Horizontal structure function and vertical correlation analysis of mesoscale water vapor variability observed by airborne lidar

Abstract

Analysis is presented of airborne lidar measurements of water vapor, covering a height range from 1.5 to 10.4 km, from three field campaigns (midlatitude summer, polar winter, and subtropical summer). The lidar instrument provides two‐dimensional cross sections of absolute humidity, with high accuracy (errors less than 5–7%) and high vertical (∼ 200 m) and horizontal (∼ 2 km) resolution. Structure functions, i.e., statistical moments up to the fifth‐order of absolute increments over a range of scales, are investigated, and power law scaling or statistical‐scale invariance was found over horizontal distances from 5 to 100 km. The scaling exponents are found to take different values, depending on whether or not the observations were taken in an air mass where convective clouds were present. The exponent of the first‐order structure function in nonconvective regions, H=0.63±0.10, is large indicating a smooth series with long‐range correlations, in contrast to the lower value H=0.35±0.11 found in convective air masses. Correspondingly, the moisture field in the convective regime was found to be more intermittent than for the nonconvective regime, i.e., water vapor structures in convectively influenced air mass show more jump discontinuities, which could be explained by the moistening and drying effects of updrafts and downdrafts in convective air mass. Within each regime (convective or nonconvective), the values appear to be universal, with no significant dependence on the season, latitude, or height where the observations were made. Furthermore, some evidence is found that vertical correlation lengths are longer in convective air masses.