods were generally consistent. It was demonstrated that GPS sounding signals were sensitive to convective systems and could be used to measure atmospheric water vapor. As such, it would be feasible to make use of GPS data for monitoring thunderstorm development associated with mesoscale convective systems.

Land vegetation categories and physical parameters retrieved from NOAA satellite remote sensing data in the HUAMEX project during May and June 1998 was tested by using the PSU/NCAR mesoscale model MM5 to simulate an IOP case of heavy rainfall during 23-24 May 1998 and compared with that by using NCAR land vegetation data-set. Results show that with our land vegetation data, which represents in more detail the complex land surface characteristics of China and near area, the simulated 24h accumulated rainfall was increased about 12-14% for grid spacing 5-15km compared with that using NCAR data. The dynamic structures on the low levels are also affected. For larger grid spacing, 45km, the effects are not obvious.

Since the retrieve of land vegetation, especially for the associated physical parameters, is hardly accurate, it needs more studies to enhance the accuracy of the retrieved data and test in various area and synoptic situations.

During the sixth intensive observation period (IOP#6) of Torrential Rainfall Experiment over the Both Sides of the Taiwan Strait and Adjacent Area (HUAMEX), localized intense precipitation occurred on June 9,1998 in Changle area in Fujian province. The primary objective of this paper is to analyze this case using single-Doppler weather radar data, in particular the wind field structures retrieved by VAD (Velocity Azimuth Display) and V-D (Vorticity-Divergence) methods. The results showed that the event was related to wind shear at 3-5 km height and the activity of a mesoscale cyclone wave as seen on the radar echo images.

After the onset of the South China Sea Monsoon, a large area of rainstorm affected South China coastal region on 24 May 1998. Although in Macao, the rainstorm process only last from 1700UTC 23 to 1000UTC 24, the precipitation recorded, 100.4mm, was the maximum daily rainfall of that month. The red rainstorm warnings were then issued.

The Fifth-Generation NCAR / Penn State Mesoscale Modeling System, MM5, was introduced to the Macao Meteorological and Geophysical Services, and used to simulate this rainstorm process. The data of SCSMEX (South China Sea Monsoon Experiment) in 1998 was used as initial and boundary conditions for the model. Cumulus Parameterization Schemes were then applied to the simulation. As the simulative results were compared to the observation, the overall outcome shows that, in this case, the Anthes-Kuo scheme gives rather accurate descriptions of rainfall in Macao and the area over the Pearl River Estuary region.

Based on the data from South China Rainstorm Experiment and South China Sea Monsoon Experiment in 1998, we analyzed the features and variation rules of the physical quantities of large-scale, synoptic-scale and meso-scale systems of six rainstorms, which had happened during the experiments period. The counter-Hadley cells was found having established during the rainstorm periods in some degree. The system distribution of the MCCs in South China was also given in this study.

When Taiwan is not directly under typhoon circulation and not in Meiyu, extremely heavy rainfall exceeding 130 mm per day may occasionally occur over the northeast region during the northeast monsoon period, especially from October to December, and over the southwest region during the southwest monsoon period. In this study, effort is made to find out the key factors that cause such heavy rainfalls, based on synoptic maps as well as other data. It is found that flow confluence and orographic lifting play very important roles in producing the heavy rainfalls over northern, southern or central Taiwan. The confluent flows that cause the heavy rainfalls over northern Taiwan mostly result from certain combinations of a continental cold high and a tropical depression. The confluent flows that cause the heavy rainfalls over southwestern Taiwan depend on the position and orientation of the subtropical high, typhoon activity and the movement of the Yangtze-River low. Several such synoptic patterns are identified.

Heavy rain during spring and summer in Hong Kong often causes considerable disruptions in social and economical activities. The Hong Kong Observatory (HKO) is running a high resolution Regional Spectral Model (RSM) to improve short-term rainfall forecasts. For operational use at the HKO, if rainfall forecasts over model grid points covering Hong Kong territory exceed certain threshold values, model rainfall alerts will be issued to the forecasters.

The RSM system, which is adapted from the Japan Meteorological Agency (JMA), implements physical initialization to incorporate hourly rainfall amount based on (1): rainfall analysis using radar reflectivity against raingauge data and (2): cloud cover and cloud top temperature from Geostationary Meteorological Satellite. Numerical simulations for past heavy rainstorm cases using 20-km horizontal resolution reveal that the model can give a significant improvement over the 60-km resolution in prediction of rainfall in the initial hours of forecasts.

In this paper, we will discuss the model performance in rainfall forecasts for April – June 1999. Objective verification of rainfall forecasts using scores like Probability of Detection (POD) and False Alarm Ratio (FAR) will be presented. Model performance will also be illustrated using rainstorm cases. Rainfall amount, spatial distribution and its temporal trend during forecast hours will be compared with actual observations.

In this paper, the EC/TOGA Advanced 1.125° × 1.125° analysis data for May and June of 1992, 1993, 1995 and 1996 are used to investigate the heavy rainfall during the Mei-Yu season in Taiwan. With a criteria of 100 mm per day for at least two surface stations and three automatic rain gauges of the Central Weather Bureau, twelve heavy-rain cases are selected. For the first part of this study, a composite analysis based on nine cases is carried out (three cases in 1994 are not included). The composite analysis shows that a weak baroclinic zone associated with the Mei-Yu front forms a northern boundary to the southwesterly flow and the embedded mesoscale convective systems. As the result of the interaction between the tropical and extratropical weather systems, the convectively ageostrophic winds not only increase the Mei-Yu frontal intensity but also strengthen the low-level jet (LLJ). The LLJ plays an important role in the development of Mei-Yu frontal systems through the water vapor supply and vertical motion enhancement. A vertical cross-section analysis reveals the intensification of the Mei-Yu front and its transition from an “anafront” to a “katafront”. The heavy rainfall event is closely associated with this transformation.

In the second part of this study, nine cases are simulated with the PSU/NCAR Mesoscale Model MM5 and the simulation results show that the composite warm moist flow mainly comes from the Bay of Bengal. But the moisture flux comes from the sea west of Philippines as the return flow of the subtropical Pacific High. Another branch of moisture flux is the cross-equatorial flow from the west of Australia. The numerical sensitivity experiments show that as the latent heating term is turned off or the relative humidity field over the Bay of Bengal is reduced to forty percent, the LLJ and the corresponding heavy rainfall does not exist.

The following sections are included:

• INTRODUCTION

• EXPERIMENT DESIGN AND DATA PROCESSING

• MAJOR RESULTS

• DISCUSSION

• Acknowledgements

A rainstorm nowcasting system SWIRLS (Short-range Warning of Intense Rainstorms in Localized Systems) is currently under development at the Hong Kong Observatory. Phase I of SWIRLS went through operational trial in 1998 and was officially put into service in April 1999 (Li et al. 2000). Movement and intensity change of individual radar echo are analyzed by TREC (Tracking Radar Echoes by Correlation). Extrapolation based on the respective TREC vector provides the forecast echo position and intensity, but development of the latter has the option of being constrained by an idealized profile. Meanwhile, making use of the dense raingauge data over Hong Kong, SWIRLS adjusts the reflectivity-rainfall (Z-R) relationship in real time. Combining the echo forecast and the time-adjusted Z-R relationship, SWIRLS generates short-range quantitative precipitation forecasts (QPF) over Hong Kong every 6 minutes, i.e. time interval of radar scans.

QPFs by SWIRLS during several heavy rain episodes in 1998 and 1999 are objectively evaluated using common performance measures such as NAP (No Alarm Probability), FAR (False Alarm Rate), Critical Success Index (CSI) and Heidke Skill Index (HSI). Results from this study help to: (a) assess the reliability and robustness of SWIRLS QPFs; (b) reveal strength and weaknesses in SWIRLS QPFs for forecasters' guidance and reference in operating the rainstorm warning system; (c) focus researchers’ attention to rain characteristics peculiar to Hong Kong, hence pointing the way forward in SWIRLS development work.

The inventory of aircraft emissions over the Taipei Flight Information Region (FIR) has been developed with 1997 as the base year. We first analyze the spatial and temporal distributions of flight frequency base on the official civil aviation monthly reports, airline schedules, pilots' on air reports. Fuel consumption and trace-gas emissions are then calculated by using the engine performance database of the International Civil Aviation Organization (ICAO). Compared with the official records, the amount of fuel consumption estimated in this study is within 0.5% accuracy. The emission inventory is built for different flight types (domestic or international), aircraft types (commercial jets, cargo jets or propeller aircraft), as well as the state of aircraft operation (idle, takeoff, climb-out, cruise and approach/landing). Engine types, number of engine per aircraft, as well as engine performance at different operation states are all considered in estimating the emission rates.

The emission rates of six trace species (NO_x, CO, CO₂, Sulphur, HC and H₂O) are estimated for major flight routes and airports, and further separated into three height levels (ground, near surface, and cruising altitudes). To perceive their relative importance, we took the ratios of the annual emissions from aircraft over the Taipei FIR with those from ground transportation in Taiwan. The most significant ratios are 14.9% for NO_x, 9.7% for Sulphur, 0.87% for CO, and 0.43% for HC. Over 80% of the NO_x were emitted into the upper troposphere, whereas most of the CO and HC were produced on the surface. A further comparison shows that the amount of aircraft-produced NO_x over the Taiwan area is about 70% of those produced over China mainland.

An updated version of the Regional Acid Deposition Model (RADM) driven by meteorological fields derived from the Chinese Regional Climate Model (CRegCM) is used to simulate seasonal variation of tropospheric ozone over eastern China. The results show that:

Peak O₃ concentration area moves from southern to northern China in response to the shifting of solar perpendicular incidence point from south to north.

In eastern China, the highest O₃ month-average concentration occurs in July, the lowest in January, and intermediate values in April and October. The pattern mainly depends on solar radiation, the O₃ concentration precursors of NO_x and NMHC, and the ratio of NMHC/NO_x.

Diurnal variations of O₃ over eastern China are well defined. O₃ concentration increases with the rising sun, reaching its maximum at noon and then falling off. The range of variation is highest in summer (40 × 10⁻⁹ in volume fraction) and lowest in winter (20 × 10⁻⁹ in volume fraction).

Diurnal variations of O₃ over western China are not as clear. The range of variation is less than 10 × 10⁻⁹ in volume fraction.

NO_x concentration increases as its emission increases. In January, surface O₃ decreases as NO_x increases. In July, O₃ generally increases over the country. In April and October, O₃ decreases in northeastern China, northern China and Sichuan province but increases in other areas.

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