Projecting Wintertime Newly Formed Arctic Sea Ice through Weighting CMIP6 Model Performance and Independence

Precipitous Arctic sea-ice decline and the corresponding increase in Arctic open-water areas in summer months give more space for sea-ice growth in the subsequent cold seasons. Compared to the decline of the entire Arctic multiyear sea ice,changes in newly formed sea ice indicate more thermodynamic and dynamic information on Arctic atmosphere–ocean–ice interaction and northern mid–high latitude atmospheric teleconnections. Here, we use a large multimodel ensemble from phase 6 of the Coupled Model Intercomparison Project(CMIP6) to investigate future changes in wintertime newly formed Arctic sea ice. The commonly used model-democracy approach that gives equal weight to each model essentially assumes that all models are independent and equally plausible, which contradicts with the fact that there are large interdependencies in the ensemble and discrepancies in models' performances in reproducing observations. Therefore, instead of using the arithmetic mean of well-performing models or all available models for projections like in previous studies, we employ a newly developed model weighting scheme that weights all models in the ensemble with consideration of their performance and independence to provide more reliable projections. Model democracy leads to evident bias and large intermodel spread in CMIP6 projections of newly formed Arctic sea ice. However, we show that both the bias and the intermodel spread can be effectively reduced by the weighting scheme. Projections from the weighted models indicate that wintertime newly formed Arctic sea ice is likely to increase dramatically until the middle of this century regardless of the emissions scenario.Thereafter, it may decrease(or remain stable) if the Arctic warming crosses a threshold(or is extensively constrained).

NUMERICAL SIMULATION AND DYNAMIC STUDY OF THE WINTERTIME CIRCULATION OF THE BOHAI SEA

This numerical study of the Bohai Sea wintertime circulation by means of a two dimensional barotropic model with resolution of 1/24° in longitude and latitude showed that the Bohai Sea wintertime circulation is dominated by local monsoon winds. The major current components include the Bohai Warm Current, the North Shandong Coastal Current, and the Liaodong Gyre. The Bohai Warm Current originates from the Yellow Sea Warm Current at the northern part of Bohai Strait, meanders westwards and finally enters the northern part of Bohai Bay. The North Shandong Coastal Current flows along the southwest shore of Bohai Bay and Laizhou Bay and exits from the Bohai Sea through the south Bohai Strait. The anticyclonic Liaodong Gyre is located in the north of Liaodong Bay. A pair of eddies and the small scale Jinzhou Gyre are found between the Bohai Warm Current and the Liaodong Gyre. The computed volume transport for both the Bohai Warm Current and North Shandong Coastal Current is about 0.03 Sv (1 Sv=1×10 6 m 3/s). The numerical experiments showed that the combined effect of local monsoon winds and bottom topography dominate the formation of the circulation pattern. The Coriolis force and the wind stress curl are of certain importance. The beta effect, the momentum advection and the open boundary condition have little influence on the circulation pattern.

THE ROLE OF COLD AIR AND CHARACTERISTICS OF WATER VAPOR IN BOTH TCS NANMADOL (0428) AND IRMA (7427) MAKING LANDFALL ON CHINA IN WINTERTIME

The NCEP/NCAR reanalysis data are used to investigate the role of cold air and moisture characteristics during the evolution of two cases of tropical cyclones (Nanmadol and Irma) which made landfall on China in wintertime. The results are shown as follows. (1) The East Asia trough steered the cold air into the tropical ocean in early winter. The tropical cyclones moved in opposite directions with a high moving out to sea and the enhancement of the pressure gradient at the periphery played a role in maintaining and strengthening the intensity of the storms. The intrusion of weak cold air into the low levels of the tropical cyclones strengthened them by improving the cyclonic disturbance when they were still over the warm sea surface. When the cold air was strong enough and intruded into the eyes, the warm cores were damaged and stuffed before dissipation. (2) The tropical cyclones were formed in a convergence zone of moisture flux and their development could enhance the disturbance of water vapor convergence, thus strengthening the moisture convergence zone. However, when they were outside the moisture zone, the storms could not gain sufficient water vapor and became weak. There were no belts of strong moisture transportation during the wintertime tropical cyclone processes.

Impacts of Aerosol−Radiation Interactions on the Wintertime Particulate Pollution under Different Synoptic Patterns in the Guanzhong Basin,China

The effects of aerosol-radiation interactions(ARI)are not only important for regional and global climate,but they can also drive particulate matter(PM)pollution.In this study,the ARI contribution to the near-surface fine PM(PM_(2.5))concentrations in the Guanzhong Basin(GZB)is evaluated under four unfavorable synoptic patterns,including“northlow”,“transition”,“southeast-trough”,and“inland-high”,based on WRF-Chem model simulations of a persistent heavy PM pollution episode in January 2019.Simulations show that ARI consistently decreases both solar radiation reaching down to the surface(SWDOWN)and surface temperature(TSFC),which then reduces wind speed,induces sinking motion,and influences cloud formation in the GZB.However,large differences under the four synoptic patterns still exist.The average reductions of SWDOWN and daytime TSFC in the GZB range from 15.2%and 1.04°C in the case of the“transition”pattern to 26.7%and 1.69°C in the case of the“north-low”pattern,respectively.Furthermore,ARI suppresses the development of the planetary boundary layer(PBL),with the decrease of PBL height(PBLH)varying from 18.7%in the case of the“transition”pattern to 32.0%in the case of the“north-low”pattern.The increase of daytime near-surface PM_(2.5)in the GZB due to ARI is 12.0%,8.1%,9.5%,and 9.7%under the four synoptic patterns,respectively.Ensemble analyses also reveal that when near-surface PM_(2.5)concentrations are low,ARI tends to lower PM_(2.5)concentrations with decreased PBLH,which is caused by enhanced divergence or a transition from divergence to convergence in an area.ARI contributes 15%-25%toward the near-surface PM_(2.5)concentrations during the severe PM pollution period under the four synoptic patterns.

Wintertime Stratospheric Anomalies-Part II: Sudden Warmings

The process of stratospheric sudden warmings from development of planetary waves to.the sudden cooling after reversal of mean zonal circulation will be studied with the primitive equations of heat and momentum balances. It will be explained that the sudden warmings may occur only in the polar regions of winter stratosphere where zonal mean temperature decreases poleward. The heating rate in the order of major warmings is produced by developed planetary waves in the stratospheric breaking layers. The particular perturbation structure characterized by large amplitude of wave 1 together with minimum of wave 2 discovered by Labitzke (1977) is crucial for initiation of major warmings. The cooling by the same mechanism can be produced in the regions with reversed mean temperature gradient.

Wintertime Stratospheric Anomalies-Part I: Warm Pools

The stratospheric worm pools, called the 4-day wave also, are mainly the temperature anomalies in the polar regions of winter hemisphere. It will be shown that their occurrence, propagation speed and specific structure can be explained by the lower frequency coherent heating resulting from the wave interaction in the breaking layers of the stratosphere. Although their vertical phase slope is negligibly small, the warm pools cannot be considered as a barotropic anomaly.

PRELIMINARY STUDY ON STRUCTURE OF WINTERTIME TYPHOON NANMADOL IN 2004

Using NCEP/NCAR reanalysis, the structure of a wintertime typhoon named Nanmadol that landed on Taiwan 4 December, 2004 has been examined in this paper. It is found that Nanmadol looks similar in structure and time evolution to summer typhoons;the central part of it is warm and humid, and the convergence is observed in the lower troposphere while there is divergence in the upper troposphere. The differences between wintertime and summertime typhoons are found. The southwest stream flow in the lower troposphere and cyclonic disturbance in the upper troposphere seem significantly weaker in Nanmadol than in summertime typhoons. The EOF analysis performed for sea level pressure (SLP) of Nanmadol shows that about 90% of the total variance of temporal changes in typhoon circulation can be explained by two leading EOF modes of EOF1 and EOF2. EOF1 shows the structure and intensity variations of Nanmadol while EOF2 shows the changes in environmental SLP distributions that influences the moving direction of Nanmadol.

A NUMERICAL STUDY OF THE WINTERTIME CIRCULATION IN THE BOHAI AND HUANGHAI SEAS

The paper presents a numerical two-dimensional model (with a realistic sea basin and wind fields as exter nal forcing) to simulate the basic features of the wintertime circulation in the Bohai and Huanghai (Yellow) Seas (BHS) and to show how the circulation can be driven by wind. The main results can be summarized as follows (1) The basic features of the BHS wintertime circulation can be depicted by the wind-driven barotropi’c motion. (2) The traditionally named Huanghai Sea Warm Current (HSWC) is actually generated by the north wind field, at least in winter. (3) The southward coastal current off the Korean west coast plays a more significant role in the southern Huanghai Sea wintertime circulation than traditionally believed. (4) Though the coastal landform and bottom topography play important roles in the wintertime BHS circulation pattern, the wind is a primary forcing.

Some numerical experiments of the dynamics of the wintertime circulation in the Northern Huanghai Sea and the Bohai Sea

-Some numerical experiments have been developed to explain dynamically the mechanism of the formation of the wintertime circulation in the Northern Huanghai Sea and the Bohai Sea. From the results of the numerical experiments with Coriolis parameter having positive and negative values respectively, it is found that the current pattern characterized by flowing into the Bohai Sea in the northern part of the Bohai Straits and flowing out of the Bohai Sea in the southern part of the Bohai Straits is controlled basically by the bottom topography with 'the deepness on the northern side and shallowness on the southern side'in the Bohai Straits, and is not related to Coriolis parameter having positive and negative values. It also shows that the Huanghai Warm Current regarded as the inllow condition passing through the in-ports at the water boundary of the numerical experiments only influences the region near the water boundary and the eastern region of the Northern Huanghai Sea. In addition, the mechanism of the formation of several mesoscale eddies and the influence of the bottom topography on the wintertime circulation in the basin have been discussed.

Responses of the Zhe-Min coastal current adjacent to Pingtan Island to the wintertime monsoon relaxation in 2006 and its mechanism

In conjunction with synchronous remotely sensed winds and sea surface temperature （SST）, the spatiotemporal features of the Zhe-Min coastal current （ZMCC）, especially responses of the ZMCC adjacent to Pingtan Island （PT） to the wintertime mon- soon relaxation in 2006 and corresponding mechanism are investigated based on the field observations. In situ data are ac- quired from Conductivity-Temperature-Depth （CTD） cruise and Bottom-Mounted Moorings （BMM）, which are conducted during a comprehensive survey for the Chinese Offshore Investigation and Assessment Project in winter 2006. It is revealed that the ZMCC is well mixed vertically in winter 2006. The ZMCC （〈14℃） recedes during the relaxation of the wintertime monsoon and is accompanied by the enhanced northward shift of the warm, saline Taiwan Strait Mixed Water （TSMW, higher than 14~C and is constituted by the Taiwan Strait Warm Water and the Kuroshio Branch Water）. And greatly enhanced south- ward intrusion of the ZMCC can be detected when the wintertime monsoon restores. Correspondingly, the thermal interface bounded by the ZMCC and the TSMW moves in the northwest/southeast direction, leading to periodic warm/cold reversals of the near-seabed temperature adjacent to the PT. By EOF （Empirical Orthogonal Function） analysis of the large-scale wind fields and wavelet power spectrum analysis of the water level, ocean current and the near-seabed temperature, responses of the ZMCC off the PT to wintertime monsoon relaxation are suggested to be attributed mainly to the southward propagating coast- ally trapped waves triggered by the impeding atmospheric fronts. As a result, ocean current and near-seabed temperature demonstrate significant quasi-5 d and quasi-10 d subtidal oscillations. By contrast, the onshore/offshore water accumulation resulted from Ekman advection driven by the local winds has minor contributions.

Interannual Variability of the Wintertime Northern Branch High Ridge in the Subtropical Westerlies and Its Relationship with Winter Climate in China

The high ridge in the northern branch of the subtropical westerly （hereafter referred to as northern branch ridge） extends from the north of the Tibetan Plateau to the north of Barr Kashmir Lake （32.5°-47.5°N, 70°-95°E） during wintertime. The intensity index and zonal position index of the wintertime northern branch ridge （WNBR） are defined in this study by using the NCEP-NCAR reanalysis data and precipitation and temperature observations at 160 stations in China. Interannual variation features of the WNBR and its relation with precipitation and surface air temperature in China are discussed based on wavelet analysis, correlation analysis, and composite analysis. The results indicate that the intensity and zonal position of the WNBR exhibit significant interannual variability. The intensity of the WNBR gradually weakens and varies in periodic cycles of 4-6, quasi-2, and quasi-16 yr. Its zonal position shifts westward slightly and varies in periodic cycles of quasi-8 and quasi-16 yr. The WNBR is highly correlated with precipitation and surface air temperature in China. When the WNBR is strong （weak） and its zonal position shifts eastward （westward）, winter precipitation in Northeast China and the northern region of Northwest China increases （decreases）, whereas precipitation decreases in central China, South China, and eastern regions of Northwest and Southwest China. Meanwhile, surface air temperatures in most areas of China are higher （lower） than normal. Further investigation has revealed that the two indexes are closely related to variations in general atmospheric circulation, which explains why there exists a close linkage between the variation of the WNBR and climate in China. It is believed that the WNBR is also tied to the Rossby wave, the North Atlantic Oscillation, the East Asian trough, and the North Pacific jet stream.

Volatile organic compounds in wintertime North China Plain:Insights from measurements of proton transfer reaction time-of-flight mass spectrometer(PTR-ToF-MS)

The characteristics of wintertime volatile organic compounds(VOCs)in the North China Plain(NCP)region are complicated and remain obscure.VOC measurements were conducted by a proton transfer reaction time-of-flight mass spectrometer(PTR-ToF-MS)at a rural site in the NCP from November to December 2018.Uncalibrated ions measured by PTRToF-MS were quantified and the overall VOC compositions were investigated by combining the measurements of PTR-ToF-MS and gas chromatography-mass spectrometer/flame ionization detector(GC-MS/FID).The measurement showed that although atmospheric VOCs concentrations are often dominated by primary emissions,the secondary formation of oxygenated VOCs(OVOCs)is non-negligible in the wintertime,i.e.,OVOCs accounts for 42%±7%in the total VOCs(151.3±75.6 ppbV).We demonstrated that PTR-MS measurements for isoprene are substantially overestimated due to the interferences of cycloalkanes.The chemical changes of organic carbon in a pollution accumulation period were investigated,which suggests an essential role of fragmentation reactions for large,chemically reduced compounds during the heavy-polluted stage in wintertime pollution.The changes of emission ratios of VOCs between winter 2011 and winter 2018 in the NCP support the positive effect of“coal to gas”strategies in curbing air pollutants.The high abundances of some key species(e.g.oxygenated aromatics)indicate the strong emissions of coal combustion in wintertime of NCP.The ratio of naphthalene to C8 aromatics was proposed as a potential indicator of the influence of coal combustion on VOCs.

2003 vs 2002: A large amplitude wintertime temperature anomaly reversion event and its anomalous atmospheric circulation

It is revealed that there happened a large am-plitude wintertime temperature anomaly reversion event(LTAR) in Northern Hemisphere high latitude regions andSouth Australia for the year 2003 versus 2002, with the mostnotable change in Eurasia, eastern North America, NorthPacific, and South Australia. The accompanying atmosphericgeneral circulation anomalies are analyzed and the dominantchanges in the circulation arc found out. The anomalouscyclonic eddies over the north Pacific and the anomalousanticyclonic eddies over North Europe that cooperate eachother are mostly linked with the event. Both anomalous eddysystems are connected with the remote teleconnection pat-terns respectively. The research also suggests the existence ofa cross-equator teleconnection pattern, namely, the NorthernPacific-Western Pacific-Australia teleconnection pattern(NPWPA). It is not only related to LTAR, but also associatedwith the temperature transition from cold episode to warmepisode in the year 2003 versus 2002 during Decem-ber-January-February.

Wintertime peroxyacetyl nitrate(PAN) in the megacity Beijing: Role of photochemical and meteorological processes

Previous measurements of peroxyacetyl nitrate （PAN） in Asian megacities were scarce and mainly conducted for relative short periods in summer. Here, we present and analyze the measurements of PAN, O3, NOx, etc., made at an urban site （CMA） in Beijing from 25 January to 22 March 2010. The hourly concentration of PAN averaged 0.70 × 10^-9 mol/mol （0.23 × 10^9-3.51 × 10^9 mol/mol） and was well correlated with that of NOa but not O3, indicating that the variations of the winter concentrations of PAN and O3 in urban Beijing are decoupled with each other. Wind conditions and transport of air masses exert very significant impacts on O3, PAN, and other species. Air masses arriving at the site originated either from the boundary layer over the highly polluted N-S-W sector or from the free troposphere over the W-N sector. The descending free-tropospheric air was rich in O3, with an average PAN/O3 ratio smaller than 0.O31, while the boundary layer air over the polluted sector contained higher levels of PAN and primary pollutants, with an average PAN/O3 ratio of 0.11. These facts related with transport conditions can well explain the observed PAN-O3 decoupling. Photochemical production is important to PAN in the winter over Beijing. The concentration of the peroxyacetyl （PA） radicai was estimated to be in the range of 0.0014 × 10^-12～0.0042 × 10^-12 mol/mol. The contributions of the formation reaction and thermal decomposition to PAN＇s variation were calculated and found to be significant even in the colder period in air over Beijing, with the production exceeding the decomposition.

Characterization of peroxyacetyl nitrate(PAN)under different PM_(2.5) concentration in wintertime at a North China rural site

As a secondary pollutant of photochemical pollution,peroxyacetyl nitrate(PAN)has attracted a close attention.A four-month campaign was conducted at a rural site in North China Plain(NCP)including the measurement of PAN,O_(3),NO_(x),PM_(2.5),oxygenated volatile organic compounds(OVOCs),photolysis rate constants of NO_(2)and O_(3)and meteorological parameters to investigate the wintertime characterization of photochemistry from November 2018 to February 2019.The results showed that the maximum and mean values of PAN were 4.38 and 0.93±0.67 ppbv during the campaign,respectively.The PAN under different PM_(2.5)concentrations from below 75μg/m^(3)up to 250μg/m^(3),showed different diurnal variation and formation rate.In the PM_(2.5)concentration range of above 250μg/m^(3),PAN had the largest daily mean value of 0.64 ppbv and the fastest production rate of 0.33 ppbv/hr.From the perspective of PAN’s production mechanism,the light intensity and precursors concentrations under different PM_(2.5)pollution levels indicated that there were sufficient light intensity and high volatile organic compounds(VOCs)and NO_(x)precursors concentration even under severe pollution level to generate a large amount of PAN.Moreover,the bimodal staggering phenomenon of PAN and PM_(2.5)provided a basis that PAN might aggravate haze through secondary organic aerosols(SOA)formation.

UU_* filtering of nighttime net ecosystem CO_2 exchange flux over forest canopy under strong wind in wintertime

The mechanism of the negative nighttime net CO2 flux in wintertime and reasonable treatment with it is of great importance in evaluating the carbon metabolism of boreal forest. Results, based on the data obtained with open-path eddy covariance system and CO2 profile measurement system from Nov. 2002 to Apr. 2003 and that obtained with five sonic anemometers in Nov. 1999, show that (1) the negative net ecosystem CO2 exchange flux (NEE) always appeared under conditions of strong wind; (2) the pressure fluctuation and horizontal advection flow are dominantly responsible for the negative NEE. Operable upper bound u* filtering method (UU* filtering) was introduced since the difficulties in real-time measuring of pressure fluctuation and horizontal advection fluxes under conditions of strong wind. Nighttime upper u*. threshold for the broad-leaved Korean pine mixed forest of the Changbai Mountains is 0.4 ms-1 and can be applied to the daytime filtering; and (3) the UU* filtering corrected the nighttime 'problem' of negative NEE under strong wind and made the estimates more ecologically reasonable.

THE STUDY ON THE WINTERTIME STRATOCUMULUS CLOUDS OVER URUMQI,XINJIANG OF CHINA——NUMERICAL MODELING OF STRATOCUMULUS CLOUDS

On the basis of the conceptual model,the development of wintertime stratocumulus cloud has been sim- ulated by using a planetary boundary layer/cloud physics model.The main characteristics of the cloud struc- ture and evolution obtained from the simulation are consistent with those from observations.By analyzing the modeled results,some details of the cloud structures and microphysics processes are discussed.

Study on numerical simulation and dynamic mechanism of win-ter- time circulation in the eastern China seas

An MOM2 based 3-dimentional prognostic baroclinic Z-ordinate model was established to study the circulation in eastern China seas, considering the topography, inflow and outflow on the open boundary, wind stress, temperature and salinity exchange on the sea surface. The results were consistent with observation and showed that the Kuroshio intrudes in large scale into the East China Sea continental shelf East China, during which its water is exchanged ceaselessly with outer sea water along Ryukyu Island. The Tsushima Warm Current is derived from several sources, a branch of the Kuroshio, part of the Taiwan Warm Current, and Yellow Sea mixed water coming from the west of Cheju Island. The water from the west of Cheju Island contributes ap-proximately 13% of the Isushima Warm Current total transport through the Korea Strait. The circulation in the Bohai Sea and Yellow Sea is basically cyclonic circulation, and is comprised of coastal currents and the Yellow Sea Warm Current. Besides simulation of the real circulation, numerical experiments were conducted to study the dynamic mechanism. The numerical experiments indicated that wind directly drives the East China Sea and Yellow Sea Coastal Currents, and strengthens the Korea Coastal Current and Yellow Sea Warm Current. In the no wind case, the kinetic energy of the coastal current area and main YSWC area is only 1% of that of the wind case. Numerical experiments also showed that the Tsushima Warm Current is of great importance to the formation of the Korea Coastal Current and Yellow Sea Warm Current.

Comparison of Snowfall Variations over China Identified from Different Snowfall/Rainfall Discrimination Methods

Based on the snowfall observations at 836 surface weather stations in China and the Daily Surface Climate Variables of China version 3.0 dataset for 1961–2013, capability of five methods with different objective criteria for identifying wintertime snowfall is evaluated, to provide reference for application of these methods in snowfall/rainfall discrimination. Methods Ⅰ, Ⅱ, Ⅲ, Ⅳ, and Ⅴuse the daily average surface air temperature(Ta), wet-bulb temperature(Tw), dynamic threshold Tw, 0-cm ground temperature, and 700–850-hPa thickness, respectively, to identify the snowfall. The results show that the climatological distribution of snowfall can be well produced by Methods Ⅰ, Ⅱ, and Ⅲ. Method Ⅳ underestimates the snowfall days in eastern Tibetan Plateau(ETP), and Method Ⅴ cannot yield the actual large numbers of snowfall days and amounts. Accordingly, the linear trends of snowfall days estimated from Methods Ⅰ, Ⅱ, and Ⅲ largely agree with the observations, while a discrepancy is found in the linear trend of snowfall amounts over southeastern China(SEC). For interannual and decadal variations of snowfall, Method Ⅴ shows the worst performance. It is more reasonable to use Tw to distinguish snowfall from rainfall instead of Ta, 0-cm ground temperature, and 700–850-hPa thickness;and the reference thresholds of Tw in northeastern China(NEC), northwestern China(NWC), ETP, and SEC are-1.5,-1.5,-0.4, and-0.3°C, respectively. The above results are beneficial to identifying snowfall in short-term climate prediction.