Borivoj Rajković

A resistance representation of schemes for evaporation from bare and partly plant-covered surfaces for use in atmospheric models

Abstract In the parameterization of land surface processes, attention must be devoted to surface evaporation, one of the main processes in the air-land energy exchange. One of the most used approaches is the resistance representation which requires the calculation of aerodynamic resistances. These resistances have been calculated using K theory for different morphologies of plant communities, then the performance of the evaporation schemes within the “α”, “β”, and their combination approaches that parameterize evaporation from bare and partly plant-covered soil surfaces are discussed. Additionally, a new “α” scheme is proposed based on an assumed power dependence α on volumetric soil moisture content and its saturated value. Finally, the performance of the considered and the proposed schemes is tasted based on time integrations using real data. The first set was for 4 June 1982, and the second for 3 June 1981 at the experimental site in Rimski Sansevi, Yugoslavia, on chernozem soil, as representative for ...

Sensitivity of soil surface temperature in a force‐restore equation to heat fluxes and deep soil temperature

The ‘force-restore’ approach is commonly used in order to calculate the surface temperature in atmospheric models. A critical point in this method is how to calculate the deep soil temperature which appears in the restore term of the ‘force-restore’ equation. If the prognostic equation for calculating the deep soil temperature is used, some errors in surface temperature calculation and consequently in partitioning the surface energy and land surface water can be introduced. Usually, these errors should appear as a result of incorrect parameterization of surface energy terms in the prognostic equation based on ‘force-restore’ approach. In this paper, the sensitivity of the ‘force-restore’ model for surface temperature to the: (a) changes of soil heat flux; (b) variations of deep soil temperature and (c) changes in soil water evaporation is examined. In addition, the impact of the deep soil temperature variations on partitioning the surface energy and land surface water is discussed. Finally, a new procedure for calculating the deep soil temperature based, on climatological data of soil temperature and its exponential attenuation in the deep soil layers is suggested. All numerical experiments with the LAPS land surface scheme were performed using two data sets, obtained from the micrometeorological measurements over a bare soil at Rimski Sancevi (Yugoslavia), RS, and Caumont (France), HAPEX. Copyright

Wind profile within the forest canopy and in the transition layer above it

Abstract The forest as an underlying surface has to be considered in atmospheric models of different scales. Experimental evidence shows that there can be a significant variation of the wind profile within the forest and in the so-called transition layer above it. Experimentally observed wind speed is often found below as indicated by the wind speed profile obtained by: (a) the logarithmic relationship in the transition layer and (b) K theory within the forest. This situation can seriously disturb the real physical picture concerning the transfer of momentum, heat and water vapour from the surface into the atmosphere. In order to minimise the foregoing problems, we have suggested an empirical expression for the wind profile in the transition layer above the forest as well as the expressions for the wind profile and turbulent momentum transfer coefficient within the forest canopy layer. Additionally, for the proposed wind profiles, the expressions for the displacement height, roughness length and parameters are determined as functions of the forest structural characteristics using continuity conditions and a simple mass conservation hypothesis. The validity of the proposed expressions was checked using the micrometeorological measurements from the experimental sites in the Thetford Scots pine forest in Norfolk, United Kingdom and in the Ponderosa pine forest at the Shasta Experimental Forest, California, USA.

Turbulent Transfer Coefficients and Calculation of Air Temperature inside Tall Grass Canopies in Land–Atmosphere Schemes for Environmental Modeling

Abstract A method for estimating profiles of turbulent transfer coefficients inside a vegetation canopy and their use in calculating the air temperature inside tall grass canopies in land surface schemes for environmental modeling is presented. The proposed method, based on K theory, is assessed using data measured in a maize canopy. The air temperature inside the canopy is determined diagnostically by a method based on detailed consideration of 1) calculations of turbulent fluxes, 2) the shape of the wind and turbulent transfer coefficient profiles, and 3) calculation of the aerodynamic resistances inside tall grass canopies. An expression for calculating the turbulent transfer coefficient inside sparse tall grass canopies is also suggested, including modification of the corresponding equation for the wind profile inside the canopy. The proposed calculations of K-theory parameters are tested using the Land–Air Parameterization Scheme (LAPS). Model outputs of air temperature inside the canopy for 8–17 Jul...

A Roughness Sublayer Wind Profile Above A Non-Uniform Surface

In atmospheric models for different scales the underlying surface consists of patches of bare soil and plant communities with different morphological parameters. Experimental evidence indicates that there is a significant departure of the wind profile above a vegetative surface from that predicted by the logarithmic relationship, which gives values that are greater than those observed. This situation can seriously disturb the physical picture concerning the transfers of momentum, heat and water vapour from the surface into the atmosphere.The intention of this paper is to generalise the calculation of exchange of momentum between the atmosphere and a non-homogenous vegetative surface, and to derive a general equation for the wind speed profile in a roughness sublayer under neutral conditions. Furthermore, these results are extended to non-neutral cases. The suggested expression for the wind profile is compared with some earlier approaches and the observations obtained above a broad range of plant communities.

The validation of various schemes for parameterizing evaporation from bare soil for use in meteorological models: A numerical study using in situ data

Parameterization of evaporation from a non-plant-covered surface is very important in the hierarchy strategy of modelling land surface processes. One of the representations frequently used in its computation is the resistance formulation. The performance of the evaporation schemes using the “α”, “β”, and their combination resistance approaches to parameterize evaporation from bare soil surfaces is discussed. For that purpose, the nine schemes, based on a different dependence of α and β on volumetric soil moisture content and its saturated value, are used.The tests of performances of the considered schemes are based on time integrations by the land surface module (BARESOIL) using observed data. The 23 data sets at a bare surface experimental site in Rimski Šančevi, Yugoslavia on chernozem soil, were used for the resistance algorithm evaluation. The quality of the schemes was compared with the observed values of the latent heat flux using several statistical parameters.

The main features of the hydrological module in the Land Air Parameterization Scheme (LAPS)

Abstract This paper describes the hydrological module of the biophysical scheme named LAPS (Land-Air Parameterization Scheme) which is designed as a software package which can be included as part of an atmospheric model or as a stand-alone model. It includes: hydrological processes, bare soil transfer processes and canopy transfer processes. In the hydrological module the direct loss of liquid water across the scheme domain boundaries is considered as separate processes which can be summarised as follows: overland flow (when a precipitation excess is over infiltration capacity or when the surface becomes saturated), subsurface runoff (horizontal drainage from unsaturated flow) and vertical drainage through the lower scheme boundary. Moving from the top to the bottom of the soil, the water column has the three layers where the vertical water flow is considered according to Darcys law. The performance of this module through long term integration, using HAPEX-MOBILHY data set are examined.

Some Indicators of the Present and Future Climate of Serbia According to the SRES-A1B Scenario

According to the last IPCC report, there are several indications that the area of Southeastern Europe might experience large climate changes due to the increase of the concentration of greenhouse gases. These include results of several regional climate studies. In order to objectively study the climate change, precipitation and temperature indices can be used. Climate indices can be calculated either from the local observations of temperature and precipitation or from climate simulations. In this study, we use the results of dynamical downscaling of global simulations obtained by the atmosphere–ocean global circulation model SINTEX-G (AOGCM SX-G/INGV) using a regional climate model, the EBU-POM. The EBU-POM is the combination of the Eta/NCEP as the atmospheric component and Princeton Ocean Model (POM) as the ocean component. Global simulations were done for the very long period 1771–2100, while downscaling was done for the 1961–1990 and 2071–2100 periods according to the SRES-A1B scenario.

Calculation of Standard Deviation of Concentration Using a Second-Order Closure Theory

The quality of the simulation of the motion and spread of a passive substance in a puff model depends on the ability to estimate the speed of the wind at the source height and standard deviation of concentration both in time and space. The wind profile is calculated using the standard Monin-Obukhov similarity theory and standard deviation of concentration, (“sigmas” in the further text) is calculated using a scheme based on the second-order closure theory. To asses the success of the model comparison was made with two other approaches for the calculation of the “sigmas”, the Briggs empirical method and a semi-empirical scheme based on the statistical theory. All of these methods are based on the various parameters of turbulence which were calculated from standard measurements using the so called meteorological pre-processing. Finally the results were compared with the results from the measurements from Tracer Dispersion Experiments in the Copenhagen Area during 1978/1979. First we have analyzed the wind extrapolation results and when we were satisfied with those we did comparisons with the observed concentration.

Method for efficient prevention of gravity wave decoupling on rectangular semi-staggered grids

Generation of short gravity wave noise often occurs on semi-staggered rectangular grids as a result of sub-grid decoupling when there is a strong forcing in the mass field. In this study a numerical scheme has been proposed to prevent the generation of the gravity wave decoupling. The proposed numerical method provides efficient communication between decoupled gravity wave finite-difference solutions by a simple averaging of a term in the height tendency in the continuity equation. The proposed method is tested using a shallow water sink model developed for the purpose of this study. It has been demonstrated that this method outperforms other existing approaches. The new scheme is time efficient, based on explicit time integration and can be easily implemented. The proposed method is applicable in hydrodynamic models specified on semi-staggered B or E grids.