Estimating the components of the sensible heat budget of a tall forest canopy in complex terrain

Ultrasonic wind measurements, sonic temperature and air temperature data at two heights in the advection experiment MORE II were used to establish a complete budget of sensible heat including vertical advection, horizontal advection and horizontal turbulent flux divergence. MORE II took place at the long-term Carbo-Europe IP site in Tharandt, Germany. During the growing period of 2003 three additional towers were established to measure all relevant parameters for an estimation of advective fluxes, primarily of CO₂. Additionally, in relation to other advection experiments, a calculation of the horizontal turbulent flux divergence is proposed and the relation of this flux to atmospheric stability and friction velocity is discussed. In order to obtain a complete budget, different scaling heights for horizontal advection and horizontal turbulent flux divergence are tested. It is shown that neglecting advective fluxes may lead to incorrect results. If advective fluxes are taken into account, the sensible heat budget based upon vertical turbulent flux and storage change only, is reduced by approximately 30%. Additional consideration of horizontal turbulent flux divergence would in turn add 5–10% to this sum (i.e., the sum of vertical turbulent flux plus storage change plus horizontal and vertical advection). In comparison with available energy horizontal advection is important at night whilst horizontal turbulent flux divergence is rather insignificant. Obviously, advective fluxes typically improve poor nighttime energy budget closure and might change ecosystem respiration fluxes considerably.     

Mesoscale moisture variability and its impact on energy transfer through the boundary layer

The impact of mesoscale moisture variability on the vertical energy transfer through a pre-frontal boundary layer is studied with NOPEX aircraft data. The moisture variability relates to a cold front that passed the area 2 1/2 hours after the observations. We find a density front ahead of the cold front. The large vertical divergence of the turbulent moisture flux in the surface layer is partly related to this moisture variability. Large scale horizontal advection contributes to the observed vertical turbulent flux divergence. The estimated horizontal mesoscale advection term in the budget of sensible heat and moisture is on average small but locally it can be large. This term acts to re-distribute moisture in the boundary layer and leads to sub-grid variations of relative humidity, which is an important quantity for boundary-layer cloud models. The distinct spatial variations of specific humidity are mainly related to synoptic forcing and not to heterogeneity in the surface energy balance.     

鄱阳湖地区复杂地表条件下一次强降水过程的近地面边界层特征

使用鄱阳湖北部70 m气象塔湍流和梯度观测数据,分析了2011年6月6日夜间一次暴雨过程中近地面边界层特征.结果表明,此次过程是在高空低槽和西南急流的天气背景下,受鄱阳湖复杂地表影响产生的局地性强降水.强降水发生前受东南暖平流影响,近地面边界层中水汽累积,不稳定性增加;强降水过程中,近地层感热、潜热通量迅速增加,同时,近地面层湍流动量通量下传和水平输送增加,鄱阳湖的水汽输送加强降水强度.另外,强降水过程中,近地面湍流动能迅速增大并达到最大值,而平均动能的增大发生在强降水结束后,表明地表作用明显,近地面边界层的湍流场为暴雨提供动力条件.尺度分析表明,强降水前,中尺度动量通量占主要地位,降水过程中湍流动量通量显著加强.     

Airborne Investigation Of Arctic Boundary-Layer Fronts Over The Marginal Ice Zone Of The Davis Strait

Three aircraft-based studies of boundary-layer fronts (BLFs) werecarried out during the experiment KABEG in April 1997near the sea-ice edge over the Davis Strait. The zone of strongestcross-front horizontal gradients showed a typical length scaleof 20 km, while the along-front scale was observed to beseveral hundreds of kilometres.The observed BLFs were stronger than the few previously reportedcases. Horizontal gradients of potential temperature and specifichumidity ranged up to 3 K or 0.25 g kg^-1over 20 km, respectively.Low-level winds were around 15 m s^-1 parallel to the ice edge.The capping inversion sloped from between250 and 400 m over sea ice to between 400 and 700 m over ocean.For two BLF cases turbulent fluxes and energy budgets are calculated.Turbulent energy fluxes show a factor 2 to 3 contrast acrossthe ice edge and range from 15 to 50 W m^-2 over sea iceand from 50 to 100 W m^-1 over open ocean.The mean boundary-layer energy budgets are dominated bycold, dry horizontal advection, which is exceededby vertical heat flux convergence.The momentum budgets are dominated by pressure gradient force,Coriolis force and momentum flux divergence.     

Atmospheric Response to Mesoscale Ocean Eddies over the South China Sea

The South China Sea(SCS) is an eddy-active area. Composite analyses based on 438 mesoscale ocean eddies during 2000–2012 revealed the status of the atmospheric boundary layer is influenced remarkably by such eddies. The results showed cold-core cyclonic(warm-core anticyclonic) eddies tend to cool(warm) the overlying atmosphere and cause surface winds to decelerate(accelerate). More than 5% of the total variance of turbulent heat fluxes, surface wind speed and evaporation rate are induced by mesoscale eddies. Furthermore, mesoscale eddies locally affect the columnar water vapor, cloud liquid water, and rain rate. Dynamical analyses indicated that both variations of atmospheric boundary layer stability and sea level pressure are responsible for atmospheric anomalies over mesoscale eddies. To reveal further details about the mechanisms of atmospheric responses to mesoscale eddies, atmospheric manifestations over a pair of cold and warm eddies in the southwestern SCS were simulated. Eddy-induced heat flux anomalies lead to changes in atmospheric stability. Thus, anomalous turbulence kinetic energy and friction velocity arise over the eddy dipole, which reduce(enhance) the vertical momentum transport over the cold(warm) eddy, resulting in the decrease(increase) of sea surface wind. Diagnoses of the model's momentum balance suggested that wind speed anomalies directly over the eddy dipole are dominated by vertical mixing terms within the atmospheric boundary layer, while wind anomalies on the edges of eddies are produced by atmospheric pressure gradient forces and atmospheric horizontal advection terms.     

不均匀地表产生的中尺度通量的数值试验

采用北京大学三维的复杂地形中尺度模式,结合陆面过程模式(SiB),模拟了草原和沙漠并存的下垫面的边界层大气运动.利用SiB模式计算了地表辐射、感热、潜热通量,并且预报地表温度.中尺度模式则模拟了沙漠地区受热抬升,形成的辐合运动,垂直速度的分布,不同高度上水平流场的变化以及中尺度动量和热量通量,把中尺度通量跟湍流通量进行了比较,以确定这种中尺度运动在GCM模式的参数化过程中的重要性.试验表明中尺度通量尤其是热量通量要比湍流通量大很多.     

A study of multiple stable layers in the nocturnal lower atmosphere

The structure of nocturnal inversions in the first 300 m of the atmosphere is analyzed using observational data from the Boulder Atmospheric Observatory (BAO) from March through June 1981. The temperature profiles show more than one inversion layer 41% of the time during the observational period. The vertical distributions of wind speed and moisture also show evidence of stratification during these multiple-layer events. The relation between the radiative cooling rate in time and height, including moisture, and the vertical structure of the multiple layers is calculated. The vertical distribution of eddy kinetic energy and the turbulent vertical fluxes of heat and momentum are also calculated. Turbulent structure in the elevated inversion layers is more complicated than that in the single-layer, stable nocturnal boundary layer. The total heat budget for a multiple-layer case is calculated, and turbulent cooling is found to be negligible relative to radiative cooling and to horizontal advection and/or horizontal divergence of heat flux.     

Boundary-layer heat and moisture budgets from fife

Aircraft stacks were flown upwind and downwind of the First ISLSCP Field Experiment (FIFE) site in Kansas to measure the heat and moisture budgets of the boundary layer under fairly clear skies for four daytime periods. In this paper, we evaluate the terms in the conservation equation. The vertical flux divergence and advection do not account for the difference between surface and low-level aircraft flux estimates. Budget estimates of the surface fluxes using the aircraft data agree well with surface flux measurements, but extrapolation of the aircraft fluxes gives surface fluxes that are too low. With the 5 km cutoff filter used, the aircraft underestimate for sensible heat flux is about 40%, and for the latent heat flux about 30%. Part of the underestimation is attributable to long-wavelength contributions (longer than the 5 km filter), but more investigation is needed.     

Structure and Dynamics of Katabatic Flow Jumps: Idealised Simulations

For the first time, results from a high-resolution numerical simulation (with horizontal grid spacing of 35m) were used to reveal the detailed structure near an atmospheric katabatic jump over an idealized slope. The simulation represents flow over the slopes of Coats Land, Antarctica for austral winter conditions. The katabatic jump is characterised by an updraft with vertical velocities of order 1ms⁻¹ and serves as a possible forcing mechanism for the gravity waves frequently observed over the ice shelves around the Antarctic. Results also indicate that strong turbulence is generally confined within a mixing zone near the top of the katabatic layer upstream of the jump and extends downstream through the top of the strong updraft associated with the jump. Detailed analyses of momentum and heat budgets across the katabatic jump indicate that, upstream of the jump, turbulent mixing is important in decelerating the upper part of the katabatic layer, while within the jump the upslope pressure gradient force associated with the pool of cold air plays a role in decelerating the flow near the surface. The heat budget near the jump reveals a simple two-term balance: the turbulent heat flux divergence is balanced by the advection. A comparison of model results with available theories indicates that mixing between layers of different potential temperature structure indeed plays some role in the development of katabatic flow jumps, especially for strong jumps. Theories used to study katabatic jumps should include this mixing process, of which the amount depends on the intensity of the jump. A conceptual model of a katabatic jump, including the main dynamical processes, is constructed from these detailed analyses.     

Subgrid-scale vertical heat fluxes for the African region

Summary A diagnostic model for complete heat budgets in the free atmosphere is presented and is applied to the African-Atlantic sector between 35°S–30°N for May 1979. The model is based on the conservation equations for latent and sensible heat. These are evaluated in a form integrated over 24 hours in time and over atmospheric boxes of 2.5°×2.5° in horizontal and 100 hPa in vertical direction. Grid-scale input data are the 3D-ECMWF-diagnoses of the FGGE period plus parameterized fields of surface rain, evaporation and sensible Heat flux. This leads to an overspecification of latent and sensible heat budgets for any atmospheric column between surface and top of the atmosphere and thus yields an objective column imbalance. In order to separate the vertical subscale fluxes of rain, moisture and heat in the free atmosphere the model uses a closure assumption for the coupling between moisture and sensible heat flux as well as one for the vertical imbalance profiles; it is demonstrated that the budgets are not too sensitive with respect to these parameterizations.Results are presented in terms of vertical profiles of the subscale vertical fluxes of rain, moisture and heat. These are interpeted as measures of convective activity, with particular emphasis on the ITCZ. May 1979 averages as well as results for a respresentative single day are discussed. The imbalance (=the error) can be sufficiently well separated from the signal. It is shown that the low-level mass flux divergence does not coincide with the position of the ITCZ while the maximum of the subscale fluxes does coincide. Over the continent, it is not the horizontal mass flux convergence which feeds the ITCZ and the rainbelt but rather the subscale moisture flux and its convergence in the vertical. Over the Saharan latitudes, there is considerable convective flux of sensible heat, but not of latent heat. Over the ocean, deep convection in the ITCZ is weaker than over Africa, and it is consistently correlated with upward converging subscale moisture flux. The fields of the subscale vertical fluxes are coherent in space and time. It is argued from these results that the presented diagnostic model is potentially useful for testing parameterizations of convection in general circulation and climate models.With 19 Figures     

Vertical Heat Transfer in the Lower Atmosphere over the Arctic Ocean During Clear-sky Periods

A diagnostic study of heat transfer within the lower atmosphere and between the atmosphere and the surface of the Arctic Ocean snow/ice pack during clear-sky conditions is conducted using data from the Surface Heat Budget of the Arctic Ocean (SHEBA) field experiment. Surface heat budgets computed for four cloudy and four clear periods show that, while the net turbulent heat fluxes at the surface are small during the cloudy periods, during the clear-sky periods they are a considerable source of surface heating, balancing significant portions of the conductive heat fluxes from within the snow/ice pack. Analysis of the dynamics and thermodynamics of the lower atmosphere during the clear-sky periods reveals that a considerable portion of the heat lost to the surface by turbulent heat fluxes is balanced by locally strong heating near the atmospheric boundary-layer (ABL) top due to the interaction of subsiding motions with the strong overlying temperature inversions surmounting the ABL. This heat is then entrained into the ABL and transported to the surface by turbulent mixing, maintained by a combination of vertical wind shear and wave-turbulence interactions. The frequency of stable, clear-sky periods, particularly during the winter, combined with these results, suggests that the downward transfer of heat through the lower atmosphere and into the surface represents an important component of the heat budgets of the lower atmosphere and snow/ice pack over the annual cycle     

Modelling the Interaction Between a River Surface and the Atmosphere at the Bottom of a Valley

The role of a river of small dimensions in driving the surface exchange of sensible and latent heat fluxes at the bottom of a valley is investigated using large-eddy simulation (LES). Simulations were performed using different valley topographies, river widths and large-scale wind speed and direction. In all cases, the river acted as a sink of both sensible and latent heat during daytime. Despite the general agreement concerning the flux direction above the river surface, specific differences exist between the simulations. The topography enhances the wind divergence caused by the river, and the larger negative surface fluxes above the river occur when there are no slopes, a consequence of larger wind speeds above the river. For large-scale winds aligned with the valley axis, the surface fluxes depend on the large-scale wind speed, but this dependence is reduced if the large-scale wind is perpendicular to the valley axis. There is a minimum of temperature and a maximum of specific humidity above the river surface. The scalar budgets show that sensible heat flux converges above the river, being balanced by the warm air subsidence at the centre of the valley. Latent heat fluxes, on the other hand, converge above the river surface, and they are balanced by the horizontal advection of humidity towards the river margins.     

地气通量中存贮和平流项计算方案的探讨

从物质收支方程出发,推导了一个包含物质存贮、水平平流输送、垂直对流输送以及传统的涡度相关项的地气通量计算方程。平流项本质上是地表非均匀性的结果,不同下垫面的感热和潜热通量也不同,将会产生中尺度环流,使得辐合辐散过程得以维持,从而将体元内的物质输送到体元以外,因此可以通过计算水汽和感热的存贮,间接求出物质的水平平流输送。量纲分析和实际的资料应用都表明,存贮和平流的通量贡献是非常小的。尤其是在均匀下垫面下,方程中的存贮和平流项的通量贡献可以忽略,因此估算地气通量时仅需考虑涡度相关项和Webb修正项即可。而在非均匀下垫面下,在1 d以上的时间尺度上,为方便计算,可以忽略存贮和平流的通量贡献;而在小时这样的时间尺度上,从物质能量收支守恒的角度考虑,估算地气通量需要包括存贮和平流的通量贡献。     

天山山区及邻近地区夏季降水的环境场特征与涡度平衡

本文对天山山区及其邻近地区夏季降水过程的环境场特征、水热收支和涡度平衡做了诊断分析。结果表明，降水发生前的环境场有利于降水系统的发展。Ｑ１，Ｑ２和涡度平衡在降水发生前与降水期有一定的差异，降水期水热收支中起主作用的是水平平流项潜热和热基本上被冷平流所平衡；水汽的水平输送为降水的主要水汽来源，垂直输送也起一定的作用，涡度平衡中主要是水平平流累积的正涡度与散度项制造的负涡度相平衡，与副热带锋区上急流相     

Interdecadal variability of the Pacific Ocean: model response to observed heat flux and wind stress anomalies

Variability of the Pacific Ocean is examined in numerical simulations with an ocean general circulation model forced by observed anomalies of surface heat flux, wind stress and turbulent kinetic energy (TKE) over the period 1970-88. The model captures the 1976-77 winter time climate shift in sea surface temperature, as well as its monthly, seasonal and longer term variability as evidenced in regional time series and empirical orthogonal function analyses. Examination of the surface mixed-layer heat budget reveals that the 1976-77 shift was caused by a unique concurrance of sustained heat flux input anomalies and very strong horizontal advection anomalies during a multi-month period preceding the shift in both the central Pacific region (where cooling occurred) and the California coastal region (where warming occurred). In the central Pacific, the warm conditions preceding and the cold conditions following the shift tend to be maintained by anomalous vertical mixing due to increases in the atmospheric momentum flux (TKE input) into the mixed layer (which deepens in the model after the shift) from the early 1970s to the late 1970s and 1980s. Since the ocean model does not contain feedback to the atmosphere and it succeeds in capturing the major features of the 1976-77 shift, it appears that the midlatitude part of the shift was driven by the atmosphere, although effects of midlatitude ocean-atmosphere feedback are still possible. The surface mixed-layer heat budget also reveals that, in the central Pacific, the effects of heat flux input and vertical mixing anomalies are comparable in amplitude while horizontal advection anomalies are roughly half that size. In the California coastal region, in contrast, where wind variability is much weaker than in the central Pacific, horizontal advection and vertical mixing effects on the mixed layer heat budget are only one-quarter the size of typical heat flux input anomalies.This paper was presented at the Second International Conference on Modelling of Global Climate Variability, held in Hamburg 7–11 September 1992 under the auspices of the Max Planck Institute for Meteorology. Guest Editor for these papers is L. Dümenil     

Observations of fluxes over heterogeneous surfaces

This study analyzes data collected from repeated aircraft runs 30 m over alternating regions of irrigated and dry nonirrigated surfaces, each region on the order of 10 km across, during the California Ozone Deposition Experiment (CODE). After studying the scale dependence of the flow, the variables and their fluxes are decomposed into means for sublegs defined in terms of irrigated and nonirrigated regions and deviations from such subleg means. Since the repeated runs were flown over the same track, compositing the eight flight legs for each of the two days allows partial isolation of the influences of surface heterogeneity and transient mesoscale motions.A variance analysis is carried out to quantify the relative importance of surface heterogeneity and transient mesoscale motions on the variability of the turbulence fluxes. The momentum and ozone fluxes are more influenced by transient mesoscale motions while fluxes of heat, moisture and carbon dioxide are more influenced by surface heterogeneity. The momentum field is also influenced by a quasi-stationary mesoscale front and larger scale velocity gradients.For the present case, the mesoscale modulation of the turbulent flux is numerically more important than the direct mesoscale flux. This spatial modulation of the turbulent fluxes leads to extra Reynolds terms which act to reduce the area-averaged turbulent momentum flux and enhance the area-averaged turbulent heat flux.     

水平非均匀对流边界层热量平衡和平流输送作用的大涡模拟

采用大涡模拟所获的数据结果，分析地面热通量沿平均风方向存在 突变的条件下对流边界层的热量平衡和平流输送作用。分析表明边界层内模拟所得结果 可以很好地满足热量平衡关系。除地面热通量项以外，平流项（包括水平平流和垂直平 流）对边界层加热率的作用可达地面热通量不均匀性差值的大小，是影响边界层内热量 平衡的最重要因子，平均速度散度项对热量平衡的作用也不可忽略，但湍流通量散度项 的作用则很小。     

Statistics of shallow convection on Mars based on large-eddy simulations. Part 2: effects of wind shear

Effects of wind on quasi-steady, shallow convection in the Martian boundary layer are studied using a large-eddy simulation model. Convection in the model is generated by the radiative flux divergence and the strength of the surface heat flux, which do not vary in time. The resulting convective boundary layer exhibits transient, irregular, horizontal cellular structures, transported by wind, and a lack of well-pronounced regular horizontal rolls, observed for analogous conditions on Earth. The dimensionless statistics of turbulence are generally similar to those generated in the windless conditions, and depend on the ratio F, defined in terms of the integrated radiative and turbulent heating rates in the boundary layer. The simulations show that variations of the radiative heating influence the temperature statistics, while their effects on the wind velocity are relatively small. The horizontal velocity variances do not show a strong dependence on parameter F, in contrast with the vertical velocity variances, which are strongly dependent on F.     

Spectral Structure of Small-Scale Turbulent and Mesoscale Fluxes in the Atmospheric Boundary Layer over a Thermally Inhomogeneous Land Surface

Spectral analysis was performed on aircraft observations of a convective boundary layer (CBL) that developed over a thermally inhomogeneous, well-marked mesoscale land surface. The observations, part of the GAME-Siberia experiment, were recorded between April and June 2000 over the Lena River near Yakutsk City. A special integral parameter termed the ‘reduced depth of the CBL’ was used to scale the height of the mixed layer with variable depth. Analysis of wavelet cospectra and spectra facilitated the separation of fluxes and other variables into small-scale turbulent fluctuations (with scales less than the reduced depth of the CBL, approximately 2 km) and mesoscale fluctuations (up to 20 km). This separation approach allows for independent exploration of the scales. Analyses showed that vertical distributions obeyed different laws for small-scale fluxes and mesoscale fluxes (of sensible heat, water vapour, momentum and carbon dioxide) and for other variables (wind speed and air temperature fluctuations, coherence and degree of anisotropy). Vertical profiles of small-scale turbulent fluxes showed a strong decay that differed from generally accepted similarity models for the CBL. Vertical profiles of mesoscale fluxes and other variables clearly showed sharp inflections at the same relative (with respect to the reduced depth of the CBL) height of approximately 0.55 in the CBL. Conventional similarity models for sensible heat fluxes describe both small-scale turbulent and mesoscale flows. The present results suggest that mesoscale motions that reach up to the relative level of 0.55 could be initiated by thermal surface heterogeneity. Entrainment between the upper part of the CBL and the free atmosphere may cause mesoscale motions in that region of the CBL.     

Intra-field variability of scalar flux densities across a transition between a desert and an irrigated potato field

This paper reports on measurements of sensible and latent heat and CO₂ fluxes made over an irrigated potato field, growing next to a patch of desert. The study was conducted using two eddy correlation systems. One measurement system was located within the equilibrium boundary layer 800 m downwind from the edge of the potato field. The other measurement system was mobile and was placed at various downwind positions to probe the horizontal transition of vertical scalar fluxes. Latent (LE) and sensible (H) heat fluxes, measured at 4 m above the surface, exhibited marked variations with downwind distance over the field. Only after the fetch to height ratio exceeded 75 to 1 didLE andH become invariant with downwind distance. When latent and sensible heat fluxes were measured upwind of this threshold, significant advection of humidity-deficit occurred, causing a vertical flux divergence ofH andLE.The measured fluxes of momentum, heat, and moisture were compared with predictions from a second-order closure two-dimensional atmospheric boundary layer model. There is good agreement between measurements and model predictions. A soil-plant-atmosphere model was used to examine nonlinear feedbacks between humidity-deficits, stomatal conductance and evaporation. Data interpretation with this model revealed that the advection of hot dry air did not enhance surface evaporation rates near the upwind edge of the potato field, because of negative feedbacks among stomatal conductance, humidity-deficits, andLE. This finding is consistent with results from several recent studies.