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Testing the Effects of a New Land Surface Scheme and of Initial Soil Moisture Conditions in the Canadian Global Forecast Model

Yves DelageRecherche en prévision numérique, Atmospheric Environment Service, Dorval, Quebec, Canada

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Diana VerseghyClimate Research Branch, Atmospheric Environment Service, Downsview, Ontanrio, Canada

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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.

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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