Markku Kangas

Momentum and heat fluxes over lakes Tamnaren and Raksjo ¨ determined by the bulk-aerodynamic and eddy-correlation methods

Micrometeorological measurements made concurrently over two boreal lakes, Lake Tamnaren (surface area 37 km 2 , depth 2 m) and Lake Raksjo ¨ (1.5 km 2 , 4 m), during the NOPEX observational campaigns provided long-term latent and sensible heat fluxes determined with the bulk aerodynamic method. The turbulence transfer coefficients were verified with short-term eddy correlation runs over Lake Tamnaren. The drag coefficient and the Stanton number attained a stability dependence close to the Businger-Dyer form. During weak or moderate wind conditions, the latent heat flux determined from eddy correlation was close to that calculated with the bulk aerodynamic method using typical values of the Dalton number. However, the independent verification of the Dalton number under conditions of moderate or strong wind suffered from the poor performance of the fast-response hygrometer due to movement of the float. Measurements of the surface roughness suggested a value of the Charnock constant near 0.02. On average, the stratification of the surface layer over both lakes was unstable, but stable conditions occurred almost daily due to advection of warm air from adjacent land areas. High evaporation rates occurred even during stable stratification. The variation of the latent heat flux on an hourly or daily basis was strongly correlated with wind speed. This was also demonstrated by the higher rates of evaporation on the larger Lake Tamnaren compared to the sheltered Lake Raksjoduring strong wind conditions. Towards the end of the summer of 1995, the lake surface temperature attained somewhat higher values on Lake Raksjo ¨, attributable to its larger depth. This also resulted in higher daily average sensible and latent heat fluxes compared to Lake Tamnaren in August-September. Thus, in the long-term, the observed daily deviations between the lakes nearly compensated each other. # 1999 Elsevier Science B.V. All rights reserved.

Estimation of winter road maintenance costs using climate data

Winter road maintenance is a major task in countrieswith long winters and harsh conditions: in Finland, forexample, annual winter road maintenance costs areroughly 100 million. The year-to-year variation inmaintenance costs depends to a large extent on the vari-ation of climatological conditions during sequentialyears. Several studies aiming at modelling maintenancecosts using meteorological data have been made (e.g.Hulme 1982; Voldborg & Knudsen 1988; Thornes1991; Cornford & Thornes 1996; Gustavsson 1996;Venalainen & Helminen 1998; Laine et al. 2000).Gustavsson (1996) compared three different indices:the Hulme index (Hulme 1982) further modified byThornes (1991), the COST 309 winter index (Voldborg& Knudsen 1988) and the GAB index used in Sweden.According to Gustavsson, none of these three indiceswas able to fully predict the road maintenance activitythat occurred. Gustavsson suggested that the main rea-son for this could be that the people undertaking roadmaintenance use weather forecasts as the basis for theirdecisions, whereas the indices were calculated usingmeasured data. Cornford & Thornes (1996) examined the relationshipbetween winter road maintenance costs and weatherseverity in Scotland. Winter severity was estimatedusing the winter maximum temperature, the number ofground frosts, and the number of days with snow lyingat 09 UTC, and these variables were combined into amodified Hulme-index (Thornes 1991). According tothe study there was a link between regional expenditureand the spatialised Hulme-index. The correlation washigher over years within a given region than overregions within years. This was estimated to be due tothe uncertainty in measuring regional expenditure andalso due to different winter road maintenance practicesacross regions. The authors estimated that a betterdependence between expenditure and winter severitywould be obtainable if the meteorological variablescould be defined more precisely. Frost in particular wasfound to be a difficult variable. Another aspect requir-ing further research was deemed to be the availabilityof reliable winter regional maintenance costs. In the study of Laine et al. (2000), monthly winter roadmaintenance costs were estimated using the monthlyvalues of the following variables: snowfall amount,number of snowfall observations, number of sleetobservations, number of snow drifting cases, numberof freezing rains, number of rapid warmings leading tohoarfrost formation, number of conditions favourablefor black ice formation, number of cases when the tem-perature dropped below freezing point. For eight dif-ferent climatological regions, and also for the wholecountry, separate linear equations to describe monthlycosts were defined. According to the results, some ofthe equations gave good estimates in several years, butthere were also years when the estimates were inaccu-rate. In addition, it was found that the costs of certainroad districts could be estimated with almost anymodel, whereas the costs of other road districts werevery difficult to estimate.The results of Laine et al. (2000) support the conclu-sions of Cornford & Thornes (1996). The definition ofa winter index using a linear equation to describe main-tenance costs with climatological parameters is difficultfor several reasons. Winter maintenance practices mayvary from one road district to another, as the decisionto carry out some of the maintenance activities is still to

Heat Transport in Unsaturated Zone Thermal Energy Storage – Analysis with Two-Phase and Single-Phase Models

We analyze a field experiment where ambient air is injected into the soil during the summer and extracted again during the winter. A multiphase model accounting for the conductive transport as well as the convective transport with the moving liquid and gas phases is used along with a more simplified single-phase model where the convective transport is due to the gas alone. The latter model also accounts for subzero wintertime temperatures. The multiphase model captures well both the seasonal variations and the actual test sequence, the main calibration being in the adjustment of medium permeabilities based on the observed pressure responses. The effect of the injection pump on the temperature and humidity of the injection air needs to be known accurately. Taking into account the humidity of the injection air explicitly instead of using humidity-corrected enthalpy values also has an effect. The effect of various humidity and specific enthalpy assumptions is of the order of 1–1.5°C, while ignoring the wintertime subzero temperatures has an effect of 1–2°C. These differences are of the same order of magnitude as the heterogeneity-introduced differences in field data. Using the simplified single-phase model typically appears to cause a difference of 1–2°C, but can yield an even higher deviation of the order of 3–4°C.