Variability of Stability, Momentum and Heat Fluxes in the Stable Boundary Layer over Highveld Priority Area, South Africa

In this paper, we present the micro-spatial scale and temporal variability of Obukhov length (L), Kinematic momentum flux or turbulent friction velocity (­) and heat flux () in the Stable Boundary Layer (SBL) using observational data collected by a dense network of five Automatic Weather Stations (AWSs). The stations were in operation in the Highveld Priority Area (HPA) of the Republic of South Africa during 2008-2010. The L, ­ and  are solved using Monin Obukhov similarity theory. Analysis of spatial variability of stability regimes as presented by L suggests that HPA is dominated by strong stability regime. The analysis also revealed inter-station variability of and L while indicates no significant spatial variability between stations. Temporal variability showed that most of the low values for Obukhov length that fall within strong stable regimes (0local time (GMT+2) and about 5-10% occurs around 6-15 hours, local time (GMT+2). These observations suggest that radiative cooling and stability are stronger in the night than day time SBL and the momentum fluxes are weaker in the night than day time SBL. Finally it is found that temporal inter-station variability of L, ­ and , could play a significant role on diffusion and dispersion of tracers and contaminant and influence on local air quality at hourly to diurnal scales in the SBL.