Small-scale horizontal variability of scalars and turbulent fluxes in the stable boundary layer

In calm nights, when the air adjacent to the surface decouples from the upper boundary layer, the horizontal variability of quantities such as temperature, specific humidity and wind speed is greatly enhanced. This is a consequence of the local nature of the flow events that characterize a very stable boundary layer.

Most of the studies that addressed such small scale variability in the past focused on single terrain features, such as a given basin or valley. When more complex topographies were considered, the typical horizontal scale analyzed was on the order of a few kilometers. The main objective of the present study is to show the degree of variability that may exist over a moderately complex topography and over distances of tens to hundreds of meters.

For that purpose, a network of 11 standard meteorological stations, sampling at 2-minute intervals, was deployed over an area of 1 km by 600 m in southern Brazil, with altitudes ranging from 350 to 390 m above sea level. At this small area, many different topographical features such as small hills, valleys and basins coexist, making the site suitable to address meteorological variability over moderately complex terrain. Three of the stations, one at a hilltop, another at a low location and a third in between the other two, were equipped with fast-response sensors, which allowed the determination of turbulence-related quantities. The horizontal dimensions of the network are small enough that, in windy nights, there is virtually no variability of temperature and humidity among the sites, even between stations at contrasting locations in terms of topography. On the other hand, when the mechanical forcing is not enough to keep the entire network connected to the upper boundary layer, decoupling is favored at the lower sites, but not at the higher ones. As a consequence, temperature differences as high as 10 C have been observed at locations horizontally separated by only 300 m, with a 25-m difference in altitude between them.

The factors controlling the onset of the horizontal variability at a given night are detailed in the present study. Besides, the flow characteristics at the decoupled locations are presented. It is also shown that in some places the surface couples intermittently to the upper boundary layer and that the horizontal variability fluctuates accordingly. The analysis of the turbulence characteristics at the three sites where such observations are available shows that locations that remain turbulent throughout most of the night anchor the turbulent exchange between the surface and the atmosphere over an entire region. Implications of these facts are discussed.