Nocturnal vertical wind gradients and low-level jets analysed from RASS data

Nocturnal vertical wind gradients are relevant amongst other applications for the vertical turbulent transport of air pollutants and the operation of larger wind turbines. The level of shear-induced nocturnal turbulence is important for upward dilution of freshly emitted near-surface pollutants as well as for the downward mixing of older pollutants (e.g. ozone) from the residual layer. Larger wind turbines with hub heights of about 100 m and higher usually operate above the atmospheric surface layer at night-time. This has two important consequences in nights with considerable radiative cooling. Wind speeds are higher at night-time than at daytime and vertical wind shear across the rotor areas is close to maximum possible values at night-time.
In this presentation RASS data is obtained in Southern Germany which delivers wind and temperature profiles with high vertical resolution are analysed in order to assess the governing factors for nocturnal wind speed and shear. Bulk Richardson numbers for the layer between 60 m and 160 m above ground are derived from this data. Increasing nocturnal shear means decreasing Richardson numbers. It is found that the shear increases until a maximum Richardson number is reached. ... mehrEven higher shear would produce new mechanical turbulence which – in turn – would reduce the shear again. Therefore, nocturnal wind shear can be interpreted as an equilibrium shear for a given stable thermal stratification rather than a result from an inertial oscillation of the wind vector after sun set.