ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-7-2037-2007 A meteorological overview of the MILAGRO field campaigns Fast J. D. 1 de Foy B. 2 8 Acevedo Rosas F. 3 Caetano E. 4 Carmichael G. 5 Emmons L. 6 McKenna D. 6 Mena M. 5 Skamarock W. 6 Tie X. 6 Coulter R. L. 7 Barnard J. C. 1 Wiedinmyer C. 6 Madronich S. 6 Pacific Northwest National Laboratory, Richland, Washington, USA Molina Center for Energy and the Environment, La Jolla, California, USA Comisión Nacional del Agua, USMN-GRGC, Boca del R\'{i}o, Veracruz, México Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, México City, México University of Iowa, Iowa City, IA, USA National Center for Atmospheric Research, Boulder, Colorado, USA Argonne National Laboratory, Argonne, Illinois, USA now at: Saint Louis University, Saint Louis, Missouri, USA 13 02 2007 7 1 2037 2089 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys-discuss.net/7/2037/2007/acpd-7-2037-2007.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/7/2037/2007/acpd-7-2037-2007.pdf

We describe the large-scale meteorological conditions that affected atmospheric chemistry over Mexico during March 2006 when several field campaigns were conducted in the region. In-situ and remote-sensing instrumentation was deployed to obtain measurements of wind, temperature, and humidity profiles in the boundary layer and free atmosphere at four primary sampling sites in central Mexico. Several models were run operationally during the field campaign to provide forecasts of the local, regional, and synoptic meteorology as well as the predicted location of the Mexico City pollutant plume for aircraft flight planning purposes. Field campaign measurements and large-scale analyses are used to define three regimes that characterize the overall meteorological conditions: the first regime prior to 14 March, the second regime between 14 and 23 March, and the third regime after 23 March. Mostly sunny and dry conditions with periods of cirrus and marine stratus along the coast occurred during the first regime. The beginning of the second regime was characterized by a sharp increase in humidity over the central plateau and the development of late afternoon convection associated with the passage of a weak cold surge on 14 March. Over the next several days, the atmosphere over the central plateau became drier so that deep convection gradually diminished. The third regime began with the passage of a strong cold surge that lead to humidity, afternoon convection, and precipitation over the central plateau that was higher than during the second regime. The frequency and intensity of fires, as determined by satellite measurements, also diminished significantly after the third cold surge. The synoptic-scale flow patterns that govern the transport of pollutants in the region are described and compared to previous March periods to put the transport into a climatological context. The complex terrain surrounding Mexico City produces local and regional circulations that govern short-range transport; however, the mean synoptic conditions modulate the thermally-driven circulations and on several days the near-surface flow is coupled to the ambient winds aloft.

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