Air-sea fluxes of heat and momentum over the Yellow Sea during cold air outbreaks

The impact of sea surface waves on air-sea fluxes of heat and momentum over the Yellow Sea caused by cold fronts during cold air outbreak(CAO)events is investigated through numerical experiments with a FVCOM-SWAVE(Finite-Volume Coastal Ocean Model-Surface WAVE)wave-current coupled model.Two typical types of cold fronts,i.e.,those respectively from the north and from the west,are simulated and compared to each other and with monthly mean.During cold seasons,currents in the Yellow Sea are weaker than that during warm seasons.As a result,waves show a more prominent impact.The numerical simulations suggested that both the heat and momentum fluxes are significantly enhanced during CAO events;and they could be a few times larger than the monthly average of a five-year mean.The enhancement is highly sensitive to the features of CAOs.Specifically,it depends on the cold front orientation,intensity and evolution.One mechanism that strengthens the two fluxes is via sea waves.For the CAOs that are studied,an increase in sea wave height by 50%can double the maximal momentum flux,and cause an increase in heat flux by 10-160 W/m^2.

Understanding the physical processes in the evolution of a cold air outbreak over China in late November 2022 from a Lagrangian perspective

From 26 November to 1 December 2022,intense cold air masses swept across China from northwest to south,resulting in a nationwide cold air outbreak(CAO)case characterised by drastic and sudden temperature drops with rain,snow and strong winds.The physical processes that dominate the intensification of the cold air masses during this CAO event remain unclear.In this study,the evolution of the CAO case,which is indicated by the dry static energy(DSE),is investigated using a novel approach in the framework of Lagrangian backtracking.The dominant processes can be identified by decomposing the DSE change into four diabatic heating terms due to shortwave radiation,longwave radiation,latent heat and turbulent processes.Overall,in this case,most of the cold air parcels originated from the east of Novaya Zemlya and crossed Central Siberia before reaching China.Thus,these air parcels mainly manifested on the northwest‒southeast path.The duration of the cold air intensification differed between subregions.The cold air parcels experienced long cooling periods(approximately 9 d)before reaching northern China(i.e.Northwest,North and Northeast China),whilst the southern parts(i.e.Central,East and South China)underwent relatively short cooling periods(6-8 d).Accordingly,the cold air affecting northern China is more intense than that affecting the southern parts,especially for East and South China.For all six subregions,longwave radiative cooling is identified as the dominant contributor to the cold air intensification,and the latent heat processes as the secondary contributor.The weakening of cold air parcels as they approach and pass over these regions is driven by turbulent processes and shortwave heating.Central Siberia and Lake Baikal are identified as key areas for the intensification of cold air passing over both regions.In addition,air parcels affecting Northwest China are intensely cooled as they pass over the Junggar Basin,while the North China Plain is a key area for cooling air parcels reaching Central,East and South China.From a Lagrangian perspective,these findings provide insights into the physical processes driving the behaviour of cold air parcels,which would help understand the mechanisms involved in the past changes and future projections in CAOs.

Blocking Features for Two Types of Cold Events in East Asia

Cold air outbreaks（CAOs）always hit East Asia during boreal winter and have significant impacts on human health and public transport.The amplitude and route of CAOs are closely associated with blocking circulations over the Eurasian continent.Two categories of CAOs are recognized,namely,the ordinary cold wave events（CWEs）and the extensive and persistent extreme cold events（EPECEs）,with the latter having even stronger impacts.The blocking features associated with these two types of CAOs and their differences are investigated in this study on the intraseasonal timescale.What these two CAOs do have in common is that they are both preceded by the intensification and recurrence of a blocking high over the midlatitude North Atlantic.The difference between these events is primarily reflected on the spatial scale and duration of the corresponding blocking high.During the CWEs,blocking occurs around the Ural Mountains,and exhibits a regional feature.The resulting cold air temperature persists for only up to 8 days.In contrast,during the EPECEs,the blocking region is quite extensive and is not only confined around the Ural Mountains but also extends eastward into Northeast Asia in a southwest–northeast orientation.As a result,the cold air tends to accumulate over a large area and persists for a much longer time.The blocking activity is primarily induced by an increased frequency and eastward extension of the synoptic anticyclonic Rossby wave breaking（AWB）.Compared with the CWEs,characterized by a regional and short-lived synoptic AWB frequency,the EPE-CEs tend to be accompanied by more recurrent and eastward extensions of the synoptic AWB.