Mexico City basin wind circulation during the MCMA-2003 field campaign

[摘要] MCMA-2003 was a major field campaign investigating the atmospheric chemistry ofthe Mexico City Metropolitan Area (MCMA) in April of 2003.This paper describes the wind circulation patterns during the campaignboth 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 onwhether the day was predominantly cloudy, or whether the O₃ peak was inthe north or south of the basin.O3-South days have weak synoptic forcingdue to an anti-cyclone over the eastern Pacific. Strong solar heating leadsto northerly flows in the basin and an evening shift due to a gap flow fromthe south-east. Peak ozone concentrations are in the convergence zonein the south of the city.Cold Surge days are associated with ''El Norte'' events, with strongsurface northerlies bringing cold moist air and rain. Stable conditions leadto high concentrations of primary pollutants and peak ozone in the city center.O3-North days occur when thesub-tropical jet is closer to Mexico City. With strong westerlies aloft, thecirculation pattern is the same as O3-South days except for a wind shiftin the mid-afternoon leading to ozone peaks in the north of the city.This classification is proposed as a means of understanding pollutant transportin the Mexico City basin and as a basis for future meteorological and chemicalanalysis. Furthermore, model evaluation and design of policy recommendationswill need to take into account the three episode types.

[学科分类] 大气科学