What Determines the Nocturnal Vertical Wind Gradient?

The formation of logarithmic wind profiles requires the presence of mechanical turbulence and thus surface friction. Stable nocturnal conditions are described by the addition of a correction term following Dyer and Businger. This correction term changes the logarithmic profile into a nearly linear wind profile with a wind shear considerably larger than under neutral conditions.

The low-level jet (LLJ) is a typical nocturnal wind phenomenon which forms shortly after the evening transition. Usually a LLJ is described as an inertial oscillation around the geostrophic wind speed. This description stipulates the absence of turbulence underneath the jet.

Own observations as well as literature reports show, that wind shear underneath LLJs increases during the evolution of the jet and then does not change much any further. If nevertheless the strength of a jet is continuing to increase during the night, then the height of the jet increases keeping the shear constant.

RASS observations of nocturnal temperature and wind profiles at inland sites as well as such observations from the 100 m high offshore platform FINO 1 show that a minimum bulk Richardson number is reached at stable stratification which then stays more or less constant until conditions change again towards thermal instability (sun rise at the end of the night or the advection of warmer air over the sea). A further decrease of this bulk Richardson number due to increasing shear would mean the onset of mechanical turbulence generation which in turn would reduce the wind shear once again.

Thus, it seems that this bulk Richardson number is obviously the governing parameter for wind shear in stable conditions. This number is known to be a good indicator for the transition from laminar to turbulent flow and vice versa. The occurrence of a constant minimum Richardson number (which, by the way, is larger over land than over the sea) indicates that a new equilibrium is reached which is just at the edge between a laminar and a turbulent regime.

This presentation will show some measurement data together with some considerations how to combine the two apparently contradicting descriptions for the nocturnal wind profile (the frictional Monin-Obukhov theory and the frictionless LLJ theory). Such a merger would allow for an estimation of the maximum LLJ wind speed and the height of this maximum above ground for which there is no other relation available so far.