A numerical study of the marine stratocumulus cloud layer

A one-dimensional grid-level model including longwave radiative transfer and a level-4 second-order turbulent transfer closure which contains prognostic equations for turbulent quantities, is used to study the physics and dynamics of inversion-capped marine stratocumulus clouds.A set of numerical experiments had been performed to examined the role of sea surface temperature, large-scale vertical velocity, wind speed, and vertical wind shear in the formation and the structure of low-level clouds. For a given sea surface and geostrophic wind speed, stratocumulus clouds can grow higher with smaller large-scale subsidence as less dry air entrains into the cloud. Clouds grow higher with higher sea surface temperature for a given geostrophic wind speed and large-scale subsidence as a result of enhanced moist convection. In high wind speeds, the entire cloud deck is lifted up because of larger surface energy flux. In the budget studies of the turbulent kinetic energy (TKE), the buoyancy term is a major source term when the wind speed and the vertical shear are small across the inversion top. When the wind speed and the vertical wind shear across the inversion top become large, the mixed layer is decoupled into a cloud and a subcloud layer. In the TKE budget studies, the shear generation term becomes an important term in the budgets of the TKE and the variance of vertical velocity.     

Current problems in the stratocumulus-topped atmospheric boundary layer

Extended sheets of stratocumulus (Sc) in the upper part of the atmospheric boundary layer (ABL) often occur under appropriate meteorological conditions. These cloud decks are important both in climate studies and in weather forecasting. We review the current knowledge of the turbulent structure of the ABL capped by a cloud deck, in the light of recent observations and model studies. The most important physical processes determining this structure are longwave radiative cooling at cloud top, shortwave radiative wanning by absorption in the cloud, surface buoyancy flux, and wind shear in the ABL. As a result, turbulence can cause entrainment against the buoyancy jump at cloud top. In cases where only longwave radiative fluxes and surface buoyancy fluxes are important, the turbulent structure is relatively well understood. When shortwave radiative fluxes and/or wind shear are also important, the resulting turbulent structure may change considerably. A decoupling of the cloud from the sub-cloud layer or of the top of the cloud from the rest of the ABL is then regularly observed. In no cases are the details of the entrainment at cloud top understood well enough to derive a relatively simple formulation that is consistent with observations. Cloud-top entrainment instability may lead to the break-up of a cloud deck (but also to cloud deepening). The role of mesoscale circulations in determining fractional cloudiness is not yet well understood.     

Turbulence intensity and turbulent kinetic energy parameters over a heterogeneous terrain of Loess Plateau

A deep understanding of turbulence structure is important for investigating the characteristics of the atmospheric boundary layer, especially over heterogeneous terrain. In the present study, turbulence intensity and turbulent kinetic energy (TKE) parameters are analyzed for different conditions with respect to stability, wind direction and wind speed over a valley region of the Loess Plateau of China during December 2003 and January 2004. The purpose of the study is to examine whether the observed turbulence intensity and TKE parameters satisfy Monin-Obukhov similarity theory (MOST), and analyze the wind shear effect on, and thermal buoyancy function of, the TKE, despite the terrain heterogeneity. The results demonstrate that the normalized intensity of turbulence follows MOST for all stability in the horizontal and vertical directions, as well as the normalized TKE in the horizontal direction. The shear effect of the wind speed in the Loess Plateau region is strong in winter and could enhance turbulence for all stability conditions. During daytime, the buoyancy and shear effect together constitute the generation of TKE under unstable conditions. At night, the contribution of buoyancy to TKE is relatively small, and mechanical shearing is the main production form of turbulence.     

A wind tunnel study of air flow in waving wheat: Single-point velocity statistics

We analyse single-point velocity statistics obtained in a wind tunnel within and above a model of a waving wheat crop, consisting of nylon stalks 47 mm high and 0.25 mm wide in a square array with frontal area index 0.47. The variability of turbulence measurements in the wind tunnel is illustrated by using a set of 71 vertical traverses made in different locations, all in the horizontally-homogeneous (above-canopy) part of the boundary layer. Ensemble-averaged profiles of the statistical moments up to the fourth order and profiles of Eulerian length scales are presented and discussed. They are consistent with other similar experiments and reveal the existence of large-scale turbulent coherent structures in the flow. The drag coefficient in this canopy as well as in other reported experiments is shown to exhibit a characteristic height-dependency, for which we propose an interpretation. The velocity spectra are analysed in detail; within and just above the canopy, a scaling based on fixed length and velocity scales (canopy height and mean horizontal wind speed at canopy top) is proposed. Examination of the turbulent kinetic energy and shear stress budgets confirms the role of turbulent transport in the region around the canopy top, and indicates that pressure transport may be significant in both cases. The results obtained here show that near the top of the canopy, the turbulence properties are more reminiscent of a plane mixing layer than a wall boundary layer.     

A first order mixing–condensation scheme for nocturnal stratocumulus

A simple closure scheme for nocturnal stratocumulus is proposed. The scheme is formulated in conserved variables. Cloud fraction and cloud water amount are diagnosed assuming a top-hat distribution for total water. Conversion of cloud water into rain water is parameterized in terms of cloud water and the incoming rain flux. Turbulence transport in the cloud layer is accounted for by a first-order vertical diffusion scheme with a profile-type diffusivity. The length scale corresponds to the thickness of the cloud layer. The turbulent velocity scale is directly related to the long wave radiative flux divergence in the cloud. Entrainment at cloud top is implicitly treated by extending the in-cloud mixing profile slightly beyond cloud top. The excess height is derived from the buoyancy frequency at cloud top and a radiative–convective velocity scale. The scheme is capable of simulating realistic profiles of the conserved variables and cloud parameters for a case of nocturnal stratocumulus prepared on the basis of ASTEX data.     

2010年春季北京地区强沙尘暴过程的微气象学特征

利用北京大学校园地区PM10质量浓度观测资料、中国科学院大气物理研究所325m气象塔气象要素梯度和湍流观测资料,分析了北京地区2010年3月20～22日两次强沙尘暴过程微气象学要素和沙尘参量的时空演变以及湍流输送特征,为理解北京地区强沙尘暴天气沙尘输送规律和微气象学特征提供参考。结果表明:3月20～22日强沙尘暴过程前后不同高度温度先升后降,气压和相对湿度则相反。强沙尘暴来临时,高层风速先迅速增大,低层风速增加略有滞后,风切变明显加强,PM10浓度最大值和风速极大值出现时间较吻合。强沙尘暴过境时,不同高度向下的湍流动量输送、向上的湍流热量输送和湍流动能明显加强。与3月21日非沙尘暴日相比,强沙尘暴过程湍流动量通量增加,有利于沙尘粒子的水平和垂直输送过程;由于冷锋过境,水平热通量增大;垂直热通量因白天温度垂直梯度减小而减小,夜间因逆温层被破坏而增加;水平湍流动能对湍流动能占主要贡献,垂直湍流动能仅占水平湍流动能的10%～25%。     

Role of Shear and the Inversion Strength During Sunset Turbulence Over Land: Characteristic Length Scales

The role of shear and inversion strength on the decay of convective turbulence during sunset over land is systematically studied by means of large-eddy simulations. Different decay rates have been found for the vertical and horizontal velocity fluctuations, resulting in an increase of the anisotropy for all the studied cases. Entrainment, which persists during the decay process, favours the appearance of vertical upward movements associated with a conversion from kinetic to potential energy. Particular attention is paid to the evolution of the characteristic length scale of the various turbulent variables during this process. The length scale evolution is found to depend on the wind shear characteristics, but not on the strength of the inversion. In general the length scales of the variables grow during decay because small-scale fluctuations dissipate faster than large-scale fluctuations. Only the length scale of the vertical velocity component remains nearly constant during decay. Spectral analysis of the variance budgets shows that pressure correlations are responsible for fixing this length scale, effectively compensating the strong but oscillating influence of buoyancy. In the shear cases, after an initial period of growth, the length scales start to decrease once the buoyancy-generated variance has sufficiently subsided. Also here the effect of pressure redistribution is crucial, as it transfers the spectral influence of shear to the other velocity components.     

Radiative and Evaporative Cooling in the Entrainment Zone of Stratocumulus – the Role of Longwave Radiative Cooling Above Cloud Top

A mixing fraction determines the relative amount of above-cloud-top air that has been mixed into a cloudy air parcel. A method, based on the use of mixing fractions, to calculate the cooling effects due to mixing, longwave radiation and phase changes at cloud top is derived and discussed. We compute cooling effects for the whole range of mixing fraction for two observed cases of the stratocumulus-topped marine boundary layer. In both cases the total radiative cooling effect is found to be the most dominant contributor to the negative buoyancy excess found at cloud top. The largest radiative cooling rates are found for clear-air parcels immediately adjacent to cloud top rather than inside the cloud. With the help of a simple longwave radiation model, we show this to be caused by clear-air radiative cooling due to the temperature inversion at cloud top. Further we show that flux profiles in the entrainment zone can be computed from data obtained from a horizontal level run that is half the time in cloud and half the time out of cloud.     

Kelvin-Helmholtz waves observed by aircraft at different stages of their life-cycle in a low-level inversion

Wave-like motions within a low-level inversion capping a morning mixed layer are studied with two instrumented aircraft, one of which is equipped with a fast-response air sensing probe. Kelvin-Helmholtz waves and their different stages of development (growth, overturning and decay) are identified by means of spectral analyses of temperature and wind component records. By analysing energy conversion rate cospectra, it is found that mechanical production terms and buoyancy production terms, respectively positive and negative during growth stage, reverse signs when overturning occurs. These results and inspection of the temporal evolution of the vertical profiles of temperature suggest that the instability recurs until the initial surplus of shear is drained. Additionally, spectra computed at the top of the mixed layer are compared with those obtained within the underlying mixed layer. The results qualitatively show that the wind shear has a non-negligible effect on the entrainment of warm air through the mixed-layer top.IOPG, 12, avenue des Landais-63001, Clermont-Ferrand Cedex.Complexe Scientifique des Cézeaux, BP 45-63170 Aubière.     

Turbulence Statistics Above And Within Two Amazon Rain Forest Canopies

The turbulence structure in two Amazon rain forestswas characterised for a range of above-canopystability conditions, and the results compared withprevious studies in other forest canopies and recenttheory for the generation of turbulent eddies justabove forest canopies. Three-dimensional wind speedand temperature fluctuation data were collectedsimultaneously at up to five levels inside and abovetwo canopies of 30–40 m tall forests, during threeseparate periods. We analysed hourly statistics, jointprobability distributions, length scales, spatialcorrelations and coherence, as well as power spectraof vertical and horizontal wind speed.The daytime results show a sharp attenuation ofturbulence in the top third of the canopies, resultingin very little movement, and almost Gaussianprobability distributions of wind speeds, in the lowercanopy. This contrasts with strongly skewed andkurtotic distributions in the upper canopy. At night,attenuation was even stronger and skewness vanishedeven in the upper canopy. Power spectral peaks in thelower canopy are shifted to lower frequencies relativeto the upper canopy, and spatial correlations andcoherences were low throughout the canopy. Integrallength scales of vertical wind speed at the top of thecanopy were small, about 0.15 h compared to avalue of 0.28 h expected from the shear lengthscale at the canopy top, based on the hypothesis that theupper canopy air behaves as a plane mixing layer. Allthis suggests that, although exchange is not totallyinhibited, tropical rain forest canopies differ from other forests in that rapid, coherentdownward sweeps do not penetrate into the lowercanopy, and that length scales are suppressed. This isassociated with a persistent inversion of stability inthat region compared to above-canopy conditions. Theinversion is likely to be maintained by strong heatabsorption in the leaves concentrated near thecanopy top, with the generally weak turbulence beingunable to destroy the temperature gradients over thelarge canopy depth.     

Influence of mesoscale winds on the turbulent structure of the urban boundary layer over St. Louis

Two fair weather afternoons have been examined, where the urban boundary layer over St. Louis, though exhibiting similar thermal characteristics, had a markedly different kinematic structure. The turbulent nature of the boundary layer was examined through analysis of double theodolite wind profiles at an urban and at a rural site on each day. On 14 July 1975, the winds increased with height above the inversion at both sites and on the following day, the winds decreased above the boundary layer in the same region. While the mean wind speed in the lowest 0.8 km agl was similar on both days, the turbulence characteristics of the urban boundary-layer winds were distinctly different on these two afternoons. This was evidenced by the variance of the wind and is in agreement with simultaneous aircraft measurements reported by Hildebrand and Ackerman (1984). A similar difference in turbulence was not found over the rural site. It is suggested that the enhanced turbulence at the urban site on 14 July is likely associated with the wind profile immediately above the boundary layer, where the downward flux of high momentum air from above the inversion may have resulted in stronger mechanical mixing within the boundary layer.     

Intensive Radiosonde Measurements of Summertime Convection over the Inner Mongolia Grassland in 2014:Difference between Shallow Cumulus and Other Conditions

Using radiosonde measurements from 26 July to 30 July 2014 at Baiqi over the Inner Mongolia grassland of China, the vertical structure of shallow cumulus(SCu) clouds and associated environmental conditions were investigated. The cloud base height and the cloud top height of SCu was 3.4 km and 5 km, respectively. The temperature of the SCu layer was less than 0℃. The horizontal advection of specific humidity was smaller than the vertical transport in the atmosphere below 5 km.Above 5 km, the thermodynamic structure of the atmosphere remained stable. At the interface of the cloud layer and free air atmosphere, there was obvious wind shear and a temperature inversion(~2.9℃). Comparisons of environmental parameters associated with cumulus congestus, rain and clear days, showed that the formation of SCu was characterized by a higher Bowen ratio(high sensible heat flux and low latent heat flux), which indicated intensive turbulence in the boundary layer. The formation of SCu was associated with the boundary layer height exceeding the lifting condensation level. The maintenance of SCu was likely associated with the lower convective available potential energy, weak wind shear, and weak subsidence of the synoptic system, which did not favor the dramatic vertical development of SCu and thereby the transformation of SCu to cumulus congestus.     

Structures and characteristics of the windy atmospheric boundary layer in the South China Sea region during cold surges

An observational analysis of the structures and characteristics of a windy atmospheric boundary layer during a cold air outbreak in the South China Sea region is reported in this paper. It is found that the main structures and characteristics are the same as during strong wind episodes with cold air outbreaks on land. The high frequency turbulent fluctuations(period<1 min) are nearly random and isotropic with weak coherency, but the gusty wind disturbances(1 min相似文献   

‘Evolution of a Storm-driven Cloudy Boundary Layer in the Arctic’

To investigate the processes of development and maintenance of low-level clouds during major synoptic events, the cloudy boundary layer under stormy conditions during the summertime Arctic has been studied using observations from the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment and large-eddy simulations (LES). On 29 July 1998, a stable Arctic cloudy boundary-layer event was observed after the passage of a synoptic low pressure system. The local dynamic and thermodynamic structure of the boundary layer was determined from aircraft measurements including the analysis of turbulence, cloud microphysics and radiative properties. After the upper cloud layer advected over the existing cloud layer, the turbulent kinetic energy (TKE) budget indicated that the cloud layer below 200 m was maintained predominantly by shear production. Observations of longwave radiation showed that cloud-top cooling at the lower cloud top has been suppressed by radiative effects of the upper cloud layer. Our LES results demonstrate the importance of the combination of shear mixing near the surface and radiative cooling at the cloud top in the storm-driven cloudy boundary layer. Once the low-level cloud reaches a certain height, depending on the amount of cloud-top cooling, the two sources of TKE production begin to separate in space under continuous stormy conditions, suggesting one possible mechanism for the cloud layering. The sensitivity tests suggest that the storm-driven cloudy boundary layer is possibly switched to the shear-driven system due to the advection of upper clouds or to the buoyantly driven system due to the lack of wind shear. A comparison is made of this storm-driven boundary layer with the buoyantly driven boundary layer previously described in the literature.     

Wave-like wind fluctuations observed in the stable surface layer over a plant canopy(2)

Observations of wind velocity and air temperature fluctuations were made in the nocturnal surface inversion layer over a sorghum field. Wave-like fluctuations of temperature and wind velocity with a period of 15–20 min were observed for about 2 hours, 3 to 5 hours before sunrise. Wave-like fluctuations of temperature were observed in the air layer above and within a plant canopy and were most noticeable at the top of the plant canopy. Spectral analysis of temperature and wind velocity fluctuations reveals a separation of energy into wave-like and turbulent fluctuations. Cospectral analysis shows that for both momentum and heat, vertical transports are partitioned almost equally in the frequency ranges characteristic of wave-like and turbulent fluctuations. This suggests wave- turbulence interactions at low frequencies in the air layer near a plant canopy.     

Air mass modification over Lake Michigan

The modification of a relatively cold air mass over the warm water of Lake Michigan is studied by using a two-dimensional nonlinear mesoscale model. Considerable amounts of heat and water vapor are supplied from the water surface to the lower atmosphere by turbulent eddies. A convective mixed layer develops and grows toward the downwind region with stratocumulus clouds over the lake.The model simulates the warming and moistening of the mixed layer, the development of a boundary layer, the divergence and convergence of wind near the coastlines, and the turbulent fluxes.The model warming of the mixed layer across the lake was about 2.2 °K and the moistening of the mixed layer was about 0.8 g kg^–1, which are comparable to 2.7 °K and 0.8 g kg^–1 observed by Lenschow (1973). The convective boundary layer, which includes the cloud layer, subcloud layer, and superadiabatic layer near the water surface, is well simulated. The tilt of the inversion which coincides with the cloud top is also well reproduced. When a prescribed cooling rate is applied at the cloud top, stronger turbulence and a deeper cloud layer are generated. Without the cooling, the cloud is shallow and the shape of the cloud base is determined by surface conditions. The rise of the inversion is due to upward vertical motion, and deepening of the convective layer in the downwind region.     

高风速相干结构对通量输送影响的实验研究

切变湍流的相干结构是湍流研究中的重大发现,它表明湍流运动并非完全随机,其中具有可检测的有序结构.本文通过处理南京浦口地区大气边界层观测数据,来分析不稳定层结中高风速相干结构特征.本次观测项目包括对场地中央的气象铁塔上2 m和40 m高度上超声风速仪的脉动速度、温度测量以及风廓线雷达对边界层风速廓线的测量.对超声水平风速时间序列数据进行小波变换 (时间尺度400 s),通过阈值来识别这种高风速相干结构.与多普勒风廓线雷达测量结果对比后发现,这种方法确定的相干结构符合常规的认识,具有较长的时间尺度和较大的垂直尺度 (接近边界层厚度).分析三天相干结构特性得到无量纲空间间隔约为6,即每隔6个边界层厚度的水平位置出现一个高速相干结构.通过与垂直风速小波系数的比较,发现高风速相干结构与向下垂直风速之间有较好相关,这与湍流中 “阵风” 现象的研究结论相似.使用四象限分析方法分类得到两种动量通量输送为负的运动:较小水平风速的上扬 (ejection) 运动 (简称为上扬运动) 和较大水平风速的下扫 (sweep) 运动 (简称为下扫运动),这两种运动在整个湍流活动中处于主导地位.高风速相干结构通过促进下扫运动和抑制上扬运动来影响动量通量的输送.     

Dawn‐to‐dusk evolution of air turbulence,temperature and sensible and latent heat fluxes above a forest canopy: Concepts,model and field comparisons

Abstract

Dawn‐to‐dusk evolution of air turbulence, sensible heat and latent heat above a forest during cloud‐free or near‐cloud‐free summer conditions is modelled by way of a system of differential equations. Temperatures in and above the canopy, near canopy‐top wind velocities, early morning leaf moisture (dew) and afternoon canopy ventilation (i.e. heat released from the canopy and from below the canopy) are included in the mathematical treatment. Computed results are compared with field data for atmospheric temperature and wind speed profiles up to 1200 m, within‐canopy temperature, and canopy‐level radiation, turbulent fluxes and wind speeds. Data were collected at a central New Brunswick mixed‐wood forest site dominated by spruce (Picea spp. ) and shade‐tolerant hardwoods for four representative summer days. It was found that the effective canopy temperature was not only affected by insolation, but also by the extent of canopy ventilation and the amount of dew on the foliage. The growth of the mixing layer was affected by canopy ventilation and by above‐canopy wind speeds. Model calculations closely simulated the meteorological observations.     

日间对流边界层中的非局地动量混合

日间对流边界层最显著的结构特征是在热力作用下所形成的组织化对流。与小尺度湍涡不同的是，组织化对流具有边界层尺度的垂直相干性，可实现垂直贯穿边界层的非局地物质和能量传输。本文针对对流边界层中的动量混合，探究组织化对流对动量输送的贡献。以高精度大涡模拟数据为研究资料，通过傅里叶变换、本征正交分解和经验模态分解3种滤波方法，分离组织化对流和背景湍涡，计算与两者相关的非局地和局地动量通量，发现与组织化对流相关的非局地动量通量是总通量的重要组成部分，并主导混合层中的垂直动量输送。而后，基于协谱和相位谱分析，探究组织化对流的空间结构对动量传输的影响，发现在热力主导的不稳定环境中，单体型环流结构对动量的传输效率较低。而在风切较强的近中性环境中，滚涡型组织化结构可使垂直和水平流向扰动速度的相位差减小，从而提升动量传输效率。研究结果表明，边界层方案需要包含非局地动量通量项，其参数化应考虑整体稳定度对传输效率的影响。     

Variation of turbulence in the marine boundary layer over the Arabian Sea during indian southwest monsoon (MONEX 79)

Analysis of high frequency (20 Hz) turbulence data collected from low level flights by the National Center for Atmospheric Research (NCAR) Electra aircraft over the central Arabian Sea on 20 and 24 June, 1979 as part of MONEX 79 indicates the influence of the Somali Jet on boundary-layer turbulence. Different stages of monsoon development were evident on the two observation days from mean boundary-layer profiles. However, turbulence statistics of wind speed components and temperature in the monsoon boundary layer for both days are generally greater than those observed in laboratory experiments or tropical and trade wind boundary layers in which a strong jet was not present. Analysis of high frequency wind, temperature and humidity data was made to obtain fluxes of momentum and heat. Magnitudes of the sensible and latent heat fluxes are three to five times larger than the values observed over the monsoon boundary layer over the Bay of Bengal. The turbulent kinetic energy budget over the Arabian Sea for 24 June indicates the importance of buoyancy, and to a lesser extent shear as the dominant term. Dissipation serves as the primary sink term.