Regional climate simulation with a high resolution GCM: surface radiative fluxes

The ability of a high resolution (T106) version of the ECHAM3 general circulation model to simulate regional scale surface radiative fluxes has been assessed using observations from a new compilation of worldwide instrumentally-measured surface fluxes (Global Energy Balance Archive, GEBA). The focus is on the European region where the highest density of observations is found, and their use for the validation of global and regional climate models is demonstrated. The available data allow a separate assessment of the simulated fluxes of surface shortwave, longwave, and net radiation for this region. In summer, the incoming shortwave radiation calculated by the ECHAM3/T106 model is overestimated by 45 W m^–2 over most of Europe, which implies a largely unrealistic forcing on the model surface scheme and excessive surface temperatures. In winter, too little incoming shortwave radiation reaches the model surface. Similar tendencies are found over large areas of the mid-latitudes. These biases are consistent with deficiencies in the simulation of cloud amount, relative humidity and clear sky radiative transfer. The incoming longwave radiation is underestimated at the European GEBA stations predominantly in summer. This largely compensates for the excessive shortwave flux, leading to annual mean net radiation values over Europe close to observations due to error cancellation, a feature already noted in the simulated global mean values in an earlier study. Furthermore, the annual cycle of the simulated surface net radiation is strongly affected by the deficiencies in the simulated incoming shortwave radiation. The high horizontal resolution of the GCM allows an assessment of orographically induced flux gradients based on observations from the European Alps. Although the model-calculated and observed flux fields substantially differ in their absolute values, several aspects of their gradients are realistically captured. The deficiencies identified in the model fields are generally consistent at most stations, indicating a high degree of representativeness of the measurements for their larger scale setting.     

Relation between flow,surface‐layer armoring and sediment transport in gravel‐bed rivers

This study investigates trends in bed surface and substrate grain sizes in relation to reach‐scale hydraulics using data from more than 100 gravel‐bed stream reaches in Colorado and Utah. Collocated measurements of surface and substrate sediment, bankfull channel geometry and channel slope are used to examine relations between reach‐average shear stress and bed sediment grain size. Slopes at the study sites range from 0·0003 to 0·07; bankfull depths range from 0·2 to 5 m and bankfull widths range from 2 to 200 m. The data show that there is much less variation in the median grain size of the substrate, D_50s, than there is in the median grain size of the surface, D₅₀; the ratio of D₅₀ to D_50s thus decreases from about four in headwater reaches with high shear stress to less than two in downstream reaches with low shear stress. Similar trends are observed in an independent data set obtained from measurements in gravel‐bed streams in Idaho. A conceptual quantitative model is developed on the basis of these observations to track differences in bed load transport through an idealized stream system. The results of the transport model suggest that downstream trends in total bed load flux may vary appreciably, depending on the assumed relation between surface and substrate grain sizes. Copyright © 2007 John Wiley & Sons, Ltd.     

Modeling of Diffusion in a Wide Alpine Valley

Summary  A K-type diffusion model coupled with a massconsistent wind model is applied for one of the rural biological waste disposal sites in Austrian Alps. The site is situated in the P?ls valley in the eastern Alps, 250 km south-west of Vienna in Austria Aim of the study is to demonstrate dispersion of H₂S from the site to near by village. Model simulations are carried out each for an evening and a morning transition case characterized by flow reversals. The role of locally generated wind in changing the pollutant distribution over nearby residential area is investigated. Surface observations at two stations toward the open boundaries of the main valley are used to derive the turbulence parameters and then to obtain initial inputs of wind profiles. The turbulence parameters behave analogous to that over a plane terrain after the establishment of the valley wind. The model simulations are done for eight hours during the evening transition and eighteen hours for the morning transition by incorporating the wind field from a mass consistent wind model. The results are compared with SF₆ tracer experiments conducted during those periods. The model outputs and the observations at various points inside the valley are in good correlation except for NW part of the valley after the reversal of valley wind. The results also reveal the potential of a simple approach with minimized inputs. Received August 15, 1997 Revised August 15, 1998     

A GCM study of future climate response to aerosol pollution reductions

We use the global atmospheric GCM aerosol model ECHAM5-HAM to asses possible impacts of future air pollution mitigation strategies on climate. Air quality control strategies focus on the reduction of aerosol emissions. Here we investigate the extreme case of a maximum feasible end-of-pipe abatement of aerosols in the near term future (2030) in combination with increasing greenhouse gas (GHG) concentrations. The temperature response of increasing GHG concentrations and reduced aerosol emissions leads to a global annual mean equilibrium temperature response of 2.18 K. When aerosols are maximally abated only in the Industry and Powerplant sector, while other sectors stay with currently enforced regulations, the temperature response is 1.89 K. A maximum feasible abatement applied in the Domestic and Transport sector, while other sectors remain with the current legislation, leads to a temperature response of 1.39 K. Increasing GHG concentrations alone lead to a temperature response of 1.20 K. We also simulate 2–5% increases in global mean precipitation among all scenarios considered, and the hydrological sensitivity is found to be significantly higher for aerosols than for GHGs. Our study, thus highlights the huge potential impact of future air pollution mitigation strategies on climate and supports the need for urgent GHG emission reductions. GHG and aerosol forcings are not independent as both affect and are influenced by changes in the hydrological cycle. However, within the given range of changes in aerosol emissions and GHG concentrations considered in this study, the climate response towards increasing GHG concentrations and decreasing aerosols emissions is additive.     

Influence of Nocturnal Low-level Jets on Eddy-covariance Fluxes over a Tall Forest Canopy

Observations of low-level jets (LLJs) at the Howland AmeriFlux site in the USA and the jet’s impact on nocturnal turbulent exchange and scalar fluxes over a tall forest canopy are discussed. Low-frequency motions and turbulent bursts characterize moderately strong LLJs, whereas low-frequency motions are suppressed during periods with strong LLJs and enhanced shear. An analysis based on the shear-sheltering hypothesis seeks to elucidate the effect of LLJs on flux measurements. In the absence of shear sheltering, large eddies penetrate the roughness sublayer causing enhanced mixing while during periods with shear sheltering, mixing is reduced. In the absence of the latter, ‘upside-down’ eddies are primarily responsible for the enhanced velocity variances, scalar and momentum fluxes. The integral length scales over the canopy are greater than the canopy height. The variance spectra and cospectra from the wavelet analysis indicate that large eddies (spatial scale greater than the low-level jet height) interact with active canopy-scale turbulence, contributing to counter-gradient scalar fluxes.     

Numerical simulation of the effect of interface friction of a bounding structure on shear deformation in a granular soil

Recently, the shear behavior of a cohesionless granular strip that is in contact with a very rough surface of a moving bounding structure has been numerically investigated by several authors by using a micropolar hypoplastic continuum model. It was shown that the micropolar boundary conditions assumed along the interface have a strong influence on the deformations within the granular layer. In previous investigations, only interface friction angles for very rough bounding structures were assumed. In contrast, the focus of the present paper is on the influence of the interface roughness on the deformation behavior of the granular strip when the interface friction angle is lower than the peak friction angle of the granular material. In addition to the interface friction angle, particular attention is also paid to the influence of the mean grain diameter, the solid hardness, the initial void ratio, and the vertical stress on the maximum horizontal shear displacement within the granular layer before sliding is started. Copyright © 2011 John Wiley & Sons, Ltd.     

On the efficiency of horizontal diffusion and numerical filtering in an Arakawa‐type model

Abstract

In a series of 5‐day forecasts with a 3‐layer/2.8° hemispheric model, horizontal diffusion schemes of the second and fourth degree are compared with a numericalfilter technique. The results, which are discussed mainly in terms of spectral energetics in zonal wavenumber space, indicate that fourth‐degree diffusion is more scale selective than second‐degree and equivalent to filtering. The seventh‐order filter applied only intermittently to the prognostic variables is superior to fourth‐degree diffusion from the viewpoint of computational economy. Excessive dissipation of the long waves may inhibit the production of eddy kinetic energy from eddy available potential energy.