Onset of drainage currents in flows over forested hills

There is a huge international investment in Fluxnet: a network of towers measuring fluxes of important gases between the atmosphere and the land surface. A particular focus is on measuring carbon dioxide exchange between forests and the atmosphere because of the implications for climate. There are known problems with these direct measurements particularly with measuring night time fluxes. Here we develop theory for boundary layer flow over hills covered in forests. In particular we show the competing forces at work at night when the boundary layer becomes stably stratified and derive a condition for the onset of drainage flows. The surprise is that drainage flows can occur within the forest layer, whilst the air flow in the boundary layer above the forest remains fully turbulent and over the hill. The results could have implications for interpreting data from Fluxnet sites