A MULTIFRACTAL ANALYSIS OF LIDAR MEASURED WATER-VAPOR

Accurate and fast-response measurements of space-time observations of specific humidity were made above a drying land surface at the Univers ity of California at Davis, using the Los Alamos water Raman-lidar. In an attempt to quantify the space-time intermittency features of turbu lent flows in the lower atmosphere, a multifractal analysis of these w ater vapour measurements was performed. The structure of the specific humidity, theta(x, t), was analyzed quantifying a scalar gradient meas ure (similar to [grad theta](2)) both in time and space, for all possi ble one-dimensional cuts, i.e. chi(t)(x, t) = [partial derivative thet a(x, t)/partial derivative t](2) and chi(x)(x, t) = [partial derivativ e theta(x, t)/partial derivative x](2). The results confirm the multif ractal nature of this scalar gadient measure (a type of scalar dissipa tion rate) and show that humidity measurements at fixed times (chi(x)) are more intermittent (e.g. have less entropy dimension) than those a t fixed locations in space (chi(t)). 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.