青藏高原那曲地区夏季对流云结构及雨滴谱分布日变化特征

Characteristics of convective cloud and precipitation during summer time at Naqu over Tibetan Plateau

青藏高原对于我国大气水循环、生态环境、灾害天气产生及气候变化等均具有重要作用和影响.为进一步揭示青藏高原气象和大气物理过程,我国启动了第三次青藏高原科学试验-边界层与对流层观测（2014~2017年）重大研究项目,其中云降水物理观测试验采用了包括C波段连续波雷达、Ka波段毫米波云雷达、地面雨滴谱仪、激光云高仪等目前先进的观测仪器.本文利用2014年7月1日~8月31日期间在西藏那曲的观测数据,结合FY-2E卫星的TBB资料,分析研究了青藏高原夏季（7~8月）对流云及其降水过程和雨滴谱分布特征.研究结果表明,观测试验期间青藏高原对流活动主要集中在高原东南部和中部地区,其降水过程存在准两周的周期性;由于高原的加热效应,对流云和降水过程有着显著的日变化特征,对流活动在11：00（当地时间）由局地热对流发展,经合并增长在17：00~18：00达到最强,入夜后降水过程开始偏平流性并持续至6：00,之后逐渐消散,上午对流活动较少.高原对流云平均云顶高度为11.5 km左右（海拔高度）,最大云顶高可超过19 km;平均云底高度6.88 km.降水过程主要表现为短时阵性降水,持续时间基本小于1 h,平均降水强度在1.2 mm/h左右.另外,研究发现高原雨滴谱分布相对于同纬度和季节的平原地区较宽,导致高原对流易产生降水.Γ分布相对于M-P分布更适用于对高原上的雨滴谱分布进行拟合.

The Tibetan Plateau has an important impact on regional water cycle, ecosystem, disastrous weather formation, and climate change. China has conducted the Third Tibetan Plateau Experiment-Observation of Boundary Layer and Troposphere（2014–2017） Project in order to reveal the physical process of meteorology and atmosphere over the Tibetan Plateau. The observational instruments for cloud and precipitation processes in this field experiment include the C-band continuous wave radar, Ka-band millimetre wave cloud radar, ground-based raindrop disdrometer, lidar ceilometer etc. In this paper, the characteristics of convective clouds and precipitation as well as raindrop size distribution were analysed by using observed data and FY-2E satellite TBB data from July 1 to August 31, 2014. The result shows that the convective activities mainly distributed in the central and southeast part of the Tibetan Plateau and the precipitation had a quasi-two-week cycle during the observational period. Due to the solar heating effect over the plateau, both convective clouds and precipitation processes had obvious daily variation. There were four precipitation peaks in a day, which were 0：00–1：00, 12：00–13：00, 17：00–18：00, 19：00–22：00（LST） respectively. The convections first appeared at 11：00, and the first precipitation peak happened at around 12：00, which was mainly caused by local thermal convection with relative lower cloud-top height and wider drop spectrum. Then the convections merged and developed to their maximum at 17：00–18：00, during which cloud-base height had a sharp decrease at 17：00 and widest raindrop spectrum and the second precipitation peak happened in this period. At night the precipitation tended to be advective but remained high and lasted until 6：00 and then dissipated. During 18：00–24：00 there was a precipitation peak around 19：00–22：00 and the cloud-top height was as high as that in the afternoon and the number of smaller drops increased. After midnight, the cloud-top height as well as precipitation started to decrease and tended to be more advective, but a forth precipitation peak happened during 0：00–1：00. In the forenoon there were few convections happened. The mean cloud-top height was around 11.5 km（ASL）, and its maximum value exceeded 19 km, and the mean cloud-base height was 6.88 km（ASL） during the observation period. Precipitation was mainly short-lasting and showery, and usually lasted less than 1 h, and the mean precipitation intensity was 1.2 mm/h. The result also shows that the raindrop size distribution over the Tibetan Plateau was wider than that over plain at the same latitude and season, because of which the rainfall could be more easily produced over the plateau than that over plain. Rain happened more at night than at noon. However, there were more larger-size raindrops at daytime than at night. Γ distribution was found to be more suitable for the raindrop size distribution than M-P distribution over the Tibetan Plateau.