中纬度突发性暴雨的可能先兆特征

通过对湿位涡（MPV）不守恒过程出现条件的讨论，理论上证明了：在中纬度突发性暴雨产生前，①必定存在着深对流高度范围内δθ^＊/δz迅速趋向于零的过程。②斜压向量沿广义位温梯度有一分量。③接近饱和的不饱和层足够厚。达到深对流高度。④处于含有大量水汽和液态水的云团和云块附近。1999年合肥2次突发性暴雨确实都有体现上述4个必要条件的2种共同特征现象。⑤在暴雨生成之前，在合肥上空存在着深厚的双层干区（在多普勒雷达VWP图上表现为深对流高度范围内的两层连续无资料区），首先，随着上层干区顶部的湿冷空气开始下滑，上层千区开始从上到下消失，接着，当下滑空气停止下滑而开始上升时，下层的干区开始消失，最后在两层干区完全消失时，带来突发性暴雨的中尺度对流系统就突然出现。⑥同时在云图上，南下冷锋云带在逼近合肥市分裂成两部分，以至在断裂处几乎没有云和雾，而在周围则存在着含有大量液态水和水汽的两块云团。而在冷锋云带通过合肥后，带来突发性暴雨的中尺度对流系统就在几乎没有云和雾的断裂处爆发。

POSSIBLE PREMONITORY CHARACTERS OF DOPPLER RADAR PRODUCT AND CLOUD CHART FOR THE MID-LATITUDE SUDDEN HEAVY RAIN

By discussing the occurrence condition of non-conservation process of moist potential vorticity (MPV) according as the basal characteristic of the sudden heavy rain (SHR) in mid-latitude, it is proved theoretically as follows: The SHR in mid-latitude is possible only in the place where, just before its occurrence, (1) there must be a process that ∂θ^*/∂z within a deep convective altitude is enough and prompt approaching zero, and (2) under the condition that the baroclinic vectors have a component along (against) the direction of broad sense potential temperature gradients, (3) the atmosphere within a deep convective altitude must be in the neighborhood of saturation, but it must not be in saturation state, so there must be litter cloud or fog, and moreover, (4) the place must be in the neighborhood of cloud clusters or a cloud band containing a lot of cloud water and water vapor. By the analysis of the Doppler radar and cloud chart data related to two courses of the SHR in Hefei city collected in 1999, it is known that: Both courses have the following common possible premonitory characters embodying the four necessary conditions above: (5) There have been two deep layers of dry area in Hefei, which, on the VWP chart, are shown as two layers of continuous no-data areas within a deep convective altitude. Firstly, the upper layer disappears from the top down because the moist and cold air on top of the upper layer dry area begins to slide down. Then the lower layer begins to disappear when the air stops sliding down and begins to lift. Finally, after the two layers of the dry areas disappear entirely, a MCS (meso-scale convection System) bringing the SHR comes into being. It is a possible premonitory character. (6) In the same time, on the cloud chart, if a coming cloud band of cold front breaks down into two parts in Hefei so that there is litter cloud or fog in the break of the cloud band while there are two cloud clusters containing a lot of cloud water and water vapor around the break of the cloud band, it is in the break of the cloud band with little cloud and fog and near and between two cloud clusters containing a lot of cloud water and water vapor that the MCS comes into view after the cold front cloud band passes by. It is another possible premonitory character.