Small scale drainage front

A case study of nocturnal drainage flow is presented. The flow descends a gentle slope of a few percent extending about one half kilometer down to Roskilde Fjord in northeast Denmark. Opposing larger scale flow initially delays the advance of the drainage flow. After several hours delay, the drainage flow arrives at the coast as a sudden relatively deep surge of cold air.

This surge of cold air significantly disturbs the immediate overlying flow. Even though the surge of cold air is characterized by high Richardson number and weak turbulent activity, this drainage front induces low Richardson number and significant turbulent activity in the overlying flow resulting in an “upside down” tructure to the “boundary layer”. The drainage flow following the initial surge is thinner, weaker and nearly stationary except for occasional thicker pulses of cold air.