Overview of the Meteorological Conditions on High Ozone Days during SHARP

A main focus of the 2009 SHARP (Study of Houston Atmospheric Radical Precursors) campaign is to investigate the conditions favorable for springtime ozone formation in the Houston-Galveston area (HGA). During the campaign, which lasted from April 15 – May 31, the HGA experienced four ozone exceedance days (May 4, May 20, May 29, and May 30). Using the Ngan and Byun (2008) weather cluster analysis method, each of the four days contained favorable meteorological conditions for high ozone. May 4 and 29 belong to cluster four, which is associated with a cold air mass in the northwestern part of the analysis domain in addition to cool and dry weather associated with northerly winds. A high pressure system centered over the northeastern United States responsible for easterly to northeasterly winds over the HGA characterizes cluster two and May 20. A weak synoptic pressure gradient and stagnant conditions define cluster three, which includes May 30. In general, postfrontal conditions, including clear skies and a weak northerly wind component, characterize the synoptic features found on all four high ozone days. The weak synoptic scale winds allow the penetration of the sea breeze front on May 4, May 29, and May 30 as observed in Doppler radar, reanalysis data, and station data. This local circulation pattern aided in the recirculation of air over the HGA on May 29-30, which is evident in the backward trajectories using the University of Houston Real-Time Trajectory Analysis System (RTAS). Radiosondes, ozonesondes, and the Vaisala Celiometer CL31 are used to identify different layers of the atmosphere including the mixing height. The CL31 is especially useful due to its ability to capture the strong backscatter gradient present in the residual layer, which is present the night before each of the four high ozone days during SHARP. This layer of the lower atmosphere acts to store pollution from the previous day above the nocturnal inversion and begins to mix downward after daybreak.