Seasonal variability in mixed layer height and its impact on trace gas distribution over a tropical urban site: Ahmedabad

Altitude profiles of virtual potential temperature (θv) and specific humidity (q) derived from meteorological data obtained from balloon-borne radiosonde ascents are used to investigate the seasonal variations in mixed layer height over Ahmedabad (23.03°N, 72.54°E), an urban site located on the western part of India. A total of 82 balloon ascents were conducted fortnightly in the morning hours during a period of about four years spanning from April 2003 to July 2007. Analysis of the vertical profiles of θv and q reveals a systematic seasonal variability in the mixed layer height (MLH), showing the decreasing trend from summer–monsoon to winter season. The MLH is observed to be maximum (∼ 1170 m) in summer–monsoon while a minimum (∼ 160 m) is observed during the winter months. In general, the mixed layer height was found to be highly variant during pre-monsoon and summer–monsoon seasons. This variability is observed to be comparatively lower in the post-monsoon and winter months. Effects of MLH have been investigated on the variations in surface ozone and MOPITT derived surface and vertical distributions of CO. The reverse trend is observed in surface ozone and CO with mixed layer seasonal variability. The impact of MLH over CO vertical distributions is observed up to an altitude of 3–4 km.