吉林一次降水层状云的结构和物理过程研究

A study of the structure and microphysical processes of a precipitating stratiform cloud in Jilin

利用机载粒子测量系统的探测结果,配合雷达及地面降水资料,结合一维层状云模式,通过对吉林2004年7月1日的一例降水性层状云系的宏微观物理结构和降水机制的定量化分析,对顾震潮三层模型有了进一步的认识。观测资料表明,该降水过程为典型的层状云降水,地面降水存在不均匀性,云系结构符合顾震潮三层概念模型,其中第1层为尺度很小的冰晶;第2层为混合层,以高层云和层积云为主体,其物理过程包括冰雪晶的凝华增长、结凇、聚并以及过冷水的直接冻结等;第3层为暖层,该层存在明显的0℃层回波亮带。在此基础上进行的数值模拟表明,模拟云发展成熟时存在3层结构,第1层(7.8—10.0 km)为少量的冰雪晶,以凝华增长为主。第2层(3.8—7.8 km)的冰雪晶在该层初始时增长方式主要为贝吉龙过程,而后以结凇增长为主。第2层的雪和霰降落到第3层后的融化及进一步的碰并云水则促进了雨水的形成及降落。第3层内雨水形成后,其质量增长的50%—60%来自于有冰相粒子参与的微物理过程。总体而言,云体发展成熟时,各层之间存在播种-供应关系,其中第1层对降水的贡献0.2%—0.4%,第2层为接近75%,而第3层约25%。模拟还表明,第1层冰晶浓度减少时,可导致第2层上部雪的浓度变化约40%—90%,其影响在云体的初始阶段较大,并随云体的发展及高度的降低而减弱,可导致平均降水强度减少2%—8%,因而其重要影响不可忽视。

Based on airborne cloud particle probe observations, Doppler radar measurements and surface precipitation data, the macro- and micro-structure as well as precipitation mechanisms of one precipitating stratiform cloud occurring on 1 July 2004 in Jilin Province were studied comprehensively by using a one-dimensional stratiform cloud model. With those quantitative results, we bad a further in sight into the Koo Chenchao＇s three-layer cloud conceptual model and also got some new conclusions referring to the seeder-feeder processes. According to field observations, the non-uniform precipitation was produced by a typical stratiform cloud. The cloud struc- ture could be explained well with the three-layer cloud model where the first layer is the ice crystal layer, including primarily small amounts of ice crystals. The mixed-phase layer （the second layer）, with altostratus and stratocumulus as the main cloud region, includes such processes as depositional growth, riming, aggregation and freezing of supercooled cloud water. And the remaining layer is the warm layer, in which obvious radar bright band was observed from Doppler radar. Based on the above observations, the numerical simulations of cloud structure and precipitation formation processes were conducted. The numerical results showed the presence of the three layer structure at the mature stage of cloud development. In the first layer （7.8 - 10.0 km）, there was little ice and snow crystals which grew mainly by depositional growth. And in the second layer （3. 8 - 7.8 kin） the ice and snow crystals falling partly from the first layer grew through the Bergeron process initially and later grew predomi nantly by riming processes. After that, the grown-up snow and graupel fell to the third layer. Therefore, the melting of snow and graupel combined with subsequent collection of cloud water was the main source of rain drop mass growth. Our results indicated that 50 % - 60 % of rain drop mass growth was associated with melting of ice particles falling from the second layer. In summary, the seeder-feeder processes existed between not only the first and second layers, but also the second and the third layers. In terms of the contribution to the total surface precipitation, the first, second and third layers produced precipitation mass of a- bout 0. 2% - 0. 4%, around 75% and 25%, respectively. Besides, sensitivity tests also suggested that the decrease of ice crystals in the first layer could cause a decrease of snow concentration of about 40 % - 90 0%0 at the upper part of the second layer, which had relatively higher degree of influence at the initial stage of cloud development, and later became less important with decreasing height and time. And variations of ice crystal concentration could cause average precipitation rate changes of between 2% and 8 %. Therefore, although the first layer produced precipitation mass of less than 1%, its important influence could not be ignored.