Atmospheric Precipitation And Snow Cover

Knowledge about precipitation and its changes in the Arctic is just as important as knowledge about air temperature. This information is needed first of all to correctly estimate the mass balance of the Arctic glaciers and the Greenland Ice Sheet. In turn, the mass balance influences the recession (negative balance) or advance (positive balance) of glaciers. As a result, changes of sea level are observed. Both these processes are very important for natural environment and for human industrial activity. Therefore, during the present period of global wanning, due at least partly to human activity, the monitoring of all kinds of ice in the Arctic is crucial. Relying only on temperature investigation, it is difficult to give a credible prediction of the future behaviour of sea ice and land ice. More accurate predictions may be made when tendencies of precipitation arc also taken into account.

One can agree with the conclusion of Petterssen et al. (1956), that observations of atmospheric precipitation are 'the most unsatisfactory of all Arctic meteorological records'. This is due to the very frequent solid precipitation (snowfall) occuning in the Arctic. In addition, rainfalls in summer have a very low intensity. In winter, the snowfall is most often connected with stonn activity and typically takes the form of fine snowflakes. As a result, wind easily lifts and redistributes these snowflakes according to exposure and local topography. Arctic snow usually begins to drift at wind speeds of 7-8 m/s. During the polar night, it is sometimes difficult to distinguish between a period of snowfall and a period of blowing snow. The measurement of rainfall also encounters difficulties because rain in the Arctic occurs mainly as a light steady drizzle. Serious gauge undercatches are caused mainly by wind-induced turbulence over the gauge orifice. Other losses that decrease the gauge catch are related to evaporation from the gauge before the time of reading, and wetting losses due to moisture that adheres to the walls and funnel of the gauge. Legates and Willmott (1990) have estimated that these undercatches could rcach about 40%. On the other hand, sometimes overcatches also occur. Prik (1965) found that precipitation in Ostrov Dikson was at its highest on days with strong winds, due to snow blown into the recording gauge (particularly large during snowstorms). Bryazgin (1971) also reported that different types of gauges used in the Arctic show a different sensitivity to factors causing enors in measurement. As a result, it is rather difficult to reliably elimi-

natc these errors from the Arctic precipitation series. Hulme (1992) also came to the same conclusion.

Until now, only Bryazgin, using his own method (Bryazgin 1976a), has undertaken attempts to make adjustments of the precipitation series from the Arctic (Gorshkov 1980; Atlas Arktiki 1985). However, it is difficult to say how reliable these results are. For example, comparing the January, July, and annual totals of precipitation for the Canadian Arctic presented in the At/as Arktiki with those published by Maxwell (1980), we notice significant differences. The highest differences are for January precipitation. The map in the Atlas Arktiki shows 2-5 times greater precipitation than the map in Maxwell's work. It seems to me that such large differences cannot be explained by measurement errors. The results for July and the annual totals display fewer discrepancies, and are only higher by about 120-200%. More details about the quality of the precipitation series may be found in the following selection of papers: Prik 1965; Bogdanova 1966; Bryazgin 1969, 1976b; Bradley and England 1978; Sevruk 1982, 1986; Bradley and Jones 1985; Folland 1988; Legates and Willmott 1990; Hulme 1992; Metcalfe and Goodison 1993; Peck 1993; Marsz 1994; Hanssen-Bauer et al. 1996; Ohmura et al. 1999. More recently, Groisman et al. (1997) have provided a good brief summary of this problem. All the above limitations of the measurements of precipitation in the Arctic should be kept in mind, particularly when the data for water balance computations is used.

As has already been mentioned, precipitation measurement problems are at their greatest on the Greenland Ice Sheet, Therefore, to estimate precipitation in Greenland, two other methods arc proposed (Bromwich and Robasky 1993; Bromwich et al. 1998). The first method, the older of the two, was used initially by Diamond (1958, I960). Precipitation amounts on the Greenland Ice Sheet are computed, taking into account the accumulation of snow over one, or more typically several, years. This net build-up of snow on the surface is the end result of almost exclusively solid precipitation (due to falls of snow and/or ice crystals) minus the net runoff of mcltwater, the net flux of water vapour to the surface due to frost formation and condensation minus sublimation and evaporation, and the deposition minus the erosion of snow by drifting (Bromwich and Robasky 1993). This method is also commonly used to estimate precipitation in other glaciated areas. The second method uses indirect meteorological approaches to calculate the precipitation. One technique computes the atmospheric moisture balance. Precipitation is found as the residual from the budgeting of the fluxes of water vapour into and out of an atmospheric volume. The weakness of this method is the fact that the precipitation can be computed only for seasonal and longer time scales and for regions of at least 1 million km2 that arc monitored by a good synoptic network of radiosonde stations (e.g. Rasmusson 1977; Bromwich 1988). An other approach is to add up precipitation amounts calculated from estimates of synoptic-scale vertical motion (Bromwich et al. 1993).

A review of the literature shows that most of the geographical and climatological monographs of the Arctic give surprisingly little information about precipitation (e.g., Prik 1960; Sater 1969; Putnins 1970; Vowinckel and Orvig 1970; Sater et al. 1971; Barry and Hare 1974; Sugdcn 1982; Barry 1989). More information can be found in Rae (1951) and Petterssen et aI. (1956), but particularly in Bryazgin (1971) for the non-Soviet Arctic, in Maxwell (1980) for the Canadian Arctic, and in Przybylak (1996a, b) for the Arctic as a whole. The precipitation has only been presented in cartographic form by Bryazgin, based on the periods 1916-1973 (Gorshkov 1980) and 1930-1965 (Atlas Arktiki 1985), by Przybylak (1996a, b) based on the period 1951-1990, as well as by Bogdanova (1997) for solid precipitation. Bryazgin presents results for some months and for the year as a whole. On the other hand, Przybylak additionally provides the results for all seasons. For some parts of the Arctic, maps have also been presented by Bryazgin (1971) for the non-Soviet Arctic, Maxwell (1980) for the Canadian Arctic, and Ohmura and Reeh (1991) and Ohmura et al, (1999) for Greenland. In the case of Greenland, there are atso earlier attempts to chart the distribution of the annual accumulation (Diamond 1958, 1960; Bader 1961; Benson 1962; Mock 1967 and Barry and Kiladis 1982).

The role of snow in shaping the climate was recognised as early as at the end of the 19Ih century (see e.g. Voieikov 1889, Bruckner 1893, or Siiring 1895) and its mechanism is presented in brief in the Introduction to the present volume. The snowfall creates a snow cover when air temperature is < 0°C. A detailed consideration of the snow cover (in terms of depth and density) has often been omitted from climatological studies as being less important. It has, however, been investigated by hydrologists, who need this information for water budget computations. At present, two types of data about the snow cover are available: 1) in situ (standard observations in meteorological stations), and 2) satellite remote sensing. The first type of data gives the best information about physical characteristics of the snow cover but its main weakness is its low spatial resolution (the network of stations is sparse). On the other hand, the satellite data give very good time and spatial resolution of the extent of the snow cover, but limited (if any) information about snow depth and density. One should also add that snow-cover mapping from satellite-derived imagery (with weekly resolution) was begun in November 1966 by the National Oceanic and Atmospheric Administration (Matson 1991). Until 1972 only visible satellite charts were used, and thus there was no information from the dark season (the polar night). Moreover a major problem with snow cover mapping at this time was the difficulty of distinguishing between snow and clouds. Since 1972, however, when passive microwave sensors were introduced, mapping has been possible both in the presence of clouds and darkness (Barry 1985).

There are quite a large number of attempts to deal with the problem of snow cover in the Arctic, in the Northern Hemisphere, and in the world as a whole. A significant increase in the number of published papers has been observed since the start of the satellite era. Of most important works analysing snow cover on the global, hemispheric, and continental scales, one should mention Kotlyakov (1968), Kopanev (1978), Dewey (1987), Dudley and Davy (1989), Cess et al. (1991), Robinson (1991), Ropelewski (1991), and Kotlyakov et al. (1997). The most recent synthesis presenting different aspects connected with the snow cover in the Arctic Ocean has been provided by Radionov el at. (1997). Of the other positions, the following should also be mentioned: Bryazgin (1971), Dolgin el al. (1975), Maxwell (1980), Romanov (1991), Ice Thickness Climatology ¡961-1990 Normals (1992), Brown and Braatcn {1998), Warren et al. (1999).

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