Some evidence of vertical moisture transport during very stable conditions at the Cabauw measurement site

During clear nights the absolute wind velocity can become very weak and the temperature stratification very stable. According to simple theoretical considerations it can be shown that when certain stability criteria are violated, e.g. the bulk Richardson number exceeding its critical value of unity, vertical turbulent transport will vanish. However, during clear nights in the Netherlands one can often experience a clear, yet indirect evidence of a downward moisture transport by means of a strong dew formation at the ground surface. We performed a detailed analysis of the Cabauw tower data of situations during which the total wind velocities at 10 m were smaller than 3 m/s and the net longwave radiative loss larger than 40 W/m^2. We note that for these cases the residual in the observed surface energy balance is very large, and that the latent heat flux near the surface as observed with the eddy-correlation technique is usually negligibly small. However, in particular in the lower part of the 200 m Cabauw tower the specific humidities are frequently observed to decrease with time. We estimate the latent heat flux from the tower observations from the vertically integrated budget equation for the specific humidity. For some months we find that this approach reduces the gap in the monthly mean surface energy balance by about 10 W/m^2.