Chinese Academy of Meteorological Sciences, Beijing 100081, China
Zhongshan University, Guangzhou 510275, China
and DUAN Yihong
Shanghai Typhoon Institute, Shanghai 200030, China
Progress of the study on tropical cyclones and tropical meteorology in China has been made. A new atmospheric field experiment of tropical cyclone landfall with acronym of CLATEX was implemented in July-Aug. 2002. The boundary layer characteristics of target typhoon Vongfong and mesoscale structure features of other landfalling typhoons were studied. Some results from research program on tropical cyclone landfall, structure and intensity change, interaction between lower and mid-latitude circulation and the interaction among different motion scales were described in this paper. On the other hand, typhoon track operational forecasting errors in the last decade had been reduced because the operational monitoring and forecast techniques were improved.
The South China Sea monsoon field experiment with acronym of SCSMEX was carried out in 1998. The valuable intensive observation data have already been shared internationally. Some new findings have been published recently. Other research work in China on the tropical air-sea interaction, tropical atmospheric circulation and weather systems was reviewed in this paper. Some research results exhibit that the rainfall anomalies for different region in China were closely related to the stages of El-Nino events.
I. TROPICAL CYCLONE RESEARCH AND OPERATIONAL FORECAST
1. Tropical Cyclone Landfall
A new program of tropical cyclone landfall in China is being implemented. Under the auspices of this program, an atmospheric field experiment of tropical cyclone landfall with acronym of CLATEX (China Landfalling Typhoon Experiment) was launched in July-Aug. 2002. Real time boundary layer observation data from wind profiler and other advanced equipments were acquired.
Fig. 1a. The boundary layer thickness of Vongfong was increased strongly when it made landfall. Fig. 1b. Boundary layer of Vongfong was calmed down one day after the landfall.
Analysis shows that the thickness of boundary layer of target typhoon Vongfong (0214) was strongly increased when it made landfall (Fig. 1a) and it was calmed down one day after Vongfong made landfall (Fig. 1b). It was also found that a ground hugging jet appeared in the right semicircle of the target typhoon in the period of landfall.
Another important topic of the tropical cyclone landfalling study is the tropical cyclone sustaining mechanism. Deep land severe flooding disasters often arise from those landfalling typhoons maintaining its vortex over land a longer time period. Chen (2001) indicated that the following processes would be favorable for landfalling tropical cyclone to prolong its dissipative time period over land. Namely (1) keep the moisture transportation into the storm, (2) overlapped with an upper level strong divergence field or connected with an upper level outflow channel, (3) undergoing the transition process and gain certain baroclinic energy from weak cold air in mid-latitude, and (4) acquired the vorticity from a process of the landing storm merged with certain mesoscale strong convective system.
With 50 years data, statistics of the tropical cyclone intensity change and wind distribution pre-, and post-landfall are analyzed. It is found that tropical cyclones have a trend of decreasing its intensity in the 12 hours before landfall. Some others could maintain over 5 days and bring severe damage to in-land region after its landfall.
Using the mesoscale model MM5, the physical processes that, under idealized conditions, lead to changes in the rainfall distribution in a TC prior to, during and after landfall are studied by Chan and Liang (2003). It is suggested that cutting off the moisture flux over land have a profound effect on the TC characteristics while changing of sensible heat flux have very little impact.
The interaction between the high-level cold vortex and landfall TC is studied (Yu et al., 2001 and Wan et al., 2002). It is found that sometimes the cold vortex could be a favorable pre-condition for a TC's abrupt intensification before landfall, while it could also cause a landfall TC to weaken and become extratropical cyclone quickly after landfall.
A set of mesoscale re-analyses data is used to analyse the physical mechanism of a tropical depression-caused torrential rainfall by Yu et al. (2002). Results show that the outburst of a southerly jet in low atmosphere triggered the explosive development of cyclonic vertical vorticity in the region with steep potential temperature surfaces in front of a moist and warm air mass. That was slantwise vorticity development. While in the middle atmosphere, cyclonic vorticity increased notably as the air flow from west to east and entered a region with small vertical stability. The simultaneous sharp development of cyclonic vorticity in both lower and middle level atmosphere should be a main cause for the torrential rainfall.
2. Structure and Intensity Change
Most of tropical cyclones approaching the mainland would decrease its intensity aside from few typhoons could increase its intensity if some mesoscale vortex (MSV) merged into it. After carrying out two sets of numerical simulation of with and without the mergence of MSV. Chen and Luo (2002) exhibit that the MSV merged into a typhoon plays an important role for the great decrease of central pressure and intensifies the typhoon. It is inferred that the MSV transfers the vorticity to core region of the typhoon and strengthens its intensity.
Study on the relationship between mesoscale vortex and TC intensity shows that the interaction can intensify the maximum tangential velocity of typhoon under proper conditions (Zhou and Luo, 2002). Based on moist potential vorticity equation and the theory of slantwise vorticity development, Yu and Wu (2001) studied the possible relationship between the abrupt intensity change of TC and the evolution of its equivalent potential temperature structure. Analyses show that, due to the extreme steep moist isentropic surfaces in eye-wall region, the change of moist baroclinicity turns to be a main reason for abrupt changes of vertical vorticity. If the vertical vorticity increases rapidly in moist regions, TC may experience explosive deepening.
Chan et al. (2001) studied the interaction between TC and the underlying ocean using an atmosphere-ocean coupled model. The experiment indicates that the changes in TC intensity are sensitive to the variation of SST, the variation of TC intensity with SST is not linear. An SST of 27°C is found to be the threshold for TC development. In addition, the initial depth of the ocean mixed layer has an appreciable effect on TC intensity, which also depends on the movement of TC. Furthermore, the vertical structure of ocean (vertical temperature gradient in the stagnant layer, and temperature differential between the two layers) may play a significant role in modulating TC intensity.
A limited-area primitive equation model is used to study the effect of planetary vorticity gradient and uniform current on TC intensity change by Duan et al. (2003) . It is found that TC intensity is reduced in a non-quiescent environment compared with the case of no mean flow. A TC on a beta plane not only intensifies slower than one on an f plane, its rate of intensification also varies with the direction of the mean flow. The main physical characteristic that distinguishes the experiments i s the asymmetric thermodynamic (including convective) and dynamic structures present when either a mean flow or a planetary vorticity gradient is introduced. The magnitude of the warm core and the associated subsidence are found to be responsible for such simulated intensity changes.
Lei (2001) discussed the dynamical equilibrium features of tropical cyclones with primitive equation under the condition of adiabatic, non-friction, non-environmental steering. The results showed that the tropical cyclone intensity would be weakened by “ventilation flow” associated with “b gyres” and “shear gyres” with positive (negative) vertical wind shear at high (low) level. The positive (negative) vorticity vertical convection in high (low) level would strengthen tropical cyclone intensity.
3. Interaction between Tropical Cyclone and Mid-Latitude Systems
Tropical cyclone activities will strongly affect the intensity and distribution of rainfall associated with the mid-latitude trough and Meiyu front. Numerical simulation (Zhu et al., 2000) showed that rainfall in front of a mid-latitude trough would increase dramatically if a typhoon would approach south to the trough. Low level SE jet flow in right semicircle of the typhoon would transport abundant of moisture to the rainy region in front of the trough. Other numerical simulation (Cheng et al., 1998) demonstrated that the mid-latitude torrential rainfall of Meiyu could suddenly be ceased if a typhoon approach the South China coastal region. Meiyu heavy rainfall along the Yangtze River was maintained by the moisture transportation from the Bay of Bengal and South China Sea. Typhoon could cut off the moisture transportation if the vortex circulation appeared around the moisture channel and the moisture from Bay of Bengal and South China Sea would be drawn into the typhoon rather than transferring to Meiyu rain region. On the other hand, circulation systems in mid-latitude would affect the tropical cyclone's activities as well. Tropical cyclone would undergo a transition and restrengthening process with the influence from the mid-latitude cold wave (Xu et al., 1998). Transition process often led to tropical cyclone restructuring and strengthening with acquisition of baroclinic energy from mid-latitude weak cold wave (Chen et al., 2001) .
Tropical cyclone motion would be influenced by a upper cold low which is cut off from a deep mid-latitude trough. Tropical cyclone could suddenly turn toward northwestwards and made landfall at China east coastal area due to the influence from the upper cold low in southwest neighboring to the tropical cyclone. Studies (Xu et al., 2000) indicated that whether the tropical cyclone over Northwest Pacific would move westwards or recurving to northeastwards are closely related to the circulation pattern over mid-latitude Tibetan Plateau. 16 sets of numerical simulation were performed (Chen and Luo 2002) with a quasi-geostrophic model on b plane with a topography term. The results suggested that the tropical cyclone's track may have strong influence from the vortices originating from the western part of a large scale terrain. Probably this kind of interaction could be one of the causes to lead to the storm unusual movement.
Using scale discretion method, Meng et al. (2002) studied mesoscale characteristics of the interaction between TC and the westerly trough. Results showed that the interaction between TC and the westerly trough is apparently manifested by mesoscale activities. The distribution of divergence fields at lower and upper levels can have a kind of indication meaningful for the rainfall caused by the interaction between middle and lower latitude circulations.
A comprehensive study on the topic of the interaction between tropical cyclone and mid-latitude circulation systems was completed by Lei (2001). One of the results exhibits that in mid-latitude, tropical cyclone motion is influenced obviously by b variation with the latitude, namely “g effect”. It was suggested that g effect can not be neglected for tropical cyclone motion in mid-latitude ocean.
4. Interaction between Tropical Cyclone and Different Motion Scale Systems
With a barotropic model, the interaction between TC and the mesoscale vortex is investigated by Tian and Luo (2002). It is indicated that the different initial radial location of mesoscale vortex can cause changes in the characteristics of perturbation relative vorticity. Other studies concerning binary tropical cyclone interaction to the south of an idealized subtropical ridge (Luo and Ma, 2001) coincide with the conceptual model of binary TC interaction put forward by Carr and Elsberry in 1998. It is suggested that the interaction in the easterly current can lead to tropical cyclone unusual tracks such as abrupt changes of moving direction and velocity.
Tropical cyclone motion may be influenced by some mesoscale strong convective systems (MSS) in peripheral area of a storm. The interaction between typhoon and MSS was studied with the quasi-geostrophic barotropical model (Chen and Luo 1995). Five sets of numerical simulation were performed with a MSS in different quadrant. The experiment exhibited that the MSS in northeast quadrant led to oscillations track of typhoon movement. On the other hand, typhoon motion could have a westward deviation from the normal track (without MSS) if MSS was located near the east semicircle of the typhoon.
5. Tropical Cyclone Climate Variation
Tropical cyclone annual genesis frequency and the first landing date of TC were analyzed by wavelet technique. Results showed that the MORLET wavelet can disclose the significant period clearly. The main periods of annual frequency of TC are about 7, 3 and 10 years. The main periods of the first landing date of TC are about 5 and 10 years. Besides, with the composition analysis method, the reasons for anomalous frequency of TC affecting East China were discussed. It was shown that winter monsoon has influenced on the frequency of TCs affecting East China. A diagnostic conceptual model was constructed to forecast the anomalous frequency of TC. Furthermore, the latitudinal distribution of the climatic features of TC movement in the Northwest Pacific was analyzed including TC source area, active area, transition and dissipation area and frequencies as well as the frequency of northward motion. Some basic facts of TC activities in different latitudinal zones were revealed as well.
6. Operational Forecast
The 24 hour average error of tropical cyclone track operational forecast has been reduced from 239.8 km in 1985 to 130.5 km in 2002 (Fig.2). In the past decade, China has been strengthening the techniques of monitoring and observation systems and deploying the observing network including Doppler radar along the coastal region and neighboring provinces, developing the satellite observing and receving techniques, improving the tropical cyclone numerical prediction model (country report 2002) and warning service systems. All of those work are helpful to promote the tropical cyclone operational forecasting capabilities.
Fig. 2. 24 hour mean errors (km) during typhoon season during 1985-2002.
In NMC, the TC track prediction system was implanted to a super computer in the summer of 2002. The initial and lateral boundary conditions were from the new medium range spectral model T213L31 rather than the former T106L19. The numerical TC track prediction system of the Shanghai Typhoon Institute (STI) has also been upgraded, which is now running in parallel on Galaxy Computer. The new system is based on non-hydrostatic MM5V3, doubly nested (45km and 15km) with an enlarged horizontal domain.
In 2000, STI introduced the ensemble technique for TC track forecast (Zhou et al., 2002). Started with a barotropic model and the ensemble members were generated by perturbing the initial position and structure of TC. Testing results exhibited the average 24-hour track forecast error 117 km and 48-hour 326 km. A probability forecast method of TC track was proposed. Currently, experiments with sophisticated primitive model have been carried out.
Apart from numerical prediction models, there are several other statistical schemes being used operationally (Chen and Yu, 2003), including a consensus and a statistical-dynamical method developed by STI and a probability ellipse method developed by Jiangsu Meteorological Center. The consensus method uses correlation analysis to blend forecasts from four sub-models to produce a final forecast. It could help the forecasters to synthesize the forecasting tracks from different sources.
II. RESEARCH PERSPECTIVE IN TROPICAL METEOROLOGY
1. The South China Sea Monsoon
With the implement of SCSMEX, many meteorologists (He, Ding et al., 2001) try to make a definition for the onset indexes of the South China Sea (SCS) summer monsoon, the results (He et al., 2001) from different indexes all show that the climate mean onset time of the SCS summer monsoon is the 4th pentad of May. The results show clearly that the atmospheric circulations have notable abrupt change with the onset of the SCS summer monsoon, such as the wind field in low troposphere, the geopotential height field in upper troposphere, the humidity field and the vertical motion field all change remarkably over the South Asia and the Southeast Asia. The South Asian high moves rapidly from east of the Philippines to the north of Indochina Peninsula, the trough over India and Burma intensifies, the westerly wind over the equatorial Indian Ocean intensifies, expands and propagates eastward and northward, together with the tropics-midlatitude interaction and eastward withdraw of the western Pacific subtropical high. Those are all the large-scale characteristics of the SCS summer monsoon onset. Meanwhile, the meridional temperature contrast and zonal wind shear in the low latitudes over Asia correspond to abrupt change. The development and behaviors of 850 hPa vortexes over South Asia and Southeast Asia play an important role in the SCS summer monsoon onset. The SCS summer monsoon onset is a part of the evolution of global atmospheric circulation from winter to summer, and has the remarkable regional features.
The rainy season analyses show that there exists difference between the beginning of subtropical monsoon rainy season and that of tropical monsoon rainy season in East Asia from spring to summer. The former begins in early April over the region of north part of South China to the south of Yangtze River, and then expands southward and southwestward, reaches coastal regions of South China and the Indochina Peninsula in the late April. The rainfall belt is mainly caused by the convergence of the northerly cold surge, the southwesterly wind along the northwest edge of subtropical high, and the subtropical westerly wind in winter and spring over South Asia. After the SCS tropical monsoon onset, the subtropical monsoon rainfall belt over the coastal region of South China moves back northward, which is consistent with the northward movement of the subtropical high, the early flood season comes into the first peak period. The second peak rainfall comes in the region to the south of Yangtze River, that is the frontal rainfall period over the Yangtze River–Huaihe River region, and then further to the North China. In the mean time, the tropical monsoon rainfall belt propagates from Indo-China Peninsula to India to form the tropical monsoon rainy season (Chen et al., 2000).
The recent studies show that the low frequency oscillation is stronger in summer than in winter. Usually, the SCS summer monsoon establishes in the negative phase of the first strong low frequency oscillation in early summer. The low frequency oscillation over the South China Sea becomes stronger after the SCS summer monsoon onset, and the SCS summer monsoon onset is closely related to the eastward propagation of the low frequency oscillation in the equatorial Indian Ocean and the westward propagation of the low frequency disturbance in the western Pacific Ocean. During the SCS summer monsoon period, the low frequency circulation oscillation is characterized by the meridional oscillation of ITCZ and the zonal oscillation of the west edge of the subtropical High. The low frequency oscillation also has close relationship with the active and break/weaken phases of the SCS summer monsoon (Lin 1998; Song et al., 2000).
The summer monsoon onset date is earlier over the South China Sea region than that over India. The onset is closely related to the reversal of the meridional temperature gradient. The peak temperature gradient and the positive temperature deviation propagate westward during the transition period from winter to summer in the low latitude over Asia. Case studies show that warm advection is one of the most important factor to induce the temperature increase in the troposphere over South China before the SCS monsoon onset. The contribution of diabatic heating to the local temperature variation depends on the amount of rainfall over South China. The rapid temperature increase in the middle and high troposphere over the Indochina Peninsula before the monsoon onset is caused by the total effect of the latent and sensible heating. The distribution of the atmospheric heat sources has close relationship to the land-sea contrast. The sensible heating contrast between land and sea is the large-scale contributor to the monsoon onset. The large sensible heat fluxes are located over the Indochina Peninsula, the northeastern part of the Tibetan Plateau and the most of India Peninsula, while the small sensible fluxes are located over the oceans. The heating effect starts earlier over Indochina Peninsula than over the Tibetan Plateau, so that the sensible heating over Indochina Peninsula has notable effect on the earlier onset of the SCS summer monsoon while the Tibetan Plateau plays an important role for the maintenance of the SCS monsoon. The heating effects of the Tibet an Plateau on the summer monsoon onset over different regions of Asia are also different: it is mainly due to sensible heating during the SCS monsoon, while it is mainly due to latent heating during the Indian summer monsoon (Jiao and He, 2000; Jian and Luo, 2001; Luo and He, 1997; Shao and Qian, 2001; Wang and Qian, 2001).
2. The Tropical Air-Sea Interaction
The analyses of the SST's over tropical equatorial eastern Pacific show notable quasi-biennial and quasi-4-year oscillations. Zhu and Chen gave a figure of six phases of the quasi-4-year oscillation from La Nina to El Nino for SSTA and the 850 hPa wind field over Pacific. Wu and Ni also showed a plot for five phases from La Nina to El Nino. They conclude that the occurrence of the northerly wind anomaly over the Northwest Pacific and the southerly wind anomaly over Southwest Pacific causes the meridional wind convergence along the equator, and also the eastward propagation of the equatorial westerly wind anomalies to the eastern equatorial Pacific, resulting in the formation of El Nino event. The convergence of meridional wind anomalies needs the concurrence of the anomalies of two hemispheres which must maintain more than half a year.
Recent studies (Li and Wu et al., 1998) show that eastward propagating (quasi-steady) waves, whose periods are longer than 90 days, play an important role for the eastward propagation of the westerly wind anomalies and inducing El Nino events. After the occurrence of El Nino event, the intensities of the above-mentioned two oscillations decrease suddenly, and then propagating westward and inducing the La Nina event. It is also found (Wu et al., 1998 and Meng et al., 2000) that there are good positive correlations between the equatorial SST's in Indian Ocean and that in Pacific which is controlled by two Walker circulations. The two circulations anomalies look like a pair of gear existing in equatorial Indian Ocean and Pacific Ocean. When one changes in clockwise direction, the other changes in counter-clockwise direction. The merging point of the two circulations propagates eastward into the Pacific Ocean, leading to the changes in SST's and zonal winds to its east side and inducing the appearance of El Nino event.
Some meteorologists studied the influences of the warm pool in the west Pacific. They (Huang and Zhang et al., 2000) found that the SST increase occurred first over the warm pool and then propagated eastward during the 1997/1998 El Nino event. From the observation data along 137oE , it is found (Ren and Zhou et al., 2001) that the large values of temperature variance were located over the region from 5 to 12oN in the sub-sea surface layer from 100 to 200 m. The temperature variations in this region occurred before the start of the El Nino/La Nina events. In winter, the sub-sea surface layer temperature has good correlation with the local SST in this region, while in summer, it has good correlation with the SST in Xisha due to the effect of circulation.
Many scientists (Zou et al., 1997; Zhu et al., 1998; Zhang et al., 1999; Jin et al., 1999) studied the influences of ENSO. They confirmed that it is mainly drought in summer over East China during El Nino years. They also pointed out that the summer rainfall anomalies over China depend on the stages of the El Nino event. During the mature stage of El Nino event, there exist positive rainfall anomalies over South China in autumn, winter and spring, while negative rainfall anomalies over North China and South China, positive rainfall anomalies over the mid-low reaches of Yangtze River and the Huaihe River region in summer. They also found that there are positive rainfall anomalies over the mid-low reaches of Yangtze River and the region to the south of Yangtze River in summer during the transition stage from El Nino to La Nina and vice versa.
Based on the SCSMEX data, it is found that there exist sudden changes in heat capacity of the atmosphere and oceans, and also in moisture and the momentum fluxes during the SCS summer monsoon onset. The energy is accumulated over the South China Sea before the SCS summer monsoon onset. The values of solar radiation and the latent heat flux almost half decreased, and the net energy flux between atmosphere and ocean was approximately zero (Yan et al., 2000; Jiang et al., 2002).
3. Ttropical Atmospheric Circulation
Chen et al. (2001) analyzed the inter-annual variations of the meridional circulation in January and July from 1961 to 1997, especially that averaged between 110 and 140oE and that of the East Asian monsoon circulation. They pointed out: (1) In addition to the annual variations of the meridional circulation, there are also notable inter-annual and inter-decadal variations. (2) The main characteristics of the meridional circulation averaged over 110 and 140oE are that the East Asian monsoon circulation replaces the Ferrel circulation in the Northern Hemisphere in January, while it replaces the Hadley circulation in July, and the inter-decadal variations are rather distinct. (3) The intensity of the East Asian monsoon circulation is influenced by the ENSO cycle.
The 1997/1998 ENSO event is a very strong one. Li et al. (2001) analyzed the relationship between the tropical atmospheric intra-seasonal oscillation (ISO) and ENSO, and pointed out that the occurrence of El Nino event in 1997 is strongly correlated to the abnormal intensification of the ISO in the central and western equatorial Pacific from the winter of 1996 to the spring in 1997. The abnormal ISO intensification in the upper layer over Indonesia is mainly due to the active convection in this region caused by the strong East Asian winter monsoon anomalies rather than that due to propagation from the equatorial Indian Ocean. Other scientists also found that the zonal wind anomalies in the low layer in the west Pacific can be used as a signal to forecast the warm event (Zhai et al., 2001)
Wu et al. (2001) studied the relationship between the circulation in the western equatorial Pacific in winter and the Asian monsoon circulation in the late spring and summer. The results show that the summer rainfall over China has notable influence from the circulation in the western equatorial Pacific in winter. The positive/negative sea level pressure anomalies and the corresponding cyclone/anticyclone circulation in the western equatorial Pacific in winter will cause stronger/weaker summer monsoon over South and East Asia, and more/less summer rainfall over the Yangtze-River valley in China.
Some studies show that the cross equatorial currents in summer affect the SCS monsoon and the drought/flood in East China. They found that the South Africa high, the South Indian Ocean high and Australian high all have important effects on the maintenance and the intensity of the cross equatorial currents. They also found that the ocean temperature field in the North Pacific affects the intensity of the cross-equatorial current. When the Somali cross-equatorial flow is strong in May, the SCS monsoon will start earlier. The cross-equatorial flow appears strong in the drought year over East China (Shi et al., 2001).
Based on the climate data of several decades, Song et al. (2001) found a decadal time-scale abrupt change in Asian and African monsoons in the 1960s. The results also reveal the synchronous characteristic of the decadal variations of the East Asian monsoon, the Indian monsoon and the North African monsoon, experiencing a process from strong to weak monsoons. It demonstrates that the abrupt change of summer rainfall over the arid and semi-arid regions in the Asia and Africa in 1 960s is directly related to the sudden change of the Asian and Africa monsoons.
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