The summertime energy balance of a thermokarst lake in northern Alaska: A three-year study of seasonal and interannual variability
Abstract
Shallow, thermokarst lakes - which develop atop permafrost - are a prominent landscape feature on the Arctic Coastal Plain (ACP) of northern Alaska. The ACP is vulnerable to ongoing climate change and landscape modification, as thousands of thaw lakes and ponds are impacted by changes in temperature, precipitation, thawing permafrost, and human activity. Although summer in the Arctic is short, incoming solar radiation and lake evaporation are relatively high, and both factors play a significant role in the landscape water balance. Furthermore, lake evaporation is anticipated to increase as the ice-free season lengthens and water temperatures become warmer. To improve our understanding of these processes, we performed a multi-year energy balance analysis of a thermokarst lake near Barrow, Alaska. The lake is about 4 km from the Arctic coast, and is relatively small having an area of 185.7 Ha. Timeseries of net radiation, Bowen ratio, and the rate of lake heat storage (at hourly, daily, and longer timescales) were used to calculate sensible and latent heat fluxes during the 2008-2010 ice-free periods (roughly early July through late September). Sediment heat flux estimates were included in the calculation of the total heat storage rate and were determined from a simple heat flux model (calibrated using numerous measurements of thermal conductivity and temperature gradient in the lake sediment). Hourly shortwave albedo measurements were collected during a 23-day period in 2008 to generate a diurnal albedo curve, which was found to range from roughly 0.06 in late morning to 0.17 in late evening. Results of the energy balance analysis show rapid warming of the lake water and sediments immediately following ice-off (due to high insolation), followed by similar increases in sensible and latent heat flux. Bowen ratios were typically around 0.7-1.1, indicating that the majority of the available energy was consumed by lake evaporation, which averaged around 1.3 mm/day during the ice-free period (i.e., nearly twice the mean summertime precipitation rate of 0.7 mm/day for Barrow, Alaska). Daily evaporation rates ranged from zero to greater than 4 mm/day, while seasonal patterns varied significantly from one year to the next. Much of this variability was associated with changes in cloud cover, water temperature, relative humidity, and wind speed. The mass-transfer technique was also used to provide supplementary estimates of latent and sensible heat flux, and these estimates were found to agree well with the energy balance calculations.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.C31B..08P
- Keywords:
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- 0746 CRYOSPHERE / Lakes;
- 1814 HYDROLOGY / Energy budgets;
- 1840 HYDROLOGY / Hydrometeorology