Small-Scale Horizontal Variability of Mean and Turbulent Quantities in the Nocturnal Boundary Layer

Two networks of meteorological stations are used to investigate the spatial variability of temperature, wind speed, turbulent kinetic energy, turbulent fluxes and turbulent spectra over horizontal distances of tens to hundreds of metres . The networks are located in regions of moderately complex terrain, with stations located at elevation differences as large as 37 m at Pedras Altas and 10 m at Santa Maria. The spatial temperature variability is largely enhanced on very stable, weak-wind nights. At Pedras Altas, the temperature difference reaches 12 K between stations horizontally separated by less than 500 m. Obstacles also play an important role, as obstructed sites may have temperatures a few K lower than unobstructed sites at the same elevation. The maximum wind speed over the network (Vmax) controls the horizontal variability of mean and turbulent quantities. Despite the terrain and horizontal scale differences, variables such as mean temperature, standard deviations of horizontal and vertical wind components and the turbulent fluxes of sensible heat, latent heat and carbon dioxide share the dependence on Vmax at both sites. The regime transition between the very stable and weakly-stable boundary layers occurs at much lower wind speeds at an obstructed site than at a hilltop.