M. L. Wesely

Correcting eddy-covariance flux underestimates over a grassland

Independent measurements of the major energy balance flux components are not often consistent with the principle of conservation of energy. This is referred to as a lack of closure of the surface energy balance. Most results in the literature have shown the sum of sensible and latent heat fluxes measured by eddy covariance to be less than the difference between net radiation and soil heat fluxes. This under-measurement of sensible and latent heat fluxes by eddy-covariance instruments has occurred in numerous field experiments and among many different manufacturers of instruments. Four eddy-covariance systems consisting of the same models of instruments were set up side-by-side during the Southern Great Plains 1997 Hydrology Experiment and all systems under-measured fluxes by similar amounts. One of these eddy-covariance systems was collocated with three other types of eddy-covariance systems at different sites; all of these systems under-measured the sensible and latent-heat fluxes. The net radiometers and soil heat flux plates used in conjunction with the eddy-covariance systems were calibrated independently and measurements of net radiation and soil heat flux showed little scatter for various sites. The 10% absolute uncertainty in available energy measurements was considerably smaller than the systematic closure problem in the surface energy budget, which varied from 10 to 30%. When available-energy measurement errors are known and modest, eddy-covariance measurements of sensible and latent heat fluxes should be adjusted for closure. Although the preferred method of energy balance closure is to maintain the Bowen‐ratio, the method for obtaining closure appears to be less important than assuring that eddy-covariance measurements are consistent with conservation of energy. Based on numerous measurements over a sorghum canopy, carbon dioxide fluxes, which are measured by eddy covariance, are underestimated by the same factor as eddy covariance evaporation measurements when energy balance closure is not achieved. Published by Elsevier Science B.V.

A review of the current status of knowledge on dry deposition

Abstract Dry deposition can account for a large portion of the removal of trace chemicals from the troposphere. Resistance schemes used in modeling often perform quite well in daytime conditions over flat terrain. Model results for hilly or mountainous areas, however, are considerably less reliable than those for flat terrain. Even for homogeneous atmospheric and surface conditions and flat terrain, an inadequate model description of surface properties such as vegetative species and soil moisture stress can lead to large differences between modeled and measured fluxes. Third-generation models of mesoscale meteorology and atmospheric chemistry have the potential to achieve several advances, but scaling up of local to regional flux information remains a subject of research. Also, the integrated modeling of gaseous emissions and deposition, which need to be tied together at a low level of model structure, has not yet been accomplished. Many of the processes affecting dry deposition of O3 over individual types of surfaces are fairly well understood. The role of rapid in-air chemical reactions involving NO, NO2, and O3 are difficult to quantify comprehensively, and the effects of water from rain or dew on uptake of gases can be highly variable. The influence of lipid solubility on the uptake of organic substances is not well understood. For large bodies of water, the dry deposition rate of most gases appears to be determined largely by water solubility. Parameterizations for the deposition of fine particles tend to be empirical or based on theories untested in natural settings outdoors. Direct measurements of fluxes are required for improved parameterizations for gases and particles and have been made successfully in many past experiments. Micrometeorological approaches have been used extensively, but they are sometimes limited by chemical instrumentation. Long-term flux measurements for diverse terrain and relatively large areas remain difficult.

SO2, sulfate and HNO3 deposition velocities computed using regional landuse and meteorological data

Deposition velocity fields were generated for SO2, sulfate and HNO3 over eastern United States and southeastern Canada by combining detailed landuse data with meteorological information predicted using a mesoscale meteorology model. When there is significant variation in land type within an averaging area, it was found that subgrid scale meteorological variations can significantly influence area-averaged deposition velocities. The assumption that uu is constant over the averaging area can realistically address the subgrid variations in wind speed and friction velocity. For a 3-day springtime simulation, domain-averaged mid-day SO2, sulfate and HNO3 deposition velocities at a height of approximately 40 m were found to be 0.8 cm s−1, 0.2 cm s−1, and 2.5 cm s−1, respectively. At night, the deposition velocities were approximately 50%, 45% and 70% of the corresponding daytime values for SO2, sulfate and HNO3. Using a simple parameterization to account for rainfall-wetted surfaces increased domain-averaged SO2 deposition velocities by up to a factor of two, indicating that precipitation can significantly enhance dry deposition of SO2.

Uncertainties in modeled and measured clear-sky surface shortwave irradiances

A comparison of five independent measurements of the clear-sky downward shortwave irradiance at the surface shows that they scatter within a 5% range depending on their calibration constants. When the measurements are corrected using data from two cavity radiometers, three of the five independent measurements agree within 3 W m−2 over three clear-sky days, which is well within the estimated error limit of ±1.5%. A comparison of these three sets of irradiance measurements with the computed irradiance by a δ2-stream model reveals that the model overestimates the irradiance by 5%. Detailed investigation of the approximations and uncertainties associated with the computations (including the measurement error in the water vapor and ozone amounts, neglecting the state of polarization and trace gas absorption, the 2-stream approximation, the neglect of the spectral dependence of the surface albedo, and the uncertainties associated with aerosols) demonstrates that the discrepancy is not due to these approximations. Further analysis of the modeled and measured irradiance shows that the discrepancy is almost entirely due to the difference between modeled and measured diffuse field irradiances. An analysis of narrow-band diffuse to total irradiance ratios shows that this discrepancy is the largest near 400 nm and decreases with wavelength. These results rely on the absolute calibrations of two cavity radiometers, two shaded pyranometers, and one unshaded pyranometer, as well as ratios of irradiances measured by a multifilter rotating shadow-band radiometer. Therefore, in order for instrumental error to account for the diffuse field discrepancy, three independent measurements of the diffuse field irradiance must be biased low by at least 40%. For an aerosol to account for this discrepancy, it must be highly absorbing with a single-scattering albedo as low as 0.3. The unlikelihood of instrumental errors of 40% and aerosol single-scattering albedos of 0.3 suggests a third possibility: the neglect of some gaseous absorption process at visible wavelengths.

An eddy-correlation measurement of NO2 flux to vegetation and comparison to O3 flux

Eddy-correlation measurements with a newly developed fast-response NOx sensor indicate that the deposition velocity at a height of about 6m above a soybean field has a maximum value near 0.6cms-1 for NOx and is usually about 2/3 ofthat found for ozone. In these studies, over 90% of the NOx is NO2. The corresponding minimum surface resistance for NOx calculated as the quantity remaining after atmospheric resistances are subtracted is about 1.3 s cm−1, which is larger than expected on the basis of leaf stomatal resistance alone. Emission of NO from sites in the plant canopy and soil where NO2 is deposited and reduced to NO or release of NOx as a result of biological activity may have lessened the downward fluxes of NOx as measured. During windy conditions at night, surface resistances are found to have values of about 15scm−1 for NOx (again, greater than 90% NO2) and 1.8scm−1 for O3, corresponding to deposition velocities of 0.05cms−1 and 0.3cms−1, respectively.

Area‐averaged surface fluxes and their time‐space variability over the FIFE experimental domain

The underlying mean and variance properties of surface net radiation, soil heat flux, and sensible-latent heat fluxes are examined over the densely instrumented grassland region encompassing the First ISLSCP Field Experiment (FIFE). Twenty-two surface flux stations at 20 sites were deployed during the four 1987 intensive field campaigns (IFCs). Flux variability is addressed together with the problem of scaling up to area-averaged fluxes. Successful parameterization of area-averaged fluxes in atmospheric models is based on accounting for internal spatial and temporal scales correctly. Mean and variance properties of fluxes are examined in both daily and diurnally averaged frameworks. Results are compared and contrasted for clear and cloudy situations and checked for the influence of surface-induced biophysical controls (burn and grazing treatments) and topographic controls (slope factors and aspect ratios). Examination of the sensitivity of domain-averaged fluxes to different averaging procedures demonstrates that this may be an important consideration. The results reveal six key features of the 1987 surface fluxes: (1) cloudiness variability and ample rainfall throughout the growing season led to near-consistency in flux magnitudes during the first three IFCs; (2) burn treatment, grazing conditions, and topography have clearly delineated influences on the diurnal cycle flux amplitudes but do not alter the evaporative fraction significantly; (3) cloudiness is the major control on flux variability in terms of both mean and variance properties but has little impact on the Bowen ratio or evaporative fraction; (4) spatial weighting of fluxes based on a biophysicaltopographical cross stratification generates a measurable bias with respect to straight arithmetic averaging (up to 20 W m−2 in available heating); (5) structure function analysis demonstrates significant underlying spatial autocorrelation structure in the fluxes, but the observed distance dependence is due to cloudiness controls, not surface controls; (6) Monte Carlo analysis of high resolution vegetation indices obtained from SPOT satellite measurements suggest that the mean domain amplitudes of the diurnal sensible and latent heat flux cycles can be biased up to 30–40 W m −2 by repositioning the 20 site locations within the experimental domain.

Surface flux measurements in FIFE: An overview

In 1987 the Surface Flux Group of the first ISLSCP Field Experiment (FIFE) operated 22 stations at 20 sites. In 1989, 13 sites were instrumented. A variety of sensors were employed to calculate the fluxes of mass and energy. An effort was made throughout the FIFE campaign to compare sensors. A series of papers in this special issue present these group studies and efforts. These papers principally report the 1987 campaign, although two papers report station intercomparison during 1989. Additional papers examine the time-space variability of heat, moisture, and momentum fluxes, as well as analyses of the properties of the CO2 fluxes and their relationships to water stress. In this overview paper we describe the basic methodologies of the measurements, provide details on the sensor systems used by members of the Surface Flux Group, and provide a summary of the flux articles appearing in this special issue.

Field measurement of small ozone fluxes to snow, wet bare soil, and lake water

Eddy-correlation measurements over snow, wet bare soil, and lake water indicate very small vertical ozone fluxes. Adjustments to the small vertical fluxes are needed to take into account the effect of mean Stefan flow associated with evaporation at the surface and the effects of correlation between density variations and vertical wind fluctuations. For snow, the residual resistance calculated for the surface is about 34 s cm-1, indicating that the maximum deposition velocity is abut 0.03 cm s-1. For cold bare soil well saturated with water, the surface resistance is about 10 s cm-1 (maximum deposition velocity of about 0.1 cm s-1). The highest resistances obtained are for transfer to the surface of Lake Michigan, yielding values near 90 s cm-1 for resistance (0.01 cm s-1 for deposition velocity).

An eddy-correlation measurement of particulate deposition from the atmosphere

Abstract Eddy-correlation measurements of the vertical flux of particles in the size range of 0.05-0.1 μm indicate that the deposition velocity at 5 m above a moderately rough surface varies from 1.0-0.1 cm s −1 in light winds. These velocities are only slightly less than the corresponding estimates for momentum and a few gases that are highly reactive at the surface.

Estimated dry deposition velocities of sulfur over the eastern United States and surrounding regions

Abstract Surface deposition velocities of sulfur dioxide and sulfate particles over the eastern half of the United States, southern Ontario, and nearby oceanic regions are computed from equations developed in recent field experiments, for use in studies of regional-scale atmospheric sulfur pollution. Surface roughness scale lengths and resistances to pollutant uptake by the surface are estimated from consideration of land-use characteristics and the likely biological status of the vegetation. Midsummer conditions are assumed, but other seasons can be easily considered. Average deposition velocities for grid cells corresponding to half-degree increments of longitude and latitude are presented for a range of atmospheric stabilities.