The CMC snow cover analysis differs substantially from the NOAA product in that it is based solely on in-situ daily snow depth observations, with spatial and temporal continuity provided through the temperature and precipitation fields generated by the CMC global NWP model. Details are provided below:

• Uses snow depth observations from all synoptic reports, metars (meteorological aviation reports) and sa's (special aviation reports).
• Analysis is updated every 6 hours using the method of optimum interpolation. The effect of elevation in included in the interpolation.
• Precipitation forecasts (6 hour) from the CMC global model are used to estimate the new snow between analysis times. The precipitation is assumed to be snow if the analyzed screen-level temperature is < 0°.
• A melting algorithm removes mass from the snowpack at the rate of 0.15mm SWE per hour per degree K for air temperatures above freezing.
• The analysis includes an estimate of the density of the snowpack. The density of new snow is assumed to be 100 kg/m3 and the snowpack gradually increases in density as it ages. The increase in density with aging stops when the density reaches 300 kg/m3 (210 kg/m3 for needleleaf forest as the canopy shelters the snowpack from wind and sunlight and snow densities are lower). New snow causes the density to decrease by an amount related to the mass of the new snow and the mass of the existing snowpack. During melting, the density is allowed to increase up to a maximum of 550 kg/m3.
• In regions where there are no observations of snow depth, the snow depth shown in the analysis corresponds to the accumulated precipitation from the precipitation forecasts consistent with the assumptions about density from the previous paragraph.
• The analysis has a resolution of 1/3°. The error correlations used by the analysis have an e-folding distance of about 120 km horizontally and 800 m vertically.
• The scheme makes use of an analysis of screen-level temperature produced with vertical correlations taken into account.

For further information on the analysis procedure please contact Bruce Brasnett, or consult the following reference: Brasnett, B., 1999: A global analysis of snow depth for numerical weather prediction. J. Appl. Meteorol., 38: 726-740.