Calculation of sensible and latent heat fluxes,and surface resistance from profile data

A set of semi-continuous measurements of temperature, wind and moisture gradients as well as of net radiation and ground heat flux covering a period of about one and a half years has been analysed to give a corresponding set of complete surface energy balance data on an hourly basis. An analysis of the evaporation data so obtained is given.It is shown that surface resistance r _S exhibits a diurnal trend: values are smallest (ca. 150 s m^-1) a few hours before noon and increase to as much as 800 s m^-1 towards dusk. The minimum values tend to be higher during dry periods when the soil moisture is low. There is also some indication that r _S decreases rapidly soon after rainfall.An exponential relation is found between the fraction of available energy used as evaporative flux, , and r _S for values of r _I/r_S <0.70, where r _I is the climatological resistance. On the other hand, the ratio of r _I to r _S is linearly correlated with , implying that an equilibrium state is established between the grass surface and the atmosphere, at least from mid-morning to mid-afternoon when the leaves are dry. Near-noon values calculated by Stewart and Thom for Thetford Forest also follow a linear trend.The above two regression results (In (r _S) versus r _I/r_S versus ) are combined to obtain an empirical relation of the form r _I=m exp (a-b) which is used to estimate evaporative flux. The estimates are found to be within 20% of calculated values.Guest Scientist from Department of Physics, University of Cape Coast, Cape Coast, Ghana.     

Relating remotely sensed forest damage data to wind data: storms Lothar (1999) and Vivian (1990) in Switzerland

This study compares the surface wind speed and forest damage data of two exceptionally severe winter storms, Vivian 1990 and Lothar 1999. The study area comprises the region that suffered damage in Switzerland. The wind speed data were derived from simulations of MeteoSwiss (Federal Office of Meteorology and Climatology), measurements during the storm periods and expert analyses of the data. The remotely sensed forest damage data were provided by the Federal Office for the Environment and the forest cover data by Swiss Federal Statistical Office. We compared data on the peak gust and maximum average wind speed, with data on the spatially related forest area and forest damage area, and found some clear differences in the correlations between the different wind data and forest damage. Our results point generally to the damage-causing role of near-surface gusts at maximum wind speeds during the storm. These tended to be spatially distributed on a fine scale. In only a few cases were the results statistically significant. However, these results could probably be improved with better wind data. For example, gust measurements spatially closer to forests or simulations of gusts at maximum wind speed could be produced with a spatially higher resolution.     

利用区域尺度气象模式模拟黑河地区地表能量通量的研究

应用中尺度区域模式RAMS (the regional atmospheric model system), 在40余组不同参数的条件下模拟中国干旱半干旱黑河地区的地表能量通量和土壤温度特征, 并以此探索模式在干旱半干旱地区的适用性。为了证明模拟结果的准确性和模式的稳定性, 模拟连续运行30天, 其中包含晴好和阴雨的天气过程。模拟结果表明: 即使使用较为可靠的NECP再分析气压层资料和实地探测资料, 进行初始场和参数的输入, RAMS的默认设置也很难较为合理地反演出地表能量通量, 只有合理地调整好其土壤特征参数, RAMS才能得到与实测资料符合得较好的结果。土壤特征参数对模拟结果影响较大, 影响因子的重要性依次为: 土壤含水量、 土壤层总厚度、 土壤温度。     

Variation in soil parameters: Implications for modeling surface fluxes and atmospheric boundary-layer development

Soil texture can be heterogeneous; however for land surface-atmospheric modeling purposes, it is often considered homogeneous at a particular point and described by empirical equations which have been formulated to describe average hydraulic and thermodynamic processes in the soil. Large deviations in the variables and coefficients used in these empirical equations have been previously documented. One of the coefficients is varied by plus-and-minus one standard deviation about its mean, and tested in a coupled atmospheric-plant-soil model. Results of model simulations show that the effects on surface fluxes and boundary-layer development are larges for dry to moderate values of soil moisture, particularly for bare soil conditions.     

Quadrant analysis of the scalar and momentum fluxes in the stable marine atmospheric surface layer

The quadrant technique, a conditional sampling approach that allocates Reynolds stresses into four different types of events (ejections or bursts, sweeps, inward interactions and outward interactions), is applied to stable marine atmospheric boundary-layer data, collected in the framework of the Coupled Boundary Layer Air—Sea Transfer, Low wind component experiment at Nantucket Island, Massachusetts, USA. The general properties of both scalar and momentum transport are analyzed under the scope of quadrant analysis experimentally and theoretically. It is shown that the third-order Gram–Charlier series is necessary and even sufficient in most of the cases, in describing the experimental time and flux contributions of each quadrant to the total transfer, for both scalar and momentum transport, while the ability of the Gaussian distribution is limited to outlining the general pattern of these quantities. Moreover, a threshold value is applied to the conditional analysis, separating the most important events from the less significant ones and the sensitivity of the flux and especially the time fraction of each quadrant on the choice of this value is presented and discussed. Also, a set of numerically extracted equations, completing the analytical relations, is derived, enabling the prediction of the time and flux fractions of each quadrant, for a wide range of correlation coefficient and threshold values. Finally, the sensitivity of the analysis to the atmospheric stability and the Reynolds averaging scales showed that correlated and uncorrelated motions tend to balance for increasingly stable conditions and/or for large time scales.     

Influence of irrigated agriculture on diurnal surface energy and water fluxes, surface climate, and atmospheric circulation in California

The impact of land use change on regional climate can be substantial but also is variable in space and time. Past observational and modeling work suggests that in a ‘Mediterranean’ climate such as in California’s Central Valley, the impact of irrigated agriculture can be large in the dry season but negligible in the wet season due to seasonal variation in surface energy partitioning. Here we report further analysis of regional climate model simulations showing that diurnal variation in the impact of irrigated agriculture on climate similarly reflects variation in surface energy partitioning, as well as smaller changes in net radiation. With conversion of natural vegetation to irrigated agriculture, statistically significant decreases of 4–8?K at 2?m occurred at midday June–September, and small decreases of ~1?K occurred in winter months only in relatively dry years. This corresponded to reduced sensible heat flux of 100–350?W?m^?2 and increased latent heat fluxes of 200–450?W?m^?2 at the same times and in the same months. We also observed decreases of up to 1,500?m in boundary layer height at midday in summer months, and marginally significant reductions in surface zonal wind speed in July and August at 19:00 PST. The large decrease in daytime temperature due to shifts in energy partitioning overwhelmed any temperature increase related to the reduced zonal sea breeze. Such changes in climate and atmospheric dynamics from conversion to (or away from) irrigated agriculture could have important implications for regional air quality in California’s Central Valley.     

Spatial characteristics of surface and atmospheric properties during HartX

Summary Remote sensing and ground measurements were used to evaluate the homogeneity of the Scots pine plantation of Hartheim and the interactions between the forest and its surroundings during HartX.Remote sensing data contribute to characterization of the Scots pine plantation of Hartheim in terms of surface properties and thermal and biological characteristics of the forest. The surroundings of the main experiment site in the forest has homogenous normalized difference vegetation index, microwave specle and brightness temperatures to all directions for at least 500 m. Local sensible heat flux estimates using satellite measured brightness temperatures andin situ measured wind and air temperatures showed good agreement.On some days during HartX the test area was influenced by advection of dry air from the northern Upper Rhine Valley. Agricultural surroundings close to the forest are influenced by breezes flowing out of the forest during radiative nights with weak large-scale motion.With 7 Figures     

The surface structure and properties of atmospheric ice crystals

Theories to describe the structures and properties of ice surface were critically examined. Misconceptions were clarified in regard to treatment of the liquid-like layer that is expected to exist on the surface of an ice crystal below 0°C, 1 atm under equilibrium. It was shown that the pressure in the surface layer, not the surface free energy, is responsible for the formation of the liquid-like layer. Transformation of this layer into transitional liquid layer under ice-supersaturated conditions of the environment and the roles played by the latter layer in non-dendritic and dendritic growth of ice crystals were explained.     

应用MODIS资料对城乡地表热通量的估算

为了揭示城市热岛形成机制,基于MODIS资料,结合自动气象站实测的气象资料,利用地表能量参数化方法估算了地表热通量,分析了城乡地表热通量的空间分布及变化特征.结果表明城乡地气热交换差异明显,与相关文献对比证明该方法是可行、有效的.     

Turbulent fluxes of momentum,heat and water vapor in the atmospheric surface layer at sea during ATEX

The vertical turbulent fluxes have been determined during the Atlantic Trade Wind Experiment (ATEX) both by direct and profile methods. The drag coefficient obtained from direct measurements was c _D = 1.39 × 10^–3. A distortion of the wind profile due to wave action could be demonstrated, this produced an increased drag coefficient estimated by the profile method. The dissipation technique using the downwind spectrum gave a lower drag coefficient of 1.26 × 10^–3, probably due to non-isotropic conditions (the ratio of vertical to downwind spectrum at high frequencies scattered considerably with an average of 1 instead of 4/3).From direct measurements, the sensible heat flux showed a poor correlation with the bulk parameter product U, contrary to the heat flux obtained from profiles. It is shown that this is due to the higher frequency part of the cospectrum, say above 0.25 Hz, which contributes more than 50 % of the total flux. Determination of the heat flux from temperature fluctuations by the dissipation method would be in agreement with the direct determination only if the corresponding Kolmogoroff constant were 2.1 instead of 0.8.For the vertical flux of water vapor obtained from profiles, the bulk transfer coefficient was 1.28 × 10^–3.This work was supported by the Deutsche Forschungsgemeinschaft, Schwerpunktprogramm Meeresforschung and later the Sonderforschungsbereich Meeresforschung Hamburg.     

Rainwater chemistry and bulk atmospheric deposition in a tropical semiarid ecosystem: the Brazilian Caatinga

We assessed the rainwater chemistry, the potential sources of its main inorganic components and bulk atmospheric deposition in a rural tropical semiarid region in the Brazilian Caatinga. Rainfall samples were collected during two wet seasons, one during an extremely dry year (2012) and one during a year with normal rainfall (2013). According to measurements of the main inorganic ions in the rainwater (H⁺, Na⁺, NH₄ ⁺, K⁺, Ca²⁺, Mg²⁺, Cl^?, NO₃ ^?, and SO₄ ^2?), no differences were observed in the total ionic charge between the two investigated wet seasons. However, Ca²⁺, K⁺, NH₄ ⁺ and NO₃ ^? were significant higher in the wetter year (p < 0.05) which was attributed to anthropogenic activities, such as organic fertilizer applications. The total ionic contents of the rainwater suggested a dominant marine contribution, accounting for 76 % and 58 % of the rainwater in 2012 and 2013, respectively. The sum of the non-sea-salt fractions of Cl^?, SO₄ ^2?, Mg²⁺, Ca²⁺ and K⁺ were 19 % and 33 % in 2012 and 2013, and the nitrogenous compounds accounted for 2.8 % and 6.0 % of the total ionic contents in 2012 and 2013, respectively. The ionic ratios suggested that Mg²⁺ was probably the main neutralizing constituent of rainwater acidity, followed by Ca²⁺. We observed a low bulk atmospheric deposition of all major rainwater ions during both wet seasons. Regarding nitrogen deposition, we estimated slightly lower annual inputs than previous global estimates. Our findings contribute to the understanding of rainfall chemistry in northeastern Brazil by providing baseline information for a previously unstudied tropical semiarid ecosystem.     

Similarity theory and calculation of turbulent fluxes at the surface for the stably stratified atmospheric boundary layer

In this paper we revise the similarity theory for the stably stratified atmospheric boundary layer (ABL), formulate analytical approximations for the wind velocity and potential temperature profiles over the entire ABL, validate them against large-eddy simulation and observational data, and develop an improved surface flux calculation technique for use in operational models.     

Parameterizations of surface radiation in the semiarid grasslands of Inner Mongolia under clear-sky conditions using MODIS data

A set of improved and efficient radiation parameterization schemes for surface radiation balance components under clear- sky conditions was developed by using general surface measurements and MODIS data. The set of schemes was then adapted for regions similar to the present study sites under different grazing intensities and varying degrees of drought in the semiarid grasslands of Inner Mongolia. Specifically, we mainly improved two schemes for estimating downward shortwave and longwave radiation at the surface, which could be applied to regions with certain degrees of drought. The validation datasets were from ground-based observations at various grazing sites during the growing season （May to September） of different drought years, 2005 and 2006. Through comparisons of parameterized versus measured radiation values, the increased or modified factors in the original schemes demonstrated improved estimation accuracy, and the rationalities of input parameters and variables were analyzed. The regional instantaneous net radiation estimations had root-mean-square errors of less than 30 W m-2 compared with ground measurements at the sites during the study period. The statistical results showed the improved schemes are suitable for estimating surface net radiation in regional semiarid areas during the growing season. Analyses of the sensitivity of the schemes to corresponding variables were conducted to ascertain the major error sources of the schemes and potential variables for improving the performance of the schemes in agreement with observations.     

Causes of WRF surface energy fluxes biases in a stratocumulus region

In this study, we evaluate the ability of the Weather Research and Forecasting model to simulate surface energy fluxes in the southeast Pacific stratocumulus region. A total of 18 simulations is performed for the period of October to November 2008, with various combinations of boundary layer, microphysics, and cumulus schemes. Simulated surface energy fluxes are compared to those measured during VOCALS-REx. Using a process-based model evaluation, errors in surface fluxes are attributed to errors in cloud properties. Net surface flux errors are mostly traceable to errors in cloud liquid water path (LWP_cld), which produce biases in downward shortwave radiation. Two mechanisms controlling LWP_cld are diagnosed. One involves microphysics schemes, which control LWP_cld through the production of raindrops. The second mechanism involves boundary layer and cumulus schemes, which control moisture available for cloud by regulating boundary layer height. In this study, we demonstrate that when parameterizations are appropriately chosen, the stratocumulus deck and the related surface energy fluxes are reasonably well represented. In the most realistic experiments, the net surface flux is underestimated by about 10 W m^?2. This remaining low bias is due to a systematic overestimation of the total surface cooling due to sensible and latent heat fluxes in our simulations. There does not appear to be a single physical reason for this bias. Finally, our results also suggest that inaccurate representation of boundary layer height is an important factor limiting further gains in model realism.     

Latent heat fluxes simulated with a non-hydrostatic weather forecast model using actual surface properties from measurements and remote sensing

In this study the influence of land-surface parameters on latent heat fluxes simulated with the numerical weather prediction model Lokalmodell (LM) of the German Meteorological Service is investigated. The area of interest is the LITFASS area during the LITFASS-2003 campaign. Based on simulations with varying soil and vegetation properties, we confirm that simulated latent heat fluxes strongly depend on soil moisture and leaf area index. Both parameters are difficult to obtain from in situ measurements with sufficient spatial resolution over heterogeneous land surfaces. Therefore, a procedure is proposed to determine area average values of soil moisture from time domain reflectometer measurements performed at a limited number of sites. The area averages cover the 7 × 7 km² grid cells of the LM around Lindenberg (south-east of Berlin). Furthermore, satellite inferred plant parameters from NOAA–AVHRR are used to initialise model runs; the derived vegetation parameters show notable differences with those in the standard input of LM. The latent heat fluxes from the LM are compared with the aggregated eddy-covariance-measurements, and while the operational LM shows a strong overestimation of latent heat fluxes, it is demonstrated that the application of land-surface parameters derived from measurements can significantly reduce the deviation between the simulated and measured latent heat fluxes.     

Ensemble formulation of surface fluxes and improvement in evapotranspiration and cloud parameterizations in a GCM

The influence of some simple modifications to the physical parameterizations in the current GLAS climate GCM is examined. The aim of these modifications was to eliminate strong occasional bursts of 2 - t oscillations in the PBL fluxes. The PBL of the current GLAS climate model was modified by invoking concepts of ensemble averaging of PBL eddies in a grid cell of the GCM. This resulted in smoothly varying bulk aerodynamic friction and heat transport coefficients. An arbitrary function to account for diffusion of moisture from stomatal cavities found in vegetation was also incorporated. Simultaneously some modifications to the cloud parameterizations were made.Two integrations, one with the old model and the other with the modified model, were made to simulate 47 days, starting from the NMC analysis for June 15, 1979. Their comparisons showed that the surface fluxes and cloudiness in the modified model simulations are far better. The planetary albedo in the modified model is also realistic. However, some weaknesses still persist, including an ITCZ (Inter-tropical convergence zone) that is too far northward in Sahelian Africa, polar regions that are too cold, and a rather strong ITCZ. It is pointed out that these weaknesses are primarily caused by model deficiencies, e.g., the cloud parameterization and the uniformly prescribed land surface roughness height.In another simulation with the modified model using a realistic value of surface roughness for deserts, the precipitation in the Sahara Desert reduced significantly, which effectively pushed the ITCZ southward to a more realistic location as compared to observations.M/A Com Sigma Data Through contract No. NASA 25900.     

Formulation and verification of a land surface parameterization for atmospheric models

The need for a well-defined lower boundary condition for atmospheric numerical models is well documented. This paper describes the formulation of a land surface parameterization, which will be used in atmospheric boundary-layer and mesoscale numerical models. The land surface model has three soil layers for the prediction of soil moisture and soil temperature. Model soil properties depend on soil texture and moisture content. A homogeneous distribution of vegetation is also included, so that transpiration may be included, as well as the interception of precipitation by vegetation elements. The simulated vegetation also affects the mean surface albedo and roughness characteristics.First ISLSCP Field Experiment (FIFE) data are used to verify the model. Three cases during the growing season were chosen, each case having different amounts of vegetation cover. Stand alone simulations, where observations of atmospheric and radiation variables are input to the land surface model, were performed. These simulations show that the model is able to reproduce observed surface energy budgets and surface temperatures reasonably well. The RMS differences between modeled and obsered turbulent fluxes of heat and moisture are quite comparable to those reported by more detailed land surface models.