西北地区大风日数的时空分布特征

利用选取的西北5省(区)以及内蒙古西部(110°E以西)分布均匀的127个气象站1960-2000年41a逐月大风出现日数资料, 分析研究了西北地区大风的空间、时间特征, 并具体分析了大风与沙尘暴的时空关系, 揭示了西北地区大风分布的一些新事实。西北大风天气可划分为较少区(年均大风日数小于10d)、较多区(年均大风日数10~50d)、多发区(年均大风日数50~100d)和频发区(年均大风日数大于100d)。西北地区大部分区域为大风较多区, 占总站数的614%, 大风频发区分布最小; 大风最频繁发生的地方在新疆西北部的阿拉山口, 年平均大风日数超过160d, 平均不到3d就有一次大风天气, 大风日数最少的地方是陕西北部延安, 平均每年发生大风天气的日数不到1d。大风日数空间分布与地形有很大关系, 两山之间的峡谷地带以及高山和青藏高原极易出现大风天气。西北地区多数台站近40a来大风呈减少趋势, 其中新疆西北部、甘肃河西走廊西部和陕西东部等地区减少最为明显, 大风增加的区域主要集中在新疆东北部到青海西部地区, 其代表站年均大风日数从20世纪60年代到80年代中期以后增加了近3倍, 达到190d。总体来讲, 西北地区大风天气最多的季节是春季, 以5月最多, 其次是夏季, 秋、冬季特别是秋季大风最少; 陕西、甘肃中南部夏季大风较多, 青海东南部则夏季最最少, 冬季大风更多一些。进一步分析表明, 西北地区大风频发区与沙尘暴频发区并不完全重合, 例如, 南疆是西北乃至我国沙尘暴最频发区之一, 但是南疆却是西北地区大风的较少区, 年大风日数远少于沙尘暴日数。同样是沙源丰富的沙漠地区, 也都是我国沙尘暴的主要频发区, 但是塔克拉玛干沙漠及其附近地区的沙尘暴日数远多于大风日数, 而巴丹吉林沙漠地区的大风日数却比沙尘暴日数明显偏多。最近40 a西北地区大风与沙尘暴发生次数随时间变化趋势一致, 基本呈线性减少特征, 这说明在下垫面状况不变或变化不大的情况下, 近年来沙尘暴次数减少可能主要是由于大风天气(沙尘暴驱动因子)减少而造成的。大风的时间变化可以决定沙尘暴随时间的变化。

Temporal and Spatial Characteristics of Gale Weather over Northwest China

The temporal and spatial characteristics of gale and the relationships between gale weather and sand-dust storm over northwest China are analyzed in detail by using the gale and sand-dust storm data for the period from 1960 to 2000 of 127 observational stations which are proportionally located in northwest China and west part of the Inner Mongolia. Some significant results and new facts are obtained. It can be divided into four sub-regions according to the annual frequency of gale weather in northwest China, which are fewer gale region(where annual gale weather are less than 10 days), more gale region(where annual gale weather are between 10 to 50 days), much more gale region(where annual gale weather are between 50 to 100 days) and frequent region(where annual gale weather are more than 100 days). Most parts of northwest China are more gale regions, and the most frequent region of gale weather is the Ala Mountain pass of northwest Xinjiang which yearly gale weather are more than 160 days. Yanan in Shanxi province is the region where gale weather is the least in northwest China, which the annual gale weather is less than 1 day. It has close relationship between gale spatial distribution and terrain that gorges, mountains and Qinghai-Xizang Plateau have more gale weathers than other areas. The reducing trend of gale is clear in most regions of northwest China in recent 40 years, whereas the gale increasing region mainly locates from northeast part of Xinjiang to west part of Qinghai, where the annual gale days of representative station in 1980s were 190 days, three times more than 1960s. In general, gale weather in northwest China occurs mostly in spring, especially in May, and the least is in autumn. There are more summer gale weathers in middle-south parts of Shanxi and Gansu provinces. Southeast part of Qinghai has more winter gale.Further investigations show that, some gale frequent regions are not completely the same as the sand-dust storm regions. But in recent 40 years, the trend of temporal variation of gale is consistent with sand-dust storm in northwest China, all which shows the same time feature of linear decreasing. Therefore, under such a fact that the landuse over northwest China do not or will not change better now or latter, the decreasing of gale weather may be the only and real reason to reduce sand-dust storm here.