Estimation of atmospheric water vapour flux profiles in the nocturnal unstable urban boundary layer with Doppler sodar and Raman lidar

The vertical wind profiles determined by Doppler sodar and the water vapour mixing ratio profiles obtained by Raman lidar are used to estimate the atmospheric water vapour flux profiles in the nocturnal urban boundary layer under unstable conditions. The experiment was conducted for several nights in the central area of Rome under a variety of moisture conditions and different urban boundary-layer flow regimes. Despite some scatter in the profiles, the latent heat flux is found to be positive throughout the depth of the nocturnal urban boundary-layer. The layer-averaged flux shows a variation between -4 to +40 W m(-2), while individual values of flux in excess of +150 W m(-2) pertain to a case of free convection during cold air advection caused by the sea breeze. The quality of flux estimates is found to be highly limited by the low sampling rates employed in the experiment resulting in errors to the order of 60%. Therefore, the results must be viewed as estimates rather than precise measurements. The skewness profiles of the turbulent fluctuations of vertical velocity and water vapour mixing ratio are also positive.