Measurements of a gust front passage and its effect on ozone levels (Formerly Paper P5A.1)

A gust front produced by a thunderstorm to the northwest of Nashville, TN passed through the Cornelia Fort instrumentation site on the evening of 22 June 1999, during the Southern Oxidant Study (SOS) field campaign. As a National Oceanic and Atmospheric Administration/Environmental Technology Laboratory (NOAA/ETL) high-resolution Doppler lidar (the mini-MOPA) scanned to the southeast, into the ambient southeasterly flow, the nose of the gust front became apparent as it moved into the lidar s scanning path. The lidar continued its pattern of scanning to the southeast, from the surface to 30° above the horizon, while the gust front continued to the southeast. These scans were occasionally interrupted to perform a nearly horizontal scan around the compass to map out the horizontal variability of the wind. Over 1.5 h, high-resolution lidar measurements documented the changes in the wind field from the surface to 500 m above ground level (AGL). Measurements showed winds with a wavy structure behind the gust front evolving into a shallow, elevated nocturnal jet.

A preliminary look at the surface chemistry (instruments deployed by the NOAA/Aeronomy Laboratory) showed sudden changes in concentrations of various chemical species (O3, NOx, and CO) coinciding with the passage of the gust front. The surface ozone levels decreased gradually before the front passage, and quickly dropped at least 5 ppb when the front passed. Interestingly enough, ozone levels quickly recovered to the pre-frontal values within a few minutes of the front passage.

Possible mechanisms for the quick ozone disappearance and recovery, such as vertical mixing, will be explored. This strategy includes analyzing data from the NOAA/ETL ground-based ozone lidar, collocated with the Doppler lidar, to document changes in the ozone layers present before the gust front passage. Sodar measurements also show the changing vertical structure of the gust front, and give insight into the vertical mixing between pre- and post-frontal air masses.