Snowpack Parameters Research Articles

Data for snowmelt model development, calibration, and verification at an Alpine Site, Colorado Front Range

Logistical constraints have caused data collection in seasonally snow covered areas to generally be on a campaign basis with limited instrumentation. The problems of winter access, cold air temperatures, and blowing snow cause both equipment malfunctions and problems with consistent and timely maintenance. At the Long‐Term Ecological Research program network site in an alpine area of Colorado we have been operating a meteorological station and subnivean (below snow) laboratory at 3517 m since the spring of 1994 to collect information that will allow us to better understand snow‐surface energy exchanges and the mass flux of water during snowmelt. This unique and high‐quality data set was designed to measure the meteorologic and hydrologic parameters necessary to compute the surface energy and snowpack mass balances at a point. All meteorological parameters are directly measured. Turbulent fluxes are calculated using the aerodynamic profile method. The timing and magnitude of snowmelt is measured with 18 snow lysimeters. Meteorologic parameters and energy fluxes are available at 10‐min, hourly, and daily time steps. Complementary information includes a high‐resolution digital elevation model, snowpack parameters, and stream discharge.

EFFECTS OF FOREST ON THE SNOW PARAMETERS DERIVED FROM MICROWAVE MEASUREMENTS DURING THE BOREAS WINTER FIELD CAMPAIGN

Passive microwave data have been used to infer the areal snow water equivalent (SWE) with some success. However, the accuracy of these retrieved SWE values have not been well determined for heterogeneous vegetated regions. The Boreal Ecosystem-Atmosphere Study (BOREAS) Winter Field Campaign (WFC), which took place in February 1994, provided the opportunity to study in detail the effects of boreal forests on snow parameter retrievals. Preliminary results reconfirmed the relationship between microwave brightness temperature and snow water equivalent. The pronounced effect of forest cover on SWE retrieval was studied. A modified vegetation mixing algorithm is proposed to account for the forest cover. The relationship between the microwave signature and observed snowpack parameters matches results from this model.

The temporal and spatial variation of snow pressure on structures

Measurements of snow creep pressure on an avalanche-defence structure in western Norway are presented. Two different types of measurement methods are described and evaluated. Pressure data from four winters are correlated with the following measured snowpack properties: density, snow depth, snowpack temperature, and snowpack stiffness. The results show that maximum and average pressures are strongly dependent on the product of density and snow depth as well as snowpack stiffness. The highest pressures were observed in spring prior to melting of the snowpack. Key words: snow pressure, snowpack parameters, slope inclination, structures.

Comparison of snow pressure measurements and theoretical predictions

Measurements of snow creep pressures from the centre section of a 3.4 m high and 15 m long avalanche-defence supporting structure located on a mountain in western Norway are presented. The site has a deep snow cover and an average slope angle of 25°. The measurement configuration corresponds to plane-strain conditions and the data, along with measured snowpack parameters, allow comparison with simple theoretical predictions. The analysis shows that the average pressure on the structure may be calculated fairly accurately using linear, viscous modelling for the snow deformation. The maximum pressures proved to be higher than that provided by a linear model and this is considered characteristic of nonlinear material. The implications of these results for estimates of design loads are discussed. Keywords: snow pressure, creep, measurements, viscous, plane-strain, finite element.

Optical properties of snow

Measurements of the dependence of snow albedo on wavelength, zenith angle, grain size, impurity content, and cloud cover can be interpreted in terms of single‐scattering and multiple‐scattering radiative transfer theory. Ice is very weakly absorptive in the visible (minimum absorption at λ = 0.46 µm) but has strong absorption bands in the near infrared (near IR). Snow albedo is therefore much lower in the near IR. The near‐IR solar irradiance thus plays an important role in snowmelt and in the energy balance at a snow surface. The near‐IR albedo is very sensitive to snow grain size and moderately sensitive to solar zenith angle. The visible albedo (for pure snow) is not sensitive to these parameters but is instead affected by snowpack thickness and parts‐per‐million amounts (or less) of impurities. Grain size normally increases as the snow ages, causing a reduction in albedo. If the grain size increases as a function of depth, the albedo may suffer more reduction in the visible or in the near IR, depending on the rate of grain size increase. The presence of liquid water has little effect per se on snow optical properties in the solar spectrum, in contrast to its enormous effect on microwave emissivity. Snow albedo is increased at all wavelengths as the solar zenith angle increases but is most sensitive around λ =1 µm. Many apparently conflicting measurements of the zenith angle dependence of albedo are difficult to interpret because of modeling error, instrument error, and inadequate documentation of grain size, surface roughness, and incident radiation spectrum. Cloud cover affects snow albedo both by converting direct radiation into diffuse radiation and also by altering the spectral distribution of the radiation. Cloud cover normally causes an increase in spectrally integrated snow albedo. Some measurements of spectral flux extinction in snow are difficult to reconcile with the spectral albedo measurements. The bidirectional reflectance distribution function which apportions the reflected solar radiation among the various reflection angles must be known in order to interpret individual satellite measurements. It has been measured at the snow surface and at the top of the atmosphere, but its dependence on wavelength, snow grain size, and surface roughness is still unknown. Thermal infrared emissivity of snow is close to 100% but is a few percent lower at large viewing angles than for overhead viewing. It is very insensitive to grain size, impurities, snow depth, liquid water content, or density. Solar reflectance and microwave emissivity are both sensitive to various of these snowpack parameters. However, none of these parameters can be uniquely determined by satellite measurements at a single wavelength; a multichannel method is thus necessary if they are to be determined by remote sensing.

Investigations on snow parameters by radiometry in the 3‐ to 60‐mm wavelength region

We report on a 2‐year period of monitoring parameters of a natural snowpack by ground‐based microwave radiometry on a high‐altitude Alpine test site. The microwave brightness temperatures are compared to a large set of ground‐truth data. Three stages in the seasonal development of the snow cover are easily distinguishable which allow the prediction of the beginning of the snow melting. The moisture content of the melting surface layer is estimated by the aid of the typical daily variations of microwave brightness temperatures in spring. The test site was composed of two snow fields. The first one was lying on slightly reflecting soil, and the second one was lying on a completely reflecting metal foil. By measuring on both fields some microwave snow parameters can be determined. The damping coefficients for microwaves between 5 and 100 GHz were estimated by comparing the results of two extreme theories. Both theories gave results from less than 1 dB/m to more than 30 dB/m depending on the snow state, especially its liquid water content.

Stratigraphic Studies of the Winter Snow Layer, Mount Logan, St. Elias Range

A traverse of King Trench, which rises from Upper Ogilvie Glacier (2300 m) to King Col (4500 m) a distance of 16 km, Yukon Territory, was made for US Army CRREL, June 1965, to see if techniques of stratiographic analysis of snow and firn could be successfully applied to studies of the alpine climate, as they have been on the Greenland ice sheet, Data on near-surface snow accumulation, temperature and density from eight pits along the traverse provide a climatological model which relates the variations in snow properties to localized topographic obstruction or enhancement of katabatic air drainage. Limited conclusions suggest directions for future study rather than for application to other large ranges or even other parts of St.Elias Range: topography may be the most important control in determining the areal distribution of near-surface climatic elements in mountains; extrapolation of climatological parameters to mountain slopes from nearby valley stations or free-air soundings is not a valid approach: analysis of pertinent stratiographic parameters of the alpine snow pack may be a useful approach to study of local mountain climates.

Estimated frequencies and extreme values of snow‐pack water equivalent at major cities in the United States

The limited number of direct observations of weight of snow pack, particularly in populated localities, has made it necessary to study relations between observed or estimated weight of snow pack and other parameters. Fair relations have been established at stations having suitable data and have been generalized on the basis of the good areal distribution of two parameters that are available in convenient published form; depth of snow on the ground and accumulated seasonal snowfall. The results seem to be consistent with the limited snow‐survey data available in the area of interest and with other sources.The water equivalent of accumulated seasonal snow pack that may be equaled or exceeded once in 50 years (about the maximum of record) is about seven per cent of the maximum seasonal snowfall and about 20 pct of the maximum observed snow depth in inches. The values of water equivalent to be equaled or exceeded one year in ten are 75 pct, and one year in two are about 50 pct, of the maximum of record.