Separating remote fetch and local mixing influences on vertical radon measurements in the lower atmosphere

Two-point radon gradients provide a direct, unambiguous measure of near-surface atmospheric mixing. A 31-month data set of hourly radon measurements at 2 and 50 m is used to characterize the seasonality and diurnal variability of radon concentrations and gradients at a site near Sydney. Vertical differencing allows separation of remote (fetch-related) effects on measured radon concentrations from those due to diurnal variations in the strength and extent of vertical mixing. Diurnal composites, grouped according to the maximum nocturnal radon gradient (Delta C-max), reveal strong connections between radon, wind, temperature and mixing depth on subdiurnal timescales. Comparison of the bulk Richardson Number (Ri(B)) and the turbulence kinetic energy (TKE) with the radon-derived bulk diffusivity (K-B) helps to elucidate the relationship between thermal stability, turbulence intensity and the resultant mixing. On nights with large Delta C-max, K-B and TKE levels are low and Ri(B) is well above the 'critical' value. Conversely, when Delta C-max is small, K-B and TKE levels are high and Ri(B) is near zero. For intermediate Delta C-max, however, Ri(B) remains small whereas TKE and K-B both indicate significantly reduced mixing. The relationship between stability and turbulence is therefore non-linear, with even mildly stable conditions being sufficient to suppress mixing.