Using Slow-Response Observations from Oklahoma Mesonet to Assess the Spatial Variability of Stable Boundary Layer Turbulence Regimes

Spatial variability of mean and turbulent quantities may be specially large in the nighttime stable boundary layer (SBL), under low mean wind speeds. On the other hand, under high wind speed, the enhanced mixing causes homogeneous conditions over fairly large horizontal areas. A good portion of such variability may be attributed to the stable boundary layer turbulence regime. The case with low wind speeds and large spatial variability refers to the very stable regime, while large wind speed and more horizontally homogeneous conditions occur in the weakly stable regime.

Recent studies have identified that there is a wind speed threshold between the two regimes, and that such a threshold is variable from site to site and from night to night at a single site. An important control is exerted by net radiation: larger wind speeds are necessary to maintain the SBL in the weakly stable regime when the radiative loss from the surface is large. Other controls are exerted by surface properties, such as the soil heat capacity. Sites with different surface thermal properties may be at different regimes, even with similar conditions of mean wind speed and net radiation. This may lead to differences in turbulence regime over a horizontal heterogeneous area that complicates the determination of area-averaged fluxes and of representative atmospheric quantities over an area.

In this study, it is shown that the average relationship between mean wind speed and net radiation may be used to fully characterize how the surface properties control the turbulence regime at a given site. It is shown that average wind speed increases linearly with net radiation for large values of radiative loss. Moreover, the linear relationship may be extrapolated to low values of low wind speed with a zero-intercept. The slope of such a linear dependence is a measurement of how large wind speed are necessary to maintain the surface at the weakly stable regime for a given value of radiative loss, and this slope is defined as the surface coupling strength.

The method introduced allows inferring the turbulence regime without the need of fast-response observations of turbulent fluctuations. Therefore, it provides a mapping of turbulence regime. The analysis presented uses 7 years of data at 80 stations of Oklahoma Mesonet during Oklahoma Atmospheric Surface-Layer Instrumentation System (OASIS) program. At the same time, 10 super-sites with turbulence observations during the same program are used to validate the method.

Implications of the findings for the representation of turbulence fluxes using a formulation that takes in account the dependence of the turbulence regimes on net radiation and surface properties are discussed.