Björn Bringfelt

Plume rise measurements at industrial chimneys

Abstract The results of about 70 measurements of smoke plume rise from industrial chimneys are described. The experimental technique was as follows: The smoke plume was photographed repeatedly from a small aeroplane during a period of about 1 hr. The mean plume rise was evaluated at fixed distances from the source using several photographs taken during the test. A survey and a comparison are given of the various very different methods which have previously been used for measuring plume rise. When plotting all available plume rise data measured at fixed distances and a sampling period of 1 2 –1 hr. against chimney gas heat emission significant regression lines are obtained. The scatter of the points is large but this representation is useful for a first general estimation of plume rise.

Important factors for the sulphur dioxide concentration in central Stockholm

Abstract Mean daily concentrations of sulphur dioxide averaged over four measuring sites in central Stockholm for the winter periods 1967–1968 and 1968–1969 are compared with temperature, wind speed and mixing height by multiple regression analyses. Methods of extracting the values of meteorological predictors from routine weather data are compared. The temperature predictor was obtained as the difference of the daily mean temperature at the rather centrally located airport Bromma below the datum value of 25°C. The mixing height was calculated from the night and day soundings (01 and 13 hours) and the minimum and 13 h temperatures in the city. The wind speed at ground level at Bromma was used in preference to the speed averaged over the mixing layer. The multiple correlation coefficient was found to be as high as 0.84. The daily SO2-levels can be predicted with a standard error of about 25 per cent. The diurnal curve for SO2-concentration has two maxima, one at 6–10 h (early in spring and autumn, later in midwinter) and one in the late evening. These maxima are chiefly due to maxima in the emission rate of SO2, but stability effects may be responsible for seasonal variations. During the winter 1967–1968 the SO2-concentrations were high and the Swedish air quality standards were frequently exceeded. The next winter showed much lower values. This improvement is shown to depend on the fact that the permitted fuel sulphur content was only 1 per cent in central Stockholm during the later winter. In the days 3–5 of stagnation periods the measured SO2-levels were significantly higher than the levels predicted by the statistical relations by a factor 1.2. The reason is that the “statistical model” uses only day-to-day values of these variables. It does not consider accumulation of pollutants in the city air.

A study of buoyant chimney plumes in neutral and stable atmospheres

Abstract Most of the Swedish plume rise observations in 1966 and 1967 were made in neutral or stable conditions. Many of the plumes were found to reach a maximum height at a definite distance downwind after which they levelled off or descended. These heights and distances are plotted nondimensionally using also wind speed, stability and stack gas buoyancy flux. The results agree with a model of “turbulent entrainment” for hot plumes in their first phase (atmospheric turbulence neglected), but the maximum heights observed in slightly stable or windy conditions are smaller than predicted by the model. This is probably due to atmospheric turbulence. Values of the entrainment constant α were 1. (1) evaluated from plume radii on photographs 2. (2) calculated from the plume rises observed. The two averages agree well with each other. Heights and distances to the transition to a second phase were evaluated for all plumes and are found to agree with values expected from the influence of atmospheric turbulence.

The role of aerodynamic roughness for runoff and snow evaporation in land-surface schemes—comparison of uncoupled and coupled simulations

This paper describes the impact of changes in aerodynamic roughness length for snow-covered surfaces in a land-surface scheme (LSS) on simulated runoff and evapotranspiration. The study was undertaken as the LSS in question produced widely divergent results in runoff, depending on whether it was used in uncoupled one-dimensional simulations forced by observations from the PILPS2e project, or in three-dimensional simulations coupled to an atmospheric model. The LSS was applied in two versions (LSS1 and LSS2) for both uncoupled and coupled simulations, where the only difference between the two versions was in the roughness length of latent heat used over snow-covered surfaces. The results show that feedback mechanisms in temperature and humidity in the coupled simulations were able to compensate for deficiencies in parameterizations and therefore, LSS1 and LSS2 yielded similar runoff results in this case. Since such feedback mechanisms are absent in uncoupled simulations, the two LSS versions produced very different runoff results in the uncoupled case. However, the magnitude of these feedback mechanisms is small compared to normal variability in temperature and humidity and cannot, by themselves, reveal any deficiencies in a parameterization. The conclusion we obtained is that the magnitude of the aerodynamic resistance is important to correctly simulate fluxes and runoff, but feedback mechanisms in a coupled model can partly compensate for errors.

A new land-surface treatment for HIRLAM - comparisons with NOPEX measurements

In order to improve the accuracy of forecasting near-surface atmospheric variables over a heterogeneous landscape, a framework of subgrid surface types and the ISBA parameterisation scheme for land surfaces have been tested in the operational weather forecast model HIRLAM, using a 5.5 km grid resolution. Surface energy fluxes measured during a single summer day at six fixed sites in the NOPEX area, representing agricultural fields, boreal forests and lakes, were used for verification. Both, in-situ field measurements and the HIRLAM simulation indicated that the Bowen ratio over forests was about twice as large as that of adjacent agricultural fields. This difference could be explained by the more effective turbulent mixing and larger surface resistance associated with the forest, thus making the sensible heat flux relatively large there. The use of initial soil moisture from a routine hydrological model gave improved agreement with measured surface fluxes and radiosonde temperature and humidity profiles compared to initialising from routine HIRLAM surface data. The differences in heat fluxes between the various surface types were also demonstrated by airborne flux measurements flown along a track at a height of ca. 100 m above the terrain. Modelled heat fluxes along the flight track were considerably smoothed due to the grid resolution used, e.g. the effect of a lake in reducing grid-averaged sensible heat flux could only be weakly detected, because the lake surface represented only 10% of the grid area. When the proportion of a contrasting surface type (lake) was altered from 10 to 100%, the surface fluxes calculated for the lake surface were almost unchanged; the results of the comparison did not provide evidence that more complex aggregation schemes for heat fluxes than straightforward areaweighted averaging would be required. The hourly variation of the modelled and simulated heat fluxes during the day studied could not be directly compared, because the simulated cloudiness did not exactly match that observed at the field sites. When the simulated net radiation was replaced with direct measurements, the model-based estimates of sensible and latent heat fluxes were closer to the corresponding field measurements. The divergence of sensible heat flux with height, as inferred from the tower measurements made over the forest, were supported by the aircraft measurements and the HIRLAM simulations. # 1999 Elsevier Science B.V. All rights reserved.

Synoptic evapotranspiration model applied to two northern forests of different density

Abstract The aim of this study was to develop and test a model at one site and apply it to another site where validation data such as transpiration and interception were available. The model was based on the Penman combination equation for transpiration and on Rutters equation for interception evaporation. The transpiration was estimated from energy balance/Bowen ratio measurements at both sites and the interception evaporation was estimated from measurements of throughfall and gross precipitation. Total evapotranspiration calculated by the model was within 10% of the evapotranspiration of the application site when using parameters estimated from the test site data. However, this agreement was apparent in the sense that the model underestimated the transpiration by about 20% and overestimated the interception evaporation by about 60%. An attempt was made to derive an independent parameter set on basis of the test site values. The parameter determining the magnitude of the surface resistance was scaled with respect to the difference in leaf area index between the forests and with respect to the relative differences in stomatal resistance between the different species. Using this modified parameter set resulted in a 50% underestimation of the transpiration. It was concluded that factors other than forest density and species were responsible for the scaling of the surface resistance between different forests.

Air humidity and radiation influence on forest transpiration

The transpiration from a dry-canopy coniferous forest was evaluated hourly by the Bowen ratio-energy balance method using measurements above the canopy for two summer seasons. Average values for 52 days were determined of the flux rates of transpiration (E) and global radiation and of the vapour concentration deficit (VCD). Values of the daily canopy surface conductance ks (calculated from EVCD) were found not to correlate to global radiation or VCD but correlated remarkably well with values produced by a physiologically-based function of global radiation and VCD. A smaller transpiration rate was not found during periods with less soil water content, so probably the soil did not become dry enough to reduce transpiration.

A numerical air pollution dispersion model for central Stockholm

Abstract The model construction is described as well as the preparation of emission and meteorological data for the test period. For this period SO2-concentration statistics were calculated by the model at the positions of four downtown measuring sites. The best agreement between observed and predicted concentrations was found if the stability over the city was always taken as independent of the surrounding rural area and corresponding to an unstable Pasquill-Turner class. Some residual scatter, probably due to measurement and source estimate inaccuracies remains, but the agreement is sufficiently good to justify further study.

A Forest Evapotranspiration Model Using Synoptic Weather Data

A model to calculate daily actual evapotranspiration from synoptic weather data has been developed using results from a forest meteorological measuring site in southern Sweden. The following submodels are included: Model for evaporation of rain water intercepted by the forest canopy. Parameters are the free throughfall coefficient and water storage capacity of the canopy. Model for transpiration from the dry forest canopy using a physiological expression for the surface resistance.