Abstract
Abstract. Interpretation of observed diurnal carbon dioxide (CO2) mixing ratios near the surface requires knowledge of the local dynamics of the planetary boundary layer. In this paper, we study the relationship between the boundary layer dynamics and the CO2 budget in convective conditions through a newly derived set of analytical equations. From these equations, we are able to quantify how uncertainties in boundary layer dynamical variables or in the morning CO2 distribution in the mixed-layer or in the free atmosphere (FA) influence the bulk CO2 mixing ratio. We find that the largest uncertainty incurred on the mid-day CO2 mixing ratio comes from the prescribed early morning CO2 mixing ratios in the stable boundary layer, and in the free atmosphere. Errors in these values influence CO2 mixing ratios inversely proportional to the boundary layer depth (h), just like uncertainties in the assumed initial boundary layer depth and surface CO2 flux. The influence of uncertainties in the boundary layer depth itself is one order of magnitude smaller. If we "invert" the problem and calculate CO2 surface exchange from observed or simulated CO2 mixing ratios, the sensitivities to errors in boundary layer dynamics also invert: they become linearly proportional to the boundary layer depth. We demonstrate these relations for a typical well characterized situation at the Cabauw site in The Netherlands, and conclude that knowledge of the temperature and carbon dioxide profiles of the atmosphere in the early morning are of vital importance to correctly interpret observed CO2 mixing ratios during midday.
Highlights
Surface turbulent fluxes and boundary layer dynamics determine the daily evolution of temperature, moisture and other scalar quantities in the atmospheric boundary layer (Lemone et al, 2002)
Vila-Guerau de Arellano et al (2004) used aircraft and surface observations combined with mixed-layer theory to analyze the role of the entrainment of air on the distribution and evolution of carbon dioxide in the convective boundary layer (CBL), and compared the ratio of entrainment to surface CO2 fluxes with the values obtained for heat and humidity
Based on mixed-layer theory, we have derived analytical expressions to quantify the dependence of the key components of CO2 budget on boundary layer dynamics
Summary
Surface turbulent fluxes and boundary layer dynamics determine the daily evolution of temperature, moisture and other scalar quantities in the atmospheric boundary layer (Lemone et al, 2002). Where h0 and h are the initial and the instantaneous boundary layer depth; C is the carbon dioxide mixing ratio vertically integrated between the surface and h; CFA is the CO2 mixing ratio in the free atmosphere, just above the inversion; and w c |s is the time-dependent surface flux of CO2. In this equation all the variables except h0 are time dependent. In the Appendix we provide a full derivation of the classical mixed layer equation from the budget Eq (1)
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