Sensible heat budgets for stable nocturnal boundary layers over weakly heterogeneous terrain

The stable nocturnal boundary layer (SNBL) is commonly viewed or modeled with a one-dimensional approach. That is, local cooling is assumed to be balanced by the vertical heat flux divergence although the radiative flux divergence is sometimes also included in the heat budget. This perspective has dictated the design of field experiments for investigating SNBLs. Tower-based micrometeorological data from three field campaigns are analyzed to investigate the validity of the one-dimensional heat balance over surfaces with weak heterogeneity.

Observed cooling/warming (tendency term) approximately balances the vertical heat flux divergence with confidence for a small fraction of the entire records. The values of the radiative flux divergence are generally too small and sometimes of the wrong sign to explain the residual. An analysis of random flux errors and uncertainties in the tendency term indicate that such errors cannot explain large imbalances, suggesting the importance of advection of temperature or possibly the divergence of mesoscale fluxes. Even weak surface heterogeneity can create significant horizontal gradients in stable boundary layers. However, it is shown that existing field data and observational strategy do not allow adequate evaluation of advection and mesoscale flux divergence terms.