Surface Heterogeneity Effects on Regional-Scale Fluxes in the Stable Boundary Layer: Aerodynamic Roughness Length Transitions

The effects of abrupt streamwise transitions of the aerodynamic roughness length ( $z_\mathrm{o}$ z o ) on the stable atmospheric boundary layer are evaluated using a series of large-eddy simulations based on the first Global Energy and Water Cycle Experiment Atmospheric Boundary Layer intercomparison study (GABLS1). Four $z_\mathrm{o}$ z o values spanning three orders of magnitude are used to create all possible binary distributions with each arranged into patches of characteristic length scales equal to roughly one-half, one, and two times the equivalent homogeneous boundary-layer height. The impact of the heterogeneity on mean profiles of wind speed and temperature, on surface fluxes of heat and momentum, and on internal boundary-layer dynamics are considered. It is found that $z_\mathrm{o}$ z o transitions do not significantly alter the functional relationship between the average surface fluxes and the mean profiles of wind speed and potential temperature. Although this suggests that bulk similarity theory is applicable for modelling the stable boundary layer over $z_\mathrm{o}$ z o heterogeneity, effective surface parameters must still be specified. Existing models that solve for effective roughness lengths of momentum and heat are evaluated and compared to values derived from the simulation data. The existing models are unable to accurately reproduce both the values of the effective aerodynamic roughness lengths and their trends as functions of patch length scale and stability. A new model for the effective aerodynamic roughness length is developed to exploit the benefits of the other models tested. It accurately accounts for the effects of the heterogeneity and stratification on the blending height and effective aerodynamic roughness length. The new model provides improved average surface fluxes when used with bulk similarity.     

Incorporation of a high-roughness lower boundary into a mesoscale model for studies of dry deposition over complex terrain

For flow over natural surfaces, there exists a roughness sublayer within the atmospheric surface layer near the boundary. In this sublayer (typically 50z ₀ deep in unstable conditions), the Monin-Obukhov (M-O) flux profile relations for homogeneous surfaces cannot be applied. We have incorporated a modified form of the M-O stability functions (Garratt, 1978, 1980, 1983) in a mesoscale model to take account of this roughness sublayer and examined the diurnal variation of the boundary-layer wind and temperature profiles with and without these modifications. We have also investigated the effect of the modified M-O functions on the aerodynamic and laminar-sublayer resistances associated with the transfer of trace gases to vegetation. Our results show that when an observation height or the lowest level in a model is within the roughness sublayer, neglect of the flux-profile modifications leads to an underestimate of resistances by 7% at the most.     

On the consideration of mesoscale transports in climate modelling

Summary The dynamical effect of land surface heterogeneity on heat fluxes in the atmospheric boundary layer (ABL) is investigated using numerical simulations with a non-hydrostatic model over a wide range of grid resolutions. It is commonly assumed that mesoscale or dynamical fluxes associated with mesoscale and convective circulations simulated by a high-resolution model (subgrid (SG) model) on the subgrid scale of a climate model (large-scale (LS) model) represent additional processes in the ABL, which are not considered by the turbulence scheme of the LS-model, and which can be parameterized using the SG-model. The present study investigates the usefulness of this methodology for small-scale and large-scale idealized heterogeneities using a SG-model resolving mesoscale or even microscale circulations to compute the mesoscale fluxes on the scale of the LS-model. It is shown that the dynamical transports as derived from the SG-model should not be used to correct the parameterized turbulent fluxes of the LS-model. The reason is that the subgrid circulations simulated by the SG-model interact with the fields of wind and scalars in the ABL, which results in reduced turbulent fluxes in the ABL. Thus the methodology of previous studies to use mesoscale/dynamical fluxes for the correction of flux profiles simulated by climate models seems to be questionable.     

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.     

Horizontal variability and boundary-layer modeling

Micrometeorologists have traditionally set aside consideration of horizontal variability and have studied boundary-layer structure with horizontal homogeneity. The numerical forecasting of boundary-layer structures, over normally varying terrain and including normal disturbances such as fronts, requires selection of an appropriate horizontal scale.A simple analysis of steady-state balance between horizontal advection and vertical diffusion provides estimates of the vertical scale (or depth) of surface-induced features. The scale height is a function of the horizontal scale of the variations. Models neglecting important terrain scales of length below ~ 1000 km can predict down to levels of ~ 0.5 to 1 km while those that neglect important terrain scales below ~ 100 km can predict down to ~ 0.2 to 0.6 km. Below these levels, any predicted features will be dominated by the vertical diffusion so that they are solutions of a one-dimensional boundary-value problem.The boundary-induced advection effects dominate free atmosphere advection effects in the lowest few hundred meters as well. This means that if mesoscale advections are resolved and terrain influences are strong, the predictions in the layer ~ 0.2 to 0.8 km can provide mesoscale detail without mesoscale initial conditions above the surface, because the surface forcing will dominate the solution.     

陆面面积平均通量的参数化问

目前,气候和大气边界层物理研究中一个十分重要的研究方向就是面积平均通量的参数化问题。本文对这一研究方向中存在的问题、可能的解决方法和目前的研究进展情况进行了阐述,简要介绍了非均匀陆面影响的高度及其尺度划分,阐述了混合高度、参考层高度（观测高度和模式第一层高度）、近地层高度、内边界层高度、平衡层高度、粗糙度副层和边界层高度等之间的关系及其在非均匀尺度划分中的作用,并且阐述了整体输送公式在不同尺度的非均匀陆面中存在的问题及相应的可能解决办法。同时还对中尺度的非均匀陆面驱动的一类非经典中尺度环流的参数化,即中尺度通量的参数化问题进行了评述。最后针对内蒙古草原实验和青藏高原实验等具体问题,提出了边界层观测和非均匀陆面参数化方法的几点问题。     

Influence of heterogeneous land surfaces on surface energy and mass fluxes

Summary Land-surface heterogeneity affects surface energy fluxes. The magnitudes of selected land-surface influences are quantified by comparing observations with model simulations of the FIFE (First ISLSCP Field Experiment) domain. Several plausible heterogeneous and homogeneous initial and boundary conditions are examined, although soilmoisture variability is emphasized. It turns out that simple spatial averages of surface variation produced biased flux values. Simulated maximum latent-heat fluxes were approximately 30 to 40 W m^–2 higher, and air temperatures 0.4 °C lower (at noon), when computations were initialized with spatially averaged soil-moisture and leaf-area-index fields. The planetary boundary layer (PBL) height and turbulent exchanges were lower as well. It additionally was observed that (largely due to the nonlinear relationship between initial soil-moisture availability and the evapotranspiration rate), real latent-heat flux can be substantially less than simulated latent-heat flux using models initialized with spatially averaged soil-moisture fields. Differences between real and simulated fluxes also vary with the resolution at which real soil-moisture heterogeneity is discretized.With 8 Figures     

On Modeling Dry Deposition of Long-Lived and Chemically Reactive Species over Heterogeneous Terrain

An explicit multi-layer subgrid-scheme was developed for ameso-/-scale model to consider subgrid-scale surface heterogeneity, dry deposition, biogenic and anthropogenic emission of trace gases. Since dry deposition measurements of highly reactive trace species are scarce we try to evaluate this scheme by heuristic principles. The results of simulations conducted for a 5×5 km² resolution with and without thisscheme are evaluated by using results of a model run with 1×1 km²resolution, which is taken as a `grand thruth' and which has the same resolution as the subgrid. The explict multi-layer subgrid scheme provides a similar distribution of dry deposition fluxes as the much more computationally expensive simulation with the 1×1 km² resolution.Dry deposition fluxes determined from observations give evidence that the explicit multi-layer subgrid scheme which does not require a constant flux approximation for a layer of several decameters leads to an improvement in determining the exchange between the atmosphere and the ground.Results of simulations with a microscale model show that the inhomogeneity at forest edges leads to an increase of the turbulent transports of up to a factor 4 compared to horizontally homogeneous terrain, which is assumed to be the conditions of the subgrid cells (and which is usually the assumption for the entire grid cell in mesoscale models). Inhomogeneity inside an extended stand of trees causes an overall increase of 5–10% withhigh local extremes, i.e. such an inhomogeneity results to an underestimation of dry deposition in meso-/-scale models. The effects are most pronounced for a wind direction perpendicular to the forest edge.     

Effects of land-surface heterogeneity on numerical simulations of mesoscale atmospheric boundary layer processes

Landscape heterogeneity that causes surface flux variability plays a very important role in triggering mesoscale atmospheric circulations and convective weather processes. In most mesoscale numerical models, however, subgrid-scale heterogeneity is somewhat smoothed or not adequately accounted for, leading to artificial changes in heterogeneity patterns (e.g., patterns of land cover, land use, terrain, and soil types and soil moisture). At the domain-wide scale, the combination of losses in subgrid-scale heterogeneity from many adjacent grids may artificially produce larger-scale, more homogeneous landscapes. Therefore, increased grid spacing in models may result in increased losses in landscape heterogeneity. Using the Weather Research and Forecasting model in this paper, we design a number of experiments to examine the effects of such artificial changes in heterogeneity patterns on numerical simulations of surface flux exchanges, near-surface meteorological fields, atmospheric planetary boundary layer (PBL) processes, mesoscale circulations, and mesoscale fluxes. Our results indicate that the increased heterogeneity losses in the model lead to substantial, nonlinear changes in temporal evaluations and spatial patterns of PBL dynamic and thermodynamic processes. The decreased heterogeneity favor developments of more organized mesoscale circulations, leading to enhanced mesoscale fluxes and, in turn, the vertical transport of heat and moisture. This effect is more pronounced in the areas with greater surface heterogeneity. Since more homogeneous land-surface characteristics are created in regional models with greater surface grid scales, these artificial mesoscale fluxes may have significant impacts on simulations of larger-scale atmospheric processes.     

Effects of land-surface heterogeneity upon surface fluxes and turbulent conditions

Summary In this paper, we examine the effects of land-surface heterogeneity on the calculation of surface-energy and momentum fluxes in a meso-scale atmospheric model. A series of numerical experiments has been carried out with a combination of different resolutions for the atmosphere and the land surface, which allows an examination of the aggregation and dynamic effects associated with land-surface heterogeneity. The numerical results show that for a given atmospheric model resolution, increased land-surface resolution leads to better estimates of surface-energy and momentum fluxes, and for a given land-surface resolution, increased atmospheric model resolution also improves the estimates of these fluxes. This latter result contradicts the prevailing view that subgrid variation in atmospheric data plays only a minor role in estimating the fluxes. It is also shown that subgrid land-surface heterogeneity leads to increased turbulent fluctuations. The responsible mechanisms of this effect are both the subgrid variation of surface-energy fluxes and their impact upon the development of convective cells. It is suggested that subgrid atmospheric motions induced by surface heterogeneity may be an important factor which needs to be considered in subgrid closure schemes for atmospheric models. Received August 28, 2000/Revised June 1, 2001     

地表热力非均匀性对近地层相似理论适用性影响的大涡模拟

利用非均匀地表加热的大涡模拟试验,研究了不稳定条件下地表热力非均匀性对近地层相似理论适用性的影响。结果发现,边界层的平均廓线基本不受地表热力非均匀性的影响。进一步分析发现,较大尺度的地表非均匀加热可以激发出有组织的大尺度次级环流,冷暖斑块的通量直到边界层上部才混合均匀;而当地表非均匀尺度较小时,次级环流难以形成有组织的结构,冷暖斑块的通量很快就可以混合均匀。然而,不管是哪种尺度的非均匀地表,非均匀斑块间的平流都对各斑块近地层结构产生重要影响,进而斑块近地层通量—梯度关系与相似理论产生偏差,其中风速梯度关系的偏差更为明显。最后,对目前大气模式中常用的基于相似理论的次网格非均匀地表通量参数化方法——Mosaic方法提出了改进思路。     

Temporal Oscillations in the Convective Boundary Layer Forced by Mesoscale Surface Heat-Flux Variations

A theoretical approach suggests that the surface heterogeneity on a scale of tens of kilometres can generate mesoscale motions that are not in a quasi-stationary state. The starting point of the theoretical approach is the equations of horizontal velocity and potential temperature that are low-pass filtered with a mesoscale cut-off wavelength. The transition of the generated mesoscale motions from a quasi-stationary state to a non-stationary state occurs when horizontal advection is strong enough to level out the potential temperature gradient on the surface heterogeneity scale. Large-eddy simulations (LES) suggest that the convective boundary layer (CBL) changes to a non-stationary state when forced by a surface heat-flux variation of amplitude of 100W m⁻² or higher and a wavelength of the order of 10 km. Spectral analysis of the LES reveals that when the mesoscale motions are in a quasi-stationary state, the energy provided by the surface heat-flux variation remains in organized mesoscale motions on the scale of the surface variation itself. However, in a non-stationary state, the energy cascades to smaller scales, with the cascade extending down into the turbulence scale when the wavelength of the surface heat-flux variation is on a scale smaller than 100 times the CBL height. The energy transfer from the generated mesoscale motions to the CBL turbulence results in the absence of a spectral gap between the two scales. The absence of an obvious spectral gap between the generated mesoscale motions and the turbulence raises questions about the applicability of mesoscale models for studies on the effect of high-amplitude surface heterogeneity on a scale of tens of kilometres. The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

A Case Study on the Effects of Heterogeneous Soil Moisture on Mesoscale Boundary-Layer Structure in the Southern Great Plains, U.S.A. Part II: Mesoscale Modelling

The importance of soil moisture inputs and improved model physics in the prediction of the daytime boundary-layer structure during the Southern Great Plains Hydrology Experiment 1997 (SGP97) is investigated using the non-hydrostatic fifth-generation Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR) Mesoscale Model MM5. This is Part II of a two-part study examining the relationship of surface heterogeneity to observed boundary-layer structure. Part I focuses on observations and utilizes a simple model while Part II uses observations and MM5 modelling. Soil moisture inputs tested include a lookup table based on soil type and season, output from an offline land-surface model (LSM) forced by atmospheric observations, and high-resolution ( 800 m) airborne microwave remotely sensed data. Model physics improvements are investigated by comparing an LSM directly coupled with the MM5 to a simpler force-restore method at the surface. The scale of land surface heterogeneities is compared to the scale of their effects on boundary-layer structure.The use of more detailed soil moisture fields allowed the MM5 to better represent the large-scale (hundreds of km) and small-scale (tens of km) horizontal gradients in surface-layer weather and, to a lesser degree, the atmospheric boundary-layer (ABL) height, which was evaluated against observations measured by differential absorption lidar (DIAL). The benefits of coupling an LSM to the MM5 were not readily evident in this summertime case, with the model having particular difficulty simulating the timing of maximum surface fluxes while underestimating the depth of the mixed layer.     

Numerical simulations of wind and temperature structure within an Alpine lake basin, Lake Tekapo, New Zealand

Summary High-resolution numerical model simulations have been used to study the local and mesoscale thermal circulations in an Alpine lake basin. The lake (87km²) is situated in the Southern Alps, New Zealand and is located in a glacially excavated rock basin surrounded by mountain ranges that reach 3000m in height. The mesoscale model used (RAMS) is a three-dimensional non-hydrostatic model with a level 2.5 turbulence closure scheme. The model demonstrates that thermal forcing at local (within the basin) and regional (coast-to-basin inflow) scales drive the observed boundary-layer airflow in the lake basin during clear anticyclonic summertime conditions. The results show that the lake can modify (perturb) both the local and regional wind systems. Following sunrise, local thermal circulations dominate, including a lake breeze component that becomes embedded within the background valley wind system. This results in a more divergent flow in the basin extending across the lake shoreline. However, a closed lake breeze circulation is neither observed nor modelled. Modelling results indicate that in the latter part of the day when the mesoscale (coast-to-basin) inflow occurs, the relatively cold pool of lake air in the basin can cause the intrusion to decouple from the surface. Measured data provide qualitative and quantitative support for the model results.     

In the Core of the Night-Effects of Intermittent Mixing on a Horizontally Heterogeneous Surface

Intermittent breakdowns that accompany wind gusts at the surface are responsible for a large fraction of the turbulent exchange between the surface and the upper boundary layer in the core of clear nights. Vertical and horizontal structure of the breakdowns are investigated using data from a network of 26 stations in an area of 30 km × 30 km. Surface heterogeneity in the area includes complex terrain with different types of land cover. We treat the fine-scale landscape structure near sensors (sheltering) as a separate component of heterogeneity. These features have important consequences on the spatial distribution of mean variables and surface fluxes. We found that breakdowns connect the surface layer to a higher level (level H_C). Weak wind gusts below a threshold (approximately 1.5 m s^-1) mix the air down to the colder ground, cooling the surface layer. On the other hand, wind gusts above this threshold promote mixing with upper levels, warming the surface layer. The spatial maximum of surface temperature over the network can be used as an estimate of the temperature at H_C, allowing vertical gradients and stability to be approximated. Minimum temperature is a function of topography and sheltering. Appreciable surface fluxes at night occur primarily at high, open locations, and can be large enough there to influencearea-averaged values. Surface-fluxparameterizations currently used in mesoscale models were tested first by estimating fluxes at each station and aggregating, and then by formingarea-averages before estimating fluxes. Results show that these formulations underestimate the average surface fluxes over a region for most of the nights.     

Near Wall Flow over Urban-like Roughness

In this study, comprehensive measurements over a number of urban-type surfaces with the same area density of 25% have been performed in a wind tunnel. The experiments were conducted at a free stream velocity of 10 m s^-1 and the main instrumentation was 120 ° x-wire anemometry, but measurement accuracy was checked using laser Doppler anemometry.The results haveconfirmed the strong three-dimensionalityof the turbulent flow inthe roughness sublayer and the depths of the inertial sublayer (log-law region) and roughness sublayer for each surface have been determined. Spatial averaging has been used to remove the variability of the flow in the roughness sublayer due to individual obstacles and it is shown that the spatially averaged mean velocity in the inertial sublayer and roughness sublayer can,together, be described by a single log-law with a mean zero-plane displacement and roughness length for the surface, provided that the proper surface stress is known. The spatially averaged shear stresses in the inertial sublayer and roughness sublayer are compared with the surface stress deduced from form drag measurements on the roughness elements themselves.The dispersive stress arising from the spatial inhomogeneity in the mean flow profiles was deduced from the data and is shown to be negligible compared with the usual Reynolds stresses in the roughness sublayer. Comparisons have been made between a homogeneous (regular element array) surface and one consisting of random height elements of the same total volume. Although the upper limits of the inertial sublayer for both surfaces were almost identical at equivalent fetch, the roughness sublayer was much thicker for the random surface than for the uniform surface, the friction velocity and the roughness length were significantly larger and the `roughness efficiency' was greater. It is argued that the inertial sublayer may not exist at all in some of the more extreme rough urban areas. These results will provide fundamental information for modelling urban air quality and forecasting urban wind climates.     

Field And Wind-Tunnel Studies Of Aerodynamic Roughness Length

The aerodynamic roughness length (z₀) values of three Gobi desert surfaces were obtained by measurement of the boundary-layer wind profile in the field. To clarify the factors affecting the Gobi surface aerodynamic roughness length, a wind-tunnel experiment was conducted. The wind-tunnel simulation shows that z₀ values increase with increasingsize and coverage of roughness elements. Especially, the shape and height of roughnesselements are more important than other factors in affecting roughness length. The roughness length increases with decreasing values of the geometric parameter (the ratio of element horizontal surface area to height, ) of roughness elements. But at a higher free stream velocity, the height is more important than the shape in affecting roughness length.     

Effect of Surface Heterogeneity on the Boundary-Layer Height: A Case Study at a Semi-Arid Forest

We investigate the effects of an isolated meso-\(\gamma \)-scale surface heterogeneity for roughness and albedo on the atmospheric boundary-layer (ABL) height, with a case study at a semi-arid forest surrounded by sparse shrubland (forest area: \(28~\text{ km }^2\), forest length in the main wind direction: 7 km). Doppler lidar and ceilometer measurements at this semi-arid forest show an increase in the ABL height over the forest compared with the shrubland on four out of eight days. The differences in the ABL height between shrubland and forest are explained for all days with a model that assumes a linear growth of the internal boundary layer of the forest through the convective ABL upwind of the forest followed by a square-root growth into the stable free atmosphere. For the environmental conditions that existed during our measurements, the increase in ABL height due to large sensible heat fluxes from the forest (\(600~\text {W~m}^{-2}\) in summer) is subdued by stable stratification in the free atmosphere above the ABL, or reduced by high wind speeds in the mixed layer.     

The spectral composition of fluxes and variances over land and sea out to the mesoscale

We discuss the accuracy requirements for measuring mesoscale (roughly horizontal scales > 10 km or 5 to 10 times the planetary boundary-layer (PBL) depth) fluxes in the convective PBL, and the ability of current research aircraft to achieve this accuracy. We conclude that aircraft equipped with inertial nagivation systems capable of < 3 km hr⁻¹ navigational accuracy are able to resolve mesoscale fluctuations in velocity, and thus variances and fluxes on the mesoscale. We then discuss measurements of velocity and scalar spectra, and cospectra of vertical velocity with horizontal velocity components and scalars, obtained from long flight legs with the National Center for Atmospheric Research Electra aircraft over the boreal forest of Canada in summer during the BOreal Ecosystem-Atmosphere Study (BOREAS), over the tropical Pacific Ocean from the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE), and over the East China Sea during wintertime cold-air outbreaks from the Air Mass Transformation Experiment (AMTEX). Each of these studies has somewhat different forcings and boundary conditions, so we can compare their consequences on the spectra and cospectra. On average, we found no significant scalar or momentum fluxes for horizontal scales > 10 km. We also develop a simple model based on observed thermal structure to explain the phase angle between vertical velocity and the along-wind horizontal velocity as a function of height, which shows good agreement with the observed phase angle in AMTEX. The National Center for Atmospheric Research is sponsored by the National Science Foundation.