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

Sonde-based climatologies of tropospheric ozone (O) are vital for developing satellite retrieval algorithms and evaluating chemical transport model output. Typical O climatologies average measurements by latitude or region, and season. Recent analysis using self-organizing maps (SOM) to cluster ozonesondes from two tropical sites found clusters of O mixing ratio profiles are an excellent way to capture O variability and link meteorological influences to O 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; 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 O profiles with similar tropopause height, 500 hPa height/temperature, and amount of tropospheric and total column O. Cluster means are compared to monthly O climatologies. For all four sites, near-tropopause O is double (over +100 parts per billion by volume; ppbv) the monthly climatological O mixing ratio in three clusters that contain 13 - 16% of profiles, mostly in winter and spring. Large mid-tropospheric deviations from monthly means (-6 ppbv, +7 - 10 ppbv O 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 O (fall-winter, high tropopause) profiles, respectively. As for tropical profiles previously analyzed with SOM, O averages are often poor representations of U.S. O profile statistics.