Mexico City basin wind circulation during the MCMA-2003 field campaign B. de Foy1, E. Caetano2, V. Magaña2, A. Zitácuaro2, B. Cárdenas3, A. Retama4, R. Ramos4, L. T. Molina1, and M. J. Molina1 1Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, USA 2Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, Mexico 3General Direction of the National Center for Environmental Research and Training (CENICA), National Institute of Ecology (INE), Mexico 4Secretaría del Medio Ambiente, Gobierno del Distrito Federal, Mexico
Abstract. MCMA-2003 was a major field campaign investigating the atmospheric chemistry of
the Mexico City Metropolitan Area (MCMA) in April of 2003.
This paper describes the wind circulation patterns during the campaign
both within the Mexico City basin and on the regional scale.
''Time roses'' are introduced to concisely analyze the diurnal wind patterns.
Three episode types were identified that explain the conditions encountered:
''O3-South'', ''Cold Surge'' and ''O3-North''.
These can be diagnosed from a combination of synoptic and basin observations based on
whether the day was predominantly cloudy, or whether the O3 peak was in
the north or south of the basin.
O3-South days have weak synoptic forcing
due to an anti-cyclone over the eastern Pacific. Strong solar heating leads
to northerly flows in the basin and an evening shift due to a gap flow from
the south-east. Peak ozone concentrations are in the convergence zone
in the south of the city.
Cold Surge days are associated with ''El Norte'' events, with strong
surface northerlies bringing cold moist air and rain. Stable conditions lead
to high concentrations of primary pollutants and peak ozone in the city center.
O3-North days occur when the
sub-tropical jet is closer to Mexico City. With strong westerlies aloft, the
circulation pattern is the same as O3-South days except for a wind shift
in the mid-afternoon leading to ozone peaks in the north of the city.
This classification is proposed as a means of understanding pollutant transport
in the Mexico City basin and as a basis for future meteorological and chemical
analysis. Furthermore, model evaluation and design of policy recommendations
will need to take into account the three episode types.
Citation: de Foy, B., Caetano, E., Magaña, V., Zitácuaro, A., Cárdenas, B., Retama, A., Ramos, R., Molina, L. T., and Molina, M. J.: Mexico City basin wind circulation during the MCMA-2003 field campaign, Atmos. Chem. Phys., 5, 2267-2288, doi:10.5194/acp-5-2267-2005, 2005.