Influence of terrestrial ecosystems and topography on coastal CO₂ measurements: A case study at Trinidad Head, California

Abstract

[1] Coastal stations are critical for interpretation of continental-scale CO 2 exchanges although the impacts of land and sea breezes, local topography, katabatic winds, and CO 2 transport from nearby terrestrial ecosystems are not well characterized. We applied a modeling framework that couples meteorological (MM5), land-surface (LSM1), and tracer models to investigate the impact of these factors on coastal CO 2 measurements. Model predictions compared well with measurements over 4 months at our case study site (Trinidad Head, California). We predicted that during midday and under strong onshore wind conditions, positive and negative CO 2 anomalies from the assumed ''background'' marine layer air were sampled at the station. These anomalies resulted from two classes of mechanisms that couple transport and recent terrestrial ecosystem exchanges. First, and most important, are local and large-scale recirculation of nighttime positive CO 2 anomalies resulting from katabatic flows off the coastal mountain range. Second, negative anomalies generated by daytime net ecosystem uptake can be transported offshore in the residual layer and then entrained in the marine boundary layer. We predicted monthly averaged CO 2 anomalies associated with terrestrial exchanges of 0.53, 0.34, 3.1, and 0.05 ppm during March, June, September, and December of 2002. Positive anomalies from nighttime ecosystem respiration were more likely to be sampled than are negative anomalies associated with daytime net ecosystem uptake. Current atmospheric models used in continental-scale inverse studies do not resolve these two classes of mechanisms and therefore may infer incorrect CO 2 exchange rates.

In situ measurement of atmospheric CO<sub>2</sub> at the four WMO/GAW stations in China

“…In winter, in addition to the lower boundary layer, the increase of fossil fuel consumption may also partly contribute to the higher CO 2 mole fractions. The maximum CO 2 mole fraction occurrence time at WLG, a global WMO background site, agrees with the MBL and those observed at sites of similar latitudes (Nakazawa et al, 1993;Riley et al, 2005;Thoning et al, 1989).…”

Comparing high resolution WRF-VPRM simulations and two global CO<sub>2</sub> transport models with coastal tower measurements of CO<sub>2</sub>

“…A recirculation of respired nighttime CO 2 fields, for which the term "3-D rectifier effect" was established (Riley et al, 2005;Pérez-Landa et al, 2007;Ahmadov et al, 2007) is demonstrated for the case 20 May, 2005 (Fig. 5).…”

“…The conclusions made by Geels et al (2007) impose severe limitations on the usability of continental CO 2 concentration data from short towers and mountain stations in inversions. Moreover one may also add coastal stations to the "difficult sites" list (Riley et al, 2005). As a result, the CO 2 inversions performed by the TransCom 3 inversion community down-weighted continental observations, assuming that the data contained too much "noise" (Gurney et al, 2004;Wang et al, 2007).…”

Modelling regional scale surface fluxes, meteorology and CO<sub>2</sub> mixing ratios for the Cabauw tower in the Netherlands

“…Continental scale studies show that the forward simulation of CO 2 improved by increasing the horizontal resolution from a number of degrees (Gurney et al, 2002) to one degree or less (Geels et al, 2007;Parazoo et al, 2008). Further increasing the horizontal resolution to just a few kilometres in more limited domain studies (Dolman et al, 2006) was shown to improve the CO 2 mixing ratio simulation at observation stations in uneven and coastal terrain, because of the models ability to simulate mesoscale circulations, like sea breezes and topography induced katabatic flows (Nicholls et al, 2004;Riley et al, 2005;Van der Molen and Dolman, 2007;Sarrat et al, 2007;Ahmadov et al, 2009). This also avoids representation errors by resolving a larger part of the variability in the CO 2 mixing ratio Tolk et al, 2008).…”