Gradient-based similarity in the atmospheric boundary layer

The “flux-based” and “gradient-based” similarity in the stable boundary layer and also in the interfacial part of the convective boundary layer is discussed. The stable case is examined on the basis of data collected during the CASES-99 experiment. Its interfacial counterpart is considered in both the quasi-steady (mid-day) and non-steady states, utilizing the results of large-eddy simulations. In the stable regime, the “gradient-based” approach is not unique and can be based on various master length scales. Three local master length scales are considered: the local Monin-Obukhov scale, the buoyancy scale, and the Ellison scale. In the convective “quasi-steady” (mid-day) case, the “mixed layer” scaling is shown to be valid in the mixed layer and invalid in the interfacial layer. The temperature variance profile in non-steady conditions can be expressed in terms of the convective temperature scale in the mixed layer. The analogous prediction for velocity variances is not valid under non-steady conditions.     

Simulation of extreme precipitation over the Yangtze River Basin using Wakeby distribution

Based on the daily observational precipitation data at 147 stations in the Yangtze River Basin during 1960–2005 and projected daily data of 79 grid cells from the ECHAM5/ MPI-OM model in the 20th and 21st century, time series of precipitation extremes which contain AM (Annual Maximum) and MI (Munger Index) are constructed. The distribution feature of precipitation extremes is analyzed based on the two index series. Three principal results were obtained, as stated in the sequel. (i) In the past half century, the intensity of extreme heavy precipitation and drought events was higher in the mid-lower Yangtze than in the upper Yangtze reaches. Although the ECHAM5 model still can’t capture the precipitation extremes over the Yangtze River Basin satisfactorily, spatial pattern of the observed and the simulated precipitation extremes are much similar to each other. (ii) For quantifying the characteristics of extremely high and extremely low precipitation over the Yangtze River Basin, four probability distributions are used, namely: General Extreme Value (GEV), General Pareto (GPA), General Logistic (GLO), and Wakeby (WAK). It was found that WAK can adequately describe the probability distribution of precipitation extremes calculated from both observational and projected data. (iii) Return period of precipitation extremes show spatially different changes under three greenhouse gas emission scenarios. The 50-year heavy precipitation and drought events from simulated data during 1951–2000 will become more frequent, with return period below 25 years, for the most mid-lower Yangtze region in 2001–2050. The changing character of return periods of precipitation extremes should be taken into account for the hydrological design and future water resources management.     

Joint effects of subgrid-scale diffusion and truncation errors in large-eddy simulations of the convective boundary layer

Two runs of a large-eddy simulation model with Deardorff's and Schumann's subgrid parametrizations have been compared in order to analyze spurious effects at the top of the mixed layer due to a presence of a strong temperature jump. Simulations performed showed that Schumann's subgrid eddy viscosity was sufficient to spreads out sharp ripples which appeared in the numerical solution due to numerical dispersion. Deardorff's subgrid eddy viscosity was found too small near the top of the mixed layer, and as a result truncation dispersion errors caused unphysical solutions in this region.     

Statistics of shallow convection on Mars based on large-eddy simulations. Part 1: shearless conditions

Results of large-eddy simulations of shallow, quasi-steady, shear-less convection in the Martian boundary layer are presented and discussed. In the considered three cases, turbulence is forced by the radiative flux divergence, prescribed as given functions of height, and the strength of the surface heat flux. It is constrained by the temperature inversion at the boundary-layer top. The resulting convective boundary layer exhibits horizontal cellular structures. The presence of radiative heating causes dimensionless statistics of turbulence to depend on the parameter F, defined in terms of the integrated radiative and turbulent heating rates in the boundary layer.     

Farewell,HSJ!—address from the retiring editor

On the eve of my retirement as co-editor of the International Association of Hydrological Sciences (IAHS) and Hydrological Sciences Journal (HSJ), I would like to take the opportunity to address the HSJ audience with some reflections about the journal and the publishing system.     

On the speciation of metals in the water column of a polluted estuary

The dissolved (<0.40 γm) fraction of water samples from Newark Bay, New Jersey was analysed for Zn, Cu and Pb content by differential pulse anodic stripping polarographic techniques. In the dissolved fraction, non-labile forms of Zn, Cu and Pb exist as shown by differential pulse anodic stripping voltammetry analysis on acidified samples and acidified-UV irradiated samples. The particulate fraction (>;0.40 μm) contains metals in the form of sulphides (Fe, Zn, Cu, Mn), oxides and oxyhydroxides (Si, Al, Fe, Cu, Ni, Sn), phosphate (Ca, Ce, La), clay minerals (Fe, Zn, Cu, Ti) and carbonaceous material (Fe, Cu, Zn) as demonstrated by X-ray microanalysis. The solid phases are likely present in colloidal form in the dissolved fraction of the water column as well. The forms of the metals in the water column are partially due to the resuspension of bottom sediments by dredging and natural processes, to sewage outfall and to natural geochemical processes.     

Special Section on Ecohydrology—Editorial

Abstract

This paper makes use of a water balance study of a mountainous area with a wide range of average annual rainfall in an arid and semiarid region to illustrate the development of both a statistical model of daily rainfall and a rainfall-runoff model. The models are appropriate for these conditions and may be relevant to similar areas. Comparisons of mean rainfall and runoff at the arid end of the scale suggest that runoff coefficients do not conform to common assumptions.