A Simple Parametrization for the Concentration Variance Dissipation in a Lagrangian Single-Particle Model

A new quasi-analytical mixed-layer model is formulated describing the evolution of the convective atmospheric boundary layer (ABL) during cold-air outbreaks (CAO) over polar oceans downstream of the marginal sea-ice zones. The new model is superior to previous ones since it predicts not only temperature and mixed-layer height but also the height-averaged horizontal wind components. Results of the mixed-layer model are compared with dropsonde and aircraft observations carried out during several CAOs over the Fram Strait and also with results of a 3D non-hydrostatic (NH3D) model. It is shown that the mixed-layer model reproduces well the observed ABL height, temperature, low-level baroclinicity and its influence on the ABL wind speed. The mixed-layer model underestimates the observed ABL temperature only by about 10 %, most likely due to the neglect of condensation and subsidence. The comparison of the mixed-layer and NH3D model results shows good agreement with respect to wind speed including the formation of wind-speed maxima close to the ice edge. It is concluded that baroclinicity within the ABL governs the structure of the wind field while the baroclinicity above the ABL is important in reproducing the wind speed. It is shown that the baroclinicity in the ABL is strongest close to the ice edge and slowly decays further downwind. Analytical solutions demonstrate that the \(\mathrm{e}\)-folding distance of this decay is the same as for the decay of the difference between the surface temperature of open water and of the mixed-layer temperature. This distance characterizing cold-air mass transformation ranges from 450 to 850 km for high-latitude CAOs.     

亚洲夏季风结构和变动与大气运动的斜压和正压特征:斜压模分析

通过定义流场的斜压性度量参量，利用观测资料，对大气运动的斜压特征量与夏季风变动的联系进行了探讨。结果表明：夏季风流场具有较强的斜压性；纬向气流的斜压强弱的时间变动与夏季风进退相吻合；强斜压区的范围可反映夏季风活动的基本范围。经向斜压运动强弱的水平分布及时间－经度剖面上斜压运动动能的纬向传播都与亚洲季风体系中季风子系统有关     

The Role of Southern High Latitude Wind Stress in Global Climate

In this paper, the role of westerly winds at southern high latitudes in global climate is investigated in a fully coupled ocean-atmosphere general circulation model. In the model, the wind stress south of 40°S is turned off with ocean and atmosphere fully coupled both locally and elsewhere. The coupled model explicitly demonstrates that a shutdown of southern high latitude wind stress induces a general cooling over the Antarctic Circumpolar Current (ACC) region, with surface Ekman flow and vertical mixing p...     

Geostrophic departure and the functions A and B of Rossby-number similarity theory

A new empirical assessment of the functions A and B of Rossby-number similarity theory is made based on the Wangara data. Variations of these functions with stability, baroclinicity and time of day are discussed. It is found that B is dependent on stability in agreement with older data but contradicting the prediction of Csanady (1972). Coefficients expressing the variation of A and B with the two components of baroclinicity have been derived from the data, and these are claimed to be correct in regard to sign and approximately in regard to magnitude. Longer period time changes, represented by the diurnal cycle, are shown to result in systematic differences in A and B between the case of increasing stability and that of decreasing stability, for the same value of the stability parameter.The first attempt, to our knowledge, to present the actual functional form of the wind departure components (based on field data) is made. As the surface layer is approached in near-neutral conditions, the departure component in the direction of the surface wind assumes the expected logarithmic form.     

Spectra of CO<Subscript>2</Subscript> and Water Vapour in the Marine Atmospheric Surface Layer

Spectra of CO₂ and water vapour fluctuations from measurements made in the marine atmospheric surface layer have been analyzed. A normalization of spectra based on Monin–Obukhov similarity theory, originally developed for wind speed and temperature, has been successfully extended also to CO₂ and humidity spectra. The normalized CO₂ spectra were observed to have somewhat larger contributions from low frequencies compared to humidity spectra during unstable stratification. However, overall, the CO₂ and humidity spectra showed good agreement as did the cospectra of vertical velocity with water vapour and CO₂ respectively. During stable stratification the spectra and cospectra displayed a well-defined spectral gap separating the mesoscale and small-scale turbulent fluctuations. Two-dimensional turbulence was suggested as a possible source for the mesoscale fluctuations, which in combination with wave activity in the vertical wind is likely to explain the increase in the cospectral energy for the corresponding frequency range. Prior to the analysis the turbulence time series of the density measurements were converted to time series of mixing ratios relative to dry air. Some differences were observed when the spectra based on the original density measurements were compared to the spectra based on the mixing ratio time series. It is thus recommended to always convert the density time series to mixing ratio before performing spectral analysis.     

Estimating the surface wind speed over drifting pack ice from surface weather charts

Routine surface wind speed data, vital for the study of drifting pack ice, are not available for the polar oceans. Over sea, it has been demonstrated by Hasse that estimates of surface wind speed may be obtained from the geostrophic wind speed using linear formulae. Comparing estimated with observed data for six sites in Canada, it is found that the formulae may also be applied to obtain estimates of surface wind speed over pack ice.     

A Study of the Stable Boundary Layer Based on a Single-Column K-Theory Model

We document numerical experiments with a single-column, high-resolution model of the stable boundary layer. The model resolves the logarithmic layer, and does not require inverting the Monin–Obukhov similarity functions in order to calculate the surface fluxes. The turbulence closure is based on the K-theory approach, with a new form of stability functions of the Richardson number, evaluated by using the Surface Heat Budget of the Arctic Ocean (SHEBA) and the Cooperative Atmosphere-Surface Exchange Study (CASES-99) data. A comparison with two, high-resolution large-eddy simulation models shows very good agreement. The reported numerical experiments test the effects of shear, surface cooling, the Coriolis parameter, subsidence, and baroclinicity. The time evolution of the drag coefficient, the heat-transfer coefficient, and the cross-isobar angle is also evaluated.     

Estimating the Surface Wind Speed Over Drifting Pack Ice from Surface Weather Charts

Routine surface wind speed data, vital for the study of drifting pack ice, are not available for the polar oceans. Over sea, it has been demonstrated by Hasse that estimates of surface wind speed may be obtained from the geostrophic wind speed using linear formulae. Comparing estimated with observed data for six sites in Canada, it is found that the formulae may also be applied to obtain estimates of surface wind speed over pack ice.     

东亚持续强冬季风影响赤道海表温度初始异常的数值试验研究

通过数值模拟和理论分析 ,文中指出在强东亚季风期间不仅在欧亚大陆和北印度洋出现强大的反气旋环流异常 ,而且通过海气相互作用在北太平洋西部和西北部形成异常气旋式流场 ;在其东南部产生异常反气旋式流场。在这种流场异常的驱动下赤道西太平洋西风加强 ,海面升高 ,海表温度上升 ,赤道中东印度洋和东太平洋东风加强 ,海面降低 ,海表温度下降。证明由于海表温度异常及海表温度变化趋势存在积分关系 ,因此持续的强东亚冬季风所强迫的沿赤道海表温度变化趋势的上述分布的强讯号可以在海洋中存在近一年之久 ,为尔后赤道太平洋 ENSO事件的可能发展提供初始条件 ,也为跨季度气候预测提供前期讯号     

冬季东亚-西太平洋西风急流基本结构及其异常的诊断分析

利用ECMWF再分析资料ERA40中的逐月风场和温度场资料,对冬季位于东亚-西太平洋上空的西风急流(WJS)基本结构和异常特征进行了分析.结果表明,一年当中最为强盛的冬季200 hPa WJS主体稳定地分布于东亚至西太平洋上空,向上向下均迅速减小.在对流层中高层,从东亚沿海到太平洋上空的西风系统较为深厚,低层对应着显著的大气斜压区.通过对东亚-西太平洋地区冬季200 hPa纬向风异常的EOF分解得到三种异常空间分布型,第一模态的中纬度西风异常主要发生于日界线以东的中东太平洋上空,第二、三模态则分别对应WJS主体发生增强/减弱和位置的北/南移动.WJS的这三种空间异常形态与中纬度低层大气斜压性的三种异常分布型相对应.通过对冬季WJS长期趋势的初步分析发现,近二十年来,洋面上空西风急流有增强趋势,这是由于急流轴南侧洋面上空大气存在显著增暖趋势、而急流轴北侧洋面上空却有变冷的趋势所造成的.     

Sea surface winds derived from cloud motion vectors over the tropical Atlantic

The relationship between satellite-derived low-level cloud motion, surface wind and geostrophic wind vectors is examined using GATE data. In the trades, surface wind speeds can be derived from cloud motion vectors by the linear relation: V = 0.62 V _s + 1.9 m s^–1 with a mean scatter of ±1.3 m s^–1. The correlation coefficient between surface and satellite wind speed is 0.25. Considering baroclinicity, i.e., the influence of the thermal wind, the correlation coefficient does not increase, because of the uncertainty of the thermal wind vectors. The ratios of surface to geostrophic wind speed and surface to satellite wind speed are 0.7 and 0.8, respectively, with a statistical uncertainty of ±0.3. Calculations of the ratio of surface to geostrophic wind speed on the basis of the resistance law yield V/V _g = 0.8 ± 0.2, in agreement with experimental results. The mean angle difference between the surface and the satellite wind vectors amounts to - 18 °, taking into account baroclinicity. This value is in good agreement with the mean ageostrophic angle - 25 °.     

The Influence of Transient Flow Distortion on Turbulence in Stable Weak-Wind Conditions

Transport by very weak turbulence near the surface, particularly with strong stratification, often violates existing similarity theory due partly to transient distortion of the wind profile by nonstationary mesoscale motions. This problem is examined with 4 months of flux data at seven levels on a 30-m tower. Difficulties with calculation of fluxes and shear with weak winds and weak turbulence are examined. The distortion of the wind profile assumes a wide variety of forms, but sometimes occurs with a transient wind maximum in the lowest 10 m. Though the vertical mixing of momentum is weak for weak-wind nonstationary conditions, it is often more efficient than predicted by existing similarity theory. In fact for weak-wind stable conditions, the flux-gradient relationship depends more on the curvature of the wind profile than the value of the stability parameter. This statistical study suggests a new direction for future work, but does not unravel the physics of the complex and varied structure for stratified weak-wind conditions.     

The structure of the convective atmospheric boundary layer as revealed by lidar and Doppler radars

Results on the structure of the convective atmospheric boundary layer based on the analyses of data from the instrumented NSSL-KTVY tower, airborne Doppler lidar, and ground-based Doppler radars are presented. The vertically averaged wind over the boundary layer was found to be insensitive to baroclinicity, supporting the hypothesis of Arya and Wyngaard (1975). The computed momentum flux profiles were affected by baroclinicity. Horizontal wind spectra from lidar, radar, and tower data compared well with each other both in shape and magnitude. A consistent peak found near 4 km in all the computed spectra might have been caused by horizontally symmetric cells with horizontal wavelength 4 times the boundary-layer height as shown in Kuettner (1971) for the case of weak wind shear.     

Regional features of precipitation over Asia and summer extreme precipitation over Southeast Asia and their associations with atmospheric–oceanic conditions

The variations of both total and extreme precipitations over Asia are characterized by large regional features and seasonality. Extreme precipitation mainly occurs in summer and then in autumn over South Asia but it is a prominent phenomenon in all seasons over Southeast Asia. It explains above 40% of the total precipitation in winter over India, while the ratio of extreme precipitation to total precipitation is 30% or smaller in all seasons over southern-central China. Over Southeast Asia, the largest ratio appears in winter. The extreme precipitation over Southeast Asia (EPSEA) exhibits significant positive trends in all seasons except autumn. The long-term increase in summer EPSEA is associated with significant surface warming over extratropical Asia and the Indo-Pacific oceans and linked to a large-scale anomalous cyclonic pattern over Southeast Asia. An increase in de-trended summer EPSEA is associated with less significant surface warming. However, it is still clearly linked to an anomalous cyclonic pattern over Southeast Asia, contributed by intensifications of monsoon flow from the west, trade wind from the east, and cross-equatorial flow over Indonesia. The antecedent features of increased summer EPSEA include an overall warming over the tropical–subtropical northern hemisphere and an anomalous cyclonic pattern over Southeast Asia in winter and spring. When the large-scale Asian monsoon (measured by the Webster-Yang monsoon index) or the South Asian monsoon is strong, summer extreme precipitation mainly increases over tropical Asia. When monsoon is strong over Southeast Asia or East Asia, extreme precipitation increases over Southeast Asia and decreases over East Asia. A strong summer monsoon over Southeast Asia or East Asia is also followed by decreased autumn extreme precipitation over Southeast Asia.     

我国北方地区一次沙尘暴天气特征分析

使用美国大气中心6h一次的NCEP再分析格点资料(1.0°×1.0°)对2002年3月18—22日发生在我国北方的大范围沙尘暴天气成因进行分析。从环流形势、物理量诊断、高空急流等方面进行研究分析,结果表明:蒙古气旋是这次沙尘暴天气的主要影响系统,这次气旋发生发展在斜压区,气旋的发展阶段温度平流作用明显。沙尘暴过程主要是由气旋冷锋及锋后地面大风触发的,地面大风的形成与气旋发展、锋后冷平流及高空急流动量下传有关。     

A subgrid-scale model for large-eddy simulation of planetary boundary-layer flows

A long-standing problem in large-eddy simulations (LES) of the planetary boundary layer (PBL) is that the mean wind and temperature profiles differ from the Monin-Obukhov similarity forms in the surface layer. This shortcoming of LES has been attributed to poor grid resolution and inadequate sub-grid-scale (SGS) modeling. We study this deficiency in PBL LES solutions calculated over a range of shear and buoyancy forcing conditions. The discrepancy from similarity forms becomes larger with increasing shear and smaller buoyancy forcing, and persists even with substantial horizontal grid refinement. With strong buoyancy forcing, however, the error is negligible.In order to achieve better agreement between LES and similarity forms in the surface layer, a two-part SGS eddy-viscosity model is proposed. The model preserves the usual SGS turbulent kinetic energy formulation for the SGS eddy viscosity, but it explicitly includes a contribution from the mean flow and a reduction of the contributions from the turbulent fluctuations near the surface. Solutions with the new model yield increased fluctuation amplitudes near the surface and better correspondence with similarity forms out to a distance of 0.1–0.2 times the PBL depth, i.e., a typical surface-layer depth. These results are also found to be independent of grid anisotropy. The new model is simple to implement and computationally inexpensive.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

1998梅雨锋的结构特征及形成与维持

应用1998年6月中旬的分析资料, 对1998年6月16~17日暴雨时段的梅雨锋结构及梅雨锋的形成与维持进行了诊断分析, 确认了暴雨发生时期梅雨锋结构的一些普遍特征, 如在对流层低层表现为θ_se锋而不是温度的强烈对比, 梅雨锋区是一个低层正涡度带以及风和水汽的辐合带等。同时揭示了1998年6月暴雨时期梅雨锋结构的典型特征:锋区从近地面可伸展到600 hPa层 (一般认为梅雨锋只存在于800 hPa以下), 近乎垂直、略向北倾; 锋区900 hPa以下是一个大气弱对流不稳定区, 向上气层变为潮湿中性直至400 hPa, 等θ_se线基本上呈垂直分布; 锋区斜压性相当弱等。研究表明, 出现强降水时的梅雨锋结构已经变性或者说它是介于温带锋面结构和ITCZ结构之间的副热带锋系结构。分析还认为, 在强盛的南海季风涌和频繁的西风带扰动组配的大尺度有利背景形势下, 低层空气的水平运动和地转偏差风对1998梅雨锋的形成和维持有明显的正贡献。     

Simulating tropical instability waves in the equatorial eastern Pacific with a coupled general circulation model

Satellite observations of SSTs have revealed the existence of unstable waves in the equatorial eastern Pacific and Atlantic oceans. These waves have a 20-40-day periodicity with westward phase speeds of 0.4-0.6 m s-1 and wavelengths of 1000-2000 km during boreal summer and fall. They are generally called tropical instability waves (TIWs). This study investigates TIWs simulated by a high-resolution coupled atmosphere-ocean general circulation model (AOGCM). The horizontal resolution of the model is 120 km in...     

Estimation of the Exchange Coefficient of Heat During Low Wind Convective Conditions

For the first time, the exchange coefficient of heat C_H has been estimated from eddy correlation of velocity and virtual temperature fluctuations using sonic anemometer measurements made at low wind speeds over the monsoon land atJodhpur (26°18' N, 73°04' E), a semi arid station. It shows strong dependence on wind speed, increasing rapidly with decreasing wind speed, and scales according to a power law C_H = 0.025U₁₀ ^-0.7 (where U₁₀ is the mean wind speed at 10-m height). A similar but more rapid increase in the drag coefficient C_Dhas already been reported in an earlier study. Low winds (<4 m s^-1) are associated with both near neutral and strong unstable situations. It is noted that C_H increases with increasing instability. The present observations best describe a low wind convective regime as revealed in the scaling behaviour of drag, sensible heat flux and the non-dimensional temperature gradient. Neutral drag and heat cofficients,corrected using Monin–Obukhov (M–O) theory, show a more uniform behaviour at low wind speeds in convective conditions, when compared with the observed coefficients discussed in a coming paper.At low wind convective conditions, M-O theory is unable to capture the observed linear dependence of drag on wind speed, unlike during forced convections. The non-dimensional shear inferred from the present data shows noticeable deviations from Businger's formulation, a forced convection similarity. Heat flux is insensitive to drag associated with weak winds superposed on true free convection. With heat flux as the primary variable, definition of new velocity scales leads to a new drag parameterization scheme at low wind speeds during convective conditionsdiscussed in a coming paper.