Vertical wind velocity observations at the Cabauw tower

To calculate the spatially averaged vertical turbulent flux of momentum, heat or scalars with the eddy-correlation technique the horizontal mean value of the vertical wind speed above a flat land surface can be assumed to be negligibly small. However, measurements are typically collected at a fixed position in space, and mean values are calculated from high-frequency time series. We have investigated the dependency of the temporal mean value of the vertical velocity on the averaging time. We used high-frequency data that were collected at 3, 60, 100 and 180 m height from the Cabauw meterological tower. Above 60 m height, and for averaging times less than 30 minutes, it is found that the temporal mean vertical velocity during convective conditions is typically larger than 10 cm/s. In addition, time series of the vertical velocity were sampled at a fixed position in the domain of a large eddy simulation model. These results corroborate the experimental findings.