Snow accumulation and deposition on a west coast midlatitude mountain
Creators: name: Fitzharris, B. B. (Brian Blair)
Publisher: name: University of British Columbia
Year: 2010

description: The first objective of this study is to measure, describe, and attempt to predict variations of net snow accumulation with elevation over a mesoscale area of 14 km² on a west coast midlatitude mountain. The second objective is to measure and describe similar variations of snow deposition after each storm for two consecutive winters, so defining the snow input system of the hydrologic cycle. The winters sampled (1969-70, 1970-71) represent a wide range of probable conditions. A related goal is the development of a climatology of winter storms. The final objective is to estimate snow deposition over a west coast midlatitude mountain after a storm. Input to the deterministic model which is evolved is restricted to precipitation and temperature data measured at the base of the mountain. All measurements are made on Mount Seymour, British Columbia, within a carefully structured experimental design using a double stratified random sampling scheme. Measurements are confined to a partly forested terrain segment of constant aspect and slope. Precipitation data are presented for open areas at 12 elevations from 120 m to 1260 m for 138 storms. These include data for 82 snow storms, where additional measurements are made within defined positions of the forest. Frequent net snow accumulation measurements are also made of the snowpack. Continuous temperature measurements at six elevations and on a TV mast define the thermal regime during storms. Several types of snowline are recognised and their elevations monitored every few days. The first objective cannot easily be achieved because simple empirical relationships between net snow accumulation and elevation are not reliable on a west coast midlatitude mountain. This is a consequence of the formation of a snow wedge on -the mountain, whose shape and slope is largely controlled by the frequency of winter storm types, except at the end of the season, or at low elevations, where melt processes are important. Snow accumulation in the forest can be estimated from that in the open with good precision, provided the snowpack is greater than 100 cm water equivalent. Snow deposition from each storm increases with elevation in wedge like form. The new snowlines and shape of this new snow wedge are mainly determined by the orographic increase in precipitation, and by the elevation of the freezing level. Large fluctuations of the freezing level among and within storms are a feature of west coast midlatitude mountains. Summation of snow deposition from each storm defines the total winter snow input to the snowpack (the second objective). This input also increases with elevation in wedge like form, hence explaining, with snow melt, the similar distribution of net snow accumulation. Contrary to earlier findings, total winter precipitation increases linearly with elevation, with no evidence of a consistent storm maximum at intermediate elevations. The input of water by rime is indexed, and found to be substantial. Ways of improving the efficiency of the sampling network of this study are proposed. The climatology of winter storms (the third objective) is developed from surface synoptic maps and upper air data by examining storm types, tracks, freezing levels and atmospheric fluxes of water vapour. The different snow regime of the two studied winters is explained in terms of this climatology. The relative importance of storm type in providing snowfall is assessed for each elevation on Mount Seymour. Good agreement is found when the deterministic model, which is developed to estimate storm snow deposition variations with elevation as in the final objective, is tested by comparing simulated snow deposition with independent data. Confidence limits about the predicted estimates are difficult to assess, but do not appear to be small. Throughfall of snow in the forest is predicted from that in the open with the relationships changing with elevation and storm characteristics. Evidence is presented that management of the seasonal snow cover on west coast midlatitude mountains is likely to be most effective at intermediate elevations.