Distinct wind convergence patterns in the Mexico City basin due to the interaction of the gap winds with the synoptic flow B. de Foy1,*, A. Clappier2, L. T. Molina1,*, and M. J. Molina1 1Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, USA 2Air and Soil Pollution Laboratory, Swiss Federal Institute of Technology, Lausanne (EPFL), Switzerland *now at: Molina Center for Energy and the Environment (MCE2), La Jolla, CA 92037, USA
Abstract. 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 gap wind 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
gap wind. 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 gap wind is found to occur on most days of the campaign
and is the result of a temperature gradient across the southern
basin rim 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 flow 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 both stronger gap flows
due to channelling and winds over the southern and western basin rim.
This results in 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.
Citation: de Foy, B., Clappier, A., Molina, L. T., and Molina, M. J.: Distinct wind convergence patterns in the Mexico City basin due to the interaction of the gap winds with the synoptic flow, Atmos. Chem. Phys., 6, 1249-1265, doi:10.5194/acp-6-1249-2006, 2006.