How do we define Atmospheric Stability in Complex Terrain? Suggestions from comparing the XPIA and Perdigão Campaigns

Quantification of atmospheric stability is critical for a number of applications ranging from air quality and dispersion to wind energy forecasting. Low-level stability may also influence the development of severe storms. However, numerous metrics for quantifying or classifying atmospheric stability populate the academic literature. For some applications, quantifying stability is meant to provide a proxy for ambient turbulence that would affect dispersion of contaminants or dissipation of wind turbine wakes. Complex terrain and its accompanying dynamics of flow separation, recirculation, canopy flows, and drainage flows, further complicates the assessment of atmospheric stability.

Two recent field campaigns - one in flat terrain, one in complex terrain - provide opportunities for comparing and contrasting stability metrics measured with both in situ sonic anemometers and remote sensing instruments. The XPIA campaign in Colorado's Front Range collected sonic anemometer measurements at six heights along a 300-m meteorological tower co-located with several profiling lidars. The Perdigão field campaign in central Portugal employed 49 meteorological towers and numerous lidars over a 3km x 3 km region in complex terrain. For both of these campaigns, we calculate stability metrics including static stability, bulk Richardson Numbers (calculated over several different choices of heights), gradient Richardson Numbers, flux Richardson Numbers, and Obukhov Lengths. We then compare these stability metrics to turbulence metrics including turbulent intensity, turbulent kinetic energy, and turbulent dissipation rate to assess the power of stability metrics to suggest turbulent behavior relevant to wind energy, dispersion, and other applications.