A study of horizontal convective rolls occurring within clear-air convective boundary layers.

Also issued as thesis (Ph. D.)--University of California, Los Angeles, 1995.

Includes bibliographical references (p. 168-179).

Data from the Convection and Precipitation/Electrification (CaPE) project were combined with numerical simulations to examine horizontal convective rolls. Four primary topics were addressed: (i) the environmental conditions necessary for sustaining rolls, (ii) horizontal thermodynamic variability within the convective boundary layer (CBL) due to rolls, (iii) thunderstorm initiation by rolls and (iv) the cause of along-roll periodicities. Both observations and numerical model simulations suggested that a moderate surface heat flux and some vertical wind shear was necessary for roll existence. Unlike previous studies, however, it was shown that rolls occurred within very low shear conditions (~2times10 ^{-3} s^{-1 }. In addition, the low-level (i.e., ~200 m) shear seemed to be more important than the CBL shear in roll sustenance. The horizontal potential temperature variations caused by rolls were 0.5 K while mixing ratio variations of 1.5-2.5 g kg^{-1} were observed at all heights throughout the CBL. The observed cloud base heights, measured with photogrammetry, were produced from the highest moisture values which occurred within the roll updraft regions. Since the roll updrafts contained the moisture values which were most representative of cloud base, it is likely that measurements from within roll updrafts are most representative of the potential for deep convection. It was documented that thunderstorms were initiated by rolls in Florida. Detailed comparisons were made between the days on which thunderstorms were initiated by rolls to days on which rolls existed but did not produce thunderstorms. No apparent differences were identified between the two types of days in any of the parameters examined. Along-roll irregularities were often apparent along the cloud streets, as well as within the CBL motions. The observed periodicities within the CBL had the same wavelength along adjacent roll updraft regions and were continuous in the cross-roll direction. The most likely source of these features was found to be shearing instabilities. A significant amount of knowledge about rolls has been obtained from analyzing the CaPE dataset although many assumptions and simplifications were required. These restrictions of the CaPE dataset, as well as plans for a field project during the summer of 1995 chiefly designed to sample rolls and waves, are presented.