Observations of the turbulence structure of wind, temperature and liquid water content in a foggy surface layer

Meteorological observations and turbulence measurements of wind, temperature, and liquid water content in a long lasting, high reaching boundary layer fog are presented. In contrast to observations in other fog experiments our measurements report a fog in an unstable, well mixed boundary layer with low wind shears, pseudo adiabatic lapse rates, and increasing LWC with height. To a large extent its evolution was determined by horizontal advection. Its vertical structure and also the magnitude of the observed variances for the horizontal wind components, for temperature and LWC resemble those of a stratocumulus. Only the variances of vertical wind speed are significantly weaker than in stratocumulus. A comparison of the power spectra of wind and temperature with the power spectra of LWC shows a different vertical behavior. While the surface heating causes a broadening of the power spectra of wind and temperature for eddy sizes below 200 m, identifying a convective production regime of turbulence, the LWC power spectrum decreases for these sizes with decreasing height, suggesting that the effect of evaporation dominates the LWC fluctuations in this scale. The vertical turbulent fluxes of LWC in the well mixed layer were often found to be counter-gradient.