西北东部季风过渡区夹卷率与夏季风的动力学关系

Dynamic Relationship Between Entrainment Rate and Summer Monsoon in the Transition Area of Monsoon in the East of Northwest China

夹卷率指上升单位距离卷入的周围空气质量与空气质量的比率,包括湍流夹卷和动力夹卷,应用于对流云的边界层参数化、数值模式改进、云滴谱离散度观测及热带气旋的研究中。应用西北东部季风过渡区民勤、榆中、平凉、银川和延安等5站2006—2016年5~9月逐日07时和19时每隔10 m高度的高空加密观测资料,结合地面逐日观测资料,计算不同高度夹卷率,得出不同区域夹卷率与高度、季风期降水和季风的关系。结果表明:①夹卷率与气温、饱和水汽压成正比关系,但与相对湿度成反比。有云的相对湿度阈值为65%,相对湿度阈值越大,不同量级降水的云高也越低,而云高随着雨量增加而增高。②夹卷率存在明显的早晚变化和区域差别:从地面到3 km,07时明显小于19时,近地层随高度增加而减弱,500 m以上随高度增加而增强;从小到大为季风影响区、季风摆动区和非季风区,在3 km以上无明显区域差别。③夹卷率与降水强度、性质关系较为密切:近地面至600 m以下随着雨强增强夹卷率减弱,但500 m以上至2~3 km随着雨强增强夹卷率增强;近地层700 m以下稳定性降水夹卷强,而以上对流性降水夹卷强,对流性降水饱和水汽压大、夹卷强,而稳定性降水饱和水汽密度大,水汽丰富但夹卷弱。④夹卷率与季风及其持续时间的关系:从无季风到有季风夹卷率是减弱的,夹卷最强的最大高度也在降低。在非季风区和季风影响区夹卷率的持续时间差别不明显,只有季风摆动区在近地面层持续时间越长夹卷率越小,而在高空1~2 km相反。⑤夹卷率与亚洲—太平洋涛动(APO)季风强度指数关系表明:07时在近地层随高度增强,800 m以上随高度减弱,季风强度与夹卷率相关不明显;19时在近地层随高度增强,300 m以上随高度减弱,季风越强夹卷率越小。夹卷率随边界层高度降低而减弱。

Entrainment rate refers to the ratio of surrounding air quality to air quality involved in rising unit distance,including turbulent entrainment and dynamic entrainment,which are applied to the boundary layer parametrization of convective clouds,the improvement of numerical model,the observation of cloud droplet spectral dispersion and the study of tropical cyclones.Based on the daily data at 07:00 and 19:00 every 10 m of five stations such as Minqin,Yuchong,Pingliang,Yinchuan and Yan’an from May to September during 2006-2016,combined with the daily observation data on the ground,the Entrainment Rates(ER)of different heights were calculated,and the relationships between ER and height in different regions,precipitation as well as monsoon during the monsoon period were further obtained.The main results were as follows:①The ER was proportional to air temperature and saturated water vapor pressure,but inversely proportional to relative humidity.The relative humidity threshold of cloud was 65%.The higher the relative humidity threshold was,the lower the cloud height of different orders of precipitation was,and the cloud height was higher with the increase of rainfall.②ER had obvious diurnal changes and regional differences:It was obviously smaller at 07:00 than at19:00 from ground to 3 km,which weakened with the increase of height in the near surface,but strengthened with the increase of height above 500 m;From small to large,the monsoon affected area,the monsoon swing area and the non-monsoon area were in turn,and there was no regional difference above 3 km.③ER was closely related to the intensity and property of precipitation in monsoon period.The ER weakened with the enhancement of rain intensity from near ground to below 600 m,but strengthened with the enhancement of rain intensity from 500 m to 2~3 km.From near ground to below 700 m,the ER of stable precipitation was strong,but that of convective precipitation was strong above 700 m.The convective precipitation had big saturated water vapor pressure and strong ER,while the stable precipitation had big saturated water vapor density,rich water vapor but weak ER.④The relationship between ER and monsoon as well as its duration:From no monsoon to monsoon ER was weakened,the strongest maximum height was also decreasing.There was no significant difference in the duration of ER between the non-monsoon area and the monsoon affected area,but the longer the monsoon swing area lasted in the near ground layer,the smaller the ER was,while the opposite was at 1~2 km in the high altitude.⑤The relationship between ER and the APO monsoon intensity index showed that:At07:00,the ER strengthened with height from near ground to below 800 m,but weakened with height above 800 m,and the monsoon intensity was not related to the ER.At 19:00,the ER strengthened with the height near ground but weakened with the height above 300 m,and the stronger the monsoon was,the smaller the ER was.The ER weakened with the decrease of boundary layer height.