Jacqueline Binyamin

The Role of Northern Lakes in a Regional Energy Balance

There are many lakes of widely varying morphometry in northern latitudes. For this study region, in the central Mackenzie River valley of western Canada, lakes make up 37% of the landscape. The nonlake components of the landscape are divided into uplands (55%) and wetlands (8%). With such abundance, lakes are important features that can influence the regional climate. This paper examines the role of lakes in the regional surface energy and water balance and evaluates the links to the frequency–size distribution of lakes. The primary purpose is to examine how the surface energy balance may influence regional climate and weather. Lakes are characterized by both the magnitude and temporal behavior of their surface energy balances during the ice-free period. The impacts of combinations of various-size lakes and land–lake distributions on regional energy balances and evaporation cycles are presented. Net radiation is substantially greater over all water-dominated surfaces compared with uplands. The seasonal heat storage increases with lake size. Medium and large lakes are slow to warm in summer. Their large cumulative heat storage, near summer’s end, fuels large convective heat fluxes in fall and early winter. The evaporation season for upland, wetland, and small, medium, and large lakes lasts for 19, 21, 22, 24, and 30 weeks, respectively. The regional effects of combinations of surface types are derived. The region is initially treated as comprising uplands only. The influences of wetland, small, medium, and large lakes are added sequentially, to build up to the energy budget of the actual landscape. The addition of lakes increases the regional net radiation, the maximum regional subsurface heat storage, and evaporation substantially. Evaporation decreases slightly in the first half of the season but experiences a large enhancement in the second half. The sensible heat flux is reduced substantially in the first half of the season, but changes little in the second half. For energy budget modeling the representation of lake size is important. Net radiation is fairly independent of size. An equal area of medium and large lakes, compared with small lakes, yields substantially larger latent heat fluxes and lesser sensible heat fluxes. Lake size also creates large differences in regional flux magnitudes, especially in the spring and fall periods.

Interannual and Seasonal Variability of the Surface Energy Balance and Temperature of Central Great Slave Lake

This paper addresses interannual and seasonal variability in the thermal regime and surface energy fluxes in central Great Slave Lake during three contiguous open-water periods, two of which overlap the Canadian Global Energy and Water Cycle Experiment (GEWEX) Enhanced Study (CAGES) water year. The specific objectives are to compare the air temperature regime in the midlake to coastal zones, detail patterns of air and water temperatures and atmospheric stability in the central lake, assess the role of the radiation balance in driving the sensible and latent heat fluxes on a daily and seasonal basis, quantify magnitudes and rates of the sensible and latent heat fluxes and evaporation, and present a comprehensive picture of the seasonal and interannual thermal and energy regimes, their variability, and their most important controls. Atmospheric and lake thermal regimes are closely linked. Temperature differences between midlake and the northern shore follow a seasonal linear change from 68C colder midlake in June, to 68C warmer in November‐December. These differences are a response to the surface energy budget of the lake. The surface radiation balance, and sensible and latent heat fluxes are not related on a day-to-day basis. Rather, from final lake ice melt in mid-June through to mid- to late August, the surface waters strongly absorb solar radiation. A stable atmosphere dominates this period, the latent heat flux is small and directed upward, and the sensible heat flux is small and directed downward into the lake. During this period, the net solar radiation is largely used in heating the lake. From mid- to late August to freeze up in December to early January, the absorbed solar radiation is small, the atmosphere over the lake becomes increasingly unstable, and the sensible and latent heat fluxes are directed into the atmosphere and grow in magnitude into the winter season. Comparing the period of stable atmospheric conditions with the period of unstable conditions, net radiation is 6 times larger during the period of stable atmosphere and the combined latent and sensible heat fluxes are 9 times larger during the unstable period. From 85% to 90% of total evaporation occurs after mid-August, and evaporation rates increase continuously as the season progresses. This rate of increase varies from year to year. The time of final ice melt exerts the largest single control on the seasonal thermal and energy regimes of this large northern lake.

The Influence of Lakes on the Regional Energy and Water Balance of the Central Mackenzie River Basin

The goal of this study is to define the role of lakes in the energy and water cycling of the lake-rich central Mackenzie River Basin and discuss the impacts of climate variability on the regional terrestrial water balance. This is pursued by synthesizing the results of measured data on a regional scale. Our results indicate that lake-rich regions are high energy landscapes in the thaw season. Lakes have larger net radiation and much larger water vapor fluxes and smaller sensible heat fluxes than their terrestrial surroundings. Energy exchange with the atmosphere is dominated by the annual evaporative heat flux. The presence of large lakes in a region substantially decreases the interannual variability in evaporation totals. A hypothetical region with no lakes shows a positive annual terrestrial water balance for wet and average precipitation years and only a small negative water balance for the driest years. The existing lake-rich region has a positive annual water balance only in the wettest years. Comparisons of the region with all small, all medium or all large lake scenarios indicates increased regional evaporation of 8% and 10% respectively for the latter two scenarios. Basin evaporation is a significant source for precipitation within the Mackenzie River Basin during the summer. It is hypothesized that fall and early winter evaporation from medium and large lakes enhances downwind snowfall. In response to climate warming, lake-rich high latitude basins will witness substantially increased annual evaporation.

AN ULTRAVIOLET (290 TO 325 NM) IRRADIATION MODEL FOR SOUTHERN CANADIAN CONDITIONS

A numerical model to estimate spectral and broadband ultraviolet irradiance (290 to 325 nm) for Canadian conditions is described and validated with Brewer spectro-radiometer measurements at four stations. The model applies the delta-Eddington algorithm to a 50-level, 100-km, plane-parallel atmosphere with cloud inserted between 2 and 3 km. It requires measured total atmospheric ozone depth and hourly observations of cloud amount. In the absence of ozone soundings, model ozone profiles are scaled by the ratio of measured (from the Brewer instrument) to model total atmospheric ozone depths. In the model calculations, SUSIM ATLAS 3 extraterrestrial irradiance measurements are averaged for each nanometer of wavelength to mimic the triangular filter used by the Brewer instrument. Ozone absorption is calculated from the temperature-dependent coefficients of Pauer and Bass (1985), Rayleigh optical depths after Elterman (1968), and aerosol optical properties from MODTRAN. Surface albedo is a function of snow depth and 0.05 for snow-free ground. Model and measured spectral irradiances for cloudless skies agree well, but model values are smaller than measurements for wavelengths below about 305 nm because of enhancement of the Brewer signal by stray light. Model values of daily cloudless sky irradiance using lidar aerosol optical depth measurements from York University after the Mt. Pinatubo eruption in 1991 agree well with measurements. Cloud optical depths were calculated iteratively for overcast conditions. A fixed optical depth of 45 was used to calculate cloudy sky irradiances at the four stations. These agree well with measurements. Mean bias error (MBE) is less than 5% of the mean measured daily irradiance and root mean square error (RMSE) less than 25%, decreasing to below 12% for 10-day averages. Agreement between mean daily measured and calculated spectral irradiances over a month is also good. [Key words: radiation modeling, ultraviolet irradiance, cloud optical depth.]

Modeling Lake Energy Fluxes in the Mackenzie River Basin using Bulk Aerodynamic Mass Transfer Theory

Multiple years of micrometeorological and energy flux measurements for four Canadian Shield lakes were used to develop bulk aerodynamic mass transfer coefficients (C D ) for each lake and for groups of lakes. Transfer coefficients determined from multiple years of data for the two smallest lakes were similar (1.26 x 10 -3 and 1.30 x 10 -3 ) while that for the largest lake was slightly smaller (1.10 x 10 -3 ). The coefficient for the medium-size lake was erroneously high (2.14 x 10 -3 ) likely due to generalizations in the calculation of heat storage. No strong relationships were found between the coefficient values and morphometric parameters. The linear regression comparison of measured and modeled daily fluxes using multi-year coefficients gave an average r2 of 0.78. The same coefficients performed the best at estimating cumulative latent and sensible heat loss. Absolute percent errors suggest that the multi-year coefficients give acceptable results for small and medium-size lakes only, and that these coefficients cannot be transferred from one lake to another, unless the lakes are similar in size.