3.2.8. Barow Snow Melt Date
In order to understand global climate change more fully, an assessment of the variability of the Earth's cryosphere in response to other climatic factors is necessary. An important process that occurs each spring along the arctic cryospheric boundary is the melting of the seasonal snow pack. The timing of this event can influence the net energy budget of an entire season or year, and feedbacks involving changes in surface albedo, in turn, influence the regional temperature regime. The variability of annual snow melt at any particular location depends on many interactive processes. Although temperature is an obvious primary factor that controls the timing and rate of melt, changes in cloud conditions, thermal stability of the atmosphere, and the precipitation that occurs during the preceding seasons are also very important. The depth of accumulated snow and its influence on permafrost properties must also be considered when evaluating any long-term trends in snow extent and its seasonal duration.
An interesting feature of the spring melt in northern Alaska is that once it begins it commences at a very rapid rate (see Figure 3.26). Figure 3.26a shows the relative changes in the components of the net solar radiation balance at the surface, shortwave down (SWD) and shortwave up (SWU), respectively, for 1 year at BRW. Figure 3.26b shows the derived albedo (SWD/SWU) and indicates the snow melt date which at BRW is based radiometrically on a 30% albedo threshold. This value was found to be a good objective measure because once below this value, it seldom increases again until autumn except for an occasional late snowfall of brief duration. Although the decrease in albedo occurs very rapidly, typically in a matter of a few days, the period in which this occurs each spring can vary from the third week in May to the end of June.
Fig. 3.26(a) Daily average downward and upward shortwave irradiance measured at BRW during 1994; (b) the snow melt date is determined radiometrically as the date when the surface albedo drops below 30% [adapted from Stone et al., 1996].
More important from a climate change perspective is the suggestion that in recent decades melting has occurred earlier in the season, indicating a trend possibly related to global warming. One of the main features of global climate models that simulate the effects of increasing greenhouse gases in the atmosphere is an enhancement of warming in the arctic because of a positive feedback caused by decreasing surface albedo. The decrease results initially from the melting of ice and snow. Therefore, the date of annual melt is of great interest as a potential indicator of climate change.
Foster  and Foster et al. , on the basis of historic visual observations, claimed that the disappearance of snow in spring at Barrow showed a trend manifested by a progressively earlier melt since the 1950s and speculated that this was an indication of global warming. Dutton and Endres , however, took issue with Foster's conclusion suggesting that the apparent trend was in large part attributable to local urbanization effects. Their argument was based on objective, radiometric measurements made over the open tundra similar to those illustrated in Figure 3.26b. The tundra site is not likely to be influenced by urban effects because it is in a pristine location 8 km upwind of BRW. This comparison [Dutton and Endres, 1991] was only possible for a few years of overlapping records.
The issue is revisited in Figure 3.27 where it shows the respective time series updated through 1996. The time series were fitted using a smoothing function to indicate apparent trends. It now appears that the radiometric estimates of the snow melt date made at BRW are also tending to occur earlier in recent years. Moreover, there is a temporal correlation between the two independent observations giving credence to both data records, despite recent confirmation that the site in town is no longer representative of the surrounding tundra. It is hypothesized that earlier melting of the snow pack in the vicinity of BRW, on average, may result from less than normal accumulation of snow throughout the winter, warmer spring temperatures associated with enhanced cloudiness that accelerates ablation/melt or both. More subtle changes that affect the permafrost may also be a factor, but these are very difficult to quantify. Synoptic-scale changes in circulation patterns that have influenced northern Alaskas climate in recent years [Stone, 1997] probably have contributed to what appears to be a shift toward earlier spring conditions there. The interrelation-ship between snow distribution and other climate variables is the subject of ongoing research.
Fig. 3.27. The date of snow melt in the vicinity of Barrow is apparently occurring earlier. In town, however, the disappearance of snow is occurring earlier than at the CMDL observatory located at a remote tundra site, probably because of increased use by inhabitants and other effects of urbanization [e.g., Dutton and Endres, 1991].
[BACK] [CONTENTS] [NEXT]