The climate is abnormal

Analysis and Expression of Severe Drought in China 12 Province

Since the end of June, 2008, 5438+ 10, due to the continuous lack of precipitation, there have been rare droughts in Beijing, Tianjin, Hebei, Shanxi, Shandong, Henan, Anhui, Jiangsu, Hubei, Shaanxi, Ningxia, Gansu10, and severe droughts in some areas have reached once in 50 years. 1.57 million mu of wheat suffered from drought, and China sounded the I-level drought warning for the first time. The Express made a preliminary analysis of the drought process and meteorological causes, and made a forecast of the weather in the next two weeks.

First of all, rain

Since the end of June 5438+ 10 last year, the precipitation in northern winter wheat areas such as Beijing, Tianjin, Hebei, Shanxi, Shandong, Henan, northern Anhui, northern Jiangsu, northeastern Hubei, northwestern Shaanxi, Ningxia and eastern Gansu has been 50% to 80% less than normal, and the precipitation in central Shanxi, central and southern Hebei, northeastern and central Henan, western Shandong and northwestern Anhui has been 80% less than normal. The average number of days without precipitation in most parts of North China, western Huanghuai, Jianghan and other places reached the second place in the historical period since 195 1, among which the number of days without precipitation in Henan was the highest in the historical period (Figure 1, Figure 2).

Second, drought.

The meteorological drought monitoring of China Meteorological Bureau on February 5th showed that the meteorological drought in Beijing, central and southern Hebei, most of Shanxi, northern and western Shandong, most of Henan, northern Anhui, most of Shaanxi, eastern Gansu and northern Hubei reached moderate to severe, especially in central and eastern Henan and northwestern Anhui. Fig. 3(a) shows the drought monitoring map of China on June 5,438+1October 365,438+1October 2009, and fig. 3(b) shows the drought monitoring map of Asia on February 6. As can be seen from this picture, besides the drought in China, severe droughts have also occurred in North Africa, India and Indochina Peninsula.

According to the agricultural situation report of the Ministry of Agriculture, as of February 5, the main wheat producing areas such as Henan, Anhui, Shandong, Hebei, Shanxi, Shaanxi, Gansu and Jiangsu were affected by drought/kloc-0.57 billion mu, of which 64.82 million mu were seriously affected by drought. The irrigated area of drought-affected wheat fields in 8 provinces is 78.77 million mu, accounting for 50% of the drought-affected area.

On February 4th, the National Flood Control and Drought Relief Headquarters launched the Level II emergency response to drought, and announced the launch of the Level I emergency response to drought on the 5th. This is also the first time in the history of our country to start the first-level emergency response to drought.

According to the definition of meteorology and hydrology, the return period of an abnormal climate or hydrological event is once every 30 to 50 years, which is called extreme climate or hydrological event. This drought is an extreme weather event.

Third, meteorological reasons

The severe drought in China was caused by the long-term lack of precipitation. From the monitoring and analysis of meteorological reasons, we should start with the atmospheric circulation anomaly in June 2008+June 2008+10.

There is a quasi-static circulation system in the middle and high latitudes of the northern hemisphere, and this stable circulation situation is the main reason for this drought disaster. Fig. 4a shows the average height of 500 hPa in the northern hemisphere and its anomaly 1 1 from October 2008 to February 3, 2009. Fig. 4b shows the percentage of days in which the daily height anomaly is greater than 10 gpm and the negative height anomaly is less than 10 gpm, indicating the stability of each system in the average height field of 500 hPa. The main circulation system in the middle and high latitudes of Europe and Asia is the blocking high in northern western Europe. This blocking system reached its peak in June+February, 5438, and lasted until the beginning of February, and its stability from the anomaly center reached more than 70% (that is, it was occupied by positive anomalies for more than 67 days). The west coast of Europe in the southwest is a stable trough of low pressure (negative anomaly area), and its persistence is above 70%, which is reflected in the frequent activities of the upper trough over western Europe. There is a long stationary wave train along 30 ~ 50 N in the northern hemisphere, which shows that the East Asia trough is maintained in the northeast of Japan (Thousand Islands) and there is a stable high-pressure ridge over the plateau. Chinese mainland has long been controlled by the northwest airflow behind the East Asian trough, and the cold air activities are frequent. The persistence of high pressure over the East Asian trough and plateau is also above 70%. In the latitudes around10 ~ 20 n in the northern hemisphere, it is basically controlled by the subtropical high, which is obviously strong, but affected by the trough of low pressure in the mid-latitude of120 e, the subtropical high in the western Pacific extends westward, but does not move northward obviously (Figure 4b). The monthly average height field of 500 hPa from 2008 1 1 to 2009 1 is similar to Figure 4 (omitted). It is this abnormal circulation and its stability that lead to the formation and intensification of drought. According to the distribution of the average wind speed line of 300 hectopascals from June 5438+1 October1to February 3, 2009 (Figure 5a), a jet stream extends eastward from the middle of the Atlantic, Africa and the Asian continent to the south of Japan along the 20 ~ 35 N region (North Africa, the Middle East, northern India and southwestern China). In this jet, the maximum disturbance kinetic energy of 300 hPa meridional wind is concentrated in the west of 70 E, which indicates that the disturbance system from western Asia is not easy to move eastward over East Asia, so that water vapor cannot be transported to the east of 70 E, resulting in a large-scale drought in Asia.

We give the horizontal wind anomaly field of 200 hPa in June 2009 at 5438+1October, which represents the circulation in the upper troposphere during the most severe drought period (fig. 6a). There are several anticyclone anomaly centers in the northern hemisphere along 30 N, which are located in the middle of the Atlantic Ocean, the Mediterranean Sea and the Qinghai-Tibet Plateau, and there is a cyclone circulation area along the coast of China. These anomaly centers correspond to the positive and negative anomaly areas of 500 hPa in the middle troposphere. It can be seen from the horizontal wind anomaly of 700 hPa in the lower troposphere in June 5438+1October 2009 (Figure 6b) that northerly winds prevailed in most parts of North Africa, Indian subcontinent, Indo-China Peninsula and China, which indicated that winter winds were extremely prevalent in these areas, and water vapor from the ocean could not easily reach these areas.

Some areas with severe drought were once snowy areas in June 5438+1October, 2008, and the snow disaster was serious. These two processes are worthy of comparative analysis. Fig. 7 is a cross-sectional view of the monthly average vertical velocity (fig. 7a) and relative humidity (fig. 7b) near116 e, 33 N) in Fuyang, Anhui. During the snowstorm in June 5438+1October, 2008, the middle and lower troposphere (below 600 hPa) in Fuyang area showed an upward trend, and the relative humidity was above 55%. In June, 2009, 5438+ 10, except the whole near-surface boundary layer, the sinking speed of the middle troposphere (400~600 hPa) was close to 20 Pa× s- 1, and the air in the middle and upper troposphere sank strongly, and the relative humidity was basically below 40%, which inhibited the development of clouds and precipitation. The distribution of disturbance kinetic energy (storm path) in these two periods is also different (Figure 6b). On June 5438+ 10, 2008, three low-pressure disturbances moved eastward over China along the Qinghai-Tibet Plateau (pictured). Fig. 8 shows the average height of 5438+165438+1065438+500 hPa on June 2, 2008 and its anomaly field. It can be seen that the circulation that is most conducive to the occurrence of heavy precipitation in winter is "high in the east and low in the west", which is obviously different from the atmospheric circulation in autumn and winter in 2008-2009. The characteristics of approximate inversion of this circulation in figs. 4a and 8 and the persistence of abnormal circulation are in Hovm? The Ller diagram (300 hPa meridional wind time-longitude profile along 35 ~ 45 ON, Figure 9) is clear. Except for two strong cold waves in early February 2008 and from the end of February 2008 to the beginning of 2009 10, the eastern part of China is under the control of the northwest airflow in front of the plateau. In addition, during the 65438+ period from the end of 2008 to1October 2009, the 30-60-day oscillation (MJO) in the atmospheric season is very inactive at 60- 150 OE, which is obviously different from the strong MJO in this area during the snowstorm in early 2008 (figure omitted).

In climate prediction, people usually pay attention to the influence of SST anomalies in the equatorial eastern Pacific on global climate anomalies. We have noticed that the SST in the equatorial eastern Pacific in winter of 2008 and 2009 belongs to cold water phase (La Nina phase), while the climate in China is completely opposite. It seems that the relationship between this drought and La Nina event is very complicated and deserves further study.

Four. conclusion

The main reason for the lack of precipitation in most parts of China since winter is that although there were several strong cold air processes in the north last winter, the cold air did not bring precipitation, but only brought strong winds and cool weather. The main reason for less precipitation is that the disturbance system from West Asia is not easy to move eastward over East Asia, and the moist and warm airflow from tropical ocean surface is not strong. It only reaches the middle and lower reaches of the Yangtze River, but cannot be transported to places north of Huanghuai, North China, Henan, Shandong, Shanxi and Hebei (Figure 10).

The fifth assessment report of IPCC points out that under the climate background of global warming, the global water and energy cycles are accelerating, and extreme weather events are increasing, so is China. The severe drought occurred in China 12 provinces, which was an extreme climate event. In the summer of 2006, Sichuan and Chongqing suffered a once-in-a-century drought, and at the beginning of 2008, a serious low-temperature rain, snow and freezing disaster occurred in the south. Meteorological operations and scientific research departments should pay attention to the monitoring, diagnosis and prediction of extreme weather events.

Estimation of future weather conditions:

In mid-February, precipitation will be more frequent in southern China, and some dry areas in the north are expected to usher in precipitation. 1 13+04 There was a strong rainfall process in Jianghuai, Hetao and North China, and around 13- 14, the northeast, the middle and lower reaches of the Yangtze River, the eastern Sichuan Basin, Guizhou and Guizhou. After 15, there was another cold air heading south, but the rainfall mainly occurred in Huanghuai and its south area. In addition, according to the analysis of the medium-term forecast field of CFS by the American Climate Prediction Center (CPC) (Figure 10), by February 20th, the high-pressure dam near the plateau will still be maintained, and the atmospheric circulation conditions of "high in the west and low in the east" will not change fundamentally in a short time, and the northern region may still maintain a relatively dry situation. We should pay close attention to the adjustment of high-pressure dams near the plateau in the future.