Model Grid Scale Research Articles

Parameterization of snow-free land surface albedo as a function of vegetation phenology based on MODIS data and applied in climate modelling

The aim of this study was to develop an advanced parameterization of the snow-free land surface albedo for climate modelling describing the temporal variation of surface albedo as a function of vegetation phenology on a monthly time scale. To estimate the effect of vegetation phenology on snow-free land surface albedo, remotely sensed data products from the Moderate-Resolution Imaging Spectroradiometer (MODIS) on board the NASA Terra platform measured during 2001 to 2004 are used. The snow-free surface albedo variability is determined by the optical contrast between the vegetation canopy and the underlying soil surface. The MODIS products of the white-sky albedo for total shortwave broad bands and the fraction of absorbed photosynthetically active radiation (FPAR) are analysed to separate the vegetation canopy albedo from the underlying soil albedo. Global maps of pure soil albedo and pure vegetation albedo are derived on a 0.5° regular latitude/longitude grid, re-sampling the high-resolution information from remote sensing-measured pixel level to the model grid scale and filling up gaps from the satellite data. These global maps show that in the northern and mid-latitudes soils are mostly darker than vegetation, whereas in the lower latitudes, especially in semi-deserts, soil albedo is mostly higher than vegetation albedo. The separated soil and vegetation albedo can be applied to compute the annual surface albedo cycle from monthly varying leaf area index. This parameterization is especially designed for the land surface scheme of the regional climate model REMO and the global climate model ECHAM5, but can easily be integrated into the land surface schemes of other regional and global climate models.

Relation between sea level and bottom pressure and the vertical dependence of oceanic variability

The relation between large‐scale sea level and bottom pressure variability is studied using long (50‐yr) simulations of a general circulation model under realistic forcing. Admittance and coherence analyses are used to characterize the bottom pressure and sea level relationship as a function of period, horizontal spatial scale, and location. At the model grid scale (1°), bottom pressure is found to be essentially equivalent to sea level at periods <30 days, except in the tropics. This equivalence still holds for longer periods (∼100 days), but only at high latitudes (>60°) and in shallow depths (<200 m). Elsewhere, bottom pressure and sea level fields can differ significantly. Results indicate an increase of the importance of baroclinic signals with decreasing latitude and spatial scale, with significant baroclinic signals at intra‐seasonal and longer periods present in many subtropical and mid‐latitude regions. Variability is clearly baroclinic at inter‐annual periods, regardless of location and spatial scale. Results have broad implications for the interpretation and processing of both satellite altimetry and gravity data and for their assimilation into numerical models.

Improved simulation of flood flows using storage cell models

Storage cell codes for inundation modelling treat the floodplain as a series of discrete basins, with the flow between cells calculated using some analytical flow formulae such as the Manning equation. These codes have many of the advantages of full two-dimensional schemes but without the computational cost. However, with this class of model the propagation speed of the inundation front over the floodplain can be shown to be highly dependent on the model grid scale and insensitive to floodplain friction. To overcome these problems adaptive time step techniques have recently been proposed; however, to date these have only been tested against highly idealised analytical solutions. In this paper the authors compare fixed and adaptive time step storage cell codes for flood inundation modelling against a real world data set for the first time. The data consist of a satellite synthetic aperture radar image of inundation extent at ∼12·5 m spatial resolution taken during a 1-in-50-year flood, which occurred in 1998 on the upper River Severn in the UK; gauged flows at the upstream and downstream ends of an approximately 60 km reach of the river; and a digital elevation model developed from channel survey and an airborne laser altimeter survey of the floodplain topography. Results show that the adaptive model produces a modest improvement in simulation performance over the fixed time step code, yet is much better able to simulate realistically the drying of the floodplain in the upper reaches of the model domain. The adaptive code also demonstrates a more realistic sensitivity to floodplain friction which is likely to account for its markedly different dynamic behaviour and, in this respect, intuitively better performance.

The use of digital elevation models in the identification and characterization of catchments over different grid scales

This study examines the ability of well-known hydrological and geomorphological descriptors and statistics to differentiate between catchments with spatially varying geology, size and shape subject to the same climate in the Northern Territory, Australia. The effect of digital elevation model grid resolution on these statistics is also examined. Results demonstrate that catchment descriptors such as the area–slope relationship, cumulative area distribution and hypsometric curve can differentiate between catchments with different geology and resultant morphology, but catchment network statistics are insensitive to differences in geology. Examination of the effects of digital elevation model grid scale demonstrates that while considerable catchment information can be gained at digital elevation grids greater than 10 m by 10 m, hillslope and hydrological detail can be lost. Geomorphic descriptors such as the area– slope relationship, cumulative area distribution, width function and Strahler statistics were shown to be sensitive to digital elevation model grid scale, but the hypsometric curve was not. Consequently, caution is needed when deciding on an appropriate grid resolution as well as the interpretation and analysis of catchment properties at grid scales greater than that for optimal hillslope and area aggregation definition. Copyright  2005 John Wiley & Sons, Ltd.

Impact of Horizontal Resolution on the Numerical Simulation of a Midlatitude Squall Line: Implicit versus Explicit Condensation

The relative roles of implicit and explicit condensation schemes in the numerical representation of a squall line that occurred on 7–8 May 1995 over the southern Great Plains are examined in this study using Mesoscale Compressible Community model integrations at 2-, 6-, 18-, and 50-km resolution. Results from the 2-km model in which condensation is explicitly represented agree best with observations and are used as “synthetic” data to evaluate the performance of lower-resolution configurations. It is found that the representation of the squall system greatly deteriorates as resolution is decreased and that the relative roles of the implicit and explicit condensation schemes change dramatically. At 6-km resolution, the leading convective band is barely resolved by the model, and the implicit–explicit partition of precipitation is ambiguous because both implicit and explicit schemes are active simultaneously at the leading edge of the system. In spite of this ambiguity, it is found that use of a deep convection scheme is still beneficial to the squall-line simulation. At 18 km, the convective line is not resolved by the model, and its effect is completely due to the implicit scheme. The mesoscale circulations in the trailing anvil region of the squall system are generated at the small end of the model resolvable scales and are exaggeratedly intense. There is no ambiguity concerning the partition of condensation into implicit and explicit components at this resolution, but the relative intensity of precipitation produced by the two cloud schemes is opposite to what is observed, considering that the implicit scheme is supposed to represent subgrid-scale convection at the leading edge of the system, and the explicit scheme the grid-scale condensation in the trailing anvil. At 50 km, both the leading convection and the mesoscale circulations in the trailing anvil have to be parameterized because they are not resolved at the model grid scale. The precipitation and internal structures associated with the squall line are thus not well represented at this resolution. The results also show that all the configurations produce precipitation accumulations that are much larger than observations. This problem is most important at 18-km resolution. Grid-scale condensation is mostly responsible for this rainfall overestimation. It is suggested that this problem is linked to a misrepresentation of convective-scale processes.

Equatorial Circulation of a Global Ocean Climate Model with Anisotropic Horizontal Viscosity

Horizontal momentum flux in a global ocean climate model is formulated as an anisotropic viscosity with two spatially varying coefficients. This friction can be made purely dissipative, does not produce unphysical torques, and satisfies the symmetry conditions required of the Reynolds stress tensor. The two primary design criteria are to have viscosity at values appropriate for the parameterization of missing mesoscale eddies wherever possible and to use other values only where required by the numerics. These other viscosities control numerical noise from advection and generate western boundary currents that are wide enough to be resolved by the coarse grid of the model. Noise on the model gridscale is tolerated provided its amplitude is less than about 0.05 cm s−1. Parameter tuning is minimized by applying physical and numerical principles. The potential value of this line of model development is demonstrated by comparison with equatorial ocean observations. In particular, the goal of producing model equatorial ocean currents comparable to observations was achieved in the Pacific Ocean. The Equatorial Undercurrent reaches a maximum magnitude of nearly 100 cm s−1 in the annual mean. Also, the spatial distribution of near-surface currents compares favorably with observations from the Global Drifter Program. The exceptions are off the equator; in the model the North Equatorial Countercurrent is improved, but still too weak, and the northward flow along the coast of South America may be too shallow. Equatorial Pacific upwelling has a realistic pattern and its magnitude is of the same order as diagnostic model estimates. The necessary ingredients to achieve these results are wind forcing based on satellite scatterometry, a background vertical viscosity no greater than about 1 cm2 s−1, and a mesoscale eddy viscosity of order 1000 m2 s−1 acting on meridional shear of zonal momentum. Model resolution is not critical, provided these three elements remain unaltered. Thus, if the scatterometer winds are accurate, the model results are consistent with observational estimates of these two coefficients. These winds have larger westward stress than NCEP reanalysis winds, produce a 14% stronger EUC, more upwelling, but a weaker westward surface flow. In the Indian Ocean the seasonal cycle of equatorial currents does not appear to be overly attenuated by the horizontal viscosity, with differences from observations attributable to interannual variability. However, in the Atlantic, the numerics still require too large a meridional viscosity over too much of the basin, and a zonal resolution approaching 1° may be necessary to match observations. Because of this viscosity, increasing the background vertical viscosity slowed the westward surface current; opposite to the response in the Pacific.

The Need for Operational Forecasting During Oil Spill Response

The sensitivity of model simulations to wind data and model grid scale is discussed and a methodology for the reception of real-time UK Meteorological Office wind forecasts described. The Sea Empress incident which occurred off west Wales during 1996 is used to illustrate the techniques, and suggestions are made for the improvement of the national response to major spills. There is a need for Meteorological Office forecasts to be more readily available to the scientific community and for fine grid hydrodynamic models to be developed for regions considered to be at risk. There should be a shift in the focus of spill modelling towards real-time forecasting and away from the present emphasis on simplistic hindcasting studies. There is a need for a central forecasting service, run according to UK Meteorological Office protocols, to provide state-of-the-art forecasting at the time of a major spill. It is likely that there will continue to be a requirement for PC and workstation-based model simulations for both scientific and operational studies.

A parameterization of mixdown time for atmospheric chemicals

The finite spatial resolution of grid‐based numerical chemical transport models can lead to overestimation of chemical reaction rates that are in reality limited by the rate of mixing. A parameterization has been developed to compensate for this effect. It estimates the time taken to mix chemical features down from the model grid scale to the physical scale at which they really react, then builds an appropriate time lag into the chemistry by reducing the rate constant. The parameterization is shown to work in idealized tests, where it allows low‐resolution parameterized experiments to mimic the results of higher‐resolution unparameterized experiments in terms of the total reaction product. The parameterization is applied in a single‐layer chemical transport model of the winter stratosphere to the deactivation of ClOx. by reaction with NOx. It is found to have a large effect on the deactivation of ClOx. The resulting impact on the rate of catalytic destruction of O3 by ClOx is estimated to be significant.

Topography and the water cycle in a temperate middle mountain environment: the need for interdisciplinary experiments

Two main characteristics of mountainous regions are the large topography-driven lateral redistributions of water and energy, and the considerable topography-related heterogeneities on all scales. These features are difficult to estimate, to incorporate into hydrologic models and to aggregate on the general circulation model grid scale. On the local scale, the topography controls the spatial patterns of water and energy inputs, soil water properties and soil moisture. On the massif scale, the topography induces climatic and ecological gradients that influence the patterns of parameters and fluxes. This is illustrated by results obtained in the Ringelbach and Fecht research catchments of the Vosges massif, France. A better understanding of the water cycle in mountains requires specific interdisciplinary field experiments, as part of international programmes such as WCRP-GEWEX, IGBP-BAHC and UNESCO-IHP, using a combination of altitudinal transects and nested catchments, and including a long-term component, which is necessary for model validation. Some of the specific scientific and methodological problems that remain to be solved for these regions are given. A west-east transect across the southern part of the Vosges massif and the Rhine graben could provide a convenient site for such an experiment in a temperate middle mountain environment.

Estimating transport parameters at the grid scale: on the value of a single measurement

Distributed models of solute transport at the field and catchment scales require the specification of effective parameters at the model grid scale. The grid scale will generally be much larger than the scale at which it is possible to make measurements to derive parameter values but may be of the same order as the scale of variability of such ‘point’ values. In addition, measurements are usually expensive and time consuming to make and where, for example, ‘undisturbed’ soil cores are used, may be destructive. This paper describes a conditional probability based approach for estimating grid scale effective parameter values in the light of expected spatial heterogeneity, given only one or a small number of available measurements. Initial results show, somewhat surprisingly, that despite the integrative nature of the spatial averaging involved in moving to the grid scale, the variance of the effective grid element values does not decrease. There is a small shift in the location of the distribution, as a result of the macrodispersive effect of the grid scale variability.

A Numerical Study of the East Coast Snowstorm of 10–12 February 1983

A numerical study of the East Coast snowstorm of 10–12 February 1983 has been conducted with the NRL mesoscale model. The three-dimensional, hydrostatic, primitive equation model has 91 × 51 horizontal grid points with a half degree resolution in a verification domain of 100°W to 60°W and 25°N to 45°N. There are ten layers in the vertical of equal σ(=p/ps) thickness. The model uses a split-explicit method for temporal integration and a second-order accurate spatial finite differencing. Model physics include precipitation on the resolvable scale and parameterized boundary layer and cumulus convection. The NMC 2.5 degree hemispheric analyses are used as the basic dataset (called NMC analysis hereafter). Because significant details in the initial conditions contained in the original rawinsonde observations may have been smoothed out by the NMC analysis algorithm, the analyses are also altered to provide a closer fit to the rawinsonde data. Original rawinsonde data are used in this enhancement of the NMC analyses (called enhanced). Forecasts are made from both the NMC analysis and the enhanced analyses to determine whether the enhancement can improve the forecasts. The Barnes analysis scheme with parameters suitable for reducing the short wavelength noise on the model grid scale is used to enhance the NMC analyses with the original soundings. Three types of lateral boundary treatments—constant, tendency damping, and temporal relaxation boundary conditions—are tested and compared. Results from forecast experiments show that the boundary treatment has a great impact on the model performance. The constant boundary condition produces an unusable forecast after 12 h as judged by the S1 scores, while the relaxation boundary condition produces an excellent forecast. The enhancement of the initial conditions has a negligible effect on predictions when reasonable boundary updates are used for the snowstorm case. The enhanced dataset produces a slightly better but still useless forecast when constant boundary conditions are used. Numerical experiments have also been conducted to test the sensitivity of the cyclogenesis to physical processes by suppressing one or more physical processes in the model. It is found that the evaporation from the ocean modulates the location and amount of precipitation. Without the evaporation, the-intensity of the cyclone remains the same but the center stays on the coast instead of staying off shore. The track of the snowstorm is such that the sensible beating from the ocean dampens the development of the cyclone by reducing the low-level baroclinicity. Without the sensible heating, the minimum surface pressure of the cyclone is 11 mb lower. The latent heating is found to be important for this case in which the maximum beating rate is 15°–20°C/day. When latent heating is suppressed, the cyclone translates at a much reduced rate and its central pressure is 10 mb higher after two days of simulation. These results from the sensitivity tests are of course case-dependent.