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Wayne R. Rouse

Abstract

Three years of simultaneous measurements of incoming and reflected solar radiation for adjacent snow-free land and ice-covered sea surfaces along the Hudson Bay coastline indicate that multiple reflection enhancement can be large over high-albedo surfaces in the presence of cloud. The type and thickness of cloud is the major factor determining the magnitude of multiple reflection, and there is a wide range in cloud base albedos. When, under cloudy skies, the global solar radiation equals the magnitude for clear skies, it is due to the fact that the clouds are thin and largely transparent to the solar beam, rather than to multiple reflection enhancement.

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Christopher Spence and Wayne R. Rouse

Abstract

The objective of the study is to determine the local processes that control the partitioning of the energy budget of shield terrain in the western Canadian subarctic. The magnitude of the spring snowmelt and its potential to flood exposed bedrock portions of the landscape control the energy budget in the early part of the summer. In wet years, Bowen ratios are low and increase over the growing season. The high latent heat fluxes early in the year are promoted by water pooled in bedrock depressions and stored in the shallow soil. The high evaporation rates deplete moisture storage by the end of July after which latent heat fluxes decrease so that Bowen ratios exceed unity until the end of the growing season. This regime differs from other subarctic terrain types with similar vegetation. Exposed and shallow Precambrian bedrock keeps water close to the topographic surface and available for evaporation. The low surface resistance of ponded water on the bedrock surface and high vapor pressure deficits are possible causes for the high evaporation rates in June and July. However, in drier years, when ponded snowmelt water is minimal, evaporation is small and sensible heat dominates the early summer energy budget to a much larger degree than is observed elsewhere in the subarctic. It then becomes a very arid landscape. High evaporation-to-precipitation ratios throughout the summer are an important feature of the western Canadian Shield subarctic region, and this feature has significant hydrological implications. It is normal for a moisture deficit to be created each summer. The magnitude of this moisture deficit is the primary control on hillslope runoff response to precipitation later in the summer and autumn.

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Richard G. Wilson and Wayne R. Rouse

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Energy balance measurements of evapotranspiration from a developing corn crop are compared with daily equilibrium evapotranspiration estimates to examine the accuracy of the model and the environmental conditions under which it can be applied. Equilibrium estimates compared closely (a standard error of 6%) with the measured values when the surface was moderately dry, a condition which applied to 14 of the 24 days of the experiment. The ratio of actual evapotranspiration to available energy and the Bowen ratio are used to establish moisture and temperature limits for the model. The success of the model was related to a typical diurnal pattern of the difference between actual and equilibrium evapotranspiration which reflects expected variations of moisture stress during daytime hours. The performance of the model was nearly independent of the physical condition of the surface and the height of the required air temperature measurement. An equation is presented which permits easy calculation of equilibrium evapotranspiration from air temperature, net radiation, and soil heat flux data.

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David H. Halliwell and Wayne R. Rouse

Abstract

An analysis technique is outlined that calculates the sensible and latent heat fluxes by the Bowen ratio and aerodynamic methods, using profile measurements at any number of heights. Field measurements at two sites near Churchill, Manitoba, comprising over 4000 hours of data from June through September, are used to assess the relative accuracy of the two methods. If hourly data are eliminated when the Bowen ratio method gives the wrong sign for the fluxes, or when the Bowen ratio is close to −1, the two methods agree very well. The results also indicate that the aerodynamic method can provide acceptable results with only a single measurement of wind speed and a good estimate of the surface roughness length. The relative error associated with the use of only a single anemometer is small for unstable conditions, and only becomes significant under very stable conditions when the fluxes themselves are small.

The overall comparison of the two methods yields a mean bias error of less than 10 W m−2, and a root-mean-square error of less than 40 W m−2 for hourly averages.

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Wayne R. Rouse and Robert B. Stewart

Abstract

Energy-budget calculations and equilibrium, model estimates of evaporation from a lichen-dominated upland site in the Hudson Bay lowlands are presented. The energy-budget calculations reveal that the lichen surface is relatively resistant to evaporation with an average of only 54% of the daily net radiation being utilized in the evaporative process. Equilibrium model estimates of evaporation consistently over-estimate actual evaporation by 5 and 8% for hourly values and daily totals, respectively. A simple model, a function of the equilibrium model, is derived from a comparison of actual and equilibrium evaporation. The only inputs required for the model are net radiation, soil heat flow and screen temperatures. Tests of the model indicate that it will predict actual evaporation within 5% and that it can probably be applied to any high-latitude surface which exhibits a relatively large diffusive resistance to evaporation.

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Zuohao Cao, Jianmin Ma, and Wayne R. Rouse

Abstract

In this study, the authors have performed the variational computations for surface sensible heat fluxes over a large northern lake using observed wind, temperature gradient, and moisture gradient. In contrast with the conventional (Monin–Obukhov similarity theory) MOST-based flux-gradient method, the variational approach sufficiently utilizes observational meteorological conditions over the lake, where the conventional flux-gradient method performs poorly. Verifications using direct eddy-correlation measurements over Great Slave Lake, the fifth largest lake in North America in terms of surface area, during the open water period of 1999 demonstrate that the variational method yields good agreements between the computed and the measured sensible heat fluxes. It is also demonstrated that the variational method is more accurate than the flux-gradient method in computations of sensible heat flux across the air–water interface.

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Robert B. Stewart and Wayne R. Rouse

Abstract

Summertime latent heat flux values determined by the energy budget approach are compared to equilibrium model estimates for two shallow lakes and two sedge meadow surfaces in northern Canada. Comparison of energy budget values with equilibrium estimates for each surface show that the latent heat flux can be accurately determined by the Priestley and Taylor (1972) model, where α the ratio of actual to equilibrium evaporation equals 1.26. Results suggest that the Priestley and Taylor parameter is generally applicable to saturated surfaces in high latitudes.

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Wayne R. Rouse, Donald Noad, and John McCutcheon

Abstract

Simultaneous measurements of global solar and incoming longwave radiation are presented for roof-top sites in a heavily polluted zone and relatively clean control sites in and around Hamilton. The measurements were taken intermittently under clear skies over a 3-year period. During part of the study vertical temperature profiles were measured between 150 and 2500 m in the industrial and control atmospheres.

Solar radiation was strongly attenuated by the industrial atmosphere whereas the daytime incoming infrared flux was increased so that the latter generally compensated fully for the former and the total radiation reaching the ground was similar at both the industrial and control sites.

Vertical temperature profiles show that an elevated inversion develops anywhere between 1070 and 1980 m in the industrial atmosphere depending on the season. Prier to early afternoon the inversion weakens and moves upward but in the late afternoon it both strengthens and descends. The rate of atmospheric temperature changes beneath the elevated inversion is substantially greater than in the control atmosphere.

An hypothesis is advanced that heavy particulate concentrations in Hamilton's atmosphere play a dual role in absorbing and reflecting solar radiation and in increasing radiative emissivity and absorptivity so that the heat energy from the absorbed solar radiation and absorbed outgoing infrared radiation is rapidly dissipated. This hypothesis is supported by the fact that average atmospheric temperatures in the industrial and control atmospheres do not differ significantly.

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Claire J. Oswald and Wayne R. Rouse

Abstract

This study addresses the thermal and energy budget characteristics of four different-size Canadian Shield lakes in the Mackenzie River basin during the ice-free season of 2000. The objectives are to characterize and compare the surface temperature and thermal structures, and to quantify the magnitudes and flux rates of the energy balance components of each lake. This study highlights the variability in thermal and energy balance characteristics arising from differences in mean lake depth and surface area. The lakes exhibit similar temporal patterns for air temperature, net radiation, wind speed, and wind direction. Net radiation and wind speed are highest over the largest lake, Great Slave Lake, due to colder surface temperatures and lengthy across-lake wind fetch, respectively. During the warming phase of the summer, surface temperature is inversely related to mean depth; however, during the cooling phase this relationship reverses. The shallowest of the four lakes remains isothermal during the entire ice-free period, while the three larger and deeper lakes are all dimictic. A lag in the onset of thermal stratification in the dimictic lakes is positively correlated with mean depth and surface area. Large evaporative water losses correspond to periods of low net radiation and cold dry air over Great Slave Lake. However, over the smaller, shallower lakes periods of high evaporation occur on days with high net radiation and warm, dry air. The capacity of larger lakes to store more heat results in longer ice-free periods and higher evaporation. Maximum heat content increases and occurs later for lakes of greater depth. Maximum evaporative rates occur later and cumulative evaporation is highest for lakes of greater depth and area. The ratios of total open water evaporation for the four lakes in order of size (smallest = 1.0) are 1.0 : 1.2 : 1.3 : 1.4. Evaporation magnitudes are discussed in the context of other temperate and high-latitude lake studies.

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Peter D. Blanken, Wayne R. Rouse, and William M. Schertzer

Abstract

The turbulent exchange of water vapor and heat were measured above Great Slave Lake, Northwest Territories, Canada, using the eddy covariance method for most of the ice-free period in 1997, 1998, and 1999. In all years, evaporation tended to occur in episodic pulses, lasting 52–78 h, between which quiescent periods dominated. The contributions of these evaporation pulses to the measured total evaporation were 45%, 65%, and 47% for 1997, 1998, and 1999, respectively, yet occurred on only 24% (1997), 37% (1998), and 25% (1999) of the total number of days observed. Despite the suppression of turbulent mixing, due to the stable atmospheric conditions that dominated much of the ice-free periods, analyses of high-frequency wind, air temperature, and humidity data revealed that evaporation was enhanced by the mixing of warm, dry air down to the lake surface. Conditional sampling of turbulent measurements showed that these sweeps of warm, dry air were infrequent, yet were the dominant turbulent transfer mechanism. Because the approximately 3-day-long evaporation pulses were composed of an aggregation of sweeps, measurements of air–lake turbulent heat exchange needed to be made at a high frequency in order to capture these significant events. Implications of climate variability on the mechanisms that control short- and long-term evaporation rates were discussed, in terms of the positive feedback that developed between entrainment and evaporation.

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