| Hetao region located at the convergence zone of the Loess plateau, Inner Mongolia plateau and Erdos plateau, it is the transitional zone of Northwest China and North China, so it is very important to climatology and geopolitics. Rainstorm is the main natural disaster in this region, and it has particular characeristics and special causes. In this paper, we studied the climatic variation characteristics of rainstorm times in Hetao region, by using the daily observed data of87weather stations from1961to2010in the region, and applied linear trend analysis, composite analysis, Mann-Kendall test and wavelete analysis methods; we also analyzed the circulation background and main weather systems and the correlation between rainstorm times during june to september and height field, temperature field, wind field and sea surface temperature (shorted as SST). We wish our study can supply useful surpport for the rainstorm predicition and metearological services. The main conclusions we got are as follows:1The spatial distribution of rainstorms in Hetao region are typically as more in south-east and less in north-east; the rainstorm times declined in recent50years, and it has periodic or quasiperiodic variation characteristics in different time scales with no mutations. Around1982, the rainstorm times declined obviously in some regions and increased in other regions, the area with increasing of rainstorm times are mainly located at the juncture of Shanxi(陕西),Ningxia and Gansu Province and the loess region in middle and west Shanxi(山西) Province, the increasing of rainstorm times caused many floods and mudslides in this region.2The circulation of rainstorms in Hetao region can be divided into4types, they are2troughs and1pledge,2pledges and1trough,1trough and1pledge,1pledge and1tough; the main weather systems which caused rainstorms in this region mainly are westly trough, shear line, vortex, low-level jet and west pacific ocean subtropical high. 3In the years with more rainstorms, Hetao region are under the convergence zone of warm-wet south airflow in northwest of positive region of west pacific subtropical high and south-west airflow in the south-east of the center region of negative anomaly at500hPa height anomaly field,and under also the center positive region at500hPa temperature anomaly field. The interaction of weather systems such as vortex, trough line and so on, this makes the warm air converge and ascend there easily and benefit to the formation of rainstorms.In the years with less rainstorms, Hetao region is controlled by the north-east airflow at the south-east of negative region at north-east china at500hPa height anomaly field, and the southly airflow is weaker; and the central negative region at500hPa temperature anomaly field is just around Hetao region, it is too hard to form convergence of cold air there, and it is not benifitial to the formation of rainstorms.4In years with more rainstorms, Hetao region is under the positive region at500hPa zonal wind anomaly field, it is beneficial to the westly cold airflow to move southwardly and influence Hetao region; and Hetao region is also under the positive region at500hPa meridional wind anomaly field, it is beneficial to transport the south warm-wet airflow at the bay of Bengal to Hetao region. So the cold and warm air converges at Hetao region and forms rainstorms. On the contrary, in years with less rainstorms, Hetao region is under the edge of positive region at500hPa zonal wind anomaly field, the zonal wind is weak, it is not benifitial to the moving southwardly of cold air; and Hetao region is also under the edge of negative region at500hPa meridional wind anomaly field, the meridional wind is also weak, so it is not benifitial to the transportation of south warm-wet air at the bay of Bengal to Hetao region.5In years with more rainstorms, the positive region of SST anomaly field mainly located at coastal areas of north Europe and East China, Japan sea, and the pacific ocean east to Japan, and the SST is abnormally high especially at Novaya Zemlya, yellow sea and the pacific ocean east to Japan; the negative region of SST anomaly field mainly located at the Antarctic ocean, north India ocean and greater part of yellow sea, SST at the bay of Bengal and South china sea is abnormally low. On the contrary, In years with less rainstorms, the negative region of SST anomaly field located at coastal areas of east China, south China sea, Japan sea and pacific ocean east to Japan, and SST at yellow sea and Japan sea is abnormally low; the positive region of SST anomaly field located at the Antractic ocean, greater part of India ocean, and the SST at the region around the bay of Bengal is abnormally high.6During the flood season (from April to Octorber) in Hetao region, the correlation coefficient of rainstorm days and the landing times of typhoon of north-west pacific ocean shows negative at east and positive at east, and it shows negative at north-east and positive at south-west during July to September; comparing with other months, the positive correlation region obviously expanded during July to September, it mainly expanded eastly and northly, and it tells us that rainstorms easily occur in Hetao region during July to September due to the influence of landing of typhoon of north-west pacific ocean, but the correlation is not so obvious. |
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