Typical synoptic types of spring effective precipitation in Inner Mongolia, China

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1 METEOROLOGICAL APPLICATIONS Meteorol. Appl. 21: (2014) Published online 10 July 2012 in Wiley Online Library (wileyonlinelibrary.com) DOI: /met.1338 Typical synoptic types of spring effective precipitation in Inner Mongolia, China Tao Gao, a,b * Yaobing Si, c Wei Yan, a Lian Gao, a Xiao Yu d and Sujun Xiao e a Forestry College of Inner Mongolia Agricultural University, Hohhot, China b Inner Mongolia Meteorological Institute, Hohhot, China c Inner Mongolia Meteorological Science and Technology Service Centre, Hohhot, China d Computer Application Academy of Inner Mongolia, Hohhot, China e Inner Mongolia Meteorological Data Centre, Hohhot, China ABSTRACT: Spring (March to May) is a crucial season for crop seeding and grass growth in Inner Mongolia (IM), China. Yearly harvests of agriculture, and animal husbandry, are controlled partially by spring precipitation. Based on the observations at 104 stations in IM, the spring precipitation during was investigated and analysed. The results show that the amount of spring precipitation displays a trend associated with an increasing frequency of effective spring precipitation events (ESPE, with more than 10 stations among all observatories with over 10 mm daily precipitation). In addition, the synoptic conditions of the 141 ESPEs were studied through analysis of the sea level pressure (SLP), and 850, 700 and 500 hpa charts. They were classified into five types and named for the source positions of the frontal cyclones over the Eurasian continent on the SLP chart. Most of the Hetao, Mongolian and Huanghe cyclones in spring time, in general, bring strong wind, decreasing temperature or dust storms to IM. Sometimes they may also cause effective precipitation when the moisture transportation is favourable along the cyclone paths. The Northeast China cyclone mainly influences Northeast China and can lead to rainfall with adequate moisture supplies. In most cases, cold air from Siberia forms a frontal cyclone or a trough around Lake Baikal, and then heads eastward or southeastward, producing precipitation over large areas in IM in spring. Typical features of the synoptic evolutions of those five types are summarized and presented through analysis of representative ESPEs. KEY WORDS Spring rainfall; Inner Mongolia; frontal cyclones; classification; synoptic conditions Received 16 August 2011; Revised 28 March 2012; Accepted 21 May Introduction Inner Mongolia (IM), a large province of million km 2, lies in northern China and contains arid and semiarid regions. It spans the area from 37.3 N to 53.3 N and from 97.0 E to E. Spring (March to May) is the driest season with a 50 year average ( ) rainfall amount of 41.4 mm. When compared to a mm annual average over the same 50 year period, spring precipitation accounts for only 13.1% of the yearly total. The amount of spring precipitation in IM fluctuates over time. For instance, during the past 50 years the driest spring was in 1965 with 18.2 mm of precipitation, whereas the wettest was in 1998, which had 73.7 mm of precipitation. Spring drought is the most frequently occurring meteorological disaster in IM and insufficient spring precipitation is one cause of the drought. For example, a huge reduction of crop yields, which happened in 1965, was brought on by a lack of precipitation in the spring and summer. The anomaly percentage of the whole region s spring precipitation this year reached 49.6% compared with the 50 year mean value. The total yield of all crops decreased by 550 million kg, from 4350 million kg Correspondence to: T. Gao, Inner Mongolia Meteorological Institute, Xincheng Hailaer Str. 49, Hohhot , Inner Mongolia, China. Frautao@yahoo.com W. Yan, Forestry College of Inner Mongolia Agricultural University, Cincheng Zhaowuda Str. 306, Hohhot , Inner Mongolia, China. Yanwei 89911@163.com in the previous year to 3800 million kg in In addition, due to insufficient rainfall the number of livestock reduced by nearly 20 million head, which is 0.6% from the 1964 values, because of a lack of sufficient grass to feed the livestock (Wu, 1990). The present investigation indicates that most of the spring droughts, which occurred in IM during the past 50 years were accompanied by a lack of spring precipitation. The crop yields per unit area are estimated to vary by ± 93.4 kg hm 2 when the spring precipitation increases or decreases by 15 mm, equal to 35.3% (Chen and Gao, 2008). Therefore, spring precipitation plays an important role not only determining crop yields but also in local surface vegetation in IM in spring. Particularly in the barren deserts and sandy lands, sufficient spring precipitation may prevent sand and dust particles from being entrained by the wind, thus restraining the occurrence of spring dust storms as well. Extensive studies regarding precipitation in China have focused on annual and seasonal scales for different regions. The factors that influence the relative atmospheric circulation, such as the interseasonal oscillation over the tropical western Pacific/South China Sea, the Arctic Oscillation, the Northwestern Pacific High, the sea surface temperature (SST) and the sea level pressure (SLP) have been analysed by using statistical methods, model simulation, rotated principal component and wavelet transforms (Wang, 1994; Li and Li, 1997; Xu and Chan, 2002; Qin et al., 2005; Song et al., 2007). For the whole region of IM, the annual amount of precipitation did 2012 Royal Meteorological Society

2 Synoptic types of spring precipitation in Inner Mongolia, China 331 not exhibit an evident trend within the period , but sharply decreased during Compared to the 30 year climatological mean value ( ) the precipitation of those 7 years decreased by 35.5% (Gao et al., 2009). Most published studies about China spring precipitation have concentrated on southern China (Zhang et al., 1999; Yang and Lau, 2004; Yang and Huang, 2005). The Asian-Pacific oscillation (APO) associated zonal teleconnection pattern over the extra tropical Asian-Pacific range was identified, which reflects an out-of-phase relationship in the variability of eddy temperature between Asia and the North Pacific (Zhao et al., 2007). The APO exerts an effective control over spring precipitation in central-eastern China. It has been proved in the study of Zhou and Zhao (2010) that a positive (negative) phase of the APO tends to increase (reduce) the spring precipitation. In IM, effective spring precipitation, in any case, should play a very important role both in crop production in farming areas and livestock breeding in grasslands. Springs with one or more sufficient rain or snow event may foreshadow a rich harvest in autumn. Therefore, it is necessary to study the effective spring precipitation and its synoptic conditions in detail, for spring is the time when the atmospheric circulation transitions from winter to summer during which different synoptic situations may cause dissimilar spatial distribution and intensity of precipitation in different areas in IM. It may be useful for forecasters to predict the major spring precipitation areas if they can identify the cyclone types and learn the synoptic conditions 2 or 3 days before the rain or snow occurs. In addition, time series of the effective precipitation events and the cyclone frequency may be used to explore seasonal forecast signals among the climatic elements through statistical analysis methods. A prediction scheme may be established in the future by using those useful forecast signals and then precautionary actions to mitigate the spring drought may be taken if the local people and authorities can obtain a correct spring prediction. total area of IM. It is regarded as an effective spatial distribution within this central precipitation area, because if the areas where the precipitation is below 10 mm are accounted in the total rained-on region, the precipitation area will certainly be larger than the central area of precipitation. As mentioned above, IM has a large span in spatial distribution. The differences in climate conditions from southwest to northeast are vast. Considering spring precipitation distribution and locations of the deserts and sandy lands, agricultural cultivation, grazing and forest areas, the whole region of IM is separated into three parts (Figure 1 and ). Region 1 mainly contains the desert areas in its west part and grasslands in its east. The major farming areas are located in Region 2, where the temperature and precipitation are friendlier to the crops than those of the other two regions. Region 3 includes the Da Hinggan Mountains areas and some small farming areas in both west and east sides of the mountains. The numbers of stations with available historical precipitation records in the three regions are listed in Table Climatic features of local spring precipitation Compared to the averaged spring and yearly precipitation of the three regions, it is found that Region 1 is the driest area with the lowest amount of spring precipitation (Table 1). Region 2 has more spring precipitation than the others, although its yearly precipitation is less than that of Region 3, which has the highest annual precipitation in IM. Spring precipitation exhibits a significant increase during (Figure 1), while the yearly precipitation shows a relatively small increase compared to the spring precipitation variety in the same period. Averaged decadal spring precipitation of IM displays an obvious increase during the last three decades (Table 2), with the decade with the most increase occurring in the first decade of the 21 st Century. The most 2. Data and methods The precipitation dataset used in this study is provided by Inner Mongolia Meteorological Data Centre which contains available daily precipitation records at 104 stations scattered in the whole region of IM. All observed records were professionally checked for data quality and accuracy by using the Operational Software for Surface Meteorological Observation (OSSMO), which was issued by the China Meteorological Data Centre. Checking processes were conducted by using the extraction method in OSSMO. The precision of the observed daily precipitation is 0.1 mm and the collecting period is from 2000 to the next 2000 (Beijing Time, = UTC + 8 h). The grid data used for drawing the geopotential height, wind and relative humidity fields were downloaded from the NOAA/NCEP website. The ESPEs were calculated by using the daily rainfall amount. For the purposes of this study, 104 observation stations in IM were investigated. The stations will report a drought if there is no more than 10 mm of precipitation recorded during a 20 day period in spring (Wu, 1990). In order to describe an effective precipitation event objectively, concerning both intensity and spatial distribution at the same time, a station is flagged if it has more than 10 mm daily precipitation. When 10 or more of the 104 observation stations have been flagged, an efficient precipitation event is registered. By estimating the area of the stations and the region in their vicinities, 10 stations on average cover about km 2, which covers 8.4% of the Table year mean spring and annual precipitation of Inner Mongolia ( ). Region Amount of stations Spring P (mm) Annual P (mm) Percentage of spring P in years (%) W-R P, precipitation; W-R, whole of Inner Mongolia. Table 2. Decadal mean spring and annual precipitation anomalies of Inner Mongolia (%). Decade Spring Annual R1 R2 R3 W-R R1 R2 R3 W-R R 1, R 2, R 3 and W-R denote Region 1, Region 2, Region 3 and whole of Inner Mongolia, respectively.

3 332 T. Gao et al. Figure 1. Information about spring precipitation of Inner Mongolia. Mean spatial distribution of spring precipitation ( ), division of rainfall regions (the straight line shaded areas are farming places and the rest of the areas in Inner Mongolia are pasturing regions, sandy lands or barren deserts), temporal variations of spring precipitation anomaly during (compared with the mean precipitation of ), frequencies of more than 10 mm precipitation events during This figure is available in colour online at wileyonlinelibrary.com/journal/met apparent precipitation anomaly appeared in Region 3 which has an anomaly of more than 21.3% compared to the climatological mean value of the period from 1961 to In contrast, the smallest precipitation increase can be found in Region 2. Region 3 had more precipitation in the 1990s and 2000s, in both spring and annual precipitation compared to the 50 year climatological mean values, which is speculated to be helpful for revitalization of the surface vegetation and improvement of the local surface environment. A total of 141 effective spring precipitation events (ESPEs) have been identified among all spring precipitation processes in the past 50 years. ESPE frequencies increase as time progresses (Figure 1), which indicates that the increasing trend of spring precipitation is very likely caused by the increase of ESPEs. There were four ESPEs in 1961 and six in The biggest precipitation amounts at the precipitation central stations of the ESPEs are averaged for those 2 years. The four ESPEs mean biggest precipitation in 1961 is 25.6 and 33.5 mm for the six ESPEs in Classification of synoptic conditions All synoptic weather charts for the 141 ESPEs at four geopotential levels (surface, 850, 700 and 500 hpa) in the period are studied by analysing the evolution of the weather systems for the 2 or 3 days prior to every ESPE. It has been identified that all of the 141 ESPEs were linked to frontal cyclones and none of them were caused by squall line or other weather processes. Dissimilar surface frontal cyclone systems correspond to different circulations at the higher levels of the atmosphere, generally resulting in significant differences of precipitation in intensity and spatial scales. From this, different researchers may classify the synoptic types in dissimilar ways by focusing on various aspects, for example by attending to the weather conditions at 500 hpa or locations of the rainfall areas. Therefore, different classification results arise. In the present study, attention was paid mainly to the weather conditions at the surface. The synoptic classification is primarily based on considering the appearance of a frontal cyclone on the SLP charts 2 or 3 days before the heaviest precipitation day of the ESPEs. From that, all synoptic conditions of the 141 ESPEs are categorized into five types. They are named after the position the cyclone appears over large areas of East Asia (Figure 2). As noted in Table 3, of the five types, the first two occur with far greater frequency than the remaining three. The most frequent one is the cyclone occurring over the Hetao region and its vicinity area (38 43 N, E), named the Hetao cyclone. It is likely to appear in the winter half year. In general, it moves eastwards after its original establishment and influences the downstream regions, such as Shanxi Province, north China, the east part of IM and northeastern China. In spring, the Hetao cyclone mostly leads to strong winds, but may sometimes bring effective precipitation to IM when adequate moisture is available. Although there are no closed isolines on the surface chart, the convergence of an inverted trough and cold-warm fronts can also bring effective precipitation to most areas of IM. The next most frequent system is named the Northeast China cyclone, which is a further development of the Mongolian, Hetao or Lake Baikal cyclone. Northeastern China can receive

4 Synoptic types of spring precipitation in Inner Mongolia, China 333 Figure 2. Diagram of the five type cyclone originally create areas in East Asia during spring. Table 3. Decadal effective spring precipitation frequencies of the five type cyclones in Inner Mongolia. Decade Hetao Northeast China Mongolia Lake Baikal Huanghe All types Sum Decadal percentages (%) an effective spring precipitation when this cyclone moves into the region with an abundant supply of moisture. The Mongolian cyclone, which is the third in frequency, gets its name from its origin range (43 50 N, E) in the Mongolian Republic. In spring the Mongolian cyclone mostly results in strong winds and intensive drops of temperature in IM and can produce dust storms over Northwest China (Gu et al., 1994). If the cyclone is accompanied by sufficient moisture it also can bring effective precipitation to IM. The Lake Baikal cyclone appears in the north range, around 50 N around Lake Baikal and is the fourth most frequent system. It can be initiated with an invasion of strong cold air from Siberia, which then heads into China via the Mongolia Republic. It is regarded as one of the most influential weather systems that impact East China, as it can occasionally bring highly effective precipitation to most parts of IM in spring. The least frequent is the Huanghe cyclone. It can be identified when a cyclone appears and then develops around the middlelower branches of the Yellow River (36 41 N, E). It appears at any time of the year but is more likely to occur in the Hetao area during the winter half-year and, generally, it covers the lower reaches of the Yellow River in the summer half-year. This kind of cyclone mostly impacts north China and the southern area of northeastern China, resulting in heavy precipitation if sufficient moisture supply is available. The Hetao cyclone, as the most frequent, takes up to 41.8% out of a total 141 ESPE events. The other types account for 32.6, 14.9, 5.7 and 5.0% respectively. From another perspective, the variations of decadal frequencies of the cyclone occurrences are different for the decades from the 1960s to the 2000s. In the 1970s there were 19 cyclones which accounted for 13.5% of the total cyclone number, the lowest cyclone frequency during those five decades. Correspondingly, this decade is the driest with the lowest spring precipitation compared to the precipitation amount of the other four decades. Conversely, there were 39 cyclones observed in the 2000s, accounting for 27.7% of the 141 ESPEs, which corresponded to the wettest 10 years among the five decades (Table 3). The frequency of cyclone appearance in this decade is more than twice that in the 1970s. Evidently a close relationship exists between spring precipitation and the cyclone frequencies, with a correlation co-efficient of up to 0.75 at the 0.01 statistical significance level, which indicates more spring cyclones can result in more effective spring precipitation in IM. The causes of this climatological phenomenon of cyclone frequency increase which results in the uptrend in spring precipitation need to be explored. Primary correlation analyses indicate that the pre-summer SLP, geopotential height at 500 hpa, the temperatures both on the surface and at 500 hpa between the zones around the Equator and some other areas on the globe, display significant correlations with the spring precipitation. Additionally, indices of the subtropical high and North Polar vortex of the previous summer show close relationships with the precipitation. The atmospheric evolutionary processes of these climatic elements require further studies, but this is beyond the scope of this work. Different types of the surface cyclones associated with diverse synoptic situations at the high levels may lead to various intensities and different precipitation ranges of the ESPEs in IM when the moisture condition is favourable. Generally speaking, the circulation at 500 hpa, the westerly cold air invasion has been confirmed as the main cause of the ESPEs in the overall scale of IM. Moreover, major features of the upper level atmospheric circulations are summarized as the cold air activities which can be viewed on 800, 700 and 500 hpa charts. The sufficient moisture mass around 40 N, in general, is transferred to IM by southeasterly or southwesterly flows. 5. Typical synoptic characters It is an enormous amount of work to present all weather evolutionary characters of the ESPEs because a great number of differences exist among the situations which bring rain or snow. Therefore, five representative examples are selected and analysed in order to exhibit major evolutionary features of all synoptic types before and during the process of the ESPEs. Information about those five precipitation processes including precipitation time, region of major precipitation fall, centre of precipitation and the amount of rainfall or snowfall is shown in Table The Hetao cyclone The rainfall process of 4 6 May 1992 is regarded as a typical Hetao cyclone, with precipitation falling from west to east of IM. On 2 May, an inverted trough of low pressure (30 43 N,

5 334 T. Gao et al. Table 4. Information about spring precipitation events of the five representative cyclone types. Cyclone type Hetao Northeast China Mongolia Lake Baikal Huanghe Precipitation time 4 7 May May , May May May 1988 The heaviest precipitation day 5 May May May May May 1988 Major falling region R 1, R 2, R 3 R 3 R 1, R 2, R 3 R 1, R 2, R 3 R 1, R 2 Precipitation centre (WMO number) Zhaluteqi (54026) in R 2 Molidawaqi (50645) in R 3 Zhalantun (50639) in R3 Abagaqi (53192) in R 1 Yijinhuoluoqi (53545) in R 2 Precipitation amount of the centre (mm) Number of stations with more than 10 mm precipitation WMO represents World Meteorological Organization. R 1, R 2, R 3 denote Region 1, Region 2 and Region 3, respectively. Figure 3. Synoptic condition and precipitation of the Hetao cyclone at 0000 UTC 4 May SLP field (contour interval 150 hpa), 500 hpa geopotential height (contour interval 25 gpm), 700 hpa relative humidity and wind fields, rainfall distribution of 4 7 May 1992 (areas marked by load contours are the regions with more than 10 mm precipitation) E) can be seen over the west region of IM in the SLP chart and there was a cold high in western Siberia at the same time. The cold air accumulated in the area north of Xinjiang Uigur Autonomous Region. A continental-scale warm high settled in the eastern region of 105 E. Later that day, the inverted trough became a closed low pressure system (cyclone), with a wind speed in the eastern part of the cyclone reaching 12 m s 1, and rain began to fall in the western part of Region 1. On 3 4 May the cyclone centre was lying in the western area of Hetao and the warm front of this cyclone was right over the Hetao area (40 42 N, E). Precipitation started to appear in this region. The cyclone centre remained over the Hetao region when abundant moisture over the Pacific Ocean, transferred by the warm-wet current along the eastern rim of the cyclone, reached the cyclone warm front areas near 43 N and converged in this region (Figure 3). A large area around the convergence region received rain for the following 2 days. Aloft, a cold trough presented over the Ural Mountains at the 500 hpa level on 3 May. Ahead of the trough a ridge of high pressure occupied the region between Lake Balkhash and western Siberia. The next day, the cold trough moved to the areas of Lake Balkhash and western Siberia while the cold air entered Xinjiang along the bottom of the trough (Figure 3). The cold air constantly moved southeastward, which provided a favourable cold advection background in the upper air for precipitating in IM. Furthermore, on 3 May, there was a trough that can be found in 700 hpa chart, which covered the regions of the Qinghai-Xizang Plateau and southwest China while a warm shear presented around the Huaihe River (near 33 N, E). Over the northern area of the river, the warm-wet

6 Synoptic types of spring precipitation in Inner Mongolia, China 335 Figure 4. Synoptic condition and precipitation of the Northeast China cyclone. SLP field at 1200 UTC of 23 May 2007 (contour interval 150 hpa), 500 hpa geopotential height at 0000 UTC on 24 May 2007 (contour interval 25 gpm), 700 hpa relative humidity and wind fields at 0000 UTC on 24 May 2007, rainfall distribution of May 2007 (areas marked by load contours are the regions with more than 10 mm precipitation). air was brought northward, and then approached 40 N from a southeastern directional wind (Figure 3). During 4 6 May, a trough of low pressure remained over southeastern China, which provided helpful moisture conditions for the rainfall in IM. The spatial distribution of this precipitation process is shown in Figure 3. The major areas with more than 10 mm precipitation are located in Region 2 and the eastern part of Region The Northeast China cyclone The presented example of the Northeast China cyclone is the easterly-heading system of a strong Mongolian cyclone, the centre pressure value of which reached 990 hpa. This cyclone was originally observed over Ulan Bator the capital city of the Mongolian Republic 2 days before the precipitation process of May When the cyclone moved into northeast China on 23 May (Figure 4), it delivered an effective rainfall over this region. From the 500 hpa chart of 23 May, the zonal circulation mode was seen over the Eurasian continent. Meanwhile, a wide low pressure belt could be found from the Arctic Ocean to 60 N and between 40 and 60 N there was a Ural Mountain trough. The area of Siberia and Xinjiang Province in China was controlled by a ridge, and a low pressure trough appeared over the region of Lake Baikal and the western mountain area of the Mongolian Republic. Along with the cold air moving south, the Lake Baikal trough strengthened into a cold vortex and then headed into the northern area of IM later in the day. On the following day, the vortex continuously moved eastwards and crossed the northern part of Northeast China. Thereupon, a North China cyclone was formed (Figure 4). On 25 May the low vortex was in the northeastern region of IM, but the precipitation was over when this vortex moved outside of the region on 26 May. Furthermore, the same synoptic evolutionary process could also be viewed on the 700 hpa chart. There was a trough over Lake Baikal on 23 May while a ridge controlled the region of the Korean Peninsula and northeast China. Moisture over the East China Sea was carried to the northeastern part of IM by the southwesterly flow before the trough. On the following day, the Lake Baikal vortex moved into the northeastern area of IM while the vortex centre located in the same place appeared on the 500 hpa chart. The cold vortex was deep and produced unstable precipitation in the eastern part of IM (Figure 4). This raining process is dissimilar to the Mongolian cyclone which will be discussed next. In this case, the upper air northeastern moving vortex associated with the surface Northeast China cyclone, leading to strong and unstable converge in the vertical direction. This synoptic condition results in an effective rainfall in this region (Figure 4) The Mongolian cyclone As a Mongolian cyclone precipitation example, the synoptic evolutionary features of the May 2005 rainfall event are summed up through analysing the weather charts both on the surface and the upper air levels around these days. In the SLP chart of 28 May, a Mongolian cyclone centre

7 336 T. Gao et al. Figure 5. Synoptic condition and precipitation of the Mongolian cyclone. SLP field at 0000 UTC of 29 May 2005 (contour interval 100 hpa), 500 hpa geopotential height at 0000 UTC on 30 May 2005 (contour interval 25 gpm), 700 hpa relative humidity and wind fields at 0000 UTC on 29 May 2005, rainfall distribution of May 2005 (areas marked by load contours are the regions with more than 10 mm precipitation). covered Ulan Bator (WMO station number ) and its surrounding areas. Another cyclone appeared over the western part of IM, generally named a vice Mongolian cyclone. Later this day, the first cyclone moved eastwards and the second cyclone approached the Hetao region. The rain began to fall along the front of the first cyclones in Region 1. On the next day, the centre of the first cyclone transferred to Erlianhot (53 068), which is in the north central part of IM. The region of Mongolia was covered by a low pressure belt (Figure 5). Region 2 received precipitation on this day. On 30 May, the first cyclone was continually moving eastward and covered the area around Wulanhot (50 838), producing uneven rainfall around the cyclone centre and its front regions. Observing the weather conditions on the 500 hpa chart of 29 May, a relatively even westerly jet covered the mid-high altitude zone of East Asia while a cold vortex existed around the Altay Mountains. On the following day, the vortex moved eastwards over the Mongolian Plateau (Figure 5). Later, the centre of the vortex reached the position around 48 N to 113 E and became stationary, which provided a favourable synoptic background for precipitation in IM. In addition, the weather situation on the 700 hpa chart presented a helpful moisture condition for the rainfall (Figure 5). From the chart of 29 May, a trough around 110 E can be seen in the eastern part of the Mongolian Plateau. The significant warm advection before the trough started bringing rains to IM. Corresponding to the trough system in the region of N and E at the level of 850 hpa, an obviously cold advection appeared behind the trough. Meanwhile, a branch low air southwesterly jet formed based on the interaction between the southwesterly flows before the trough and behind the subtropical high. The wind speed of the central jet was up to 24 m s 1.On30May, a new strong baroclinic cold eddy was established around 48 N to 111 E. Therefore, in the space between 850 and 500 hpa, the low-pressure eddy upward convergence and the southwesterly flow provided a dynamic power and moisture condition to the precipitation. The rainfall covered most of the area of IM, from Region 1 to Region 3 (Figure 5) The Lake Baikal cyclone From the chart of 28 May 1970 a huge cyclone was observed around Lake Baikal (Figure 6). There was a corresponding cyclonic front lying near 55 N. On the next day, the cyclone moved eastwards approaching the position of 55 N to 125 E and the front of it approached southward, and then moved into the zone between 45 and 55 N. After that time, it was connected to a slowly easterly moving low pressure inverted trough which covered the region of the Qinghai- Xizang Plateau and southwest China. The high over North China and the Bohai Bay and the high over the Japan Sea then merged, creating a strong warm high. Furthermore, a converged upward flow formed between the warm high and the cold high over western Siberia. Due to the cold and warm air masses merging, the surface front transferred slowly and the strong converged upward flow near the front induced

8 Synoptic types of spring precipitation in Inner Mongolia, China 337 Figure 6. Synoptic condition and precipitation of the Lake Baikal cyclone. SLP field at 0000 UTC of 28 May 1970 (contour interval 150 hpa), 500 hpa geopotential height at 0000 UTC on 29 May 1970 (contour interval 25 gpm), 700 hpa relative humidity and wind fields at 0000 UTC on 29 May 1970, rainfall distribution of May 1970 (areas marked by load contours are the regions with more than 10 mm precipitation). more than 5 mm precipitation in IM during May. On 31 May the warm high lost its strength and the Mongolian cold high advanced southward and the precipitation in IM was over. On 500 hpa chart of 28 May a two trough one ridge circulation mode was recognized over the Eurasian continent (Figure 6). This mode, generally, is regarded as a meridional circulation pattern. The European plain was covered by a low pressure trough, meanwhile a high pressure ridge was hanging over the Ural Mountains and the western Siberia plain, and a comprehensive trough was present over the eastern region of the ridge. Afterwards, the high ridge strengthened and headed eastwards during the following day. The cold air before the ridge approached southwards from the Arctic, and then massed over Lake Balkhash and formed a cold trough around the Lake. On 30 May the trough remained in an inactive state while there was a temperature trough behind the geopotential trough. This circulation condition induced the cold air to head south continuously, further impacting IM. Additionally, the synoptic scenario on Lake Baikal on 28 May at the 700 hpa level was controlled by a cold trough when a low vortex appeared over southwest China. The centre of the vortex was right over Chengdu (56 294) in Sichuan Province. During this time, IM was controlled by a cold high. On the next day, the Lake Baikal trough remained stable and the cold vortex moved near Lanzhou (52 889), in Gansu Province. Meanwhile, shear lines emerged between the regions of Huaihe and the Yangtze River, which brought a heavy rain of 180 mm to this region. At the north side of the shear lines a strong southeasterly flow carrying warm-wet air reached to 41 N between 105 E and 112 E. On 30 May, the Lake Baikal trough moved fast to the east and a low level southwesterly jet before the trough appeared in the range of E with the greatest velocity at 18 m s 1. The atmospheric circulation condition between 850 and 700 hpa provided a powerful moisture condition for precipitation and the gradually fading shear also give an unblocked way for the moisture s northward transportation as well (Figure 6). The presented Lake Baikal cyclone precipitation mainly affected Region 1 and Region 2 in IM (Figure 6) The Huanghe cyclone The Huanghe cyclone example induces a wide range of precipitation, which mostly falls in the western part of IM. On the day before the rain started, a low-pressure inverted trough appeared over the Qingahi-Xizang Plateau and Xinjiang in the SLP chart of 17 May On the next day, the trough moved to the Hetao area, and then a cyclone was created over the

9 338 T. Gao et al. Figure 7. Synoptic condition and precipitation of the Huanghe cyclone. SLP field at 0000 UTC of 19 May 1988 (contour interval 150 hpa), 500 hpa geopotential height at 0000 UTC on 19 May 1988 (contour interval 25 gpm), 700 hpa relative humidity and wind fields at 0000 UTC on 20 May 1988, rainfall distribution of May 1988 (areas marked by load contours are the regions with more than 10 mm precipitation). Huanghe Valley, its centre near Dengkou (53 419), IM, and then moved slowly eastwards. Rain began to fall in the middlewest region of IM on 18 May. On 19 May, a western Siberian high approached southeastward, forcing the cyclone to move quickly eastward (Figure 7). Also, there was a secondary cold front over the Huanghe Valley which prolonged the period of precipitation in IM. The upper air synoptic situation performed a zonal circulation pattern as shown on the 500 hpa charts of May. Most of IM was controlled by a short wave trough upon the westerly belt in the medium-latitude zone (Figure 7). On 17 May, the western areas of Qinghai Province were under the control of a low-pressure trough while other areas in China were covered by a warm high. The trough then, transferred eastwards and strengthened into a deep low-pressure vortex over the regions east of 105 E. The southwesterly flow converged around Erdos (53 543) in IM. Therefore, the unstable mesoscale upward convergence system produced a favourable dynamic condition for precipitation (Figure 7). The spatial distribution of the rainfall during May is presented in Figure 7. Region 1 and the western part of Region 2 are the mainly rained on areas in this ESPE. 6. Summary All spring precipitation events in Inner Mongolia (IM) during are investigated in this study. It has been found that the variations of both spring precipitation and the effective spring precipitation event (ESPE) frequencies display increasing trends over the past 50 years. There is a total of 141 ESPEs identified and all of them were induced by different kinds of frontal cyclones which appeared in East Asia on the sea level pressure (SLP) chart. Furthermore, all synoptic statuses of the ESPEs are examined by seriously inspecting the SLP, 850, 700 and 500 hpa charts. By primarily considering the places here the frontal cyclone appeared in all 141 SLP fields, the synoptic cases are classified into five types: the Hetao, North China, Mongolian, Lake Baikal and Huanghe cyclones. For each type one representative ESPE was selected and analysed synoptically. The Hetao cyclone is the most frequent ESPE cyclone. The crucial cause of the Hetao cyclone s precipitation is its eastward movement in combination with favourable moisture transportation. Along with the cyclone s path, the rain starts to fall from the west to the east of IM. Generally, a large part of IM will receive rain or snow in this case. The North China cyclone mostly originates from a Mongolian, Hetao or Huanghe cyclone that originally formed several days before it moves into north China. The primary feature of this type is the strong convergence activity in the vertical direction which makes the atmospheric circulation become unstable over this region. A large number of spring Mongolian cyclones most probably lead to strong wind, decreasing temperature and even dust storm weather in IM, but sometimes it may also induce ESPEs in the region if the moisture transportation is favourable. The moisture movement is largely dependent on the control by southwesterly warm-wet flow over the low altitude zone. Most of the Lake Baikal cyclone precipitation is conducted by the convergent activities over the cyclone front in conjunction with advantageous moisture conditions in the region between

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