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Peng-Yun Wang and Peter V. Hobbs

Abstract

The dynamical and microphysical structures of trains of wavelike rainbands in an occlusion are described. The rainbands were located in a region of potential instability behind the cold front aloft They were 3–5 km wide, ∼80 km long, spaced 5–10 km apart, and arranged nearly parallel to the winds at their upper levels. Temperatures fluctuated in a wavelike manner in and out of the rainbands the temperatures inside the rainbands being ∼1–2°C higher than outside. The vertical air velocities also fluctuated. Near the top of the rainbands upward velocities of 1 m s−1, relative to the ambient air, were present inside the rainbands, but outside of the rainbands the relative velocities were ∼1 m s−1 downward. At lower levels the fluctuations in vertical velocities were about one-half those at the upper levels, and they showed fewer systematic variations with respect to the location of the wavelike rainbands. The convergence/divergence and airflow patterns had wavelike structures in the vicinity of the rainbands, with stronger convergence at low levels. Near the top of the rainbands the liquid water contents ranged from 0.4–1.0 g m−3 total precipitation particle concentrations from ∼4–8 L−1 and ice particle concentrations from ∼10–60 L−1, all three decreasing with decreasing height. The sizes of the ice panicles and precipitation-size waterdrops followed exponential distributions.

Horizontal roll vortices in the boundary layer, reinforced by motions produced by the release of latent beat, me discussed as one possible mechanism for this type of wavelike rainband.

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Peng-Yun Wang, David B. Parsons, and Peter V. Hobbs

Abstract

The cloud and precipitation structure and the airflow associated with wavelike rainbands in a cold-frontal zone have been investigated with Doppler radar, instrumented aircraft, rawinsondes and a network of ground stations. The rainbands were oriented perpendicular to the cold front and embedded within wide cold-frontal rainbands. The wavelike rainbands were 20–40 km long, 3–6 km wide, spaced 9–13 km apart and their tops ranged from 3-5 km in height. The radar reflectivities, convergence/divergence and airflow show regular patterns associated with the rainbands.

There is evidence that wavelike rainbands were associated with generating cells aloft. These rainbands may have been initiated by shear instability in the frontal zone, since the resonant mode for such an instability had a similar orientation, movement and spacing to those observed for the rainbands.

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Peng-Yun Wang, Jonathan E. Martin, John D. Locatelli, and Peter V. Hobbs

Abstract

The structure and evolution of a shallow but intense cold front (commonly referred to as an arctic front) and its associated precipitation features that passed through the central United States from 0000 UTC 9 March to 0000 UTC 10 March 1992 are studied with the aid of observations and outputs from a numerical simulation using the Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model MM4.

Located above the arctic front was a region of midtropospheric, frontogenetical confluence that was attended by a thermally direct vertical circulation. A large banded precipitation feature, for the most part located behind the arctic front, was produced by ice crystals from upper-level clouds (formed by the frontogenetical confluence) falling into low-level stratocumulus associated with the arctic front. The arctic front at the surface separated a region where the precipitation reaching the ground was solid from an adjacent region where the precipitation was liquid. A westward-moving, low-level jet behind the arctic front produced upslope flow over the high terrain of the northern Great Plains, which contributed to heavy snowfalls in this region.

A portion of the arctic front that moved southward, west of a low pressure center, was characterized by sharp drops in temperature and dewpoint and an increase in wind speed. However, the arctic front was not associated with either a pressure trough or much change in wind direction. The proximity of arctic fronts to such nonfrontal features as lee troughs and/or drylines often leads to the latter being misanalyzed as cold fronts.

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Jonathan E. Martin, John D. Locatelli, Peter V. Hobbs, Peng-Yun Wang, and Jeffrey A. Castle

Abstract

A convective rainband, which was approximately 1500 km in length and affected large areas of the central United States for about 16 h, developed within an evolving winter cyclone. The rainband, which will be referred to as the pre-drytrough rainband, formed approximately 400 km ahead of a developing dryline and lee trough (drytrough, for short) that created an elevated, sloping layer of convective instability. The presence of a deep pool of high-potential-temperature air in the middle troposphere over the south-central United States, advected there from the elevated terrain to the southwest (i.e., an elevated mixed layer), produced a region of warm-air advection downstream of the high terrain. This enhanced the lifting associated with a migrating short wave aloft and generated the pre-drytrough rainband.

In previous studies the dryline, the lee trough, the elevated mixed layer, and the low-level jet in the central United States have generally been viewed as isolated features. Here the authors present a more integrated view, compelled by their common dependence on the interactions of synoptic-scale disturbances with topography.

Mesoscale structures and precipitation distributions similar to those documented in this paper are common in winter cyclones in the central United States and they are responsible for much of the severe weather associated with these systems.

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Tandong Yao, Yongkang Xue, Deliang Chen, Fahu Chen, Lonnie Thompson, Peng Cui, Toshio Koike, William K.-M. Lau, Dennis Lettenmaier, Volker Mosbrugger, Renhe Zhang, Baiqing Xu, Jeff Dozier, Thomas Gillespie, Yu Gu, Shichang Kang, Shilong Piao, Shiori Sugimoto, Kenichi Ueno, Lei Wang, Weicai Wang, Fan Zhang, Yongwei Sheng, Weidong Guo, Ailikun, Xiaoxin Yang, Yaoming Ma, Samuel S. P. Shen, Zhongbo Su, Fei Chen, Shunlin Liang, Yimin Liu, Vijay P. Singh, Kun Yang, Daqing Yang, Xinquan Zhao, Yun Qian, Yu Zhang, and Qian Li

Abstract

The Third Pole (TP) is experiencing rapid warming and is currently in its warmest period in the past 2,000 years. This paper reviews the latest development in multidisciplinary TP research associated with this warming. The rapid warming facilitates intense and broad glacier melt over most of the TP, although some glaciers in the northwest are advancing. By heating the atmosphere and reducing snow/ice albedo, aerosols also contribute to the glaciers melting. Glacier melt is accompanied by lake expansion and intensification of the water cycle over the TP. Precipitation has increased over the eastern and northwestern TP. Meanwhile, the TP is greening and most regions are experiencing advancing phenological trends, although over the southwest there is a spring phenological delay mainly in response to the recent decline in spring precipitation. Atmospheric and terrestrial thermal and dynamical processes over the TP affect the Asian monsoon at different scales. Recent evidence indicates substantial roles that mesoscale convective systems play in the TP’s precipitation as well as an association between soil moisture anomalies in the TP and the Indian monsoon. Moreover, an increase in geohazard events has been associated with recent environmental changes, some of which have had catastrophic consequences caused by glacial lake outbursts and landslides. Active debris flows are growing in both frequency of occurrences and spatial scale. Meanwhile, new types of disasters, such as the twin ice avalanches in Ali in 2016, are now appearing in the region. Adaptation and mitigation measures should be taken to help societies’ preparation for future environmental challenges. Some key issues for future TP studies are also discussed.

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