祁连山南麓夏季地形云特征及山谷风影响分析

利用门源、祁连气象站2004—2013年6—8月逐时常规观测资料,分析了地形云的日变化特征,结果表明:两站夏季总、低云量的日变化呈现双峰型特征;层积云和积雨云的日变化呈反相特征。层积云出现频率最高在清晨,积雨云在午后至傍晚出现频率最高;门源站层积云出现频率高于祁连站,而祁连站积雨云出现频率高于门源站。两站山谷风环流特征明显,风速最大值出现在午后,最小值出现在清晨;门源站谷风控制时间长于山风,祁连站山风控制时间长于谷风。两站积雨云出现时间与山谷风风速最大值出现时间之间具有对应关系;有天气系统影响时形成的积雨云,持续时间较长,降水较多;仅由地形风及热力、湍流作用形成的积雨云,持续时间较短,降水较少。层积云的形成有3种类型:第1种由高层云演变而来;第2种由积雨云对流发展受到抑制而形成;第3种由局地山谷风环流形成,云的形成与山谷风环流以及边界层日变化特征相关。

Diurnal Changes of Summer Orographic Clouds on Southern Slope of Qilian Mountains and Impacts of Mountain Valley Breeze

Using the hourly meteorological data from Menyuan and Qilian weather stations from 2004 to 2013, the diurnal change characteristics of orographic clouds are analyzed. It is found that the daily changes of total and low cloud amount show a pattern of two peaks in summer clearly, and the daily change of stratocumulus is opposite to that of cumulonimbus, and the occurrence frequency of stratocumulus is the greatest in the morning. The occurrence frequency of stratocumulus is greater in Menyuan than in Qilian, but that of cumulonimbus is greater in Qilian than in Menyuan. The circulation of mountain valley breeze in both stations shows same variation characteristics: maximum wind velocity in the afternoon and minimum in the early morning, but the wind speed is greater in Qilian than in Menyuan. The dominate time of valley breeze is longer than that of mountain breeze in Menyuan, and vice versa in Qilian. The occurrence time of cumulonimbus is closely related to the occurrence time of maximum mountain breeze or valley breeze, which indicates that the mountain valley breeze is a key factor in the course of orographic cloud formation. Under the control of a weather system, cumulonimbus maintains much longer with more precipitation; however, under the control of orographic winds and thermal turbulence, it maintains shorter with less precipitation. The formation of stratocumulus is closely related to the circulation of mountain valley breeze and diurnal change of the boundary layer, and can be distinguished as three types: transformed from altostratus, derived from cumulonimbus when its growth is limited, resulted from the local circulation of mountain valley breeze.