Lidar Observations of Banded Convection during BLX83

Lidar observations of clear-air convection during the 1983 Boundary Layer Experiment (BLX83) reveal the presence of elongated, parallel regions of updrafts marked by enhanced aerosol backscattering. These linear (banded) aerosol structures were observed over a two-hour period during a cloud-free morning. During this period, the depth of the convective boundary layer (CBL) increased from 100 to 1300 m. Wind speeds averaged over the depth of the CBL varied between 0 and 2 m s−1, while the wind direction varied over a range of 160 deg. The CBL instability parameter, –Zi/L, increased from approximately 25 (weakly unstable) to 250 (strongly unstable). The spacings of the elongated, parallel plumes scaled with the CBL height. These findings suggest that secondary circulations in the form of horizontal roll vortices were present under conditions not normally associated with roll vortices. The lines of aerosol structures aligned much more closely (within 15 deg) with the direction of the vertical shear of the horizontal wind through the depth of the CBL than with either the surface wind, mean CBL wind, or the wind at an altitude of 1.1Zi.