1—3月北极涛动对北半球热带太平洋和大西洋对流活动的可能影响

利用1979—2008年日分辨率的向外长波辐射资料以及NCEP再分析资料,去除ENSO影响后,分析了1—3月北极涛动对热带太平洋和热带大西洋对流活动及降水的可能影响。结果表明北极涛动偏强(弱)时,热带太平洋和大西洋对流活动显著偏强(弱)。北半球热带大洋冬季平均向外长波辐射与北极涛动指数的相关系数存在两个显著负相关区:一个位于中太平洋区,大致包括13°—20°N、160°E—170°W;另外一个位于热带大西洋,显著区覆盖的范围大体包括5°—20°N、15°—70°W。这些区域的降水量也表现出显著的正相关。向外长波辐射、强对流面积指数、强对流强度指数、平均降水量等指标与北极涛动指数的相关均以冬季同期最高,随时间滞后相关迅速减弱。与此对应的对流层低层大气环流也有显著变化,850hPa风场的变化表现为热带太平洋有异常的气旋性环流,气旋中心区与显著强对流和降水异常区一致。而热带大西洋有显著的经向环流辐合和风切变,与异常对流和降水区吻合。海洋模式的模拟结果表明,与北极涛动有关联的海温分布,很大程度上与大气强迫有关,说明热带1—3月降水和对流活动与海温的关联较弱。北极涛动与热带太平洋、大西洋对流和降水活动之间主要是通过大气环流的变动产生联系的。     

Turbulent and Thermodynamic Structure of the Autumnal Arctic Boundary Layer Due to Embedded Clouds

We have studied the role of low-level clouds in modifying the thermodynamic and turbulence properties of the Arctic boundary layer during autumn. This was achieved through detailed analyses of boundary-layer properties in two regions, one with low-level cloud cover and the other free of clouds, using measurements from a research aircraft during the Beaufort and Arctic Storms Experiment (BASE). Both regions were measured on the same day under similar synoptic forcing. The cloudy region was characterized by strong horizontal inhomogeneity in low-level temperature and moisture that varied with the cloud-top height. The clear region was relatively homogeneous in temperature and specific humidity with a strong temperature inversion extending between heights of 100 m and 3 km. From measurements at the lowest levels, we also identified a shallow mixed layer below the deep stable layer in the clear region.Our spectral analyses revealed significant modifications of boundary-layer properties due to the presence of low-level clouds. In the cloudy region, turbulent perturbations dominated the boundary-layer flow and made large contributions to the scalar variances. In the clear boundary-layer, wave motion contributed significantly to the observed variances, while turbulent flow was relatively weak. The clear region was saturated, although no detectable clouds were measured.     

Contrasting the interleaving in two baroclinic ocean fronts

To investigate the effects of baroclinicity on frontal interleaving, we contrast the interleaving characteristics of two fronts, one in the Arctic Ocean and the other surrounding a Mediterranean salt lens (Meddy). The Meddy is broken into two parts based on the vertical temperature and salinity structure, so our comparison involves three sets of interleaving observations. The cross-front slopes of intrusions relative to horizontal and isopycnal surfaces are taken to be key diagnostics of the interleaving dynamics. Assuming the observed slopes match those that were present during the initial growth of interleaving, we use an instability theory to infer the dominant form of double diffusion that was active during the growth stage. Then, to investigate the observed interleaving, we use a steady-state model to infer the dominant form of double diffusion at the time of observation. In the Arctic Ocean front, it appears that different forms of double diffusion dominated the two stages of interleaving (salt fingering during the growth stage and diffusive convection at steady state). In contrast, in the Meddy, the same form of double diffusion appears to have dominated both stages of interleaving (salt fingering in the lower part of the Meddy, diffusive convection in the upper part). In the Arctic Ocean front, the observations suggest that interleaving was driven by baroclinicity as well as double diffusion. In both parts of the Meddy, however, driving was by double diffusion only. Motivated by these differences, we suggest a new intrusion classification scheme, based on the slopes of intrusions relative to horizontal and isopycnal surfaces.     

Sensitivity of simulated wintertime Arctic atmosphere to vertical resolution in the ARPEGE/IFS model

The current state-of-the-art general circulation models, including several of those used by the IPCC, show considerable biases in the simulated present day high-latitude climate compared to observations and reanalysis data. These biases are most pronounced during the winter season. We here employ ideal vertical profiles of temperature and wind from turbulence-resolving simulations to perform a priori studies of the first-order eddy-viscosity closure scheme employed in the ARPEGE/IFS model. This reveals that the coarse vertical resolution (31 layers) of the model cannot be expected to realistically resolve the Arctic stable boundary layer. The curvature of the Arctic inversion and thus also the vertical turbulent-exchange processes cannot be reproduced by the coarse vertical mesh employed. To investigate how turbulent vertical exchange processes in the Arctic boundary layer are represented by the model parameterization, a simulation with high vertical resolution (90 layers in total) in the lower troposphere is performed. Results from the model simulations are validated against data from the ERA-40 reanalysis. The dependence of the surface air temperature on surface winds, surface energy fluxes, free atmosphere stability and boundary layer height is investigated. The coarse-resolution run reveals considerable biases in these parameters, and in their physical relations to surface air temperature. In the simulation with fine vertical resolution, these biases are clearly reduced. The physical relation between governing parameters for the vertical turbulent-exchange processes improves in comparison with ERA-40 data.     

Intercomparison of Surface Radiative Fluxes in the Arctic Ocean

Recent satellite data analysis has provided improved data sets relevant to the surface energy budget in the Arctic Ocean. In this paper, surface radiation properties in the Arctic Ocean obtained from the Surface Radiation Budget（SRB3.0） and the International Satellite Cloud Climatology Project（ISCCP-FD） during 1984– 2007 are analyzed and compared. Our analysis suggests that these datasets show encouraging agreement in basin-wide averaged seasonal cycle and spatial distribution of surface albedo; net surface shortwave and all-wave radiative fluxes; and shortwave, longwave, and all-wave cloud radiative forcings. However, a systematic large discrepancy is detected for the net surface longwave radiative flux between the two data sets at a magnitude of ~ 23 W m–2, which is primarily attributed to significant differences in surface temperature, particularly from April to June. Moreover, the largest difference in surface shortwave and all-wave cloud radiative forcings between the two data sets is apparent in early June at a magnitude of 30 W m–2.     

从2011/2012和2015/2016年冬季大气环流异常看北极海冰以及前期夏季北极大气环流异常的作用

利用美国NCEP/NCAR、欧洲中心ERA-Interim再分析资料,以及英国哈得来中心海冰密集度资料,通过诊断分析和数值模拟试验,研究了2011/2012和2015/2016年两个冬季大气环流异常的主要特征和可能原因。结果表明,尽管热带太平洋海温背景截然不同（分别为弱的拉尼娜事件和强厄尔尼诺事件）,但这两个冬季西伯利亚高压均异常偏强,自1979年以来其强度分别排第1和第5位。前期秋季北极海冰异常偏少是导致这两个冬季西伯利亚高压偏强的主要原因。更为重要的是,前期夏季北冰洋表面反气旋风场,以及其上空对流层中、低层平均气温偏高,加强了北极海冰偏少对冬季大气变率的负反馈,进一步促进了西伯利亚高压的加强,从而有利于东亚地区冬季阶段性强严寒的出现。因此,夏季北极大气环流的动力和热力状态不仅影响夏、秋季北极海冰,而且对海冰偏少影响亚洲冬季气候变率有重要调节作用。2015/2016年冬季强厄尔尼诺事件并不能掩盖来自北极海冰和大气环流的影响。      

Case Studies of the Wintertime Convective Boundary-Layer Structure in the Urban Area of Milan, Italy

The paper describes some aspects of the convective boundary-layer structure based on simultaneous sodar and tethersonde measurements during a field experiment in the urban area of Milan in the period 8 to 20 February, 1993. During this period, fog episodes and strong low-level elevated inversions (with lower boundaries < 400 m) were observed most of the time. A close agreement in the mixing height values, derived from the sodar and tethersonde profiles, has been achieved under these conditions. The validity of the similarity relationships, which have been originally derived to describe the vertical velocity variance and heat flux profiles over horizontally homogeneous terrain under quasi-stationary conditions, was evaluated when applied to the urban boundary layer.     

1991年5-7月江淮洪涝:水汽和冷空气的作用比较

对1991年5-7月江淮流域持续性暴雨的环流异常进行分析,比较造成此次洪涝的水汽输送和冷空气活动重要性。结果表明：（1）东亚大槽维持在中国东北上空发展明显,同时鄂霍次克海上空没有建立强大阻高,这种形势有利于冷空气持续向南侵袭,盛行经向环流。此外,低纬度地区,西太平洋副热带高压主体位置于常年同期相比,明显偏西、偏强,有利于暖湿气流沿副热带高压北上到达江淮流域,与北方冷空气辐合形成强降水。（2）通过定义I_T和I_q分别表征温度与水汽对降水的贡献,发现此次江淮流域地区强降水是由对流层低层水汽异常增多和气温异常偏低共同造成的,作用基本相当。     

The Coherent Structure of Water Vapour Transfer in the Unstable Atmospheric Surface Layer

Observations of water vapour fluctuations over arice field show vapour ramps. Coherent structuresare first revealed by the frequently occurring ramp pattern in the vapourtrace. Wavelet and pseudo-wavelet analysis techniques were used inconditional sampling, and more than 100 hr of data have been analyzedto determine coherent structure characteristics. The most probablecoherent structure duration was in the range 2–12 sec andthe duration range of the most effective coherent structures shows somedifference between heat and water vapour transfers. Coherent structurescontribute to the major part of the total flux.     

低层垂直风切变和冷池相互作用影响华北地区一次飑线过程发展维持的数值模拟

利用三维数值云模式和雷达资料四维变分同化技术,通过对6部新一代多普勒天气雷达观测资料进行快速更新循环同化和云尺度数值模拟,初步分析了2009年7月23日发生在华北地区的一次飑线过程的低层动力和热动力影响机制。结果表明,这次飑线过程处在低层中等强度切变的环境条件下,低层环境垂直风切变和冷池相互作用是本次飑线过程维持发展和传播的关键机制。在飑线发展的初期,低层垂直风切变较强,但冷池偏弱,冷池传播速度(C)和垂直于飑线的低层切变分量(ΔU)的比值C/ΔU<1,飑线回波前倾。而此时环境热力条件(对流有效位能较高和自由对流高度较低)对飑线的发展加强起到了积极作用,克服了这种低层切变和冷池不平衡所形成的不利条件。在飑线的加强和成熟阶段,由于对流降水使冷空气不断下沉,从而导致冷池快速加强,使低层切变和冷池强度逐渐达到近似平衡状态(C/ΔU≈1),低层大气处于最强的垂直抬升状态,飑线发展最为强盛,飑线回波直立。随着时间的推移,降水累积效应导致冷池强度明显大于低层切变强度(C/ΔU>1),不利的形势导致飑线逐渐趋于消散,飑线回波明显变宽、后倾,回波顶高显著下降。对模拟结果的定性分析和定量计算均表明,影响这次飑线过程发展维持的低层垂直风切变和冷池相互作用机制与Rotunno和Weisman等用来解释飑线发展演变的RKW理论一致。另外,模拟结果显示,低层0—3km风切变对飑线的发展维持最为重要,但是0—6km的中层风切变也有正面作用,特别是在飑线发展旺盛阶段,应该考虑其影响。     

Comparison of Estimated Atmospheric Boundary Layer Mixing Height in the Arctic and Southern Great Plains under Statically Stable Conditions: Experimental and Numerical Aspects

The atmospheric boundary layer mixing height (MH) is an important bulk parameter in air quality (AQ) modelling. Formulating this parameter under statically stable conditions, such as in the Arctic, has historically been difficult. In an effort to improve AQ modelling capacity in North America, MH is studied in two geographically distinct areas: the Arctic (Barrow, Alaska) and the southern Great Plains (Lamont, Oklahoma). Observational data from the Atmospheric Radiation Measurement program, Climate Research Facility and numerical weather forecasting data from Environment Canada's Regional Global Environmental Multiscale (GEM15) model have been used in order to examine the suitability of available parameterizations for MH under statically stable conditions and also to compare the level of agreement between observed and modelled MH. The analysis period is 1 October 2011 to 1 October 2012. The observations alone suggest that profile methods are preferred over surface methods in defining MH under statically stable conditions. Surface methods exhibit poorer comparison statistics with observations than profile methods. In addition, the fitted constants for surface methods are site-dependent, precluding their applicability for modelling under general conditions. The comparison of observations and GEM15 MH suggests that although the agreement is acceptable in Lamont, the default model surface method contributes to a consistent overprediction of MH in Barrow in all seasons. An alternative profile method for MH is suggested based on the bulk Richardson number. This method is shown to reduce the model bias in Barrow by a factor of two without affecting model performance in Lamont.