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Yves Delage

Abstract

A parameterization of the surface nocturnal inversion of temperature is proposed to enable atmospheric circulation models to handle the surface energy budget without having to resolve the boundary layer. The scheme allows a wide range of vertical resolutions for the host model. The principle is to replace the traditional instantaneous flux-profile relationship in the surface layer by a time integrated heat conservation equation linking the surface heat flux to the amplitude and thickness of the temperature inversion. The model is able to reproduce successfully the mean diurnal cycle of the Wangara data, using the observed vertical profiles to simulate atmospheric models with various resolutions. Unbiased surface temperature results are obtained from runs in which the information from the host model is taken at a height ranging from 40 to 625 m above the surface.

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Yves Delage

Abstract

The forty-day Wangara data set is used to discuss the applicability of the traditional formulations for the surface fluxes for atmospheric circulation models. It is shown that, while in unstable conditions the surface layer relationships can be applied to the whole boundary layer with little modification, the use of the log-linear equations in stable stratification should be restricted to very near the surface. The alternative of using resistance laws for models which do not resolve the boundary layer down to a few tens of meters above ground is found to be impractical. Instead a new formulation is proposed for the surface stress in the stable case which avoids the problem of having to position the lowest level of a model close to the surface. This formulation depends explicitly on a near surface temperature and on wind and temperature at a height which can be several hundred meters above the surface. Though our expression for the surface stress is independent of the formulation of the surface beat flux, an expression for the heat flux derived from surface layer theory is found to be adequate over the same domain. The angle between the surface stress and the wind is given as a function of height and stability. The effect of baroclinity is also discussed. The benefit of the new formulation, which requires a near-surface temperature (the topic of a forthcoming paper), is to allow the calculation of surface fluxes of appropriate quality in a model regardless of its vertical resolution.

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Claude Girard and Yves Delage

Abstract

The intensity of vertical mixing in atmospheric models generally depends on wind shear and static stability, making the diffusion process nonlinear. Traditional implicit numerical schemes, which treat the variables to be diffused implicitly but the diffusion coefficients explicitly, are shown to be only conditionally stable. Instability arises in statically stable conditions with an increase of the vertical resolution or of the time step. Stable schemes are derived whose principal characteristic is to take into account the variation of the diffusion coefficient with respect to the basic variables. One scheme looks like a traditional scheme in which the parameter that determines how implicit the calculations are done is made to vary locally instead of being a constant. This insures stability and at the same time provides optimum accuracy. This scheme did remove spurious oscillations found in the Canadian spectral weather forecasting model.

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Yves Delage and Diana Verseghy

Abstract

A new land surface scheme developed for the Canadian general circulation model has been introduced into the Canadian global forecast model and tested for a summer case. It features three soil layers, a snow layer, and a vegetation layer; its behavior is compared with that of the current operational force–restore scheme, in which the evaporation is a prescribed function of the climatological soil moisture content. The most noticeable effects of replacing the operational scheme by the new scheme are a reduction of the evaporation and an increase of the sensible heat flux at the surface, a result that has also been found in other models with similar schemes. In this study, we additionally examine the impact of changing the initial soil moisture (ISM): this quantity proves to be of primary importance in setting the amplitude of the Bowen ratio for several days after the beginning of a forecast. The study also points out problems in the Canadian global forecast model that are not caused by the land surface schemes but that do have an impact on their performance, in particular, a cold and moist bias in the lower troposphere, and an excess of solar radiation at the surface. The high sensitivity of the temperature and humidity forecasts to ISM enabled the construction of a hypothetical ISM field based on forecast errors. When this field is used instead of a climatological estimate to initiate the forecast, the standard deviation of the temperature error is reduced by 20%. This suggests that national meteorological centers should produce soil moisture analyses to initiate their weather forecasts.

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Louis-Philippe Crevier and Yves Delage

Abstract

A numerical model to forecast road conditions, Model of the Environment and Temperature of Roads (METRo), has been developed to run at Canadian weather centers. METRo uses roadside observations from road weather information systems stations as input, together with meteorological forecasts from the operational Global Environmental Multiscale (GEM) model of the Canadian Meteorological Centre; the meteorologist can modify this forecast using the “SCRIBE” interface. METRo solves the energy balance at the road surface and the heat conduction in the road material to calculate the temperature evolution; it also accounts for water accumulation on the road in liquid and solid form. Radiative fluxes reaching the surface are taken from the GEM model in automatic mode or are parameterized as a function of cloud cover and temperature when run in manual mode. The road-condition forecast is done in three stages: initialization of the road temperature profile using past observations, coupling of the forecast with observations during the overlap period when the meteorological forecast and the roadside observations are both available, and the forecast itself. The coupling stage allows for adjusting the radiative fluxes to local conditions. Results for road temperature are presented for three stations in Ontario for a period of 3 months. The 24-h forecasts are issued 2 times per day at 0300 and 1500 LT. Overall, about one-half of the time the error in surface road temperature (verified every 20 min) is within ±2 K, and the nighttime rms error is about 2 K. The impact of the coupling stage is large and allows METRo to produce automatic forecasts almost as good as the manual ones, especially for the first few hours. When METRo is run in manual mode, several nearby stations can use the same meteorological input, saving preparation time for the meteorologist. METRo also contains a mechanism for correcting systematic errors at each station, and it is hoped that this capability will permit its application to many new sites without major adjustments.

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Stéphane Laroche, Monique Tanguay, and Yves Delage

Abstract

This study examines the linearization properties of a simplified planetary boundary layer parameterization based on the vertical diffusion equations, in which the exchange coefficients are a function of the local Richardson number and wind shear. Spurious noise, associated with this parameterization, develops near the surface in the tangent linear integrations. The origin of this problem is investigated by examining the accuracy of the linearization and the numerical stability of the scheme used to discretize the vertical diffusion equations. The noise is primarily due to the linearization of the exchange coefficients when the atmospheric state is near neutral static stability and when a long time step is employed. A regularization procedure based on the linearization error and a criterion for the numerical stability is proposed and tested. This regularization is compared with those recently adopted by Mahfouf, who neglects the perturbations of the exchange coefficients, and by Janisková et al., who reduce the amplitude of those perturbations when the Richardson number is in the vicinity of zero.

When the sizes of the atmospheric state perturbations are 1 m s−1 for the winds and 1 K for the temperature, which is the typical size of analysis increments, regularizations proposed here and by Janisková et al. perform similarly and are slightly better than neglecting the perturbations of the exchange coefficients. On the other hand, when the state perturbations are much smaller (e.g., 3 orders of magnitude smaller), the linearization becomes accurate and a regularization is no longer necessary, as long as the time step is short enough to avoid numerical instability. In this case, the regularization proposed here becomes inactive while the others introduce unnecessary errors.

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Lei Wen, Wei Yu, Charles A. Lin, Michel Beland, Robert Benoit, and Yves Delage

Abstract

Many studies have demonstrated the importance of land surface schemes in climate change studies using general circulation models (GCMs). However, there have not been many studies that explore the role of land surface schemes in the context of short-range and high spatial resolution precipitation forecasts. The motivation of this study is to examine the sensitivity of simulated precipitation, and sensible and latent heat fluxes, to the use of different land surface schemes at two different spatial resolutions. The meteorological model used is the Mesoscale Compressible Community (MC2) model, and the land surface schemes are the force–restore method and the Canadian Land Surface Scheme (CLASS). Parallel runs have been performed using MC2/CLASS and MC2/force–restore at spatial resolutions of 10 and 5 km to simulate the severe precipitation case of 19–21 July 1996 in the Saguenay region of Québec, Canada. Comparisons of the simulated precipitation time series and the simulated 48-h accumulated precipitation at different spatial resolutions with rain gauges indicate that MC2/CLASS at 5-km resolution gives the best simulated precipitation. The comparison results show the model accuracy of MC2/CLASS at 10 km is comparable to the accuracy of MC2/force–restore at 5 km. The mechanism responsible for this is that CLASS represents the land surface vegetation characteristics in a more sophisticated manner than the force–restore method. Furthermore, in CLASS, each grid square is divided into a maximum of four separate subareas, and subvariations of the grid surface vegetation characteristics are taken into account. Therefore, for a grid square containing different types of vegetation, the subgrid-scale information can be used by CLASS, and the computed effective variables that are fed back to MC2 on a 10 × 10 km2 grid are equivalent to computing them at a higher effective resolution than 10 km. This higher effective resolution for surface characteristics is not found in the force–restore method. The total simulated domain-averaged precipitation, and the sum of sensible and latent heat fluxes from MC2/CLASS and MC2/force–restore at different spatial resolutions, are similar. The major difference is in the partitioning of the simulated sensible and latent heat fluxes. The positioning of the simulated precipitation has been improved by using CLASS. The overall results suggest that the impact of land surface schemes is indeed significant in a short-range precipitation forecast, especially in regions with complicated vegetation variations.

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Stéphane Bélair, Louis-Philippe Crevier, Jocelyn Mailhot, Bernard Bilodeau, and Yves Delage

Abstract

The summertime improvement resulting from the operational implementation of a new surface modeling and assimilation strategy into the Canadian regional weather forecasting system is described in this study. The surface processes over land are represented in this system using the Interactions between Soil–Biosphere–Atmosphere (ISBA) land surface scheme. Surface variables, including soil moisture, are initialized using a sequential assimilation technique in which model errors of low-level air temperature and relative humidity are used to determine analysis increments of surface variables.

It was found that the magnitude and nature of the analysis increments applied to the surface variables depended on the surface and meteorological conditions observed in each region. In regions characterized by weak meteorological activity (i.e., no clouds or precipitation), model errors of low-level air characteristics are more likely to be related to an incorrect representation of surface processes due to either erroneous initial conditions or inaccurate parameterizations in the land surface scheme. In other regions characterized by more frequent and more intense precipitation events, surface corrections are mainly associated with inaccurate atmospheric forcing.

Objective evaluation against observations from radiosondes and surface stations showed that the amplitude of the diurnal cycle of near-surface air temperature and humidity is larger with the new surface system, in better agreement with observations. This type of improvement was found to extend higher up in the boundary layer (up to 700 hPa) where cold and humid biases were significantly reduced by introducing the new surface system. The model precipitation was also found to be significantly influenced by the new representation of surface fluxes. The problematic increase of a positive bias in precipitation with integration time was found to be significantly reduced with the new system, due to the warmer and drier boundary layer.

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