Use of a High-Resolution Sodar to Study Surface-layer Turbulence at Night

Measurements in the atmospheric surface layer are generally made with point sensors located in the first few tens of metres. In most cases, however, these measurements are not representative of the whole surface layer. Standard Doppler sodars allow a continuous display of the turbulent thermal structure and wind profiles in the boundary layer up to 1000 m, with a few points, if any, in the surface layer. To overcome these limitations a new sodar configuration is proposed that allows for a higher resolution in the surface layer. Because of its capabilities (echo recording starting at 2 m, echo intensity vertical resolution of approximately 2 m, temporal resolution of 1 s) this sodar is called the surface-layer mini-sodar (SLM-sodar). Features and capabilities of the SLM-sodar are described and compared with the sodar. The comparison of the thermal vertical structure given by the SLM-sodar and the sodar provides evidence that, in most cases, the surface layer presents a level of complexity comparable to that of the entire boundary layer. Considering its high vertical resolution, the SLM-sodar is a promising system for the study of the nocturnal surface layer. The nocturnal SLM-sodar measurements have shown that, depending on wind speed, the structure of the surface layer may change substantially within a short time period. At night, when the wind speed is greater than 3 m s⁻¹, mechanical mixing destroys the wavy structure present in the nocturnal layer. Sonic anemometer measurements have shown that, in such cases, also the sensible heat flux varies with height, reaching a peak in correspondence with the wind speed peak. Under these conditions the assumption of horizontal homogeneity of the surface layer and the choice of the averaging time need to be carefully treated.     

Water vapour flux profiles in the convective boundary layer

Summary Water vapour flux profiles in the atmospheric boundary layer have been derived from measurements of water vapour density fluctuations by a ground-based Differential Absorption Lidar (DIAL) and of vertical wind fluctuations by a ground-based Doppler lidar. The data were collected during the field experiment LITFASS-2003 in May/June 2003 in the area of Lindenberg, Germany. The eddy-correlation method was applied, and error estimates of ±50 W/m² for latent heat flux were found. Since the sampling error dominates the overall measurement accuracy, time intervals between 60 and 120 min were required for a reliable flux calculation, depending on wind speed. Rather large errors may occur with low wind speed because the diurnal cycle restricts the useful interval length. In the lower height range, these measurements are compared with DIAL/radar-RASS fluxes. The agreement is good when comparing covariance and error values. The lidar flux profiles are well complemented by tower measurements at 50 and 90 m above ground and by area-averaged near surface fluxes from a network of micrometeorological stations. Water vapour flux profiles in the convective boundary layer exhibit different structures mainly depending on the magnitude of the entrainment flux. In situations with dry air above the boundary layer a positive entrainment flux is observed which can even exceed the surface flux. Flux profiles which linearly increase from the surface to the top of the boundary layer are observed as well as profiles which decrease in the lower part and increase in the upper part of the boundary layer. In situations with humid air above the boundary layer the entrainment flux is about zero in the upper part of the boundary layer and the profiles in most cases show a linear decrease.     

A simple unified theory for flow in the canopy and roughness sublayer

The mean flow profile within and above a tall canopy is well known to violate the standard boundary-layer flux–gradient relationships. Here we present a theory for the flow profile that is comprised of a canopy model coupled to a modified surface-layer model. The coupling between the two components and the modifications to the surface-layer profiles are formulated through the mixing layer analogy for the flow at a canopy top. This analogy provides an additional length scale—the vorticity thickness—upon which the flow just above the canopy, within the so-called roughness sublayer, depends. A natural form for the vertical profiles within the roughness sublayer follows that overcomes problems with many earlier forms in the literature. Predictions of the mean flow profiles are shown to match observations over a range of canopy types and stabilities. The unified theory predicts that key parameters, such as the displacement height and roughness length, have a significant dependence on the boundary-layer stability. Assuming one of these parameters a priori leads to the incorrect variation with stability of the others and incorrect predictions of the mean wind speed profile. The roughness sublayer has a greater impact on the mean wind speed in stable than unstable conditions. The presence of a roughness sublayer also allows the surface to exert a greater drag on the boundary layer for an equivalent value of the near-surface wind speed than would otherwise occur. This characteristic would alter predictions of the evolution of the boundary layer and surface states if included within numerical weather prediction models.     

The Coupling State of an Idealized Stable Boundary Layer

The coupling state between the surface and the top of the stable boundary layer (SBL) is investigated using four different schemes to represent the turbulent exchange. An idealized SBL is assumed, with fixed wind speed and temperature at its top. At the surface, two cases are considered, first a constant temperature, 20 K lower than the SBL top, and later a constant 2 K h⁻¹ cooling rate is assumed for 10 h after a neutral initial condition. The idealized conditions have been chosen to isolate the influence of the turbulence formulations on the coupling state, and the intense stratification has the purpose of enhancing such a response. The formulations compared are those that solve a prognostic equation for turbulent kinetic energy (TKE) and those that directly prescribe turbulence intensity as a function of atmospheric stability. Two TKE formulations are considered, with and without a dependence of the exchange coefficients on stability, while short and long tail stability functions (SFs) are also compared. In each case, the dependence on the wind speed at the SBL top is considered and it is shown that, for all formulations, the SBL experiences a transition from a decoupled state to a coupled state at an intermediate value of mechanical forcing. The vertical profiles of potential temperature, wind speed and turbulence intensity are shown as a function of the wind speed at the SBL top, both for the decoupled and coupled states. The formulation influence on the coupling state is analyzed and it is concluded that, in general, the simple TKE formulation has a better response, although it also tends to overestimate turbulent mixing. The consequences are discussed.     

山西省大雾的中尺度数值模拟

利用中尺度模式MM5（The Fifth—Generation NCAR／Penn State Mesoscale Model）对山西省2009年发生的3场典型雾个例进行了数值模拟,探讨了物理过程参数化方案对雾数值模拟的影响,确定了基于模式模拟数据的雾判别指标,为该地区大雾数值预报系统的研制提供了理论基础。结果表明,综合考虑边界层方案和辐射方案对地表温度、高空温度、2m温度及相对湿度、10m风速、雾的空间分布、雾的生消过程、雾的发展高度等要素数值模拟的影响,边界层方案选用high-resolution planetary boundary layers cheme（HIR）方案、辐射方案选用Cloud方案时,雾数值模拟的结果与实况更为一致。综合分析多个典型雾个例的模拟结果,山西省境内雾的预报指标为：20m液态水含量为0．13～0．6g·kg^（-1）,20～1500m高度大气层存在逆温层,10m风速小于4m·s^（-1）。     

基于大涡模拟的小区气候态精细化风环境模拟试验

对具有复杂下垫面的小区精细化风环境进行数值模拟是当前城市气象研究的热点,而针对具有复杂地形的山地型城市（如重庆）的研究还比较匮乏。本文采用能显式分辨下垫面陡峭地形和复杂建筑物的计算流体力学（CFD）模式对重庆市渝北区龙湖社区气候态下的精细化风环境进行高分辨率的数值模拟。结果表明,下垫面能显著调节小区内风场的分布,风速大值区主要出现在九龙湖等开阔区域以及与中尺度背景入流方向一致的街道中。在夏季,小区整体风场以东南风为主,而其他3个季节则以偏东风为主。4个季节中,夏季小区内的风速最大,平均风速为0.3 m/s左右,局地能出现大于背景风的风速,可达0.8 m/s;其他3个季节的风速则较弱,区域平均的风速在0.2 m/s左右。不同的建筑物布局对局地风环境的影响也不同:单个孤立高层建筑迎风面的近地面存在明显地绕流,局地风速有所增加,而在背风面则形成尾流区,水平风速较低;在低矮分散的建筑群,建筑物的整体高度不高,区域内流场相对来说比较一致,风速较大,有利于小区的通风;在密集高层建筑群内,由于建筑物群本身的布局比较封闭,加之不同建筑物的环流场存在相互干扰及影响,使得小区近地面风速几乎为零,不利于小区通风和污染物扩散。建筑物的这些影响在城市冠层内尤为明显,高度越高这种影响越弱。     

Simulation of a Summer Urban Breeze Over Paris

Numerical simulations for an anticyclonic summer episode in the Paris area have been performed at the meso- scale for a 48-hour period, and compared to observations from a dense operational observational network. The meteorological stations have been classified, according to the extent of urbanization of their surroundings, into four classes (central Paris, urban, suburban, and rural). The atmospheric model, coupled with an urban surface scheme, correctly reproduces the temperature (within 1 K from the observations) and humidity. The intense urban heat island during the night is also well represented.Following the validation, the model is used to quantify atmospheric effects of Paris on the boundary layer, through a comparison with a purely rural simulation. At night, the model simulates a neutral or even slightly unstable boundary layer to a depth of 200 m over the city. In contrast, a very stable layer formed in the countryside. During the day, the boundary layer was more turbulent and 500 m deeper over Paris; vertical velocities of up to 1 m s^-1 were created over the city. This leads to an urban breeze with convergence at low levels (with winds around 5 to 7 m s^-1), and divergence at the boundary-layer top (with similar wind speeds). The horizontal extent of the breeze reaches for more than 50 km from the city centre, and could have an important impact on pollutant diffusion in the area for calm days.Finally, three other spring cases are presented briefly. These show that an urban breeze develops if the synoptic wind is weak enough or disorganized; an urban plume develops otherwise.     

An Analysis Of Secondary Circulations And Their Effects Caused By Small-Scale Surface Inhomogeneities Using Large-Eddy Simulation

A parallelized large-eddy simulation model has been used to investigate the effects of two-dimensional, discontinuous, small-scale surface heterogeneities on the turbulence structure of the convective boundary layer.Heterogeneities had a typical size of about the boundary-layer heightz_i. They were produced by a surface sensible heat flux pattern ofchessboard-type and of strong amplitude as typical, e.g., for the marginalice zone. The major objectives of this study were to determinethe effects of such strong amplitude heat flux variations and to specify theinfluence of different speeds and directions of the background wind.Special emphasis has been given to investigate the secondary circulations induced by the heterogeneities by means of three-dimensional phase averages.Compared with earlier studies of continuous inhomogeneities, the same sizeddiscontinuous inhomogeneities in this study show similar but stronger effects.Significant changes compared with uniform surface heating are only observedwhen the scale of the inhomogeneities is increased to z_i. Especially the vertical energy transport is much more vigorous and even the mean emperature profile shows a positive lapse rate within the whole mixed layer. However, the effects are not directly caused by the different shape of the inhomogeneities but can mainly be attributed to the large amplitude of the imposed heat flux,as it is typical for the partially ice covered sea during cold air outbreaks.The structure of the secondary flow is found to be very sensitive to the wavelength and shape of the inhomogeneities as well as to the heatflux amplitude, wind speed and wind direction. The main controlling parameter is the near-surface temperature distribution and the related horizontal pressure gradient perpendicular to the main flow direction. The secondary flow varies from a direct circulation with updraughts mainly above the centre of the heated regions to a more indirect circulation with updraughts beneath the centre and downdraughts above it. For background winds larger than 2.5 m s^–1 a roll-like circulation pattern is observed.From previous findings it has often been stated that moderate backgroundwinds of 5 m s^–1 eliminate all impacts of surface inhomogeneitiesthat could potentially be produced in realistic landscapes. However, this studyshows that the effects caused by increasing the wind speed stronglydepend on the wind direction relative to the orientation of theinhomogeneities. Secondary circulations remain strong, even for abackground wind of 7.5 m s^–1, when the wind direction is orientatedalong one of the two diagonals of the chessboard pattern. On the otherhand, the effects of inhomogeneities are considerably reduced, even undera modest background wind of 2.5 m s^–1, if the wind direction isturned by 45°. Mechanisms for the different flow regimesare discussed.     

北京冬季大风过程大气边界层特征分析

利用北京中国科学院大气物理研究所325 m气象观测塔的气象梯度资料和湍流资料,分析了2014年11月29日至12月5日北京两次大风过程中气象要素和湍流输送特征的变化。第一次大风过程的强度和持续时间均高于第二次大风过程。强烈的风速垂直切变主要集中在距地面100 m高度范围内,最强风速垂直切变达到0.31 s~(-1)。大风过程中,阵风系数呈现随高度减小的趋势,越接近地面,阵风系数愈大。阵风强度的变化与阵风系数相似,100 m以下高度时,阵风强度随高度增大而减小。大风过程自上而下改变边界层结构,平均动能、湍流动能和摩擦速度最先从上层(280 m)发生变化且迅速增加。近地层由于风速垂直梯度的显著差异,近地层垂直方向的湍流强度最大。大风时各功率谱在低频区(0.01 s~(-1))达到峰值,大风过后各高度的能量都有所下降。     

Some effects of deforestation on nocturnal drainage flow and local climate — A numerical study

The three-dimensional mesoscale model FITNAH has been modified to simulate effects of a tall tree canopy on airflow in complex terrain. Numerical experiments show the general features of meteorological variables inside a plant stand with low wind speeds and a nearly neutral thermal stratification during night. Available observations from the Finkenbach valley and the simulated temperatures near the ground show good agreeement.Comparison of model results for a nighttime situation for cases with and without a canopy (after complete deforestation) leads to the main results, viz., that surface wind speed will increase and the atmosphere near the ground will become colder after deforestation.However, the production rate of cooled air (expressed in m³m^–2h^–1) decreases; that means that forested slopes are more effective in ventilating a city than slopes covered with short vegetation.     

南疆沙漠腹地大气边界层气象要素廓线分析

利用塔中80m观测塔梯度系统采集的2006年8月、10月和2007年1月、4月的风、温度、湿度资料,结合气象站的同步气象资料,对南疆沙漠腹地近地层四季的晴天平均风速、温度、湿度廓线分布特征进行分析。结果表明,晴天平均风速白天随高度升高增加缓慢,夜间较快,低层风速白天比夜间大,高层则白天比夜间小,春夏季风速较大;四季平均温度廓线表现为夜间辐射型、早上过渡型、白天日射型和傍晚过渡型等四种类型,早、晚过渡时间四季各有不同,日最低、最高温度出现时间四季则相差不大;冬季夜间比湿随高度升高而增大,整个80m近地层表现为逆湿状态,其他季节逆湿一般出现在0．5—1m、1～2m、32—47m、63—80m等4个层次上,各逆湿层出现的时间各季节有所差异。     

Wind Profile Diagnosis from Surface Routine Meteorological Data Over a Coastal Area

A simple algorithm is proposed in order to transform routine surface wind speed observations near the coast to a wind at the height of the equilibrium planetary boundary layer as well as to any other height over a relatively flat coastal region. The model is based on the well known internal boundary layer (IBL) concept, Monin-Obukhov similarity theory and the resistance law, and describes the effects of the roughness transition from sea to land as well as the effect of stability on the shape of the profiles and the IBL growth. The required input weather data are no more than surface wind speed, air temperature and total cloud cover. Satisfactory agreement was found between measurements at Hellinikon airport and estimations made with the scheme. The introduction of a transition layer above the IBL did not improve the agreement to any significant extent. Mean values of the estimated wind differed by less than 1 m s ^-1 from the observed ones, a difference within the accuracy of the reported rawinsonde values. The rms error varied in the range of 17–22% of the observed average value, giving the best agreement under unstable conditions. The correlation coefficient between the observed and the estimated values of the wind, at the height of the equilibrium planetary boundary layer, ranged between 0.74 and 0.90.     

Wind regime in the lower atmosphere over Moscow from the long-term acoustic sounding data

Presented are the results of the sounding of the lower atmospheric 500-meter layer for the period of 2004–2012 carried out at the Meteorological Observatory of the Moscow State University (MSU) with the MODOS Doppler acoustic radar (sodar) produced by METEK (Germany). Discussed is the methodological basis of the sodar wind data analysis. It is demonstrated that in the air layer up to 200 m the maximum values in the annual course of the wind speed are observed more often in autumn and winter, and the minimum values, in summer; this is associated with the fact that during the cold period of the year Moscow is often located in the zones of intense gradient currents. The diurnal course of the wind speed is characterized by the daytime maximum and night-time minimum in the layer up to 40–60 m from the surface; it is poorly pronounced and characterized by the minimum in the morning in the layer of 80–120 m; and the daytime minimum and night-time maximum are observed above 140–160 m. The layer from 80 to 120 m approximately corresponds to the height of the wind rotation. The amplitude of diurnal variations of the wind speed increases from 0.3 m/s at the height of 7 m and 0.6 m/s at the height of 15 m, to 4.5 m/s at the height of 400 m; however, its secondary minimum (0.5 m/s) associated with the rotation height is registered at the altitude of 80 m. The statistical relationship between the wind speed and surface air temperature is direct during the cold season, inverse during the warm season, and is absent in April and October. The average maximum wind speed over Moscow for ten minutes in the layer up to 500 m from the surface reaches 30–35 m/s in some cases if two conditions concur: the capital is located on the periphery of vast pressure formations (usually of deep cyclones) and the local low-level jet stream is present in the wind profile.     

The wind in the baroclinic boundary layer with three sublayers incorporating the weak non-linear effect

In considering the weak non-linear effect, and using the small parameter expansion method, the analyt-ical expressions of the wind distribution within PBL (planetary boundary layer) and the vertical velocity at the top of the PBL are obtained when the PBL is divided into three layers and different eddy transfer coefficients K are adopted for the three layers. The conditions of barotropy and neutrality for the PBL are extended to that of baroclinity and non-neutral stratification. An example of a steady circular vortex is used to display the characteristics of the horizontal wind within the PBL and the vertical velocity at the top of the PBL. Some new results have been obtained, indicating that the magnitude of the speed in the lower height calculated by the present model is larger than that by the model in which k is a constant within the whole boundary layer, for example, in the classical Ekman boundary layer model and the model by Wu (1984). The angle between the wind at the top of the PBL and the wind near the surface calculated by the present model is less than that calculated by the single K model. These results are in agreement with the observations.     

Aspects Of The Atmospheric Surface Layers On Mars And Earth

The structures of mean flow and turbulence in the atmospheric surface boundary layer have been extensively studied on Earth, and to a far less extent on Mars, where only the Viking missions and the Pathfinder mission have delivered in-situ data. Largely the behaviour of surface-layer turbulence and mean flow on Mars is found to obey the same scaling laws as on Earth. The largest micrometeorological differences between the two atmospheres are associated with the low air density of the Martian atmosphere. Together with the virtual absence of water vapour, it reduces the importance of the atmospheric heat flux in the surface energy budget. This increases the temperature variation of the surface forcing the near-surface temperature gradient and thereby the diabatic heat flux to higher values than are typical on the Earth, resulting in turn in a deeper daytime boundary layer. As wind speed is much like that of the Earth, this larger diabatic heat flux is carried mostly by larger maximal values of T_*, the surface scale temperature. The higher kinematic viscosity yields a Kolmogorov scale of the order of ten times larger than on Earth, influencing the transition between rough and smooth flow for the same surface features.The scaling laws have been validated analysing the Martian surface-layer data for the relations between the power spectra of wind and temperature turbulence and the corresponding mean values of wind speed and temperature. Usual spectral formulations were used based on the scaling laws ruling the Earth atmospheric surface layer, whereby the Earth's atmosphere is used as a standard for the Martian atmosphere.     

Characteristics of Spatiotemporal Distribution of Sea Surface Wind along the East Coast of Guangdong Province

We analyzed the frequency distribution characteristics of wind speeds occurring at different offshore sites within a range of 0–200 km based on the sea surface wind data captured via buoys and oil platforms located along the east coast of Guangdong Province. The results of the analysis showed that average wind speed measured for each station reached a maximum in winter while minima occurred in summer, corresponding to obvious seasonal variation, and average wind speed increased with offshore distance. The prevailing wind direction at the nearshore site is the easterly wind, and the frequency of winds within 6–10 m s^–1 is considerable with that of winds at > 10 m s^–1. With the increase of the offshore distance, the winds were less affected by the land, and the prevailing wind direction gradually became northerly winds, predominately those at > 10 m s^–1. For areas of shorter offshore distance (< 100 km), surface wind speeds fundamentally conformed to a two-parameter Weibull distribution, but there was a significant difference between wind speed probability distributions and the Weibull distribution in areas more than 100 km offshore. The mean wind speeds and wind speed standard deviations increased with the offshore distance, indicating that with the increase of the wind speed, the pulsation of the winds increased obviously, resulting in an increase in the ratio of the mean wind speed to the standard deviation of wind speed. When the ratio was large, the skewness became negative. When a relatively great degree of dispersion was noted between the observed skewness and the skewness corresponding to the theoretical Weibull curve, the wind speed probability distribution could not be adequately described by a Weibull distribution. This study provides a basis for the verification of the adaptability of Weibull distribution in different sea areas.     

Modelling the Meteorology at the Cabauw Tower for 2005

The meteorology at the Cabauw tower site in the Netherlands has been modelled for 2005 using a local scale prognostic meteorological and air pollution model called TAPM. A number of performance measures have been used to assess model accuracy, including comparison statistics such as root-mean-square error (RMSE) and index of agreement (IOA). Results show that the model performs very well for prediction of wind and temperature at the six tower levels that range from 10 to 200 m above the ground, as well as performing well for radiation and surface fluxes. The model simulation shows almost no bias in mean and standard deviations of wind and temperature at each tower height level, with small RMSE (e.g. RMSE of 1.2 m s⁻¹ for 10-m wind speed, and 1.6°C for 10-m temperature) and high correlation and IOA (e.g. IOA of 0.92 for 10-m wind speed and 0.98 for 10-m temperature). Results for radiation and surface fluxes also show good performance, although some biases were seen for these variables, and possibilities for future model development were identified. An examination of model sensitivity also explored several aspects of the model configuration and input.     

登陆台风边界层风廓线特征的地基雷达观测

为了分析登陆台风边界层风廓线特征,利用2004—2013年中国东南沿海新一代多普勒天气雷达收集的17个登陆台风资料,采用飓风速度体积分析方法,反演登陆台风的边界层风场结构特征。与探空观测对比表明,利用雷达径向风场可以准确地反演登陆台风的边界层风场结构,其风速误差小于2 m/s,风向误差小于5°。所有登陆台风合成的边界层风廓线显示,在近地层(100 m)以上,边界层风廓线存在类似急流的最大切向风,其高度均在1 km以上,显著高于大西洋观测到的飓风边界层急流高度(低于1 km)。陆地边界层内低层入流强度也明显大于过去海上观测,这主要是由陆地上摩擦增大引起。越靠近台风中心,边界层风廓线离散度越大,其中,径向风廓线比全风速以及切向风廓线离散度更大。将风廓线相对台风移动方向分为4个象限,分析边界层风廓线非对称特征显示,台风移动前侧入流层明显高于移动后侧。最大切向风位于台风移动左后侧,而台风右后侧没有显著的急流特征,与过去理想模拟的海陆差异导致的台风非对称分布特征一致。     

Inertial oscillations near the coast of Nova Scotia during CASP

Abstract

Analysis of current, temperature and salinity records in the nearshore region of the Scotian Shelf during the Canadian Atlantic Storms Program (CASP), reveals that the inertial wave field is highly intermittent, with comparable amplitudes in the surface and deep layers. Clockwise current energy in the surface layer is concentrated at a frequency slightly below inertial, consistent with Doppler shifting by the strong mean current and/or straining by the mean flow shear, whereas the spectral peak in deep water is at the local inertial frequency. Clockwise coherence is high (γ² ≥ 0.8) horizontally over the scale of the array (60 km × 120 km) and in the vertical, with upward phase propagation rates of 0.15–0.50 × 10^?12 ms^?1, inversely proportional to the local value of the Brunt Väisälä frequency. Clockwise current energy decreases in the onshore direction and appears to be completely inhibited on the 60‐m isobath.

A case study of the response to the CASP IOP 14 storm indicates that the inertial waves may be generated by a strong wind shift propagating onshore at a speed of 10 ms^?1. On the eastern side of the array (Liscomb line), clockwise current oscillations propagate onshore in the surface layer at a rate (8.1 ± 0.9 m s^?1) comparable with the speed of the atmospheric front, while waves in the pycnocline move offshore at a lower (internal wave) speed (1.8 m s^?1). Furthermore the temperature and salinity fluctuations are in (out) of phase with longshore current in the deep (surface) layer. However, on the western side of the array (Halifax line), the inertial waves are more complex. A sharp steepening of phase lines at the coast indicates that the phase speed of clockwise current oscillations is considerably reduced and the evidence for offshore propagation of internal waves is less clear. The discrepancies between observations on the two lines suggest that the internal wave field is three‐dimensional.

Results of simple mixed‐layer models indicate that the inertial response near the surface is sensitive to the accurate definition of the local wind field, but not to certain model physics, such as the form of the decay term. The observations also show some qualitative similarities with models for two‐dimensional response to a moving front (e.g. Kundu, 1986), but the actual forcing terms are more complicated, based on IOP 14 wind measurements.