[1] Convective turbulence within the atmospheric boundary layer (ABL) and movement of the ABL over the surface results in a large spatial (10 4 -10 5 km 2 ) integration of surface fluxes that affects the CO 2 and water vapor mixing ratios. We apply quasi-equilibrium concepts for the terrestrial ABL to measurements of CO 2 and water vapor made within the ABL from a tall tower (396 m) in Wisconsin. We suppose that CO 2 and water vapor mixing ratios in the ABL approach an equilibrium on timescales longer than a day: a balance between the surface fluxes and the exchange with the free troposphere above. By using monthly averaged ABL-to-free-tropospheric water vapor differences and surface water vapor flux, realistic estimates of vertical velocity exchange with the free troposphere can be obtained. We then estimated the net surface flux of CO 2 on a monthly basis for the year of 2000, using ABL-to-free-tropospheric CO 2 differences, and our flux difference estimate of the vertical exchange. These ABL-scale estimates of net CO 2 flux gave close agreement with eddy covariance measurements. Considering the large surface area which affects scalars in the ABL over synoptic timescales, the flux difference approach presented here could potentially provide regional-scale estimates of net CO 2 flux.
Mixing ratios of CO 2 often change abruptly in the presence of inclement weather and low pressure systems. Water vapor mixing ratio, temperature, wind speed, and wind direction data are used to infer that the abrupt changes in CO 2 mixing ratios at a site in northern Wisconsin are due to tropospheric mixing, horizontal transport, or a combination of both processes. Four different scenarios are examined: the passage of a summer cold front, a summer convective storm, an early spring frontal passage, and a late autumn low pressure system. Each event caused CO 2 mixing ratios to change rapidly when compared to biological processes. In one summer convective event, vertical mixing caused CO 2 mixing ratios to rise more than 22 ppm in just 90 s. Synoptic-scale transport was also evident in the presence of storm systems and frontal boundaries. In the cases examined, synoptic-scale transport changed CO 2 mixing ratios as much as 15 ppm in a 1-h time period. The events selected here represent extremes in the rate of change of boundary layer CO 2 mixing ratios, excluding the commonly observed venting of a shallow, stable boundary layer. The rapid changes in CO 2 mixing ratios that were observed imply that large mixing ratio gradients must exist, often over rather small spatial scales, in the troposphere over North America. These rapid changes may be utilized in inverse modeling techniques aimed at identifying sources and sinks of CO 2 on regional to continental scales.
Inference of CO2 surface fluxes using atmospheric CO2 observations in atmospheric inversions depends critically on accurate representation of atmospheric transport. Here we characterize regional‐scale CO2 transport uncertainties due to uncertainties in meteorological fields using a mesoscale atmospheric model and an ensemble of simulations with flow‐dependent transport errors. During a 1‐month summer period over North America, transport uncertainties yield an ensemble spread in instantaneous CO2 at 100 m above ground level comparable to the CO2 uncertainties resulting from 48% relative uncertainty in 3‐hourly natural CO2 fluxes. Temporal averaging reduces transport uncertainties but increases the influence of CO2 uncertainties from the lateral boundaries. The influence of CO2 background uncertainties is especially large for column‐averaged CO2. These results suggest that transport errors and CO2 background errors limit regional atmospheric inversions at two distinct timescales and that the error characteristics of transport and background errors should guide the design of regional inversion systems.
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