A multi-limit formulation for the equilibrium depth of a stably stratified boundary layer

Currently no expression for the equilibrium depth of the turbulent stably-stratified boundary layer is available that accounts for the combined effects of rotation, surface buoyancy flux and static stability in the free flow. Various expressions proposed to date are reviewed in the light of what is meant by the stable boundary layer. Two major definitions are thoroughly discussed. The first emphasises turbulence and specifies the boundary layer as a continuously and vigorously turbulent layer adjacent to the surface. The second specifies the boundary layer in terms of the mean velocity profile, e.g. by the proximity of the actual velocity to the geostrophic velocity. It is shown that the expressions based on the second definition are relevant to the Ekman layer and portray the depth of the turbulence in the intermediate regimes, when the effects of static stability and rotation essentially interfere. Limiting asymptotic regimes dominated by either stratification or rotation are examined using the energy considerations. As a result, a simple equation for the depth of the equilibrium stable boundary layer is developed. It is valid throughout the range of stability conditions and remains in force in the limits of a perfectly neutral layer subjected to rotation and a rotation-free boundary layer dominated by surface buoyancy flux or stable density stratification at its outer edge. Dimensionless coefficients are estimated using data from observations and large-eddy simulations. Well-known and widely used formulae proposed earlier by Zilitinkevich and by Pollard, Rhines and Thompson are shown to be characteristic of the above interference regimes, when the effects of rotation and static stability (due to either surface buoyancy flux, or stratification at the outer edge of the boundary layer) are roughly equally important.     

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.     

高山草甸下垫面夏季近地层能量输送及微气象特征

利用青藏高原东坡理塘站2007年6～8月的观测资料, 分析了高原东坡草甸下垫面夏季近地层气象要素和湍流通量日变化特征, 并用涡动相关法估算地面的曳力系数。结果表明: 水平风速、 动量通量、 摩擦速度等均在下午最大, 早晨最小。二氧化碳浓度表现为早晚高、 中午低的日变化特征, 比湿的最大值出现在早晨。地表辐射、 热量平衡各分量最高值出现在中午, 最低值出现在早晨。地表反照率表现出早晚高中午低的 “U” 型分布, 日平均值为0.164。夏季地面热源强度在白天午后表现为强的热源, 在夜里表现为弱的冷、 热源交替出现。夏季近地层地气热量交换中, 感热输送作用小, 潜热输送占主要地位。     

Bulk Formulation of the Surface Heat Flux

An interpretive literature survey examines different approachesfor applying the bulk aerodynamic formulato predict the surface heat flux. The surface heat flux is often predicted in terms of the surface radiation temperature, which is also used to predict the upward longwave radiation and the heat flux into the soil. In models, the thermal roughness length based on the surface radiation temperature (radiometric roughness length) is often specified to be smaller than the roughness length for momentum for a number of distinct reasons. The definition of the radiometric roughness length depends on the way that the surface temperature is measured, the choice of stability functions and displacement height and inclusion of any additional resistances.Using airborne eddy correlation data collected over eight different sites including bare soil, crops and grassland and several types of forests, the radiometric roughness length is found to vary by orders of magnitude in a manner that is difficult to formulate. Alternatively, we evaluate the approach where the thermal roughness length is equated with the better behaved roughness length for momentum and the corresponding aerodynamic surface temperature is modelled in terms of the surface radiation temperature, solar radiation, and vegetation index. The influence of wind speed and soil moisture on the difference between the aerodynamic and surface radiation temperatures is also examined.     

On the Temporal Distribution of Bursts and Events in the Stable Marine Atmospheric Surface Layer

A methodology for tracing the bursting phenomenon occurring in the turbulent boundary-layer is applied to stable marine atmospheric boundary-layer (MABL) data, collected within the framework of the Coupled Boundary Layer Air-Sea Transfer, Low wind component experiment at Nantucket Island, 2003, Massachusetts, USA. For the definition of the events contributing to the bursting phenomenon, the quadrant analysis of the momentum flux with varying threshold value is utilized. The present work aims at the better understanding of the time scales of the microstructures in the MABL, by providing the number of the events and the groups and their mean duration and the time difference of all quadrants of the quadrant analysis, for different thresholds and under different meteorological conditions. The interrelation of the different time scales as well as the correlation between the quadrants are investigated and discussed. Also, it is demonstrated that the procedure of grouping events leads to bursts with a mean duration that is independent of the threshold value, for the most important quadrants of the momentum transfer (2 and 4). It is found that the combination of near-neutral conditions with low wind speed permits the development of larger duration events within the stable MABL. Finally, it is shown that the groups of events maintain adequate time and flux coherence only for quadrants 2 and 4.     

南亚夏季风爆发前后青藏高原地表热通量的长期变化特征分析

青藏高原作为世界第三极,其热力强迫作用不仅对亚洲季风系统的发展和维持十分重要,也会对大气环流场产生深远影响。利用欧洲中期天气预报中心（ECMWF）的ERA-Interim中1979-2016年3-10月青藏高原及其周边地区的地表热通量月平均再分析资料,通过分析得出以下结论:3-5月青藏高原主体由感热占据,感热强度快速上升且呈西高东低的分布态势,潜热强度较小但随时间而增强。季风爆发后的6-8月,青藏高原感热强度减弱,潜热强度迅速增强且呈东高西低的分布特征。季风消退后的9-10月,感热与潜热强度相当,但感热呈现出西高东低的分布特征。过去38年,青藏高原地表感热总体呈现微弱下降趋势,潜热呈较弱上升趋势。青藏高原西部地区感热呈微弱下降趋势,潜热呈上升趋势。东部感热呈较为明显的下降趋势且近年来变化趋势增强,东部潜热通量则呈现较为明显的上升趋势,分析结论与近期全球变暖条件下青藏高原气候变暖变湿这一变化状况一致,通过对青藏高原地表热通量的变化分析为下一步运用第三次青藏高原大气科学试验所获资料分析青藏高原上空大气热源的变化以及地表加热场如何影响大气环流奠定基础。      

丘陵区土壤热通量遥感估算模型适应性分析

利用具有丘陵区典型特征的四川省乐至县气象站的土壤热通量和净辐射观测数据,分析了二者的变化特征,并验证了土壤热通量遥感估算模型的适用性。分析结果表明：晴空土壤热通量G与净辐射Rn存在明显日变化,最大值出现13时左右;其比值G／Rn受土壤湿润程度和地表覆盖的影响,地表湿润、覆盖率高,比值小。在干旱时,瞬时比值可高达0．7,而湿润情况,可低至0．05。对目前广泛使用的G／Rn卫星遥感模型估算结果与实测值的对比分析表明,不同的G／Rn卫星遥感估算模型估算结果存在明显的差异,仅依靠植被指数的模型不适合南方丘陵区。本研究认为Bastiaanssen等的模型较适合丘陵区的土壤热通量遥感估算模型。     

Large-eddy simulations of thermally forced circulations in the convective boundary layer. Part II: The effect of changes in wavelength and wind speed

This paper extends previous large-eddy simulations of the convective boundary layer over a surface with a spatially varying sensible heat flux. The heat flux variations are sinusoidal and one-dimensional. The wavelength is 1500 or 4500 m (corresponding to 1.3 and 3.8 times the boundary-layer depth, respectively) and the wind speed is 0, 1 or 2 m s^-1.In every case the heat flux variation drives a mean circulation. As expected, with zero wind there is ascent over the heat flux maxima. The strength of the circulation increases substantially with an increase in the wavelength of the perturbation. A light wind weakens the circulation drastically and moves it downwind. The circulation has a significant effect on the average concentration field from a simulated, elevated source.The heat flux variation modulates turbulence in the boundary layer. Turbulence is stronger (in several senses) above or downwind of the heat flux maxima than it is above or downwind of the heat flux minima. The effect remains significant even when the mean circulation is very weak. There are effects too on profiles of horizontal-average turbulence statistics. In most cases the effects would be undetectable in the atmosphere.We consider how the surface heat flux variations penetrate into the lower and middle boundary layer and propose that to a first approximation the process resembles passive scalar diffusion.The research reported in this paper was conducted while the first author was on study leave at Colorado State University.     

Three-Dimensional Calculation of Photolysis Frequencies in the Presence of Clouds and Impact on Photochemistry

For atmospheric photochemistry, clouds can significantly affect actinic flux distributions. In this paper, we examine the effects of convective clouds on the three-dimensional distribution of the spectral actinic flux and on photolysis frequencies for various chemical species. Three-dimensional solutions of the UV-VIS radiative transfer equation are produced using the Spherical Harmonic Discrete Ordinary Method solution technique. This solver uses as input the 3-D cloud characteristics simulated by a dynamical cloud model. The ultraviolet and visible spectra are divided into 5 intervals in order to explore the wavelength dependency of the cloud effect on the actinic flux. Results show that the distribution of the actinic flux over the cloud domain is far from homogeneous and depends primarily on the cloud extinction associated with the hydrometeors. Maximum actinic flux is found at the top edge of the cloud and is related to scattering by ice crystals. The actinic flux is enhanced by a factor of 2 to 5, compared to clear air values, above, at the top edge, and around the cloud. The 3-D actinic flux is used to calculate the photolysis rates for some chemical species (e.g. NO₂, O₃, and HCHO). Forcomputing photolysis rates, a discretized spectral representation of the absorption wavelengths is used in the model. The calculated photolysis rates are distributed inhomogeneously throughout the cloud, and maxima are found in regions where the actinic flux is enhancement is large. Temperature effects on absorption are found in the photolysis frequencies of some species. Finally, the potential importance of this photolysis enhancement on photochemistry is studied using box model simulations. Results show that enhanced OH concentrations are found in the upper troposphere (120–200%) overthe clouds and changes in ozone production rates (+15%) are obtained in quasi-steady state conditions.     

热带西太平洋海气交换系数订正

在计算海-气感热通量、潜热通量和动量通量时，因气象要素的测量高度不一致而带来的热交换系数cT、蒸发系数cE和摩擦系数 cυ的偏差，给计算结果带来一定的影响。文章介绍一种高度订正方法，对cT、cE、cυ的值加以订正，从而提高了计算准确性。计算结果表明，系数订正后的误差可减小9%—11%。     

南极瑞穗站雪面热量平衡特征

利用南极瑞穗站(日本)1979年近地面层微气象资料及净辐射、本站气压等资料进行了统计分析。采用鲍文比-能量平衡法求得月平均感热通量和潜热通量,采用热含法计算得出月平均雪面热通量,使用了直接测量的净辐射通量,研究了该站雪面热量平均特征。其中突出的特征是3—12月雪面净辐射值为负值,主要靠感热通量由大气向雪面补充热量。将本文结果与苏联少先队站1956年的结果进行了比较,得出相当一致的年变化规律。最后,得出瑞穗站雪面为一强冷源。     

On the relationship between the eddy covariance,the turbulent flux,and surface exchange for a trace gas such as CO<Subscript>2</Subscript>

In the context of CO₂ surface exchange estimation, an analysis combining the basic principles of diffusion and scalar conservation shows that the mixing ratio is the appropriate variable both for defining the (eddy covariance) turbulent flux and also for expressing the relationship between the turbulent flux and surface exchange in boundary-layer budget equations. Other scalar intensity variables sometimes chosen, both the CO₂ density and molar fraction, are susceptible to the influence of surface exchange of heat and water vapour. The application of a hypsometric analysis to the boundary-layer “control volume” below the tower measurement height reveals flaws in previously applied approaches: (a) incompressibility cannot be assumed to simplify mass conservation (the budget in terms of CO₂ density); (b) compressibility alone makes the analysis of mass conservation vulnerable to uncertainties associated with resultant non-zero vertical velocities too small to measure or model over real terrain; and (c) the WPL (Webb et al. (1980) Quart J Roy Meteorol Soc 106:85–100) “zero dry air flux” assumption is invalidated except at the surface boundary. Nevertheless, the definition and removal of the WPL terms do not hinge upon this last assumption, and so the turbulent CO₂ flux can be accurately determined by eddy covariance using gas analysers of either open- or closed-path design. An appendix discusses the necessary assumptions and appropriate interpretations for deriving the WPL terms.     

Where does all the energy go? Surface energy partitioning in suburban Christchurch under stable wintertime conditions

Summary Typical observations of surface energy fluxes in urban areas usually employ the eddy covariance approach with measurements from a tower at a height well above the roughness elements. Net radiation and the turbulent fluxes are directly measured, the anthropogenic flux may be estimated and the storage flux is calculated as an energy balance residual. This paper reports both measurement and modelling of energy fluxes during wintertime in suburban areas of Christchurch, New Zealand. Under settled anticyclonic conditions, a strong inversion can occur over the city which severely restricts turbulent mixing. Even after sunrise the turbulent fluxes are small, and if one assumes advection is negligible, this means the storage flux increases in importance to very high levels. This paper suggests that these high storage rates are physically unrealistic for this environment. Rather, it is likely that some energy, which is assumed to be dissipated as heat storage, is more likely lost through mesoscale advection or attributed to errors caused by unsuitable measurement techniques under conditions with low friction velocity. However, even these two processes cannot fully account for flux loss. A full study is recommended to resolve this issue and meanwhile, caution is advised when applying current research methodology to estimate storage fluxes in urban areas in stable wintertime conditions.     

On the dependence of ENSO simulation on the coupled model mean state

Systematic model error remains a difficult problem for seasonal forecasting and climate predictions. An error in the mean state could affect the variability of the system. In this paper, we investigate the impact of the mean state on the properties of ENSO. A set of coupled decadal integrations have been conducted, where the mean state and its seasonal cycle have been modified by applying flux correction to the momentum-flux and a combination of heat and momentum fluxes. It is shown that correcting the mean state and the seasonal cycle improves the amplitude of SST inter-annual variability and also the penetration of the ENSO signal into the troposphere and the spatial distribution of the ENSO teleconnections are improved. An analysis of a multivariate PDF of ENSO shows clearly that the flux correction affects the mean, variance, skewness and tails of the distribution. The changes in the tails of the distribution are particularly noticeable in the case of precipitation, showing that without the flux correction the model is unable to reproduce the frequency of large events. For the inter-annual variability the momentum-flux correction alone has a large impact, while the additional heat-flux correction is important for the teleconnections. These results suggest that the current forecasts practices of removing the forecast bias a-posteriori or anomaly initialisation are by no means optimal, since they can not deal with the strong nonlinear interactions. A consequence of the results presented here is that the predictability on annual time-ranges could be higher than currently achieved. Whether or not the correction of the model mean state by some sort of flux correction leads to better forecasts needs to be addressed. In any case, flux correction may be a powerful tool for diagnosing coupled model errors and predictability studies.     

青藏高原东部草甸下垫面涡旋相关观测数据的质量控制及评价研究

目前利用涡旋相关仪观测地表通量的方法已被广泛采用, 但由于涡旋相关法是建立在方程假设简化的基础上, 这对观测数据的质量提出了一定的要求, 所以对观测数据的处理及通量结果的质量状况分析就显得尤为重要。本文以理塘站2006年7月中到8月中的数据为例, 对其进行质量控制, 并将质量控制后的通量计算结果与原始值进行比较。质量控制后感热通量和潜热通量总和有所增长, 动量通量值则受质量控制影响较大。质量评价的结果为:动量通量、感热通量、潜热通量中高质量数据所占比例为68.2%、60.6%、63.3%。     

青藏高原西部地表热通量输送的低频特征

以1997年9月-1998年10月青藏高原西部改则地区自动气象站(AWS)近地层连续观测的梯度资料为基础,计算了高原西部地面感热通量、蒸发潜热通量及地面热源强度,应用Marr小波变换重点分析了地表热通量输送以及与此相关的降水量、土壤湿度和土壤热通量的周期振荡特征.结果表明:地面感热具有明显的30～60天低频振荡,并且在夏季存在准8天的中期振荡;蒸发潜热和降水量以准双周振荡为主.土壤热通量以30～50天低频振荡为主,夏季还存在准8天的中期振荡;土壤湿度在冬季呈现明显的30～50天低频振荡,夏季则为20～30天的低频振荡.     

白洋淀水陆不均匀地区能量平衡特征分析

应用2005年9月在河北白洋淀地区进行的大气边界层综合观测实验资料, 对水陆不均匀地表条件下的白洋淀地区陆地的能量平衡特征进行了分析。结果表明: (1) 该地区存在能量不闭合现象。涡动相关法得到的感热、 潜热之和仅为有效能的75%, 其中涡动相关法得到的潜热通量为Bowen比法得到的潜热通量的70%, 而涡动相关法得到的感热通量为Bowen比法得到的感热通量的77% 。 (2) 地表潜热通量和感热通量随着净辐射的变化而变化。但潜热通量明显比感热通量大, 净辐射主要消耗于地表的水汽蒸发。 (3) 该地区白天的Bowen比平均在-0.4~0.4之间, 总体平均为0.131。受天气条件影响较大, 有明显的日变化, 午后15:00以后近地面层会出现逆温, Bowen比变为负值。 (4) 能量闭合程度有一定的日变化, 随着太阳高度角的增大而增大。     

塔中涡动资料不同坐标旋转方法比较分析

坐标旋转是涡动协方差(EC)通量观测数据后处理当中必须进行的步骤,本文利用塔克拉玛干沙漠腹地塔中西面标校站自然沙丘上3 m高度和塔中本站32 m高度2009年4—10月湍流通量数据,对比分析了不同坐标旋转方法在塔中通量观测数据中的应用。结果表明,塔克拉玛干沙漠腹地塔中湍流各通量在旋转订正前后差别不显著,不订正造成的通量计算误差1%。塔中32 m和塔中西站3 m通量计算误差,与两个站点的采集仪器架设高度差别较大有关。即使对于相对平坦的下垫面,也必须要进行倾斜校正,以修正地势、仪器振动、安装倾斜等产生的虚假信息。本研究对塔中涡动资料进行旋转时采用了二次坐标旋转方法。此结果为塔克拉玛干沙漠腹地EC通量观测的风速仪倾斜校正提供参考。     

A Parameterization of Dry Thermals and Shallow Cumuli for Mesoscale Numerical Weather Prediction

For numerical weather prediction models and models resolving deep convection, shallow convective ascents are subgrid processes that are not parameterized by classical local turbulent schemes. The mass flux formulation of convective mixing is now largely accepted as an efficient approach for parameterizing the contribution of larger plumes in convective dry and cloudy boundary layers. We propose a new formulation of the EDMF scheme (for Eddy Diffusivity\Mass Flux) based on a single updraft that improves the representation of dry thermals and shallow convective clouds and conserves a correct representation of stratocumulus in mesoscale models. The definition of entrainment and detrainment in the dry part of the updraft is original, and is specified as proportional to the ratio of buoyancy to vertical velocity. In the cloudy part of the updraft, the classical buoyancy sorting approach is chosen. The main closure of the scheme is based on the mass flux near the surface, which is proportional to the sub-cloud layer convective velocity scale w _*. The link with the prognostic grid-scale cloud content and cloud cover and the projection on the non- conservative variables is processed by the cloud scheme. The validation of this new formulation using large-eddy simulations focused on showing the robustness of the scheme to represent three different boundary layer regimes. For dry convective cases, this parameterization enables a correct representation of the countergradient zone where the mass flux part represents the top entrainment (IHOP case). It can also handle the diurnal cycle of boundary-layer cumulus clouds (EUROCS\ARM) and conserve a realistic evolution of stratocumulus (EUROCS\FIRE).