北京雷暴大风日环境特征分析

Analysis on Environmental Condition for Wind Gust in Beijing Area

雷暴大风是指由对流活动带来的除龙卷以外的地面灾害性强阵风。根据北京地区21个观测站2000-2007年的观测资料，将出现在该期间的雷暴大风按强阵风出现时降水量的大小划分为干、湿两种类型，探讨了两类雷暴大风日环境大气的热力稳定度条件、环境风垂直分布及演变等特征。结果表明，绝大多数干型雷暴大风产生在对流有效位能较小但对流层中低层环境风垂直切变却比较大的环境中，因此反映热力不稳定性的对流参数在干型雷暴大风的预报中具有一定的局限性，给对流初生的预报带来了一定难度。而湿型雷暴大风则多发生在热力不稳定的条件下。两种类型雷暴大风日环境大气温湿廓线有较大差别是造成热力不稳定性不同的原因之一。因此在预报雷暴大风时，除了环境大气的热力不稳定性外，还应考察环境风垂直切变等因素。下沉气流的热力稳定性和对流层中下层环境风速的演变是判断对流活动能否给地面带来短时强阵风的两个重要因素。下沉对流有效位能（DCAPE）的分析结果表明，多数雷暴大风日临近时刻的下沉对流有效位能大于600J·kg^-1，而且86％的干型个例和59％的湿型个例在地面大风出现前DCAPE呈现增加的趋势，这对雷暴大风特别是干型雷暴大风的潜势预报具有一定的意义。在雷暴大风来临前，抬升凝结高度以下的环境温度直减率明显增加，这种演变趋势也可为临近预报提供有用的信息。此外，风廓线仪观测资料是对常规探空的有效补充。分析表明，有一些雷暴大风的产生与高空水平动量下传有关。在雷暴大风出现前，高空环境风陡增，具有较高数值的等风速线连续下落，在雷暴大风产生时到达地面。有效地使用风廓线仪观测资料，将有利于提高雷暴大风的临近预报和预警水平。

Wind gust is caused by the convective cloud and is different from the tornado. Based on the rainfall, the wind gust events are divided into two kinds, i. e. , the dry and the wet ones. The instability and environmental wind perpendicular distribution within the troposphere in the wind gust days are analysed respectively. The results indicate that the environmental condition is some different for these two kinds of events. In dry wind gust days, the value of convective available potential energy is very low but the vertical wind shear below 6 km is great. So, the dynamic instability due to the strong wind shear plays an important role in the convective development in these days. And the utilization of instability indexes in convection development forecast is restricted. However, the convection initiation causing the wet wind gust is easier for forecast because the environment is normally instable. Mainly, the difference in soundings leads to above dissimilarity. Therefore, the wind perpendicular distribution should be taken into consideration while making the wind gust potential forecasting. Most of the wind gust events （not only dry but also wet ones） develop when the downdraft convective available potential energy is higher than 600 J·kg^-1 and the value increases＇before the strong wind approach, which is also useful. Prior to the wind gust, the environmental lapse rate of temperature below the lifting condensation level also rises obviously. Besides, the profiler data are supplementary to the sounding. In some events, the isotach in higher value goes down continuously and reaches the surface while the gust breaks out. If the profiler data could be used effectively, the nowcasting and warning for the wind gust might be improved.