Comparing mountain breezes and their impacts on CO2 mixing ratios at three contrasting areas

Abstract

This work presents the characterisation and comparison of daytime and nighttime mountain breezes observed at three sites through the analysis of tower data. The sites are located: (i) in the foothills of the Guadarrama Mountains in Spain, (ii) on a plateau adjacent to the Pyrenees in France, and (iii) in the Salt Lake Valley (SLV) in the southwest of the United States. The thermally-driven winds are detected through a systematic algorithm which considers both synoptic and local meteorological conditions. The characteristics of the mountain breezes depend on the scale of the breeze at each site. Nighttime events are associated with stronger wind speeds at the two sites located farther away from the mountains due to larger-scale phenomena (valley winds and mountain-plain winds). The arrival of both nighttime and daytime flows to the sites are observed approximately when the buoyancy heat flux changes sign, being a few hours delayed at the sites farther from the mountains.

In addition, the impacts of these breezes on CO2 mixing ratios are analysed. The characteristic increase of CO2 mixing ratio observed during the evening transition takes place approximately when the nocturnal breeze arrives at the site. Nonetheless, both processes are not always simultaneous, indicating that CO2 advection is not the main mechanism controlling the drastic CO2 increase. An analogous result is obtained for the CO2 decrease at the morning transition. However, we have found that the CO2 mixing ratio is sensitive to wind direction (horizontal advection) in highly heterogeneous areas like the SLV, where CO2 emissions from the nearby city centre play an important role.

Finally, a clear relationship is found between the CO2 mixing ratio and near-surface turbulence at night. Maximum CO2 mixing ratios are found for specific turbulence thresholds, which depend on the height of the CO2 sensor. Conditions associated with both stronger and weaker turbulence levels lead to reduced CO2 mixing ratios at the local measurement height due to excessive and ineffective mixing, respectively.