Distinct wind convergence patterns in the Mexico City basin due to the interaction of the gap winds with the synoptic flow

[摘要] Mexico City lies in a high altitude basin where air qualityand pollutant fate is strongly influenced by local winds.The combination of high terrain with weak synoptic forcingleads to weak and variable winds with complex circulation patterns.A gap wind entering the basin in the afternoon leads tovery different wind convergence lines over the city dependingon the meteorological conditions.Surface and upper-air meteorological observations are analysedduring the MCMA-2003 field campaign to establish the meteorologicalconditions and obtain an index of the strength and timing of thegap wind. A mesoscale meteorological model (MM5) is used in combinationwith high-resolution satellite data for the land surface parametersand soil moisture maps derived from diurnal ground temperature range.A simple method to map the lines of wind convergence both in the basinand on the regional scale is used to show the different convergencepatterns according to episode types.The gap wind is found to occur on most days of the campaignand is the result of a temperature gradient across the southernbasin rim which is very similar from day to day.Momentum mixing from winds aloft intothe surface layer is much more variable and can determineboth the strength of the flow and the pattern of the convergencezones. Northerly flows aloft lead to a weak jet with an east-westconvergence line that progresses northwards in the late afternoonand early evening. Westerlies aloft lead to both stronger gap flowsdue to channelling and winds over the southern and western basin rim.This results in a north-south convergence line through themiddle of the basin starting in the early afternoon.Improved understanding of basin meteorology will lead to betterair quality forecasts for the city and better understanding ofthe chemical regimes in the urban atmosphere.

[学科分类] 大气科学