Estimating Boundary Layer Depths in Mexico City

Planetary boundary layer (PBL) processes control water, energy and pollutant fluxes from the surface into the atmosphere. Nevertheless, with the exception of a few short intense field campaigns, there are no continuous direct observations of PBL heights and fluxes in Mexico City which is located at a high-altitude valley (2200 m ASL) surrounded by mountains. These singular geographical conditions, impact significantly daily energy exchanges, PBL evolution and air quality in the Mexico City Basin. In this study, a microwave thermodynamic profiler (radiometer) a wind profiler radar and an instrumented tethered balloon are used in combination to characterize the diurnal evolution of the PBL during the dry winter season when surface-based temperature inversions and events of downgraded air quality are most prominent. The instruments are located within the urban area to the northwest of the city. Additionally, meteorological surface station data and an eddy covariance system are used to obtain surface energy and water vapor fluxes at the same experimental site. Comparisons between in-situ measurements of wind, temperature and relative humidity from the tethered balloon and those obtained from the radiometer and wind radar profiler reveal some important differences as far as the ability to detect temperature inversions near the surface from 200 to 500 m AGL in the early hours. Above 500 m, both sets of measurements show good agreement during the daily evolution of the PBL. Boundary layer heights computed from equivalent potential are compared to several Richardson number-based models. Comparison between models and measurements reveals differences that can be partly attributed to the lack of good estimates of turbulent energy near the surface.