A multifractal analysis of lidar measured water vapour

Accurate and fast-response measurements of space-time observations of specific humidity were made above a drying land surface at the University of California at Davis, using the Los Alamos water Raman-lidar. In an attempt to quantify the space-time intermittency features of turbulent flows in the lower atmosphere, a multifractal analysis of these water vapour measurements was performed. The structure of the specific humidity, θ(x, t), was analyzed quantifying a scalar gradient measure {Mathematical expression} both in time and space, for all possible one-dimensional cuts, i.e. {Mathematical expression} and {Mathematical expression}. The results confirm the multifractal nature of this scalar gradient measure (a type of scalar dissipation rate) and show that humidity measurements at fixed times (χx) are more intermittent (e.g. have less entropy dimension) than those at fixed locations in space (χt). Similar multifractal behaviour of the spatial data, with and without a transformation from the observed wind velocities, supports the validity of Taylor's hypothesis for the studied fields.