Rachel E. Jordan

Validation of the energy budget of an alpine snowpack simulated by several snow models (SnowMIP project)

Abstract Many snow models have been developed for various applications such as hydrology, global atmospheric circulation models and avalanche forecasting. The degree of complexity of these models is highly variable, ranging from simple index methods to multi-layer models that simulate snow-cover stratigraphy and texture. In the framework of the Snow Model Intercomparison Project (SnowMIP), 23 models were compared using observed meteorological parameters from two mountainous alpine sites. The analysis here focuses on validation of snow energy-budget simulations. Albedo and snow surface temperature observations allow identification of the more realistic simulations and quantification of errors for two components of the energy budget: the net short- and longwave radiation. In particular, the different albedo parameterizations are evaluated for different snowpack states (in winter and spring). Analysis of results during the melting period allows an investigation of the different ways of partitioning the energy fluxes and reveals the complex feedbacks which occur when simulating the snow energy budget. Particular attention is paid to the impact of model complexity on the energy-budget components. The model complexity has a major role for the net longwave radiation calculation, whereas the albedo parameterization is the most significant factor explaining the accuracy of the net shortwave radiation simulation.

Snow ablation modeling at the stand scale in a boreal jack pine forest

The purpose of this study is to predict spatial distributions of snow properties important to the hydrology and the remote sensing signatures of the boreal ecosystem. This study is part of the Boreal Ecosystems Atmosphere Study (BOREAS) of central Saskatchewan and northern Manitoba. Forested environments provide unique problems for snow cover process modeling due to the complex interactions among snow, energy transfer, and trees. These problems are approached by coupling a modified snow process model with a model of radiative interactions with forest canopies. Additionally, a tree well model describes the influence of individual trees on snow distribution on the ground. The snow process and energy budget model calculates energy exchange at the snow surface, in-pack snow processes, melting and liquid water flow, heat conduction, and vapor diffusion. The surface radiation model provides input on the radiation receipt at the snow surface for model runs in the jack pine forest. Field data consisted of measured meteorological parameters above and within the canopy, spatial variability of snow properties, and variations of incoming solar irradiance beneath the forest canopy. Results show that the area beneath tree canopies accumulated 60% of the snow accumulated in forest openings. Peak solar irradiance on the snow cover was less than one half that measured above the canopy. Model runs are compared between the open and the forested sites and show the open area ablating four days before areas beneath the canopy and eight days before forest openings and compare favorably with measured data. Physically based modeling of snow ablation was successful at the forested site and nearby open area.

Heat budget of snow‐covered sea ice at North Pole 4

The Russian drifting station North Pole 4 (NP-4) was within 5° latitude of the North Pole from April 1956 to April 1957. We use a wide-ranging set of snow and meteorological data collected at 3-hourly intervals on NP-4 during this period to investigate energy and mass transfer in the snow, sea ice, and atmospheric surface layer in the central Arctic. SNTHERM, a one-dimensional energy and mass balance model, synthesizes these diverse NP-4 data and thereby yields energetically consistent time series of the components of the surface heat budget. To parameterize the sensible heat flux during extremely stable stratification, we replace the usual log-linear stability function with the “Dutch” formulation and introduce a windless coefficient in the bulk parameterization. This coefficient provides sensible heat transfer at the surface, even when the mean wind speed is near zero, and thereby prevents the surface temperature from falling to unrealistically low values, a common modeling problem when the stratification is very stable. Several other modifications to SNTHERM introduce procedures for creating a realistic snowpack that has continuously variable density and is subject to erosion and wind packing. The NP-4 data provide for two distinct simulations: one on 2-year ice and one on multiyear ice. We validate our modeling by comparing simulated and observed temperatures at various depths in the snow and sea ice. Simulations for both sites show the same tendencies. During the summer, the shortwave radiation is the main term in the surface heat budget. Shortwave radiation also penetrates into the snow and causes a subsurface temperature maximum that both the data and the model capture. During the winter, the net longwave balance is the main term in the surface heat budget. The snow and sea ice cool in response to longwave losses, but the flux of sensible heat from the air to the surface mitigates these losses and is thus nearly a mirror image of the emitted longwave flux.

Variation of snow cover ablation in the boreal forest: A sensitivity study on the effects of conifer canopy

The duration and meteorological history of winter and thaw periods in the boreal forest affect carbon exchange during the growing season. Characteristics of conifer canopies exert important control on the energy exchange at the forest floor, which in turn controls snow cover processes such as melting. This analysis investigated the role of the conifer tree characteristics, including height and canopy density. Canopy and snow models estimated radiation incoming to the snow surface, the net energy budget of the snow, and melting rates of snow cover under conifer forests with different canopy density and tree height. This analysis assumed that canopy effects dominated snow surface energy exchange under conifers in the boreal forest. We used data layers of forest characteristics from the Boreal Ecosystem-Atmosphere Study (BOREAS) modeling subareas in Saskatchewan and Manitoba to guide the choice of modeled tree height and canopy density. Modeled stand characteristics assumed random location of trees and used a uniform tree height within a stand and regular crown geometry scaled to tree height. Measurements during winter and thaw in 1994 of incoming solar and longwave radiation, humidity, and wind speed above the forest canopy provided input to the models, along with air temperature measured in the canopy. Results showed the importance of canopy density and tree height as the first-order controls on cumulative incoming solar radiation at the forest floor for the range of these variables in the BOREAS test area. The combined canopy and snow models showed a large range of snow ablation within conifers, which showed the trade-offs between canopy density and tree height. Solar fluxes dominated the net transfer of energy to the snow in the north, while sensible heat exchange, net solar, and net longwave radiation played important roles in the south.

Evaluations of the von Kármán constant in the atmospheric surface layer

The von Karman constant

Snow ablation modelling in a mature aspen stand of the boreal forest

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Parameterizing turbulent exchange over sea ice in winter

relates the flow speed profile in a wall-bounded shear flow to the stress at the surface. Recent laboratory studies in aerodynamically smooth flow report

Incorporating effects of forest litter in a snow process model

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Air permeability and capillary rise as measures of the pore structure of snow: an experimental and theoretical study

values that cluster around 0.42–0.43 and around 0.37–0.39. Recent data from the atmospheric boundary layer, where the flow is usually aerodynamically rough, are similarly ambiguous:

Simulations of Snow, Ice, and Near-Surface Atmospheric Processes on Ice Station Weddell

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