Richard Essery

An evaluation of snow accumulation and ablation processes for land surface modelling

This paper discusses the development and testing of snow algorithms with specific reference to their use and application in land surface models. New algorithms, developed by the authors, for estimating snow interception in forest canopies, blowing snow transport and sublimation, snow cover depletion and open environment snowmelt are compared with field measurements. Existing algorithms are discussed and compared with field observations. Recommendations are made with respect to: (a) density of new and aged snow in open and forest environments; (b) interception of snow by evergreen canopies; (c) redistribution and sublimation of snow water equivalent by blowing snow; (d) depletion in snow-covered area during snowmelt; (e) albedo decay during snowmelt; (f) turbulent transfer during snowmelt; and (g) soil heat flux during meltwater infiltration into frozen soils. Preliminary evidence is presented, suggesting that one relatively advanced land surface model, CLASS, significantly underestimates the timing of snowmelt and snowmelt rates in open environments despite overestimating radiation and turbulent contributions to melt. The cause(s) may be due to overestimation of ground heat loss and other factors. It is recommended that further studies of snow energetics and soil heat transfer in frozen soils be undertaken to provide improvements for land surface models such as CLASS, with particular attention paid to establishing the reliability of the models in invoking closure of the energy equation. #1998 John Wiley & Sons, Ltd.

A distributed model of blowing snow over complex terrain

Physically-based models of blowing snow and windflow are used to develop a distributed model of blowing snow transport and sublimation over complex terrain. The model is applied to an arctic tundra basin. A reasonable agreement with results from snow surveys is obtained, provided sublimation processes are included; a simulation without sublimation produces much greater snow accumulations than were measured. The model is able to reproduce some observed features of redistributed snowcovers: distributions of snow mass, classified by vegetation type and landform, can be approximated by lognormal distributions, and standard deviations of snow mass along transects follow a power law with transect length up to a cut-off. The representation used for the downwind development of blowing snow with changes in windspeed and surface characteristics is found to have a large moderating influence on snow redistribution.

Turbulent fluxes during blowing snow: field tests of model sublimation predictions

Sublimation fluxes during blowing snow have been estimated to return 10-50% of seasonal snowfall to the atmosphere in North American prairie and arctic environments. These fluxes are calculated as part of blowing snow two-phase particle transport models with provision for phase change based upon a particle-scale energy balance. Blowing snow models have normally been evaluated based upon their ability to reproduce diagnostic mass flux gradient measurements and regional-scale snow redistribution patterns and snow mass. Direct evidence is presented here that large latent heat fluxes (40-60 W m -2 ) that result in sublimation rates of 0.05-0.075 mm snow water equivalent hour -1 , are associated with mid-winter, high-latitude blowing snow events. For events with wind speeds above the threshold level for snow transport, these fluxes are in the range of those predicted by the Prairie Blowing Snow Model. The fluxes are well in excess of those found during spring snowmelt and which can be predicted by standard bulk aerodynamic transfer equations, suggesting that blowing snow physics will have to be incorporated in land surface schemes and hydrological models in order to properly represent snow surface mass and energy exchange during blowing snow events.

Effect of covariance between ablation and snow water equivalent on depletion of snow-covered area in a forest.

The influence of the spatial distribution of snow water equivalent and covariance between spatial distributions of ablation and snow water equivalent on depletion of snowcover was investigated in the boreal forest of central Saskatchewan. Canada. Changes in the spatial distributions of snow water equivalent were measured before and during melt in five stands, ranked by canopy density as: black spruce, jack pine, mixed wood, burned and recent clear-cut. The pre-melt frequency distribution of snow water equivalent within forest stands was found to fit a log-normal distribution. Higher variability in snow water equivalent resulted in earlier exposure of ground under spatially uniform melt simulations, confirming the previous findings of others for open environments. The spatial distribution of daily ablation within stands was found, however, to be correlated inversely to the distribution of snow water equivalent. This negative covariance between snow water equivalent and ablation further accelerated snow cover depletion. The combined acceleration as a result of variance of snow water equivalent and covariance with ablation was greatest in mixed-wood stands and smallest in burned and spruce stands. Simulations that included the within-stand covariance of ablation and snow water equivalent showed improved fit with measured data over those that only considered the effect of the distribution of snow water equivalent on snow-cover depletion.

Effects of needleleaf forest cover on radiation and snowmelt dynamics in the Canadian Rocky Mountains

Radiation is the main energy source for snowpack warming and melt in mountain needleleaf forests, and runoff from these forests is the main contributor to spring river flows in western North America. Utilizing extensive field obser- vations, the effect of needleleaf forest cover on radiation and snowmelt timing was quantified at pine and spruce forest sites and nearby clearings of varying slope and aspect in an eastern Canadian Rocky Mountain headwater basin. Compared with open clearing sites, shortwave radiation was much reduced under forest cover, resulting in smaller differences in melt timing between forested slopes relative to open slopes with different aspects. In contrast, longwave radiation to snow was substantially enhanced under forest cover, especially at the dense spruce forest sites where longwave radiation dominated total energy for snowmelt. In both pine and spruce environments, forest cover acted to substantially reduce total radiation to snow and delay snowmelt timing on south-facing slopes while increasing total radiation and advancing snowmelt timing on north-facing slopes. Results strongly suggest that impacts on radiation to snow and snowmelt timing from changes in mountain forest cover will depend much on the slope and aspect at which changes occur.