Stratosphere‐to‐troposphere transport revealed by ground‐based lidar and ozonesonde at a midlatitude site

This paper presents ozone structures measured by a ground‐based ozone lidar and ozonesonde at Huntsville, Alabama, on 27–29 April 2010 originating from a stratosphere‐to‐troposphere transport event associated with a cutoff cyclone and tropopause fold. In this case, the tropopause reached 6 km and the stratospheric intrusion resulted in a 2‐km thick elevated ozone layer with values between 70 and 85 ppbv descending from the ∼306‐K to 298‐K isentropic surface at a rate of ∼5 km day−1. The potential temperature was provided by a collocated microwave profiling radiometer. We examine the corresponding meteorological fields and potential vorticity (PV) structures derived from the analysis data from the North American Mesoscale model. The 2‐PVU (PV unit) surface, defined as the dynamic tropopause, is able to capture the variations of the ozone tropopause estimated from the ozonesonde and lidar measurements. The estimated ozone/PV ratio, from the measured ozone and model derived PV, for the mixing layer between the troposphere and stratosphere is ∼41 ppbv/PVU with an uncertainty of ∼33%. Within two days, the estimated mass of ozone irreversibly transported from the stratospheric into the troposphere is between 0.07 Tg (0.9 × 1033 molecules) and 0.11 Tg (1.3 × 1033molecules) with an estimated uncertainty of 59%. Tropospheric ozone exhibited enormous variability due to the complicated mixing processes. Low ozone and large variability were observed in the mid‐troposphere after the stratospheric intrusion due to the westerly advection including the transition from a cyclonic system to an anticyclonic system. This study using high temporal and vertical‐resolution measurements suggests that, in this case, stratospheric air quickly lost its stratospheric characteristics once it is irreversibly mixed down into the troposphere.