Abstract: Cloud occurrence frequency (COF, the ratio between the number of records with detected clouds with respect to the total available records) and the distribution of COF within one day below 8 kilometers in summer from 2007 to 2009 over Beijing is studied. Cloud base height (CBH) of one layer clouds, the characteristics of convective CBH and the relationship between convective CBH and relative humidity (RH) measured on the surface are further studied. Results show that cloud occurrence frequency is about 12% in summer over Beijing. Clouds are more likely to occur at night than in the afternoons. The distribution of CBH has one peak at 700 (±50) meters and low clouds (ranging up to 3000 meters) account for 82% of all clouds. Convective CBH ranges from 400 to 2000 meters for the cases studied, but do not vary much within each day. CBH can be conveniently derived from linear relationship between CBH and surface RH based on Adiabatic Parcel Model. Mean relative standard deviation of CBH is 0.321.

Key words: ceilometer, convective cloud, cloud base height, cloud occurrence frequency, relative humidity

摘要： 研究北京地区2007—2009 年夏季8 km 高度以下云出现概率(探测到云的次数与总探测次数之比)及云出现概率在一天内的分布规律, 并研究三年中夏季单层云的云底高度分布规律, 典型对流云的云底高度特征, 以及对流云云底高度与地面相对湿度的关系。结果表明: 北京地区夏季云出现概率约为12%, 一天内云最容易出现在夜间, 下午较夜间云出现得少; 云底高度分布呈单峰, 最容易出现在700±50 m 高度, 低云(云底高度在3000 m 以下)出现次数占云总出现次数的82%; 典型对流云云底高度出现在400~2500 m 高度, 一天之内云底高度变化不大。利用绝热气团模型得到云底高度与地面相对湿度的线性关系, 并以此方便地给出对流云云底高度, 平均相对标准差为0.321。

关键词: 云高仪, 对流云, 云底高度, 云出现概率, 相对湿度