engleski

Thermodynamical and airflow conditions within the lower troposphere during the tropopause fold event leading to the elevated surface ozone concentrations

We investigated conditions in the lower troposphere during the episode of a winter stratospheric ozone intrusion over Zagreb, Croatia on February 6, 1990. This particular event was accompanied with elevated ozone values at two measuring sites in the broader Zagreb area. Sudden spiky increase of ozone concentration from 2 ppbv to 100 ppbv was at first recorded at 11.45 LST at the site RBI, which is situated in the northern residential part of the city of Zagreb at 180 m asl. About one hour latter, a rise from 40 ppbv to out of instrument scale (i.e. higher than 100 ppbv) was recorded at the mountain site P (980 m asl), which is situated about 10 km northward from RBI. We employed an Eulerian nonhydrostatic prognostic mesoscale model (MEMO) in order to reproduce a fine-scale thermodynamic and airflow features within the first 6 km of the troposphere during the particular tropopause fold event. According to the model results, a potential vorticity (PV) anomaly, with the values up to more than 1.6 10-6 K m Pa-1 s-3 was found in the lower troposphere in times from 1.5 to 1 day prior to measured ozone peaks. It consisted of the wave-like train of the vertically stretched vortices. In time, PV anomaly moved southward. Simultaneously, it gradually vanished due to the irreversible mixing with the adjacent tropospheric air. Considering model results together with the ozone measurements we conclude that the air of stratospheric origin, which was found in the troposphere, retained its chemical signature about one day longer than the thermodynamic features.

mesoscale; ozone; tropopause fold; stratospheric intruzion

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