Interpreting concentration measurements above bare and forested surfaces in the presence of a nocturnal low-level jet

This study reports on the location of sources contributing to a point concentration measurement in the stable boundary layer. Since experimental studies focused on spatial scales of such measurements seem to be unlikely possible, because of demanding logistical consideration, the present study resorts to numerical modelling. Concentration footprints for instrumentation located at different heights during the night are examined using the combination of both atmospheric boundary layer and Lagrangian simulations models: characteristics of the flow required to initialize the backward-in-time random-walk model were obtained using the model SСADIS (Sogachev et al., 2002, 2008). The model captures the most prominent features of a low-level jet, including its vertical structure as well as its diurnal phase and amplitude. Simulations of air parcel transport in a no-steady-state atmospheric boundary layer above a bare surface and a forest under different insolation suggest that sources located as far as several hundred kms away contribute to a concentration measurement made at levels as high as 500 m. The origin of the maximal contribution area shifts during the night depending on the wind direction at the sensor location, a feature most prominent in the presence of decoupling between sensor levels and surface sources. Simulations suggest that atmospheric static stability alone is not a sufficient criterion to trigger flow decoupling. The presence of the low-level jet provides vertical mixing of air parcels even for stable boundary layers with Richardson numbers higher than the critical. This finding is in agreement with earlier observations suggesting that the level of the low-level jet nose acts as a strong lid prohibiting vertical gas propagation. These findings have important implications in the interpretation of eddy-flux measurements in nocturnal conditions.