Tropopause fold structure determined from airborne lidar and in situ measurements

Abstract

The first remote measurements of O 3 and aerosols across a tropopause fold are presented in this paper. An airborne differential absorption lidar (DIAL) system was used to obtain profiles of O 3 and aerosols along a cross section of a fold on April 20, 1984, over southern Nevada and California. The DIAL measurements across the tropopause fold show a 2.0‐km‐deep layer, with high O 3 concentrations and enhanced aerosol backscattering, that slopes downward from north of Las Vegas, Nevada, to the top of the planetary boundary layer (PBL) over Yuma, Arizona. This is the first continuous mapping of a tropopause fold from the upper troposphere to the PBL. Mixing ratios of O 3 in excess of 200 parts per billion by volume (ppbv) were measured remotely in the fold, and these values were corroborated by in situ measurements of O 3 through the fold at an aircraft altitude of 6.7 km. Enhanced aerosol back‐scattering was observed in the tropopause fold and attributed to stratospheric aerosols from the El Chichon eruption. An analysis of the potential vorticity distribution along the DIAL flight track was performed using radiosonde data. A high positive correlation was found between the DIAL O 3 mixing ratios and the potential vorticity values in the fold. The average ratio between O 3 and potential vorticity was found to be 50.2 ppbv/10 −5 cm 2 deg g −1 s −1 in the fold. The decrease in layer thickness, potential vorticity, ozone mixing ratio, and aerosol backscatter down the axis of the fold is consistent with a convergent entrainment of tropospheric air across both boundaries of the fold and subsequent irreversible mixing by small‐scale turbulent motions.