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Danhong Fu and Xueliang Guo

Abstract

The cloud-resolving fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) was used to study the cloud interactions and merging processes in the real case that generated a mesoscale convective system (MCS) on 23 August 2001 in the Beijing region. The merging processes can be grouped into three classes for the studied case: isolated nonprecipitating and precipitating cell merging, cloud cluster merging, and echo core or updraft core merging within cloud systems.

The mechanisms responsible for the multiscale merging processes were investigated. The merging process between nonprecipitating cells and precipitating cells and that between clusters is initiated by forming an upper-level cloud bridge between two adjacent clouds due to upper-level radial outflows in one vigorous cloud. The cloud bridge is further enhanced by a favorable middle- and upper-level pressure gradient force directed from one cloud to its adjacent cloud by accelerating cloud particles being horizontally transported from the cloud to its adjacent cloud and induce the redistribution of condensational heating, which destabilizes the air at and below the cloud bridge and forms a favorable low-level pressure structure for low-level water vapor convergence and merging process. The merging of echo cores within the mesoscale cloud happens because of the interactions between low-level cold outflows associated with the downdrafts formed by these cores.

Further sensitivity studies on the effects of topography and large-scale environmental winds suggest that the favorable pressure gradient force from one cloud to its adjacent cloud and stronger low-level water vapor convergence produced by the topographic lifting of large-scale low-level airflow determine further cloud merging processes over the mountain region.

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Shichao Zhu, Xueliang Guo, Guangxian Lu, and Lijun Guo

Abstract

Ice crystal habits and growth processes in two cases of stratiform clouds with embedded convection are investigated using data observed simultaneously from three aircraft on 18 April 2009 and 1 May 2009 as part of the Beijing Cloud Experiment (BCE). The results show that the majority of ice crystal habits found in the two cases at temperatures between 0° and −16°C included platelike, needle column, capped column, dendrite, and irregular. A mixture of several ice crystal habits was identified in all of the clouds studied. However, the ice crystals recorded in the embedded convection regions contained more dendrites and possessed heavier riming degrees, and the ice crystals identified in the stratiform clouds contained more hexagonal plate crystals. Both riming and aggregation processes played central roles in the broadening of particle size distributions (PSDs), and these processes were more active in embedded convection regions than in stratiform regions. However, riming was more prevalent in the 18 April case than aggregation, though aggregates were evident. In contrast, the 1 May case had a more dominant aggregation processes, but also riming. With the decrease in height, PSDs broadened in both embedded convection regions and stratiform regions, but the broadening rates between 4.8 km (T ≈ −11.6°C) and 4.2 km (T ≈ −8°C) were larger than those between 4.2 km (T ≈ −8°C) and 3.6 km (T ≈ −5°C). In addition, the broadening rates of PSDs in the embedded convection regions were larger than those in the stratiform clouds, as the aggregation and riming processes of ice particles in embedded convection regions were active. High supercooled water content is critical to enhancing riming and aggregation processes in embedded convection regions.

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Ping Zhao, Xiangde Xu, Fei Chen, Xueliang Guo, Xiangdong Zheng, Liping Liu, Yang Hong, Yueqing Li, Zuo La, Hao Peng, Linzhi Zhong, Yaoming Ma, Shihao Tang, Yimin Liu, Huizhi Liu, Yaohui Li, Qiang Zhang, Zeyong Hu, Jihua Sun, Shengjun Zhang, Lixin Dong, Hezhen Zhang, Yang Zhao, Xiaolu Yan, An Xiao, Wei Wan, Yu Liu, Junming Chen, Ge Liu, Yangzong Zhaxi, and Xiuji Zhou

Abstract

This paper presents the background, scientific objectives, experimental design, and preliminary achievements of the Third Tibetan Plateau (TP) Atmospheric Scientific Experiment (TIPEX-III) for 8–10 years. It began in 2013 and has expanded plateau-scale observation networks by adding observation stations in data-scarce areas; executed integrated observation missions for the land surface, planetary boundary layer, cloud–precipitation, and troposphere–stratosphere exchange processes by coordinating ground-based, air-based, and satellite facilities; and achieved noticeable progress in data applications. A new estimation gives a smaller bulk transfer coefficient of surface sensible heat over the TP, which results in a reduction of the possibly overestimated heat intensity found in previous studies. Summer cloud–precipitation microphysical characteristics and cloud radiative effects over the TP are distinguished from those over the downstream plains. Warm rain processes play important roles in the development of cloud and precipitation over the TP. The lower-tropospheric ozone maximum over the northeastern TP is attributed to the regional photochemistry and long-range ozone transports, and the heterogeneous chemical processes of depleting ozone near the tropopause might not be a dominant mechanism for the summer upper-tropospheric–lower-stratospheric ozone valley over the southeastern TP. The TP thermodynamic function not only affects the local atmospheric water maintenance and the downstream precipitation and haze events but also modifies extratropical atmospheric teleconnections like the Asia–Pacific Oscillation, subtropical anticyclones over the North Pacific and Atlantic, and temperature and precipitation over Africa, Asia, and North America. These findings provide new insights into understanding land–atmosphere coupled processes over the TP and their effects, improving model parameterization schemes, and enhancing weather and climate forecast skills.

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