B. J. J. M. van den Hurk

A verification of some methods to determine the fluxes of momentum, sensible heat, and water vapour using standard deviation and structure parameter of scalar meteorological quantities

A set of micro-meteorological data collected over a horizontal, uniform terrain (the plain of La Crau, France) in June 1987 is analysed. Conditions were predominantly sunny and arid, while due to the “Mistral” the wind speed could exceed 10 m/s. Verification of several methods to evaluate surface fluxes of heat, momentum and water vapour from the standard deviation of temperature, wind and specific humidity is presented. Also, a similar approach using the structure parameter of temperature is considered. These methods are all based on Monin-Obukhov (M-O) similarity theory. It is found that the standard deviation of temperature, vertical and horizontal wind speed as well as the structure parameter for temperature behave according to M-O similarity. It is shown that the sensible heat flux and friction velocity can be determined from a fast response thermometer and a cup anemometer. Also, it appears that the analytic solution of the set of governing equations as derived by the first author yields good results. M-O theory does not appear to work for the standard deviation of specific humidity. This may be due to the relative importance of large eddies.

Effect of anthropogenic land-use and land cover changes on climate and land carbon storage in CMIP5 projections for the 21st century

AbstractThe effects of land-use changes on climate are assessed using specified-concentration simulations complementary to the representative concentration pathway 2.6 (RCP2.6) and RCP8.5 scenarios performed for phase 5 of the Coupled Model Intercomparison Project (CMIP5). This analysis focuses on differences in climate and land–atmosphere fluxes between the ensemble averages of simulations with and without land-use changes by the end of the twenty-first century. Even though common land-use scenarios are used, the areas of crops and pastures are specific for each Earth system model (ESM). This is due to different interpretations of land-use classes. The analysis reveals that fossil fuel forcing dominates land-use forcing. In addition, the effects of land-use changes are globally not significant, whereas they are significant for regions with land-use changes exceeding 10%. For these regions, three out of six participating models—the Second Generation Canadian Earth System Model (CanESM2); Hadley Centre Glo...

The Second Phase of the Global Land–Atmosphere Coupling Experiment: Soil Moisture Contributions to Subseasonal Forecast Skill

AbstractThe second phase of the Global Land–Atmosphere Coupling Experiment (GLACE-2) is a multi-institutional numerical modeling experiment focused on quantifying, for boreal summer, the subseasonal (out to two months) forecast skill for precipitation and air temperature that can be derived from the realistic initialization of land surface states, notably soil moisture. An overview of the experiment and model behavior at the global scale is described here, along with a determination and characterization of multimodel “consensus” skill. The models show modest but significant skill in predicting air temperatures, especially where the rain gauge network is dense. Given that precipitation is the chief driver of soil moisture, and thereby assuming that rain gauge density is a reasonable proxy for the adequacy of the observational network contributing to soil moisture initialization, this result indeed highlights the potential contribution of enhanced observations to prediction. Land-derived precipitation forec...

The Rhône-Aggregation Land Surface Scheme Intercomparison Project: An Overview

The Rhone-Aggregation (Rhone-AGG) Land Surface Scheme (LSS) intercomparison project is an initiative within the Global Energy and Water Cycle Experiment (GEWEX)/Global Land-Atmosphere System Study (GLASS) panel of the World Climate Research Programme (WCRP). It is a intermediate step leading up to the next phase of the Global Soil Wetness Project (GSWP) (Phase 2), for which there will be a broader investigation of the aggregation between global scales (GSWP-1) and the river scale. This project makes use of the Rhone modeling system, which was developed in recent years by the French research community in order to study the continental water cycle on a regional scale. The main goals of this study are to investigate how 15 LSSs simulate the water balance for several annual cycles compared to data from a dense observation network consisting of daily discharge from over 145 gauges and daily snow depth from 24 sites, and to examine the impact of changing the spatial scale on the simulations. The overall evapotranspiration, runoff, and monthly change in water storage are similarly simulated by the LSSs, however, the differing partitioning among the fluxes results in very different river discharges and soil moisture equilibrium states. Subgrid runoff is especially important for discharge at the daily timescale and for smaller-scale basins. Also, models using an explicit treatment of the snowpack compared better with the observations than simpler composite schemes. Results from a series of scaling experiments are examined for which the spatial resolution of the computational grid is decreased to be consistent with large-scale atmospheric models. The impact of upscaling on the domain-averaged hydrological components is similar among most LSSs, with increased evaporation of water intercepted by the canopy and a decrease in surface runoff representing the most consistent inter-LSS responses. A significant finding is that the snow water equivalent is greatly reduced by upscaling in all LSSs but one that explicitly accounts for subgrid-scale orography effects on the atmospheric forcing.

Some Practical Notes on the Parameter kB−1 for Sparse Vegetation

This paper deals with the parameter kB21, the logarithm of the ratio between momentum and heat roughness length, of sparsely vegetated surfaces and bare soil. The bare soil surface is included as a reference, since it is fairly homogenous and smooth, having no distinguishable roughness elements. The mean value of kB21 is about 8 for the vineyard and 12 for the savannah. These values are significantly greater than kB 21 5 2, which is usually assumed to hold for vegetation. The mean value of kB21 for bare soil is small and negative, which agrees with the literature. A large variation of kB 21 during the day is measured for all three surfaces. This behavior has been observed for sparse vegetation in previous studies. Some authors explained the phenomenon with a vertical movement of the source of heat through the day as solar angle varies, or with the use of an inappropriate value of effective surface temperature to calculate kB21. For the first time, this diurnal variation is measured for a smooth surface, the bare soil, for which neither explanation is valid. A sensitivity study reveals that the calculated kB21 is very sensitive to measuring errors in the micrometeorological variables and errors in the roughness length for momentum. This explains the large range in observed kB21 values for one particular surface type. In addition, several semiempirical expressions for kB21 from the literature are tested. Two well-established formulas, both based on a simple combination of Reynolds and Prandtl numbers, appear to produce the best estimates of daily averaged kB21 values. None of the formulas are able to describe the diurnal variation. The authors conclude that the concept of kB21 is questionable as it is based upon extrapolating a theoretical profile through a region where this profile does not hold, toward a ‘‘surface temperature’’ that is difficult to define and to measure. It should therefore be avoided in meteorological models, for example, by applying canopy boundary layer resistances. Unfortunately, in remote sensing, the bulk transfer equations are up to now the only option, which therefore requires the use of kB21.

The scintillation method tested over a dry vineyard area

Measurements of a scintillometer device mounted at 4 m above a dry vineyard area in La Mancha, Spain, are used to obtain an average sensible heat flux densityH. Averaging is over a rectangular area determined by the distance between the scintillometer light source and receptor (875 m) and some upwind distance governed by the horizontal wind speed perpendicular to that line. Using similarity relations obtained from La Crau, a good correspondence betweenH measured with the scintillometer and an eddy-correlation device in the centre of a vineyard is obtained. The friction velocityu* was either measured directly using a sonic anemometer or obtained indirectly from two wind speeds and known values of the roughness length zo and displacementd. The free convection formulation underestimates the sensible heat flux by about 30%. This is due to a significant contribution of mechanically generated turbulence to the total turbulent transport, which was caused by relatively strong winds and rough terrain.

A 'Lagrangian' revision of the resistors in the two-layer model for calculating the energy budget of a plant canopy.

This paper completes the construction of a two-layer model for the energy balance of sparse canopies begun by Van den Hurk and McNaughton (1994). The model is based on the Lagrangian theory of Raupach (1989). In the earlier work, we showed that the average effect of the near-field component of the scalar concentration profile can be represented by a ‘near-field’ resistor. Here we calculate values for the upper and lower ‘far-field’ resistors using Raupachs expression for the far-field diffusivity and his empirical descriptions of the profiles of vertical velocity variance and Lagrangian integral time scale (Raupach, 1988). The boundary-layer resistance for the foliage of the overstorey canopy is also reassessed. Calculations are carried out for a range of possible canopies. Representative values are chosen for use where a detailed description of the vegetation is unavailable.The resistors of the new model are compared with those of earlier two-layer models. The new ‘far-field’ resistors are smaller than the corresponding ‘aerodynamic’ resistors of the earlier models of Shuttleworth and Wallace (1985) and Choudhury and Monteith (1988) while the new boundary-layer resistor can be somewhat larger, depending on the choices made in the calculation. We could not find experimental data on canopy energy balances suitable for deciding which model is best. Instead, we have attempted to verify the model using measurements of surface temperature and calculated values of the ‘excess’ resistance, but the results are not definitive. A major conclusion is that turbulent transport near the ground beneath an overstorey canopy is poorly known and therefore is poorly represented in all models. Another conclusion is that current models for the excess resistance are inadequate.

A New Methodology for Assimilation of Initial Soil Moisture Fields in Weather Prediction Models Using Meteosat and NOAA Data

In this study, a simple method is described and tested for deriving initial soil moisture fields for numerical weather prediction purposes using satellite imagery. Recently, an algorithm was developed to determine surface evaporation maps from high- and low-resolution satellite data, which does not require information on land use and synoptic data. A correction to initial soil moisture was calculated from a comparison between the evaporation fields produced by a numerical weather prediction model and the satellite algorithm. As a case study, the method was applied to the Iberian Peninsula during a 7-day period in the summer of 1994. Two series of short-term forecasts, initialized from a similar initial soil moisture field, were run in parallel: a control run in which soil moisture evolved freely and an experimental run in which soil moisture was updated daily using the simple assimilation procedure. The simple assimilation resulted in a decrease of the bias of temperature and specific humidity at 2-m height during the daytime and a small decrease of the root-mean-square error of these quantities. The results show that the surface evaporation maps, derived from the satellite data, contain a signal that may be used to assimilate soil moisture in numerical weather prediction models.

Thermal soil properties for vineyard (EFEDA-I) and savanna (HAPEX-Sahel)sites

This paper describes the course of topsoil thermal conductivity, λ, diffusivity, α, and heat capacity, Ch, during two measurement campaigns, conducted in semi-arid areas—the EFEDA-I experiment and HAPEX-Sahel. For the derivation of α, five methods (the Amplitude, Phase, Arctangent, Logarithmic and Harmonic equation) were compared. Values of Ch were derived from measurements of soil moisture content, θ, and dry bulk density. λ was either measured directly (the non-stationary probe method) or calculated from λ = α· Ch. Thermal soil properties were clearly related to θ (and thus rainfall) throughout the measurement campaigns: hardly any changes occurred during the EFEDA-I experiment where continuous dry conditions prevailed, whereas for HAPEX-Sahel a clear decrease in all thermal properties was observed after the last rainfall. For calculation of α, the Amplitude and the Harmonic equation gave the best results. Calculation of α and λ beneath the vegetation plots yielded unreliable results, mainly due to shading effects causing more than one temperature maximum. Direct measurement of A yielded unrealistically low values for the dry soil conditions as encountered in Spain, due to poor contact between probe and soil. A correction for the contact resistance is necessary to obtain better estimates. For HAPEX-Sahel, measured and calculated λ values were much closer, mainly for reasons of higher θ values and a high soil compaction which ensured better contact between soil and probes.

Soil Control on Runoff Response to Climate Change in Regional Climate Model Simulations

Simulations with seven regional climate models driven by a common control climate simulation of a GCM carried out for Europe in the context of the (European Union) EU-funded Prediction of Regional scenarios and Uncertainties for Defining European Climate change risks and Effects (PRUDENCE) project were analyzed with respect to land surface hydrology in the Rhine basin. In particular, the annual cycle of the terrestrial water storage was compared to analyses based on the 40-yr ECMWF Re-Analysis (ERA-40) atmospheric convergence and observed Rhine discharge data. In addition, an analysis was made of the partitioning of convergence anomalies over anomalies in runoff and storage. This analysis revealed that most models underestimate the size of the water storage and consequently overestimated the response of runoff to anomalies in net convergence. The partitioning of these anomalies over runoff and storage was indicative for the response of the simulated runoff to a projected climate change consistent with the greenhouse gas A2 Synthesis Report on Emission Scenarios (SRES). In particular, the annual cycle of runoff is affected largely by the terrestrial storage reservoir. Larger storage capacity leads to smaller changes in both wintertime and summertime monthly mean runoff. The sustained summertime evaporation resulting from larger storage reservoirs may have a noticeable impact on the summertime surface temperature projections.