Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 5 6 2005 10.5194/acpd-5-11055-2005 http://www.atmos-chem-phys-discuss.net/5/11055/2005/ http://www.atmos-chem-phys-discuss.net/5/11055/2005/acpd-5-11055-2005.html http://www.atmos-chem-phys-discuss.net/5/11055/2005/acpd-5-11055-2005.pdf 11055 11090 2005-11-01 Distinct wind convergence patterns due to thermal and momentum forcing of the low level jet into the Mexico City basin B. de Foy bdefoy@ucsd.edu A. Clappier L. T. Molina M. J. Molina Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, USA Air and Soil Pollution Laboratory, Swiss Federal Institute of Technology, Lausanne (EPFL), Switzerland Mexico City lies in a high altitude basin where air quality and pollutant fate is strongly influenced by local winds. The combination of high terrain with weak synoptic forcing leads to weak and variable winds with complex circulation patterns. A low level jet entering the basin in the afternoon leads to very different wind convergence lines over the city depending on the meteorological conditions. Surface and upper-air meteorological observations are analysed during the MCMA-2003 field campaign to establish the meteorological conditions and obtain an index of the strength and timing of the jet. A mesoscale meteorological model (MM5) is used in combination with high-resolution satellite data for the land surface parameters and soil moisture maps derived from diurnal ground temperature range. A simple method to map the lines of wind convergence both in the basin and on the regional scale is used to show the different convergence patterns according to episode types. The low level jet is found to occur on most days of the campaign and is primarily due to thermal forcing which is very similar from day to day. Momentum mixing from winds aloft into the surface layer is much more variable and can determine both the strength of the jet and the pattern of the convergence zones. Northerly flows aloft lead to a weak jet with an east-west convergence line that progresses northwards in the late afternoon and early evening. Westerlies aloft lead to stronger jets and a north-south convergence line through the middle of the basin starting in the early afternoon. Improved understanding of basin meteorology will lead to better air quality forecasts for the city and better understanding of the chemical regimes in the urban atmosphere.