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David M. Mocko and Y. C. Sud

Abstract

Refinements to the snow-physics scheme of the Simplified Simple Biosphere Model (SSiB) are described and evaluated. The upgrades include a partial redesign of the conceptual architecture of snowpack to better simulate the diurnal temperature of the snow surface. For a deep snowpack, there are two separate prognostic temperature snow layers: the top layer responds to diurnal fluctuations in the surface forcing, while the deep layer exhibits a slowly varying response. In addition, the use of a very deep soil temperature and a treatment of snow aging with its influence on snow density is parameterized and evaluated. The upgraded snow scheme produces better timing of snowmelt in Global Soil Wetness Project (GSWP)-style simulations using International Satellite Land Surface Climatology Project (ISLSCP) Initiative I data for 1987–88 in the Russian Wheat Belt region.

To simulate more realistic runoff in regions with high orographic variability, additional improvements are made to SSiB's soil hydrology. These improvements include an orography-based surface runoff scheme as well as interaction with a water table below SSiB's three soil layers. The addition of these parameterizations further helps to simulate more realistic runoff and accompanying prognostic soil moisture fields in the GSWP-style simulations.

In intercomparisons of the performance of the new snow-physics SSiB with its earlier versions using an 18-yr single-site dataset from Valdai, Russia, the revised version of SSiB described in this paper again produces the earliest onset of snowmelt. Soil moisture and deep soil temperatures also compare favorably with observations.

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David M. Mocko and Y. C. Sud

Abstract

Four different methods of estimating land surface evapotranspiration are compared by forcing each scheme with near-surface atmospheric and soil- and vegetation-type forcing data available through International Satellite Land Surface Climatology Project Initiative I for a 2-yr period (1987–88). The three classical energy balance methods by Penman, by Priestley–Taylor, and by Thornthwaite are chosen; however, the Thornthwaite method is combined with a Mintz formulation of the relationship between actual and potential evapotranspiration. The fourth method uses the Simplified Simple Biosphere Model (SSiB), which is currently used in the climate version of the Goddard Earth Observing System II GCM. The goal of this study is to determine the benefit of using SSiB as opposed to one of the energy balance schemes for accurate simulation of surface fluxes and hydrology. Direct comparison of sensible and latent fluxes and ground temperature is not possible because such datasets are not available. However, the schemes are intercompared. The Penman and Priestley–Taylor schemes produce higher evapotranspiration than SSiB, while the Mintz–Thornthwaite scheme produces lower evapotranspiration than SSiB. Comparisons of model-derived soil moisture with observations show SSiB performs well in Illinois but performs poorly in central Russia. This later problem has been identified to be emanating from errors in the calculation of snowmelt and its infiltration. Overall, runoff in the energy balance schemes show less of a seasonal cycle than does SSiB, partly because a larger contribution of snowmelt in SSiB goes directly into runoff. However, basin- and continental-scale runoff values from SSiB validate better with observations as compared to each of the three energy balance methods. This implies a better evapotranspiration and hydrologic cycle simulation by SSiB as compared to the energy balance methods.

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