长江上游中小洪水天气学机理分析及致洪特征

基于1981~2012年长江上游128个中小洪水历史个例及NCEP/NCAR再分析资料,采用普查及天气学分型方法,建立了纬向型、经向型、偏东气流型以及两高之间型4种致洪降水天气学概念模型,研究了各天气型致洪降水发生机理及相应中小洪水特征。得到以下结论:纬向型中高纬环流相对平直多波动,伴有明显冷平流南下,地面锋面位置略偏北。该类型强降水过程多,强度大,持续时间长,对应中小洪水多为双峰或多峰型,平均洪峰流量、过程增幅最强,洪水过程时间也最长。经向型环流中高纬贝加尔湖和东北地区为深厚低槽,中低层常伴有暖式切变线或低涡发展,中上层急流出口处的辐散以及冷平流四类型中最强。该类型雨带多呈东北-西南走向,中小洪水一般以单峰为主,其洪峰流量及过程增幅均较大,造成的洪水涨水较快,过程时间最短。纬向和经向型均为全流域降水型,但在金沙江北部、岷沱江、嘉陵江以及宜宾-宜昌常出现较高频次的60 mm以上较强面雨量。偏东气流型副高与热带气旋外围环流汇合北进,其强降水前后冷暖平流变化不明显,受地形强迫抬升影响,最易产生准静止型、团状、突发性强降水。该类型中小洪水以单峰为主,涨水快,洪峰流量及过程增幅均最小,强降水主要分布在嘉陵江和岷沱江两大流域。两高之间型多为"鞍"型场的环流配置,青藏高压与副高在流域上空形成南北向切变线,其动力和水汽条件均较弱。该类型降水强度较弱,稳定少动,累积降水量较大,洪水以单峰为主,双峰偶有发生,其洪峰流量、过程增幅均较大,洪水过程时间较长,强降水多位于岷沱江、嘉陵江和宜宾-重庆中部流域。

SYNOPTIC ANALYSIS OF MEDIUM-SMALL FLOOD AND THE CHARACTERS OF FLOOD-CAUSING IN THE UPPER REACHES OF YANGTZE RIVER

Based on the medium and small flood data from 1981-2012 in the upper reaches of Yangtze River and the NECP/NCAR reanalyzed data, the primary weather types of medium and small flood are summarized. They are zonal type, meridional type, easterly flow type and shear line between Tibetan high and subtropical high. The occurrence mechanism of flood-causing and the characters of medium and small flood have been researched by using the diagnostic analysis. The results show that the circulation of the zonal type is relatively flat in mid-high latitudes. Low trough in the northern XinJiang leads cold air flow to low-mid latitudes, and often accompanying with southwest jet at 850 hPa and 700 hPa, but the ground front position is slightly north. This type often has stronger precipitation processes, which are more severer and have long durations. And the medium and small flood presents double peaks or multiple peaks. The average peak discharge, the increasing discharge is the strongest. And the maintenance time of the flood process is the longest, about 14 days. The circulation of meridional type has deeper trough in Baikal lake and Northeast China, and has warm and humidity shear line or vortex development. The divergence in the exit area of the jet and cold advection is the most outstanding. The rainbelt of this type often appears from southwest to northeast. The medium and small floods present single peaks. The average peak discharge and the increasing discharge is medium. And the maintenance time of the flood process is the shortest, about 12 days and rising flow is faster. The high frequency of surface rainfall often occurs in the northern Jinshajiang River, Mingtuojiang River, Jialingjiang River and Yibin-Yichang watershed. The strong precipitation could occur in the whole watershed. Subtropical High over the Western Pacific and tropical cyclone meet toward to the upper reaches of Yangtze River with water vapor of the easterly flow type. The cold advection does not change significantly before and after the heavy rain. Influenced by topographic forcing uplift, the spatial distribution of heavy rain is frequently quasi-stationary, appeared as gobbets and sudden character. The medium and small flood of this type presents single peak. The average peak discharge and the increasing discharge is minimum and the rising flow is fastest. The strong precipitation occurs in the Mingtuojiang River and Jialingjiang River watershed. The circulation of shear line between Tibetan high and subtropical high is obvious saddle field. The power and water vapor conditions are weaker. This intensity of the precipitation is weaker, less stable, but accumulated precipitation is larger. The flood is given priority single peak, though bimodal peak occasionally occurred. The average peak discharge, the increasing discharge and the flood process is medium. The strong precipitation occurs in the Mingtuojiang River, Jialingjiang River and Yibin-Chongqing watershed.