Role of low-level jets and boundary-layer properties on the NBL budget technique

The atmospheric properties of nights suited to the use of the nocturnal boundary layer budget technique to assess nocturnal CO2 fluxes are presented and discussed. This paper demonstrates that the nocturnal boundary layer technique can be successfully used when nights are characterized by both thermal inversions and the presence of either a low level jet (LLJ) or a wind maximum, since these conditions lead to the confinement of gases within the nocturnal boundary layer. When used alone, thermal inversions and stability parameters are not criteria sufficient to determine the successful use of the nocturnal boundary layer budget technique despite earlier suggestions to the contrary. For the cases studied, it appears that the combination of both stability conditions above the critical gradient Richardson number and the location of either a LLJ or well-defined wind maximum provides conditions leading to the successful application of the nocturnal budget technique. This technique, when used in the above conditions, has the potential to provide estimates of nocturnal fluxes in conditions where other methods such as eddy-covariance fail.