大气对黑潮延伸区中尺度海洋涡旋的响应——冬季暖、冷涡个例分析

采用动态合成、带通滤波等方法,通过对冬季黑潮延伸区暖、冷两个中尺度海洋涡旋的分析,研究了大气对中尺度海洋涡旋的响应特征。结果表明,海表温度（SST）与近海面风速的正相关关系在涡旋的动态合成图上清晰可见,暖（冷）涡上空对应10 m风速的极大（小）值,即海洋对大气的强迫作用在日时间尺度上表现显著;SST高低值中心基本对应10 m风无辐散区,暖（冷）涡上空为异常正（负）涡度分布;暖（冷）涡上空潜热、感热通量增大（减小）,降低（增大）大气稳定度,从而加强（减弱）边界层垂直混合作用,使得海洋大气边界层增厚（变薄）。暖（冷）涡旋上空对应摩擦速度极大（小）值,反映了湍流粘性力在高（低）海温中心增大（减小）的特征,表明动量垂直混合机制在中小尺度海气相互作用中起着主要作用。中尺度海洋涡旋能够影响大气瞬变扰动,大气瞬变扰动强度在暖（冷）涡下游上空出现极大（小）值,该影响不仅表现在海洋大气边界层,在自由大气中低层也有较为清晰的反映。此外,从能量转换的角度入手,发现斜压能量转换在中尺度海洋涡旋影响大气瞬变扰动强度中贡献明显。     

Impacts of Increased SST Resolution on the North Pacific Storm Track in ERA-Interim

This study examines the artificial influence of increasing the SST resolution on the storm track over the North Pacific in ERA-Interim. Along with the mesoscale oceanic eddies and fronts resolved during the high-resolution-SST period, the low-level storm track strengthens northward, reaching more than 30% of the maximum values in the low-resolution-SST period after removing the influence of ENSO. The mesoscale structure firstly imprints on the marine atmospheric boundary layer, which then leads to changes in turbulent heat flux and near-surface convergence, forcing a secondary circulation into the free atmosphere, strengthening the vertical eddy heat, momentum and specific humidity fluxes, and contributing to the enhancement of the storm track. Results from a high-resolution atmospheric model further indicate the changes in the storm track due to the mesoscale SST and their relationship.     

Synoptic Controls on Boundary-Layer Characteristics

We report the characteristics of the three-dimensional, time evolving, atmospheric boundary layer that develops beneath an idealised, dry, baroclinic weather system. The boundary-layer structure is forced by thermal advection associated with the weather system. Large positive heat fluxes behind the cold front drive a vigorous convective boundary layer, whereas moderate negative heat fluxes in the warm sector between the cold and warm fronts generate shallow, stably stratified or neutral boundary layers. The forcing of the boundary-layer structure is quantified by forming an Eulerian mass budget integrated over the depth of the boundary layer. The mass budget indicates that tropospheric air is entrained into the boundary layer both in the vicinity of the high-pressure centre, and behind the cold front. It is then transported horizontally within the boundary layer and converges towards the cyclone’s warm sector, whence it is ventilated out into the troposphere. This cycling of air is likely to be important for the ventilation of pollution out of the boundary layer, and for the transformation of the properties of large-scale air masses.     

Atmospheric Response to Mesoscale Ocean Eddies over the South China Sea

The South China Sea(SCS) is an eddy-active area. Composite analyses based on 438 mesoscale ocean eddies during 2000–2012 revealed the status of the atmospheric boundary layer is influenced remarkably by such eddies. The results showed cold-core cyclonic(warm-core anticyclonic) eddies tend to cool(warm) the overlying atmosphere and cause surface winds to decelerate(accelerate). More than 5% of the total variance of turbulent heat fluxes, surface wind speed and evaporation rate are induced by mesoscale eddies. Furthermore, mesoscale eddies locally affect the columnar water vapor, cloud liquid water, and rain rate. Dynamical analyses indicated that both variations of atmospheric boundary layer stability and sea level pressure are responsible for atmospheric anomalies over mesoscale eddies. To reveal further details about the mechanisms of atmospheric responses to mesoscale eddies, atmospheric manifestations over a pair of cold and warm eddies in the southwestern SCS were simulated. Eddy-induced heat flux anomalies lead to changes in atmospheric stability. Thus, anomalous turbulence kinetic energy and friction velocity arise over the eddy dipole, which reduce(enhance) the vertical momentum transport over the cold(warm) eddy, resulting in the decrease(increase) of sea surface wind. Diagnoses of the model's momentum balance suggested that wind speed anomalies directly over the eddy dipole are dominated by vertical mixing terms within the atmospheric boundary layer, while wind anomalies on the edges of eddies are produced by atmospheric pressure gradient forces and atmospheric horizontal advection terms.     

Isolating mesoscale coupled ocean–atmosphere interactions in the Kuroshio Extension region

The Kuroshio Extension region is characterized by energetic oceanic mesoscale and frontal variability that alters the air–sea fluxes that can influence large-scale climate variability in the North Pacific. We investigate this mesoscale air-sea coupling using a regional eddy-resolving coupled ocean–atmosphere (OA) model that downscales the observed large-scale climate variability from 2001 to 2007. The model simulates many aspects of the observed seasonal cycle of OA coupling strength for both momentum and turbulent heat fluxes. We introduce a new modeling approach to study the scale-dependence of two well-known mechanisms for the surface wind response to mesoscale sea surface temperatures (SSTs), namely, the ‘vertical mixing mechanism’ (VMM) and the ‘pressure adjustment mechanism’ (PAM). We compare the fully coupled model to the same model with an online, 2-D spatial smoother applied to remove the mesoscale SST field felt by the atmosphere. Both VMM and PAM are found to be active during the strong wintertime peak seen in the coupling strength in both the model and observations. For VMM, large-scale SST gradients surprisingly generate coupling between downwind SST gradient and wind stress divergence that is often stronger than the coupling on the mesoscale, indicating their joint importance in OA interaction in this region. In contrast, VMM coupling between crosswind SST gradient and wind stress curl occurs only on the mesoscale, and not over large-scale SST gradients, indicating the essential role of the ocean mesocale. For PAM, the model results indicate that coupling between the Laplacian of sea level pressure and surface wind convergence occurs for both mesoscale and large-scale processes, but inclusion of the mesoscale roughly doubles the coupling strength. Coupling between latent heat flux and SST is found to be significant throughout the entire seasonal cycle in both fully coupled mode and large-scale coupled mode, with peak coupling during winter months. The atmospheric response to the oceanic mesoscale SST is also studied by comparing the fully coupled run to an uncoupled atmospheric model forced with smoothed SST prescribed from the coupled run. Precipitation anomalies are found to be forced by surface wind convergence patterns that are driven by mesoscale SST gradients, indicating the importance of the ocean forcing the atmosphere at this scale.     

Some Characteristics of the Surface Boundary Layer of a Strong Cold Air Process over Southern China

In southern China,cold air is a common weather process during the winter season;it can cause strong wind,sharp temperature decreases,and even the snow or freezing rain events.However,the features of the atmospheric boundary layer during cold air passage are not clearly understood due to the lack of comprehensive observation data,especially regarding turbulence.In this study,four-layer gradient meteorological observation data and one-layer,10-Hz ultrasonic anemometer-thermometer monitoring data from the northern side of Poyang Lake were employed to study the main features of the surface boundary layer during a strong cold-air passage over southern China.The results show that,with the passage of a cold air front,the wind speed exhibits low-frequency variations and that the wind systematically descends.During the strong wind period,the wind speed increases with height in the surface layer.Regular gust packets are superimposed on the basic strong wind flow.Before the passage of cold air,the wind gusts exhibit a coherent structure.The wind and turbulent momentum fluxes are small,although the gusty wind momentum flux is slightly larger than the turbulent momentum flux.However,during the invasion of cold air,both the gusty wind and turbulent momentum fluxes increase rapidly with wind speed,and the turbulent momentum flux is larger than the gusty wind momentum flux during the strong wind period.After the cold air invasion,this structure almost disappears.     

SENSITIVITY TESTS OF INTERACTION BETWEEN LAND SURFACE PHYSICAL PROCESS AND ATMOSPHERIC BOUNDARY LAYER*

In this paper,an interactive model between land surface physical process and atmosphere boundary layer is established,and is used to simulate the features of soil environmental physics,surface heat fluxes,evaporation from soil and evapotranspiration from vegetation and structures of atmosphere boundary layer over grassland underlying.The sensitivity experiments are engaged in primary physics parameters.The results show that this model can obtain reasonable simulation for diurnal variations of heat balance,soil volumetric water content,resistance of vegetation evaporation,flux of surface moisture,and profiles of turbulent exchange coefficient,turbulent momentum,potential temperature,and specific humidity.The model developed can be used to study the interaction between land surface processes and atmospheric boundary layer in city regions,and can also be used in the simulation of regional climate incorporating a mesoscale model.     

SENSITIVITY TESTS OF INTERACTION BETWEEN LAND SURFACE PHYSICAL PROCESS AND ATMOSPHERIC BOUNDARY LAYER

In this paper,an interactive model between land surface physical process and atmosphereboundary layer is established,and is used to simulate the features of soil environmental physics,surface heat fluxes,evaporation from soil and evapotranspiration from vegetation and structures ofatmosphere boundary layer over grassland underlying.The sensitivity experiments are engaged inprimary physics parameters.The results show that this model can obtain reasonable simulation fordiurnal variations of heat balance,soil volumetric water content,resistance of vegetationevaporation,flux of surface moisture,and profiles of turbulent exchange coefficient,turbulentmomentum,potential temperature,and specific humidity.The model developed can be used tostudy the interaction between land surface processes and atmospheric boundary layer in cityregions,and can also be used in the simulation of regional climate incorporating a mesoscalemodel.     

Mesoscale moisture variability and its impact on energy transfer through the boundary layer

The impact of mesoscale moisture variability on the vertical energy transfer through a pre-frontal boundary layer is studied with NOPEX aircraft data. The moisture variability relates to a cold front that passed the area 2 1/2 hours after the observations. We find a density front ahead of the cold front. The large vertical divergence of the turbulent moisture flux in the surface layer is partly related to this moisture variability. Large scale horizontal advection contributes to the observed vertical turbulent flux divergence. The estimated horizontal mesoscale advection term in the budget of sensible heat and moisture is on average small but locally it can be large. This term acts to re-distribute moisture in the boundary layer and leads to sub-grid variations of relative humidity, which is an important quantity for boundary-layer cloud models. The distinct spatial variations of specific humidity are mainly related to synoptic forcing and not to heterogeneity in the surface energy balance.     

Warm and cold events in the southeast Atlantic/southwest Indian Ocean region and potential impacts on circulation and rainfall over southern Africa

Summary An important pattern of interannual variability in the southern African region is one where sea surface temperature (SST) in neighbouring waters, particularly in the Agulhas Current, its retroflection region and outflow across the southern midlatitudes of the Indian Ocean, is anomalously warm or cool. Evidence exists of significant rainfall anomalies over large parts of southern Africa during these warm or cool SST events. Here, a general circulation model is used to study the response of the atmosphere in the region to an idealised representation of these SST anomalies. The induced atmospheric circulation and precipitation anomalies over the adjacent southern African landmass on intraseasonal through to interannual time scales are investigated.A nonlinear response to the SST anomalies is found in that the changes to the model atmosphere when warm SST forcing is used are not the reverse (in either pattern or magnitude) to that when cold SST forcing is imposed. For the warm SST anomaly, it is found that the atmospheric response is favourable for enhancement of the original SST anomaly on scales up to, and including, annual. However, as the scale becomes interannual (i.e., 15–21 months after imposition of the anomaly), the model response suggests that damping of the original SST anomaly becomes likely. However, no such coherent timescale dependent response is found when the cold SST anomaly is impose. It is suggested that the relationship of the SST anomaly to the background seasonal climatology may help explain this fundamental difference in the response.Examination of the circulation and rainfall patterns under warm SST forcing indicates that there are significant anomalies over large parts of southern Africa on all scales from intraseasonal through to interannual. On the south coast, rainfall anomalies result from enhanced evaporation of moisture off the SST anomaly. Over the interior, changer in the convergence of moist air streams together with suggestions of a shift in the Walker circulations between southern Africa and the bordering tropical South Atlantic and Indian Oceans appear to be associated with the rainfall anomalies. Similar mechanisms of rainfall perturbation are found when the cold SST anomaly is imposed; however, there is a significant response only on intra-annual to interannual scales. In all cases, the magnitude of the rainfall anomalies accumulated over a 90 day season were of the order of 90–180 mm, and therefore represent a significant fraction of the annual total of many areas. These model results re-inforce previous observational work suggesting that SST anomalies south of Africa, particularly in the retroflection region of the Agulhas Current, are linked with significant rainfall anomalies over the adjacent subcontinent.With 12 Figures     

Extremely Weak Mixing in Stable Conditions

Transport by extremely weak turbulence occurring on nights with clear skies and weak winds is examined from seven tower levels of eddy-correlation data taken from each of two field programs. The very small flux is systematic, provided that the perturbations are computed from a record-dependent averaging length, which must be as small as 10 s in very stable conditions. With traditional methods for computing the flux, these fluxes were considered too small to estimate, in that the computed values behaved erratically. For extremely weak turbulence, the fluxes decrease systematically with height and often indicate very shallow boundary-layer depths on the order of 10 m. However, in one field program, the turbulence slowly increases with height above the surface flux-based boundary layer apparently due to horizontal advection of stronger turbulence driven by modest surface heterogeneity. For very weak turbulence, the eddy diffusivity for momentum is systematically greater than that for heat in both field programs. The dependence of the turbulence strength and its variability with stability is examined in some detail.     

Marine Boundary-Layer Variability Over The Indian Ocean During Indoex (1998)

The variability in boundary-layerstructure over the Indian Ocean during a north-eastmonsoon and the factors influencing it areinvestigated. This study was made possible as acomponent of the Indian Ocean Experiment (INDOEX),conducted from February 19 to March 30, 1998. The dataused are, surface-layer mean and turbulencemeasurements of temperature, humidity and wind, andvertical soundings of temperature and humidity.Significant spatio-temporal variability was observedin the boundary-layer structure throughout the cruise.The ITCZ was characterized as the region withstrongest winds and maximum surface turbulent fluxesof momentum and heat. One of the important findingsfrom this study was a strong influence of continentalair masses on the boundary-layer structure in theNorthern Hemisphere, even at a distance of 600 km offthe Indian coast. This was generally evident in theform of an elevated plume of dry continental airbetween altitudes of 1500 m and 2700 m. Advection ofcontinental aerosols in this layer presents potentialfor significant entrainment into shallow clouds inthis region, which eventually feed deeper clouds atthe ITCZ. This finding provides an explanation foranomalous higher aerosol concentrations found duringprevious studies. The structure of the marineboundary layer was influenced by various factors suchas proximity to land, an anomalous warm pool in theocean and the ITCZ. In the southern hemisphere, theboundary-layer height was primarily governed bysurface-layer sensible heat flux and was found to behighest in the vicinity of the ITCZ. North of theequator it was strongly influenced by land-air-seainteractions. In addition to this synoptic modulation,there was also a significant diurnal variability inthe boundary-layer height.     

Sensitivity of ENSO characteristics to a new interactive flux correction scheme in a coupled GCM

A fast coupled global climate model (CGCM) is used to study the sensitivity of El Ni?o Southern Oscillation (ENSO) characteristics to a new interactive flux correction scheme. With no flux correction applied our CGCM reveals typical bias in the background state: for instance, the cold tongue in the tropical east Pacific becomes too cold, thus degrading atmospheric sensitivity to variations of sea surface temperature (SST). Sufficient atmospheric sensitivity is essential to ENSO. Our adjustment scheme aims to sustain atmospheric sensitivity by counteracting the SST drift in the model. With reduced bias in the forcing of the atmosphere, the CGCM displays ENSO-type variability that otherwise is absent. The adjustment approach employs a one-way anomaly coupling from the ocean to the atmosphere: heat fluxes seen by the ocean are based on full SST, while heat fluxes seen by the atmosphere are based on anomalies of SST. The latter requires knowledge of the model??s climatological SST field, which is accumulated interactively in the spin-up phase (??training??). Applying the flux correction already during the training period (by utilizing the evolving SST climatology) is necessary for efficiently reducing the bias. The combination of corrected fluxes seen by the atmosphere and uncorrected fluxes seen by the ocean implies a restoring mechanism that counteracts the bias and allows for long stable integrations in our CGCM. A suite of sensitivity runs with varying training periods is utilized to study the effect of different levels of bias in the background state on important ENSO properties. Increased duration of training amplifies the coupled sensitivity in our model and leads to stronger amplitudes and longer periods of the Nino3.4 index, increased emphasis of warm events that is reflected in enhanced skewness, and more pronounced teleconnections in the Pacific. Furthermore, with longer training durations we observe a mode switch of ENSO in our model that closely resembles the observed mode switch related to the mid-1970s ??climate shift??.     

敦煌绿洲夏季边界层特征的数值模拟

使用美国NCAR新版MM5V3．6非静力平衡模式,采用三重嵌套方法,模拟研究了沙漠绿洲的环流及边界层特征。并且与无绿洲试验进行了比较。结果表明：沙漠中的绿洲在夏季是一个冷源,地面感热通量和潜热通量与周围的沙漠地区存在较大差异。在绿洲区,感热通量小,潜热通量大;在沙漠区,感热通量大,潜热通量小。绿洲边界层顶低,沙漠边界层顶高。绿洲可以改变沙漠地区的大气稳定度,使得原来大气层结不稳定的沙漠地区出现较稳定的大气层结。沙漠绿洲改变了原有沙漠地区的环流结构及温、湿场的分布,使得在绿洲上空大气下沉,沙漠上空大气上升,从而产生了绿洲上空大气干冷,沙漠上空大气暖湿的边界特征。绿洲边缘的沙漠形成的较大的湿气柱围绕着绿洲,起到了保护绿洲的作用。沙漠绿洲低空呈现辐散气流,使得绿洲上风方的沙漠地区风速减小,绿洲下风方的沙漠地区风速增大。     

非均匀对流边界层的地转强迫流动和动量输送

用大涡模拟方法研究地转强迫下的对流边界层流动和地表热力非均匀性影响.模拟重现了典型对流边界层的平均风廓线和动量通量垂直分布.地表热力非均匀性对区域平均风速和动量通量分布无明显影响,但边界层内的局地流动性状和湍流动量输送情况有系统性的改变.下风较热区近地面风速增强而高空流动受到阻塞,上风较冷区之上情况则正好相反.对应于平均流动场的畸变,地表较热区之上边界层大部可以出现动量向上输送的情况,较冷区成为大气动量下传的主要通道.地面应力在较热区增强、较冷区减弱的趋势明显.     

Airborne measurements of turbulent trace gas fluxes and analysis of eddy structure in the convective boundary layer over complex terrain

Using the new high-frequency measurement equipment of the research aircraft DO 128, which is described in detail, turbulent vertical fluxes of ozone and nitric oxide have been calculated from data sampled during the ESCOMPTE program in the south of France. Based on airborne turbulence measurements, radiosonde data and surface energy balance measurements, the convective boundary layer (CBL) is examined under two different aspects. The analysis covers boundary-layer convection with respect to (i) the control of CBL depth by surface heating and synoptic scale influences, and (ii) the structure of convective plumes and their vertical transport of ozone and nitric oxides. The orographic structure of the terrain causes significant differences between planetary boundary layer (PBL) heights, which are found to exceed those of terrain height variations on average. A comparison of boundary-layer flux profiles as well as mean quantities over flat and complex terrain and also under different pollution situations and weather conditions shows relationships between vertical gradients and corresponding turbulent fluxes. Generally, NO_x transports are directed upward independent of the terrain, since primary emission sources are located near the ground. For ozone, negative fluxes are common in the lower CBL in accordance with the deposition of O₃ at the surface.The detailed structure of thermals, which largely carry out vertical transports in the boundary layer, are examined with a conditional sampling technique. Updrafts mostly contain warm, moist and NO_x loaded air, while the ozone transport by thermals alternates with the background ozone gradient. Evidence for handover processes of trace gases to the free atmosphere can be found in the case of existing gradients across the boundary-layer top. An analysis of the size of eddies suggests the possibility of some influence of the heterogeneous terrain in mountainous area on the length scales of eddies.     

一次层积云发展过程对黑潮延伸体海洋锋强迫的响应研究——观测与机制分析

由于缺乏海上现场观测,对天气尺度扰动下,海表面温度锋 (海洋锋) 对海洋大气边界层 (MABL) 垂直结构和MABL内海洋性低云 (marine stratus) 的影响研究较少。2014年4月12日,中国海洋大学东方红2号科学考察船在黑潮延伸体海区的海洋锋附近捕捉到一次层积云的迅速发展。在比较稳定的天气形势下,由暖水侧向北穿越海洋锋时,云底和云顶高度升高,云区范围迅速扩大。本文利用多种大气-海洋联合观测数据,结合卫星观测和再分析资料,对此次层积云迅速发展的机理进行了综合分析。结果表明,在海上低压后部西北风控制下,在海洋锋的暖水侧 (下风方) 形成热通量大值中心和低压槽,有助于高空西风动量下传,进而又使得海气界面热通量增加,这种正反馈效应为MABL内混合层厚度加大和云底/顶高度在海洋锋的下风方升高提供有利背景条件。4月12日09:00~12:00(协调世界时),来自日本本州岛陆地的低空暖平流到达该热通量中心上空,暖平流与热通量中心的共同作用,导致该时段近海面暖中心强度异常增加,MABL中静力不稳定层加深和低压槽发展,综合作用的结果使得混合层厚度明显加深,云底高度升高,云区迅速发展。本研究有助于理解在复杂大气背景扰动下MABL对海洋强迫的响应机理。     

陆面过程和大气边界层相互作用敏感性实验

文中建立了一个研究陆面物理过程与大气边界层相互作用的模式。模拟了草原下垫面的土壤 环境物理、地面热量通量、蒸发、蒸散及大气边界层结构特征。并对主要的环境物理参数进 行了敏感性实验。结果表明，本模式能合理地模拟地表热量平衡、土壤体积含水量、植被蒸 发阻抗、地表水汽通量日变化和湍流交换系数、湍流动能、位温和比湿廓线等。该模式还可 进一步应用于研究城市陆面物理过程与大气边界层相互作用机制，及与中尺度大气模式耦合用于区域气候的研究。     

北京地区一次强沙尘天气过程的中尺度通量特征

应用大气所325 m铁塔的湍流资料和梯度资料,对2003年3月20～23日影响北京的一次强沙尘天气过程的中尺度通量和湍流通量进行了分析,结果表明:冷空气的影响是从上层开始的.沙尘爆发前,边界层有强逆温存在,动量以中尺度输送为主,湍流很弱.沙尘爆发时,向下的湍流动量通量明显加大.湍流动量通量是沙尘沉降的主要原因,但沙尘爆发前的中尺度过程不能忽略.沙尘爆发前后,感热通量均以小尺度的湍流输送为主,中尺度输送不明显.     

A numerical study of a TOGA-COARE squall-line using a coupled mesoscale atmosphere-ocean model

1. Introduction Air-sea interaction plays an important role in theglobal seasonal to inter-annual climate variability,most notably, the El Ni?no and Southern Oscillation(ENSO) phenomenon (Webster and Lukas, 1992). Be-cause of its widespread impacts on …