Snow and vegetation

Snow can easily submerge short vegetation, greatly changing the albedo and roughness of the surface, but dense forest canopies retain low albedos even when snow covered. This can have alarge influence on temperature forecasts (Viterbo and Betts, 1999). Betts (2000) has contrasted warming due to decreased albedo with cooling due to carbon sequestration if boreal forestation is used to offset anthropogenic carbon emissions. Intercepted snow on a forest canopy (Section 3.5.5) has a large exposed...

Nomenclature

Parameter, saturation vapor pressure 2 Parameter, effective thermal 4 conductivity Parameter, albedo weight function 4 Parameter, albedo weight function 4 Parameter, dimensionless sensible 3 heat flux Saltation efficiency coefficient 3 Parameter, saturation vapor pressure 2 Parameter, effective thermal 4 conductivity Parameter, saturation vapor pressure 2 Parameter, apparent change in roughness length New snow drift coefficient Mass concentration of suspended snow Bulk transfer coefficient for...

Highresolutionmultispectral data

Several polar orbiting satellite sensors have multiple spectral bands located in the visible (0.4-0.7 pm), near-infrared (0.7-1.1 pm), and shortwave infrared (1.1-3.0 pm) as well as the thermal infrared (3.0-100 pm) regions of the electromagnetic spectrum. These sensors typically have very high spatial resolution compared to the actual snow products derived from sensors such as AVHRR described Figure 5.2. Monthly Northern Hemisphere snow cover (1966-2005) and sea ice extent (1978-2005)...

References

R. and Krajeski, G. (1998). A fast, physically-based point snow melt model for use in distributed applications. Hydrol. Process., 12(11), 1809-1824. Amorocho, J. and Espildora, B. (1966). Mathematical Simulation of the Snow Melting Processes. Water Science and Engineering Papers No. 3001. Davis, CA University of California. Anderson, E. A. (1968). Development and testing of snow pack energy balance equations. Water Resources Res., 4(1), 19-37. Anderson, E. A. (1973). National Weather...

Introduction

Chapter 2 describes the complexity of snow physics. Most of the processes occurring inside the snowpack or at the snow-atmosphere interface can be represented in sophisticated snow models (see Sections 4.1 and 4.2) but not in snow param-eterizations developed for General Circulation Models (GCMs) (see Section 4.4) because of the necessary limitation of computation time. Some snow processes have little effect on snow-atmosphere energy and mass exchanges while others are critical. The first group...

Importance of snow in the climate system

Several fundamental physical properties of snow modulate energy exchanges between the surface and the atmosphere. The most important properties are the surface reflectance (albedo), the thermal insulating properties of snow, and the ability to change state (latent heat). Physical properties of a snowpack such as crystal structure, density, and liquid water content are also important for transfers of heat and water. These basic properties also determine the mechanical state of the snow cover,...

Equations of energy and mass balance

Pomeroy, Donald M. Gray, and Charles Fierz Energy balance is often formulated in terms of energy exchanges taking place at an interface (see e.g. Oke, 1987). Such an active surface has to be thought of as being infinitesimally thin and having neither mass nor specific heat. Penetration of shortwave radiation into the snowpack as well as mass movements and phase changes within the snowpack make this concept barely applicable to snow. A better approach is to consider a...

General characteristics

After being deposited on the ground or on a previous snow layer, snow crystals accumulate and give birth to a new snow layer. The initial structure of this layer depends on the shape and size of the crystals and on the stress applied to the bonds that link them together. Ice forms a solid matrix that delimits pores filled with humid air and, in the case of wet snow, with liquid water. In snow, most pores are interconnected. Thus, snow belongs to the great family of porous media whose members...

Interception by vegetation

Vegetation intercepts snow as a function of its winter leaf and stem area and the size of the snowfall event. Because sublimation and melt remove snow from canopies, interception (the snowfall trapped in the canopy, normally event-based) is distinguished from snow load (the snow held in the canopy at a particular time). Hedstrom and Pomeroy (1998) developed and field tested a model of snow interception and snow unloading of the following form (see Fig. 3.3) I he U't, h (ll Ll,o)(1 eccanP LL),...

Thermal properties

As in all aggregate materials, the thermal properties of snow depend upon the microstructure for snow this reflects on crystal type, crystal organization, and connectivity. Because snow undergoes metamorphism with time and ambient conditions, its thermal properties also change. Although the measurements of Izumi and Huzioka (1975) confirm that metamorphism induces anisotropy in physical properties, there are no quantitative analyses that can predict thermal properties based on microstructure....

Krt [1 1 S1m

At lower moisture contents and, thus, all four curves in Fig. 2.14a reach an immobile or irreducible saturation limit, si. Isolated liquid inclusions cannot be reduced below this limit except through freezing or evaporation. Water flow parameters thus scale to an effective saturation, s* (s - si) (1 - si), so that capillary pressure tends to infinity and relative permeability to 0 as s approaches si. While the retained immobile saturation after drainage is typically around 0.07, the minimum...

Middle elevation alpine snow cover

Relevance and characteristics Col de Porte is quite representative of middle elevation sites in a temperate alpine climate. Many ski resorts and hydropower stations in the Alps are located within the same elevation range. The climate is wet (2000 mm w.e. yr-1) because it is situated in the western part of the European Alps. The mean winter temperature (December-February) is -1 C. Rainfall events are common in winter. During the first part of the winter, the snowpack structure is highly variable...

Basic properties of snow

Sintering Snowpack

The following discussion of basic snow properties is presented to provide the uninitiated reader with the background to understand the discussion of snow physics and modeling in Chapters 2-4. Snow originates in clouds at temperatures below the freezing point. As moist air rises, expands and cools, water vapor condenses on minute nuclei to form cloud droplets on the order of 10 microns in radius. When cooled below 0 C such small droplets do not necessarily freeze and may super cool down to -20 C...

Snow metamorphism

Sintering Snowpack

The great variability in snow microstructure is due in a small part to the initial diversity of precipitating particles but mostly to the various transformations the ice matrix undergoes because of thermodynamic relationships among the water phases. These transformations are called snow metamorphism. Since the temperature of natural snow is generally close to the triple point, mass exchanges between vapor and ice and possibly between ice, vapor, and liquid water are very active. This activity...

Snow parameterization in GCMs

General circulation models (GCMs) are three-dimensional numerical models of the global climate system an introductory review of their use in climate modeling is given by McGuffie and Henderson-Sellers (1997). The atmospheric component of a GCM may be coupled to an ocean model or run with prescribed sea-surface temperatures and sea-ice extents to provide surface boundary conditions over oceans. Land-surface models, used to supply boundary conditions over land, have to take account of the...

Cloud formation and precipitation

Moisture in the atmosphere occurs principally in its gaseous phase, as water vapor, but also condenses to form clouds of water droplets or ice crystals. Vapor condenses when its partial pressure exceeds a saturation value, as determined by equilibrium conditions between the vapor and liquid or vapor and ice phases (Section 2.2.2). Since the saturation pressure decreases nearly exponentially with temperature, warm air at 25 C can hold about fifty times more vapor than subzero air at -25 C. Thus,...

Turbulent heat fluxes

Sensible and latent heat may be carried to or from a snow surface by the action of turbulent eddies in the surface boundary layer (Morris, 1989). Formally, these fluxes may be written as the covariance of fluctuations in vertical velocity, w, with those either in temperature, Ta, or specific humidity, Q, i.e. where the overbar denotes a mean over time, primes denote deviations from time averaged values, pa is the air density and cp,a is the specific heat of air at constant pressure (1.01 x 103...

Highelevation alpine snow cover

Christian Pluss, Charles Fierz, and Paul M. B. Fohn Relevance and characteristics High alpine seasonal snow covers are present in mountainous areas around the world. The duration and spatial distribution of alpine snow covers is extremely variable and depends mainly on the geographical location, the climatic conditions, and the elevation of the mountain range. Alpine snow covers are of large economic and social importance in many areas, for example as a water resource for hydropower or as a...

Shortwave radiation

The starting point for calculating insolation at the surface is the solar radiation flux at the top of the atmosphere, Stoa, 99.9 of which lies in the spectral band from 0.2 to 100 pm. Stoa may easily be calculated as a function of geographical location, season, and time (see, e.g., Iqbal, 1983). The part of the solar spectrum up to 4.0 pm is usually called shortwave radiation, representing about 99.2 of Stoa. However, solar radiation is absorbed by some atmospheric constituents and is...

Snow albedo

The albedo of a snow-covered surface is influenced by many factors, including the depth and grain structure of the snow, contaminants in the snow, the albedo of the underlying surface, heterogeneities in the snow cover, and masking by vegetation (Sections 2.5, 3.4). The simplest GCM snow models neglect these influences and assign a fixed albedo to any gridbox with snow cover. Snow albedos also vary greatly with the wavelength of incident radiation although they split the solar spectrum into...

Thermodynamics of phase equilibria in snow

Bulk equilibrium temperature in a pure water system Thermodynamic relationships among the three water phases determine the grain growth and metamorphism of snow. Whether the compound H2O exists as ice, water, or water vapor depends on its temperature (T) and pressure (p), as shown in the phase diagram in Fig. 2.5. The curves for evaporation (A), sublimation (B), and melting (C) trace the points where two bulk-water phases coexist in thermodynamic equilibrium. All three phases coexist at the...

Snowcover simulation models

In the 1960s with the development of digital computers, researchers were able to construct conceptual simulation models of the snow accumulation and ablation Snow and Climate Physical Processes, Surface Energy Exchange and Modeling, ed. Richard L. Armstrong and Eric Brun. Published by Cambridge University Press. Cambridge University Press 2008. process. These models were developed to solve many kinds of practical hydrologic problems. In a conceptual model, each major physical process is...

Snow compaction

Measurement of snow density and liquid water content As seen in Section 2.2.1, the mixing ratio between ice, air, and liquid water is a key parameter that explains a significant part of the variability in physical and mechanical snow properties. For dry snow, density is an accurate indicator of this ratio, while for wet snow, an additional measurement of the liquid water content is necessary. Density measurements of snow are relatively easy to perform both in the laboratory and in the field,...

Soil

Mass balance with blowing snow. Experimentally derived sublimation rates from mature evergreen forests range from 3 to about 12 kg m-2 d-1 Pomeroy et al., 1998a Nakai et al., 1999 and can return 13-40 of seasonal snowfall to the atmosphere as sublimation from northern forests Pomeroy and Gray, 1995 Pomeroy et al., 1998a . Blowing snow fluxes occur over open snow surfaces with good exposure to wind and a supply of erodible snow see Fig. 3.4 . On large, relatively uniform surfaces...