Effect of surface layer thickness on simultaneous transport of heat and water in a bare soil and its implications for land surface schemes

Abstract

A physically‐based multi‐layer numerical model is developed to determine the coupled transport of heat and water in the soil and in the soil‐atmosphere boundary layer. Using inputs of standard weather data and initial soil conditions the model is capable of predicting the surface energy balance components as well as water content and temperature profiles in the soil. It is used to predict these variables for a bare silt loam soil under two tillage treatments, viz. culti‐packed and left loose after disc‐harrowing, and the predicted results are compared with measurements. Very good agreement between the model predictions and measured evaporation and heat fluxes and soil water and temperatures for a ten‐day period shows that the model is capable of simulating the coupled transport of soil heat and soil water and their transfer across the soil surface‐atmosphere interface adequately.

Model predictions were compared with those of CLASS (Canadian Land Surface Scheme). It is shown that CLASS, version 2.6, provides good estimates of evaporation and hence the latent heat flux density, Q_E, under wetter soil conditions, but overestimates Q_E at moderately wet soil conditions and underestimates it under dry soil conditions. Under dry to moderately wet soil conditions the calculation of evaporation from bare soil is very sensitive to the thickness of the top layer particularly as the thickness approaches 10 cm.     

An Improvement of the Louis Scheme for the Surface Layer in an Atmospheric Modelling System

The Louis scheme for calculating the vertical eddy fluxes within the atmospheric surface layer is improved by broadening the original assumptions. In our approach, the momentum and heat transfer roughness lengths (z₀ and z_T respectively) can be different, and z₀ need not be negligibly small compared with the lowest height (z) in modelling. For these conditions, we choose more consistent wind and potential temperature profile forms, then derive new algorithms for calculating fluxes. Improvement is demonstrated for a wide range of z/L (L is the Obukhov length), z/z₀ and z₀ z_T, by comparing these fluxes with those derived from a theoretical surface-layer model. The improved algorithms can be used in atmospheric modelling systems for more varied surfaces and a wide range of atmospheric stability.     

Measurements Of Turbulence Transfer In The Near-Surface Layer Over The Southeastern Tibetan Plateau

Turbulent flux measurements at Qamdo site over the Tibetan Plateau during TIPEX from May 18 to June 30, 1998 are presented. Sensible heat dominated,accounting for about 66% of the available energy (the sum of net radiation and soil heat flux) prior to the monsoon(dry period), reducing to about 31%, with latent heat increased to about 56% of available energy,in the monsoon season (wet period). Surface energy budget closure on average was about 0.80 (0.85)prior to the monsoon and 0.89 (0.76) during the monsoon using eddy correlation (profile) methods. The sum of latent and sensible heat fluxes calculated from the flux-profilemethod was smaller by about 15% than that from eddy correlation. Martano's method is used toestimate the surface aerodynamic roughness length z₀ and zero plane displacement d from singlelevel sonic anemometer data, giving d = 0.12 m and z₀ = 0.08 m. The overall neutral dragcoefficient (C_DN) and scalar coefficient (C_HN) were found to be C_DN = 0.0055and C_HN = 0.0059 in the southeastern area of Tibet. Their variations with the mean wind speed at 10 m are discussed.     

A Vegetated Urban Canopy Model for Meteorological and Environmental Modelling

An urban canopy model is developed for use in mesoscale meteorological and environmental modelling. The urban geometry is composed of simple homogeneous buildings characterized by the canyon aspect ratio (h/w) as well as the canyon vegetation characterized by the leaf aspect ratio (σ _l ) and leaf area density profile. Five energy exchanging surfaces (roof, wall, road, leaf, soil) are considered in the model, and energy conservation relations are applied to each component. In addition, the temperature and specific humidity of canopy air are predicted without the assumption of thermal equilibrium. For radiative transfer within the canyon, multiple reflections for shortwave radiation and one reflection for longwave radiation are considered, while the shadowing and absorption of radiation due to the canyon vegetation are computed by using the transmissivity and the leaf area density profile function. The model is evaluated using field measurements in Vancouver, British Columbia and Marseille, France. Results show that the model quite well simulates the observations of surface temperatures, canopy air temperature and specific humidity, momentum flux, net radiation, and energy partitioning into turbulent fluxes and storage heat flux. Sensitivity tests show that the canyon vegetation has a large influence not only on surface temperatures but also on the partitioning of sensible and latent heat fluxes. In addition, the surface energy balance can be affected by soil moisture content and leaf area index as well as the fraction of vegetation. These results suggest that a proper parameterization of the canyon vegetation is prerequisite for urban modelling.     

均匀裸土地表土壤热通量计算方法对比及对能量闭合的影响

利用"内蒙古微气象观测蒸发试验"的观测资料,对6种地表土壤热通量计算方法(Plate Cal法、TDEC法、谐波法、热传导对流法、振幅法和相位法)进行比较,检验了6种方法在不同干湿地表状况下的适用性,并研究了6种方法计算地表土壤热通量的差异以及对地表能量闭合度的影响。结果表明:一般情况下,Plate Cal法计算的2 cm土壤热通量与观测值最接近,计算结果的均方差为6.9 W/m2。在不同干湿地表状况下,干燥和降水条件下适合使用Plate Cal法,计算结果的均方差分别为14.0 W/m2和30.1 W/m2;湿润条件下适合使用谐波法,计算结果的均方差为21.4 W/m2。6种方法计算的地表土壤热通量存在明显差别,最大相差178.6 W/m2,不同方法计算地表土壤热通量的最大差值超过25 W/m2的时次占样本的96.3%。不同方法计算地表土壤热通量的差异对地表能量闭合度的大小有明显影响,但不影响近地层能量闭合度随湍流混合增强而增大的规律。     

黄河源高寒湿地-大气间暖季水热交换特征及关键影响参数研究

湿地是由陆地和水体形成的自然综合体,具有重要的生态、水文和生物地球化学功能,黄河源高寒湿地作为黄河重要的水源涵养区,对其下垫面水热交换特征及关键影响参数的研究具有非常重要的意义。本文利用中国科学院西北生态环境资源研究院麻多黄河源气候与环境变化观测站2014年6~8月观测资料,分析了黄河源区高寒湿地-大气间暖季水热交换特征,并利用公用陆面模式（Community Land Model,简称CLM）模拟了热通量变化,提出针对高寒湿地的粗糙度优化方案。主要结果如下:（1）暖季向上、向下短波与净辐射的平均日变化规律一致,向上、向下长波平均日变化平缓,地表温度升高相对于向下短波具有滞后性,潜热通量始终为正值并大于感热通量;（2）温度变化显著层结为20 cm以上土壤浅层,存在明显的日循环规律,土壤中热量09:00（北京时,下同）下传至5 cm深度,温度升高,11:00至10 cm深度,13:00至20 cm深度,18:00后开始上传,温度降低,40 cm及以下深度受此影响较小,热量在土壤中整体由浅层向深层输送;（3）土壤湿度平均日变化小,5 cm深度为土壤湿度最小层,10 cm深度为最大层;（4）麻多高寒湿地动力学粗糙度Z_0m在暖季变化稳定,可作为常数,Z_0m=0.0143 m;（5）提出更加适合高寒湿地下垫面暖季附加阻尼kB-1参数化方案,使得热通量模拟效果较CLM原始方案有所提高。以上结果对于研究湿地下垫面陆面过程具有重要意义。     

Bulk Transfer Relations for the Roughness Sublayer

In the roughness sublayer (RSL), Monin–Obukhov surface layer similarity theory fails. This is problematic for atmospheric modelling applications over domains that include rough terrain such as forests or cities, since in these situations numerical models often have the lowest model level located within the RSL. Based on empirical RSL profile functions for momentum and scalar quantities, and scaling the height with the RSL height z _*, we derive a simple bulk transfer relation that accounts for RSL effects. To verify the validity of our approach, these relations are employed together with wind speed and temperature profiles measured over boreal forest during the BOREAS experimental campaign to estimate momentum and heat fluxes. It is demonstrated that, when compared with observed flux values, the inclusion of RSL effects in the transfer relations yields a considerable improvement in the estimated fluxes.     

Estimation of atmospheric surface layer parameters and numerical simulation using MM5 at Mangalore, West Coast of India

Atmospheric surface layer meteorological observations obtained from 20-m-high meteorological tower at Mangalore, situated along the west coast of India are used to estimate the surface layer scaling parameters of roughness length (z _o) and drag coefficient (C _D), surface layer fluxes of sensible heat and momentum. These parameters are computed using the simple flux–profile relationships under the framework of Monin–Obukhov (M–O) similarity theory. The estimated values of z _o are higher (1.35–1.54 m) than the values reported in the literature (>0.4–0.9 m) probably due to the undulating topography surrounding the location. The magnitude of C _D is high for low wind speed (<1.5 m s^?1) and found to be in the range 0.005–0.03. The variations of sensible heat fluxes (SHF) and momentum fluxes are also discussed. Relatively high fluxes of heat and momentum are observed during typical days on 26–27 February 2004 and 10–11 April 2004 due to the daytime unstable atmospheric conditions. Stable or near neutral conditions prevail after 1700 h IST with negative SHF. A mesoscale model PSU/NCAR MM5 is run using a high-resolution (1 km) grid over the study region to examine the influence of complex topography on the surface layer parameters and the simulated fluxes are compared with estimated values. Spatial variations of the frictional velocity (u _*), C _D, surface fluxes, planetary boundary layer (PBL) height and surface winds are noticed according to the topographic variations in the simulation.     

A Case Study Of An On-Ice Air Flow Over The Arctic Marginal Sea-Ice Zone

A case study of warm air advection over the Arctic marginalsea-ice zone is presented, based on aircraft observations with direct flux measurements carriedout in early spring, 1998. A shallow atmospheric boundary layer (ABL) was observed, which wasgradually cooling with distance downwind of the ice edge. This process was mainly connected with astrong stable stratification and downward turbulent heat fluxes of about 10–20 W m^-2, but wasalso due to radiative cooling. Two mesoscale models, one hydrostatic and the other non-hydrostatic,having different turbulence closures, were applied. Despite these fundamental differences betweenthe models, the results of both agreed well with the observed data. Various closure assumptions had amore crucial influence on the results than the differences between the models.Such an assumption was, for example,the parameterization of the surface roughness for momentum (z₀) and heat (z_T). This stronglyaffected the wind and temperature fields not only close to the surface but also within and abovethe temperature inversion layer. The best results were achieved using a formulation for z₀ that took intoaccount the form drag effect of sea-ice ridges together withz_T = 0.1z₀. The stability within theelevated inversion strongly depended on the minimum eddy diffusivity K_min. A simple ad hocparameterization seems applicable, where K_min is calculated as 0.005 timesthe neutral eddy diffusivity. Although the longwave radiative cooling was largest within the ABL, theapplication of a radiation scheme was less important there than above the ABL. This was related to theinteraction of the turbulent and radiative fluxes. To reproduce the strong inversion, it wasnecessary to use vertical and horizontal resolutions higher than those applied in most regional andlarge-scale atmospheric models.     

A simple extension of the Louis method for rough surface layer modelling

The surface flux calculation method proposed by Louis (1979) is extended by allowing momentum and heat to have different roughness lengths (z ₀ andz T respectively). Our approach is to extend the traditional analysis by a more careful consideration of potential temperature structure near the surface. This consideration leads to a redefinition of the bulk Richardson number. For bulk transfer coefficients, our method shows a significant improvement over the original Louis method when compared with the theoretical surface-layer model applied to rough surfaces. Numerical experiments simulating sea breezes in 2D show that our extension is crucially important in simulating light wind and low humidity conditions.     

Variance Method to Determine Turbulent Fluxes of Momentum And Sensible Heat in The Stable Atmospheric Surface Layer

Evidence is presented that in the stable atmospheric surface layer turbulent fluxes of heat and momentum can be determined from the standard deviations of longitudinal wind velocity and temperature, σ _u and σ _T respectively, measured at a single level. An attractive aspect of this method is that it yields fluxes from measurements that can be obtained with two-dimensional sonic anemometers. These instruments are increasingly being used at official weather stations, where they replace the standard cup anemometer–wind vane system. With methods such as the one described in this note, a widespread, good quality, flux network can be established, which would greatly benefit the modelling community. It is shown that a ‘variance’ dimensionless height (ζ _σ) defined from σ _u and σ _T is highly related to the ‘conventional’ dimensionless stability parameter ζ=z/L, where z is height and L is the Obukhov length. Empirical functions for ζ _σ are proposed that allow direct calculation of heat and momentum fluxes from σ _u and σ _T. The method performs fairly well also during a night of intermittent turbulence.     

MM5和ETA相似理论近地层方案对农田下垫面通量模拟比较研究

近地层湍流通量计算对于中尺度数值模式有重要意义, 湍流通量的参数化是当前大气边界层研究的重要课题之一。选择青藏高原东缘大理观象台边界层通量观测系统, 离线测试了WRF区域模式中的两种常用的近地层参数化方案(MM5相似理论非迭代方案A和ETA 相似理论迭代方案B), 并将参数化方案计算结果与边界层铁塔涡动相关法的观测值进行对比分析。在大理观象台观测场不同植被随季节交替的状况下, 根据边界层铁塔4层高度风速拟合, 发现近地层空气动力学粗糙度随季节变化特征明显。将拟合的空气动力学粗糙度输入模式参数化方案进行通量计算。结果表明:稳定度是影响近地层参数化方案精度的重要因素, 在不稳定条件下方案B低估了动量通量, 方案A优于方案B, 而在稳定条件下方案A低估了动量通量, 方案B优于方案A, 两种方案总体来看误差不大。对于大理边界层通量观测场地农田植被交替的环境条件, 不同季节下垫面植被类型的差异, 以及植被的稀疏对近地层参数化方案湍流通量计算结果的精度有显著影响。方案B考虑了空气动力学粗糙度z₀和热量粗糙度z_0h的差异, 不稳定条件下感热通量计算结果在裸土或稀少植被条件下明显优于方案A。针对方案B不稳定条件下感热通量计算结果在裸土下垫面仍出现高估的现象, 在使用了(Zeng, et al, 1998)提出的对于使用辐射地表温度在裸土下垫面时的订正方法后, 计算结果也有明显改善。     

MM5和ETA相似理论近地层方案对农田下垫面通量模拟比较研究

近地层湍流通量计算对于中尺度数值模式有重要意义,湍流通量的参数化是当前大气边界层研究的重要课题之一.选择青藏高原东缘大理观象台边界层通量观测系统,离线测试了WRF区域模式中的两种常用的近地层参数化方案(MM5相似理论非迭代方案A和ETA相似理论迭代方案B),并将参数化方案计算结果与边界层铁塔涡动相关法的观测值进行对比分析.在大理观象台观测场不同植被随季节交替的状况下,根据边界层铁塔4层高度风速拟合,发现近地层空气动力学粗糙度随季节变化特征明显.将拟合的空气动力学粗糙度输入模式参数化方案进行通量计算.结果表明:稳定度是影响近地层参数化方案精度的重要因素,在不稳定条件下方案B低估了动量通量,方案A优于方案B,而在稳定条件下方案A低估了动量通量,方案B优于方案A,两种方案总体来看误差不大.对于大理边界层通量观测场农田植被交替的环境条件,不同季节下垫面植被类型的差异,以及植被的稀疏对近地层参数化方案湍流通量计算结果的精度有显着影响.方案B考虑了空气动力学粗糙度z₀和热量粗糙度z_0h的差异,不稳定条件下感热通量计算结果在裸土或稀少植被条件下明显优于方案A.针对方案B不稳定条件下感热通量计算结果在裸土下垫面仍出现高估的现象,使用了Zeng等1998年提出的用辐射地表温度订正裸土下垫面感热能量方法后,计算结果也有明显改善.     

Bare-ground surface heat and water exchanges under dry conditions: Observations and parameterization

A simplified land-surface parameterization is tested against bare-soil data collected during the EFEDA experiment conducted in Spain in June 1991. A complete data set, made up of soil properties as well as hydrological and atmospheric measurements, is described and discussed. The 11-day data set is characterized by very dry conditions and high surface temperatures during the day. Large values of sensible and soil heat fluxes and small values of surface evaporation (≈1 mm/day) were observed. This data set was modelled, leading to the following conclusions:

1. In the model, the parameterization provides values of the soil thermal properties and subsequently of the predicted soil heat fluxes which are overestimated when compared with the observations.
2. Following the literature, a value of the ratio between the roughness lengths for momentumZ _oand heatZ _ohof close to 10 for fairly homogeneous areas of bare soil and vegetation is used. This value leads to a fair prediction of the surface temperature. If the roughness lengths were taken to be equal, as is often assumed in atmospheric modelling, a poorer prediction results.
3. Finally, the vapor phase transfer mode is found dominant close to the surface and a modified parameterization including this effect is proposed. It allows a fair prediction of both surface evaporation and near-surface water content.

     

Mid-21st century projections in temperature extremes in the southern Colorado Rocky Mountains from regional climate models

This study analyzes mid-21st century projections of daily surface air minimum (T_min) and maximum (T_max) temperatures, by season and elevation, over the southern range of the Colorado Rocky Mountains. The projections are from four regional climate models (RCMs) that are part of the North American Regional Climate Change Assessment Program (NARCCAP). All four RCMs project 2°C or higher increases in T_min and T_max for all seasons. However, there are much greater (>3°C) increases in T_max during summer at higher elevations and in T_min during winter at lower elevations. T_max increases during summer are associated with drying conditions. The models simulate large reductions in latent heat fluxes and increases in sensible heat fluxes that are, in part, caused by decreases in precipitation and soil moisture. T_min increases during winter are found to be associated with decreases in surface snow cover, and increases in soil moisture and atmospheric water vapor. The increased moistening of the soil and atmosphere facilitates a greater diurnal retention of the daytime solar energy in the land surface and amplifies the longwave heating of the land surface at night. We hypothesize that the presence of significant surface moisture fluxes can modify the effects of snow-albedo feedback and results in greater wintertime warming at night than during the day.     

Effects of the soil heat flux estimates on surface energy balance closure over a semi-arid grassland

Soil heat flux is important for surface energy balance (SEB), and inaccurate estimation of soil heat flux often leads to surface energy imbalance. In this paper, by using observations of surface radiation fluxes and soil temperature gradients at a semi-arid grassland in Xilingguole, Inner Mongolia, China from June to September 2008, the characters of the SEB for the semi-arid grassland were analyzed. Firstly, monthly averaged diurnal variations of SEB components were revealed. A 30-min forward phase displacement of soil heat flux (G) observed by a fluxplate at the depth of 5-cm below the soil surface was conducted and its effect on the SEB was studied. Secondly, the surface soil heat flux (G _s) was computed by using harmonic analysis and the effect of the soil heat storage between the surface and the fluxplate on the SEB was examined. The results show that with the 30-min forward phase displacement of observed G, the slope of the ordinary linear regression (OLR) of turbulent fluxes (H+LE) against available energy (R _n-G) increased from 0.835 to 0.842, i.e., the closure ratio of SEB increased by 0.7%, yet energy imclosure of 15.8% still existed in the SEB. When G _s, instead of G was used in the SEB equation, the slope of corresponding OLR of (H+LE) against (R _n-G _s) reached 0.979, thereby the imclosure ratio of SEB was reduced to only 2.1%.     

青藏高原中部聂荣半干旱草地夏季近地层能量平衡与输送分析

利用青藏高原中部聂荣地区草地下垫面2014年7~8月近地层气象要素梯度观测及湍流观测数据,分析讨论了该地区观测期间的基本气象要素特征、能量平衡特征以及能量输送特征,主要结论如下:(1)向下、向上短波辐射和净辐射日变化规律一致,向下、向上长波辐射日变化平缓。反照率呈"U"型分布,早晚大,中午小,聂荣夏季地表平均反照率为0.20。(2)在夏季白天,聂荣地区净辐射大部分以潜热的形式加热大气。考虑了土壤浅层热储存和垂直运动引起的平流输送后,能量闭合率由0.65提高到0.80,闭合率有显著的提高。(3)在不稳定层结下,动量总体输送系数CD平均值为4.7×10~(-3)和热量总体输送系数CH平均值为3.5×10~(-3);在稳定层结下,CD平均值为3.4×10~(-3),CH平均值为1.8×10~(-3);C_D和C_H在近中性层结下的平均值分别为4.30×10~(-3)和2.39 10~(-3)。     

Parameterizing turbulent exchange over sea ice: the ice station weddell results

A 4-month deployment on Ice Station Weddell (ISW) in the western Weddell Sea yielded over 2000 h of nearly continuous surface-level meteorological data, including eddy-covariance measurements of the turbulent surface fluxes of momentum, and sensible and latent heat. Those data lead to a new parameterization for the roughness length for wind speed, z₀, for snow-covered sea ice that combines three regimes: an aerodynamically smooth regime, a high-wind saltation regime, and an intermediate regime between these two extremes where the macroscale or `permanent' roughness of the snow and ice determines z₀. Roughness lengths for temperature, z_T, computed from this data set corroborate the theoretical model that Andreas published in 1987. Roughness lengths for humidity,z_Q, do not support this model as conclusively but are all, on average, within an order of magnitude of its predictions. Only rarely arez_Tand z_Q equal to z₀. These parameterizations have implications for models that treat the atmosphere-ice-ocean system.     

Towards Closing the Surface Energy Budget of a Mid-latitude Grassland

Observations for May and August, 2005, from a long-term grassland meteorological station situated in central Netherlands were used to evaluate the closure of the surface energy budget. We compute all possible enthalpy changes, such as the grass cover heat storage, dew water heat storage, air mass heat storage and the photosynthesis energy flux, over an averaging time interval. In addition, the soil heat flux was estimated using a harmonic analysis technique to obtain a more accurate assessment of the surface soil heat flux. By doing so, a closure of 96% was obtained. The harmonic analysis technique appears to improve closure by 9%, the photosynthesis for 3% and the rest of the storage terms for a 3% improvement of the energy budget closure. For calm nights (friction velocity u _* < 0.1 m s⁻¹) when the eddy covariance technique is unreliable for measurement of the vertical turbulent fluxes, the inclusion of a scheme that calculates dew fluxes improves the energy budget closure significantly.     

The surface heat flux density of a bare soil

Abstract

Half‐hourly measurements of soil surface heat flux density (G₀ ), solar irradiance (S), and the surface energy balance components were made at Agassiz, b.c., in the spring and early summer of 1978 at two adjacent bare‐soil sites, one of which was culti‐packed while the other was disc‐harrowed. G₀ was calculated using the null‐alignment procedure from half‐hourly measurements of soil temperature at 30 depths down to 1 m, and volumetric soil heat capacity calculated from measurements of bulk density, organic matter fraction, and moisture content. The latent and sensible heat flux densities were measured using the energy balance/Bowen ratio technique.

It was found that both the daily averages and diurnal variations of Go at each site were not affected as the soil surface dried, despite reductions in evaporation rate of as much as 50% at the culti‐packed site and 75% at the disc‐harrowed site on the clear dry‐soil days. Diurnal variations of G₀ at the disc‐harrowed site were about 25% less than at the culti‐packed site, although daily averages were similar at both sites. Daily and daytime averages of G₀ at each site were linear functions of S alone, or functions of net radiation and some measure of near‐surface soil water content. Night‐time averages of G₀ at each site were linear functions of a cloudiness ratio equal to the fraction received of the clear‐day S.