Characteristics of Atmospheric Circulation Associated with Cold Surge Occurrences in East Asia:A Case Study During 2005/06 Winter

The characteristics of the upper-level circulation and thermodynamical properties for the period when two distinct cold surges broke out over East Asia during the 2005/06 winter are investigated. From early December 2005 to early January 2006, exceptionally cold weather lasted for approximately one month due to two successive cold surges that took place on 2 December 2005 and 2 January 2006, respectively. This study reveals that both involve the upper-tropospheric circulation, which induces the amplification and expansion of the surface Siberian high toward East Asia, but arose from different causes： the former is caused by the upper-level blocking originated from the North Pacific and the latter is caused by the upper-level wave train across the Eurasian Continent. In particular, it is suggested that the lower-tropospheric anomalous wind caused by upper-level circulation anomalies and a steep meridional temperature gradient amplified by phase-locked annual cycle combined to induce very strong cold advection in East Asia, which resulted in exceptionally cold weather that lasted for several weeks. The present results emphasize that the characteristics of the upper-tropospheric circulation can be considered as important precursors to cold surge occurrences in East Asia.

The 2020 Summer Floods and 2020/21 Winter Extreme Cold Surges in China and the 2020 Typhoon Season in the Western North Pacific

China experienced significant flooding in the summer of 2020 and multiple extreme cold surges during the winter of 2020/21.Additionally,the 2020 typhoon season had below average activity with especially quiet activity during the first half of the season in the western North Pacific(WNP).Sea surface temperature changes in the Pacific,Indian,and Atlantic Oceans all contributed to the heavy rainfall in China,but the Atlantic and Indian Oceans seem to have played dominant roles.Enhancement and movement of the Siberian High caused a wavier pattern in the jet stream that allowed cold polar air to reach southward,inducing cold surges in China.Large vertical wind shear and low humidity in the WNP were responsible for fewer typhoons in the first half of the typhoon season.Although it is known that global warming can increase the frequency of extreme weather and climate events,its influences on individual events still need to be quantified.Additionally,the extreme cold surge during 16–18 February 2021 in the United States shares similar mechanisms with the winter 2020/21 extreme cold surges in China.

Surface and Upper-Level Features Associated with Wintertime Cold Surge Outbreaks in South Korea

The surface and upper-level features associated with a sharp drop ofwintertime daily temperature over South Korea is investigated in this study. This sharp drop indaily temperature is called a cold surge and is one of the most hazardous weather phenomena in EastAsian winters. An upper-level baroclinic wave of 60° wavelength propagating eastward at a phasespeed of 12° longitude per day across the continent of northern China from the west of Lake Baikaltoward the eastern coast of China causes the outbreak of cold air over South Korea. The coolingassociated with the upper-level baroclinic jvave is found at all altitudes under the geopotentialheight-fall center near the tropopause. The development in the ridge seems to derive the earlyevolution of the eastward-propagating sinusoidal wave, whereas the trough is connected directly withthe tropospheric temperature-drop. An enhancement of the wintertime East Asian jet stream after theoutbreak of a cold surge is a response to the steep temperature gradient associated with thedeveloping baroclinic wave.

Structures and Characteristics of the Windy Atmospheric Boundary Layer in the South China Sea Region during Cold Surges

An observational analysis of the structures and characteristics of a windy atmospheric boundary layer during a cold air outbreak in the South China Sea region is reported in this paper. It is found that the main structures and characteristics are the same as during strong wind episodes with cold air outbreaks on land. The high frequency turbulent fluctuations （period 〈 1 min） are nearly random and isotropic with weak coherency, but the gusty wind disturbances （1 rain〈period 〈 10 min） are anisotropic with rather strong coherency. However, in the windy atmospheric boundary layer at sea, compared with that over land, there are some pronounced differences： （1） the average horizontal speed is almost independent of height, and the vertical velocity is positive in the lower marine atmospheric boundary layer; （2） the vertical flux of horizontal momentum is nearly independent of height in the low layer indicating the existence of a constant flux layer, unlike during strong wind over the land surface; （3） the kinetic energy and friction velocity of turbulent fluctuations are larger than those of gusty disturbances; （4） due to the independence of horizontal speed to height, the horizontal speed itself （not its vertical gradient used over the land surface） can be used as the key parameter to parameterize the turbulent and gusty characteristics with high accuracy.

A Numerical Study on the Winter Monsoon and Cold Surge over East Asia

By using the improved regional climate model （RegCM_NCC）, a numerical study has been undertaken for the East Asia region over a period of 5 years （1998-2002） in an effort to evaluate the model＇s ability to reproduce the winter monsoon conditions that were observed. The results showed that the model can successfully simulate the basic characteristics of the winter monsoon circulations, including the location and intensity of the cold-surface, high-pressure system, as well as the wind patterns and the intensity of the winter monsoon. The simulated occurrence frequency and regions of the cold surge were consistent with the observations. The simulated rainfall distribution over China was consistent with the observations collected in South China. The features of the simulated moisture transport were also in good agreement with the observations that were derived from the NCEP reanalysis data, indicating that moisture transport coming from the Bay of Bengal trough plays a crucial role in supplying moisture needed for precipitation in South China. In addition, the moisture transport coming from the near-equatorial west-Pacific was also important. These two branches of moisture transport converged in South China, as a prerequisite for occurrence of the precipitation that was observed there. Heat budgets have shown that the development of a heat sink over the East Asian continent was remarkable and its thermal contrast relative to the neighboring seas was the important forcing factor for the winter monsoon activity. The simulation also indicated that the significant differences in circulation patterns and rainfalls during the winters of 1997/98 and 1998/99 were affected by cold and warm ENSO events, respectively. The above analysis demonstrated the model＇s ability to simulate the East Asian winter monsoon.

Spatiotemporal changes of cold surges in Inner Mongolia between 1960 and 2012

In this study, we analyzed the spatiotemporal variation of cold surges in Inner Mongolia between 1960 and 2012 and their possible driving factors using daily minimum temperature data from 121 meteorological stations in Inner Mongolia and the surrounding areas. These data were analyzed utilizing a piecewise regression model, a Sen＋Mann- Kendall model, and a correlation analysis. Results demonstrated that （1） the frequency of single-station cold surges decreased in Inner Mongolia during the study period, with a linear tendency of -0.5 times/10a （-2.4 to 1.2 times/10a）. Prior to 1991, a significant decreasing trend of-1.1 times/10a （-3.3 to 2.5 times/10a） was detected, while an increasing trend of 0.45 times/10a （-4.4 to 4.2 times/10a） was found after 1991. On a seasonal scale, the trend in spring cold surges was consistent with annual values, and the most obvious change in cold surges occurred during spring. Monthly cold surge frequency displayed a bimodal structure, and November witnessed the highest incidence of cold surge. （2） Spatially, the high incidence of cold surge is mainly observed in the northern and central parts of Inner Mongolia, with a higher occurrence observed in the northern than in the central part. Inter-decadal character- istic also revealed that high frequency and low frequency regions presented decreasing and increasing trends, respectively, between 1960 and 1990. High frequency regions expanded after the 1990s, and regions exhibiting high cold surge frequency were mainly distributed in Tulihe, Xiao＇ergou, and Xi Ujimqin Banner. （3） On an annual scale, the cold surge was dominated by AO, NAO, CA, APVII, and CQ. However, seasonal differences in the driving forces of cold surges were detected. Winter cold surges were significantly correlated with AO, NAO, SHI, CA, TPI, APVII, CW, and IZ, indicating they were caused by multiple factors. Au- tumn cold surges were mainly affected by CA and IM, while spring cold surges were significantly correlated with CA and APVII.

Impact of the cold surge event in January 2021 on the power system of China

During 6-10 January 2021,a recorded strong cold surge took place in China,with over 800 observational stations reaching their historical extremes.Unlike previous studies that focused on the response of either the power load or generation separately,this study quantitatively revealed the impacts on the balance between the demand and supply sides of the grid.On the demand side,the sensitivity of power load was found to increase substantially higher in southern China(0.533 GW°C^(−1))than in the northern region(0.139 GW°C^(−1))due to the limited municipal heating system.On the supply side,the hourly wind power generation dropped from the highest of 110 GW on 6 January to the lowest of 54 GW on 9 January due to the reduction in wind speed.In addition,a reduction in solar power generation was observed during 8-10 January.Thus,the balance of the power system was influenced by this cold event.As an effective adaptation measure,results further showed that early warning by three weeks ahead can be obtained by an operational climate model.The sensitivity of China's power system to such cold surge events may increase remarkably due to the expected increase in the proportion of wind and solar power generation in future new-type power systems.Thus,close cooperation between climate scientists and power engineers is needed to build the resilience of the power system to the cold extremes.

A NUMERICAL EXPERIMENT STUDY ON THE FORMING MECHANISMS OF LOW-LATITUDE CIRCULATION CELLS AND PRECIPITATION DURING AN EAST ASIAN COLD SURGE

The forming mechanisms of low-latitude circulation cells and precipitation during an East Asian cold surge have been studied by using a five-layer primitive equation model.Numerical experiments show that in mid-high latitudes, the propagation of the East Asian winter monsoon is nearly a dry process;in low-latitudes,the effects of cumulus con- vection intensify the northeast monsoon and northeast trade wind;however,they also prevent the intertropical convergence zone (ITCZ) from further southward invasion,and strengthen the meridional and zonal cells over East Asia. Numerical results also indicate that the winter monsoon precipitation in Indonesia and Northern Australia can exist independently.Both the South China Sea (SCS) cold surge and the West Pacific northeast cross-equatorial trade wind are of importance for the formation of the Northern Australia summer monsoon and its precipitation. Numerical results show that the development of cumulus convection,triggered by the SCS cold surge,is a crucial factor in forming the heavy monsoon precipitation and low-level stationary disturbances in Indonesia and Borneo.The tropical topography and northeast trade wind disturbances only affect the intensity of precipitation there.

SIMULATION STUDY OF THE EFFECT ON COLD SURGE OF THE QINGHAI-XIZANG PLATEAU AS A HUGE OROGRAPHY

Using a limited-area P-σ incorporated coordinate five-level primitive equation model fed with the ECMWF 5°×5°grid data,a simulation is made of a large-scale cold surge of late December 1982.Results show that the Qinghai-Xizang Plateau has no profound thermal but dynamic effect on the East-Asian winter monsoon that forces cold air to go southwards by its east side,exciting Kelvin waves behind the cold front to result in the maximum NE wind.Besides,gravitational waves independent of the Plateau occur ahead of the front advancing towards the south.They may be due to the excitation involved in the front itself. The cold surge is propagated under the Hadley cell,making it strengthened and moved southwards.

THEORETICAL STUDY ON EFFECT OF QINGHAI-XIZANG PLATEAU UPON COLD SURGE

Starting from the linear shallow-water wave equations including in themselves the effect of a large-scale orograpny, two types of gravity wave solutions are derived on the assumption that the Qinghai-Xizang Plateau serves as an orography in question inclined from west to east at the same slope. One type contains two solutions of high-frequency inertia-gravitational waves and the other solution of low-frequency topography-excited gravity waves, bearing some similarity to Kelvin mode. Both types are similar, respectively, to the gravity waves of the front of the cold surge during the rapid southward advance, and to the strong velocity center of the surge when it propagates southward slowly along the east brim of the Plateau, as shown by the numerical study. Results indicate that the low-frequency wave vanishes whereas its counterpart remains especially in the absence of the orography. In addition, we find that the propagation of the gravity-wave rays is in the form of a trochoid equation, a result in good agreement with the curved path of the strong velocity center going down the east fringe of the Plateau as shown by the numerical study. And probably a waveguide tube of topographic gravity mode is present in the coastwise area downstream of the orography.

Arctic warming and its influence on East Asian winter cold events:a brief recap

The rate of warming of Arctic surface temperature is about 2–3 times faster than the global mean surface warming. Increases of ice albedo feedback and water vapor as well as moisture intrusion from outside the Arctic all have major roles in this phenomenon. In contrast to this rapid Arctic warming, in recent decades, stronger cold air outbreaks have occurred more frequently during winter in East Asia than were recorded in the 1990s, resulting in severe socioeconomic impacts. A number of related studies have claimed the increased frequency of these stronger cold air outbreaks is linked to the amplified warming in the Arctic through complicated mechanisms. As there are time lags between the observed Arctic warming and East Asian cold weather response at various scales, understanding the entire chain of processes from the Arctic to East Asia has importance for forecasting winter weather in East Asia. There are two pathways linking Arctic warming with East Asian cold weather events. One is the synoptic-scale pathway in the lower troposphere via strengthening of the Siberian High initiated by Ural blocking. The other is the planetary-scale path through the stratosphere via activation of planetary waves and downward propagation, which weakens the polar vortex. This study briefly reviews the current understanding of the linkage mechanisms between Arctic warming and East Asian winter cold weather.

Analysis of Rain and Snow Weather Process in the Middle and East of China from December 28 to 30, 2020

The China National Climate Center and National Centers for Environmental Prediction data were used in this work. Using the knowledge of synoptic principles and synoptic analysis, the rain and snow weather process in the middle and east of China from December 28 to 30, 2020 was analyzed. The results show that: 1) This weather process was a large-scale precipitation process. Affected by the cold wave, the temperature drops sharply, and the temperature in many places dropped below the extreme value, accompanied by strong winds. 2) This process is mainly caused by the eastward movement of the upper-level trough leading the cold air southward to intersect with the warm and wet air, and the shear thread in the lower layer cooperates. 3) The cold air is from the ocean east of Novaya Zemlya. 4) From the evolution trend of 500 hPa geopotential height field and ground pressure field, the cold surge process belongs to the type of transverse trough to vertical.

The Analysis on the Features of the Atmospheric Circulation in Preceding Winters for the Summer Drought and Flooding in the Yangtze And Huaihe River Valley

Based on the annual variation of the rainfall departure percentage in summer in the lower-middle reaches of the Yangtze River and the Huaihe River valley, 7 cases for the abnormal drought and flooding summers (the drought years: 1981. 1984, 1985; the flooding yearst 1980, 1982, 1983, 1987) are selected. First we analyse the general circulation characteristics of the summer drought and flooding, and then the evolution processes of the general circulation patterns from preceding winters to summers are studied. It is found that during the two kinds of preceding winters for the drought and flooding summer, not only the general circulation patterns in the high-mid latitudes, the local Hadley cells in East Asia but also the activities of the cold surge in the lower latitude are different obviously. Spring, especially April, is the turning period of the general circulation in preceding winter for the drought or nooding summer evolution towards opposite direction. Hereafter, the drought or flooding circulation pattern is established and developed.

Analysis of recent climate change over the Arctic using ERA-Interim reanalysis data

This study investigates recent climate change over the Arctic and its link to the mid-latitudes using the ERA-Interim global atmospheric reanalysis data from the European Center for Medium-Range Weather Forecast （ECMWF）. Since 1979, sub- stantial surface warming, associated with the increase in anthropogenic greenhouse gases, has occurred over the Arctic. The great- est warming in winter has taken place offshore in the Kara-Barents Sea, and is associated with the increase in turbulent heat fluxes from the marginal ice zone. In contrast to the marked warming over the Arctic Ocean in winter, substantial cooling appears over Siberia and eastern Asia, linked to the reduction of Arctic sea ice during the freezing season （September-March）. However, in summer, very little change is observed in surface air temperature over the Arctic because increased radiative heat melts the sea ice and the amount of turbulent heat gain from the ocean is relatively small. The heat stored in the upper ocean mixed layer in summer with the opening of the Arctic Ocean is released back to the atmosphere as turbulent heat fluxes during the autumn and through to the following spring. This warming of the Arctic and the reduced sea ice amplifies surface cooling over Siberia and eastern Asia in winter.

THE EFFECTS OF VERTICAL VARIATION OF BASIC FLOW AND HORIZONTAL GRADIENT OF TEMPERATURE ON LINEAR AND NONLINEAR GRAVITY WAVES

By using two-dimensional dynamical equations in x-z plane with Boussinesq approximation,the effects of the second-order vertical shear of the basic flow ■ and the horizontal gradient of temperature (M) on the gravity wave and the isolated gravity wave are discussed.The magnitudes of ■ and M corresponding to the linear and nonlinear stabilities of the gravity waves are worked out,respectively.The results show that amplitude and width of the isolated gravity wave are closely related to ■ and M.It is indicated that the isolated gravity wave with a width of about 10 km can be motivated by the disturbance of sub-synoptic scale in the certain ranges of flow field shear and temperature gra- dient,while the motivated waves may be associated with the cold surge ahead of a cold front and the other mesoscale synoptic systems.