Tropospheric ozonesonde profiles at long-term U.S. monitoring sites: 1. A climatology based on self-organizing maps

Abstract

Sonde-based climatologies of tropospheric ozone (O₃) are vital for developing satellite retrieval algorithms and evaluating chemical transport model output. Typical O₃ climatologies average measurements by latitude or region, and season. A recent analysis using self-organizing maps (SOM) to cluster ozonesondes from two tropical sites found that clusters of O₃ mixing ratio profiles are an excellent way to capture O₃ variability and link meteorological influences to O₃ profiles. Clusters correspond to distinct meteorological conditions, e.g., convection, subsidence, cloud cover, and transported pollution. Here the SOM technique is extended to four long-term U.S. sites (Boulder, CO; Huntsville, AL; Trinidad Head, CA; and Wallops Island, VA) with 4530 total profiles. Sensitivity tests on k-means algorithm and SOM justify use of 3 × 3 SOM (nine clusters). At each site, SOM clusters together O₃ profiles with similar tropopause height, 500 hPa height/temperature, and amount of tropospheric and total column O₃ . Cluster means are compared to monthly O₃ climatologies. For all four sites, near-tropopause O₃ is double (over +100 parts per billion by volume; ppbv) the monthly climatological O₃ mixing ratio in three clusters that contain 13–16% of profiles, mostly in winter and spring. Large midtropospheric deviations from monthly means (−6 ppbv, +7–10 ppbv O₃ at 6 km) are found in two of the most populated clusters (combined 36–39% of profiles). These two clusters contain distinctly polluted (summer) and clean O₃ (fall-winter, high tropopause) profiles, respectively. As for tropical profiles previously analyzed with SOM, O₃ averages are often poor representations of U.S. O₃ profile statistics.