Diurnal boundary layer mixing patterns characterised by radon gradient observations at Cabauw

Gradient measurements of the naturally occurring trace gas radon-222 can be used to construct quantitative measures of mixing and exchange within the lower atmosphere on diurnal timescales, and are beginning to be used in the evaluation of boundary layer mixing schemes in weather and climate models. Radon, a radioactive noble gas with a half-life of 3.8-days, is emitted from terrestrial surfaces at a rate that can be assumed to be uniform and steady on diurnal timescales. Radon's decay rate is optimum for boundary layer mixing studies, as it is negligible in comparison with typical turbulent timescales, but is fast enough to constrain its concentration in the free troposphere to be typically 1-3 orders of magnitude lower than near the surface. These properties combined make radon an excellent tracer for boundary layer vertical mixing studies. Continuous measurements of radon-222 gradients are currently being conducted at two meteorological towers on seasonal to inter-annual timescales. Near-surface gradients (2-50m) are recorded on a 50m tower at Lucas Heights, New South Wales (34.053S, 150.981E), and boundary layer gradients (20-200m) are recorded on the 213m tower at the Cabauw Experimental Site for Atmospheric Research (CESAR) in The Netherlands (51.971N, 4.927E). Here we focus on results from the first year of radon gradient observations at the Cabauw site, which is located 50km inland on a polder in an agricultural region and has a simple orography with surface elevations changing by a few metres at most within a 20 km radius. Two dedicated inlets mounted on the main meteorological tower at 20m and 200m above ground level are providing continuous radon data at hourly time resolution, and meteorological data from the tower is being provided by the Royal Netherlands Meteorological Institute (KNMI). We present a series of examples of radon and corresponding meteorological time series data at two heights on the Cabauw tower at different times of year. The examples illustrate the influence on the radon signal of atmospheric processes occurring on two very different spatio-temporal scales: 1. Synoptic variability, which causes changes in regional fetch (high-radon air from continental regions and low-radon air from oceanic regions) and influences the radon signal at both heights in a similar way; and 2. Diurnal variability, which causes changes in the local vertical mixing within the ABL, leading to a strong pattern of variations in the radon gradient between the 20m and 200m levels. By comparing radon concentrations at 20m and 200m it is thus possible to disentangle influences on these two scales sufficiently to unambiguously characterise and quantify diurnal vertical mixing. The radon concentration gradient proves to be a highly sensitive direct measure of the degree of vertical turbulent mixing in the lower atmosphere. The figure illustrates the effects on the radon signal of the competing influences of suppression of turbulence by nocturnal temperature stratification, on the one hand, and its mechanical generation by low level wind shear on the other. On each of the 5 nights displayed, the temperature gradient signal indicates a moderate-to-strong suppression of vertical turbulent mixing. However, only on the first two nights does this result in a significant radon peak, because over the subsequent period increasing wind shear acts to enhance and deepen the low level mixing.