Characterization of the atmospheric boundary layer in a narrow tropical valley using remote‐sensing and radiosonde observations and the WRF model: the Aburrá Valley case‐study

Abstract

The spatiotemporal evolution of the atmospheric boundary layer (ABL), in a narrow, highly complex terrain located in the Colombian Andes, is studied using radiosondes and remote-sensing equipment. Different techniques are implemented to automatically estimate the ABL height using ceilometer backscattering profiles and a combination of a radar wind profiler and microwave radiometer retrievals. The large aerosol load from anthropogenic emissions within the valley allows the use of ceilometer-based ABL height detection methods, especially under stable atmospheric conditions. However, convective atmospheres favour aerosol dispersion, increasing the uncertainty associated with the estimation of the convective boundary layer using ceilometers. In contrast, the multisensor technique is more robust, performing better in stable and unstable conditions. All ceilometer-based methods and the multisensor scheme capture the observed ABL height diurnal cycle. The main difference among ABL height retrievals occurs in the afternoon and during the night when Richardson number estimates tend to detect the top-down contraction of the residual layer, while ceilometer-based estimates detect the sudden bottom-up onset of the nocturnal stable layer. The results also show that intra-annual and annual variations of cloudiness strongly condition the ABL expansion, leading to a modulation of the ABL height diurnal cycle. The amount of aerosol particles near the surface is influenced by the evolution of the ABL, modifying the available control volume for the pollutants to interact and disperse. The evolution of ABL over the slopes and at the valley floor differs as a result of the local thickening associated with upslope winds. Weather Research and Forecasting model simulations, from a climatological point of view, skilfully simulate the observed ABL height for both the diurnal and annual cycles; the model skill is higher over the valley floor than over the slopes. K E Y W O R D S atmospheric boundary layer; complex terrain; tropical narrow valley; remote sensing; ABL automatic retrieval methods; ABL model simulations; ABL diurnal cycle; ABL annual cycleThis is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

Effects of fireworks on particulate matter concentration in a narrow valley: the case of the Medellín metropolitan area

“…As previously mentioned, BI profiles generally depict the marked diurnal cycle of the ABL height. Around midnight, the ABL height at the base of the Aburrá Valley is typically close to 500 m, limiting the available control volume for the pollutants to disperse (Herrera-Mejía and Hoyos 2019). By the time the CBL starts to develop due to the onset of atmospheric instability, around 10:00 LT, the concentration of aerosols near the surface and within the valley decreases.…”

“…As we argue in the previous subsections, vertical dispersion and rainfall-triggered aerosol removal are especially important in narrow valleys such as the Aburrá Valley, where the complex topography limits the horizontal advection of pollution due to the magnitude of surface winds, which is usually very weak. Atmospheric stability conditions, the evolution of the atmospheric boundary layer, and, in particular, the development of a deep convective layer are considered determining factors in pollutant concentrations in the Aburrá Valley (Herrera-Mejía and Hoyos 2019). A stable atmosphere inhibits atmospheric vertical exchanges and favors pollutant accumulation, while unstable environments promote pollutant dispersion and mixing (Whiteman 2000).…”

“…1b), are used to assess changes in the vertical BI profile as a result of fireworks use. Ceilometer BI profiles are also useful to detect the height of the ABL, based on the assumption that a significant aerosol concentration reduction occurs at the top of the latter (Hayden et al 1997; Steyn et al 1999; Muenkel et al 2004; Granados-Munoz et al 2012; Emeis et al 2012; Stachlewska et al 2012; Herrera-Mejía and Hoyos 2019). During fair-weather days, BI profiles often provide a good depiction of the ABL evolution within the valley.…”

“…The generalized use of fireworks has made La Alborada even more unpopular given that the Aburrá Valley, home to approximately four million people, has endured the onset of critical air-quality episodes during the last 5 years as a result of significant anthropogenic emissions and adverse meteorological conditions for vertical dispersion of pollutants. The latter is associated with thin ABLs during certain seasons, as a result of regional scale forcing at the annual and interannual time scales (Herrera-Mejía and Hoyos 2019). In addition to La Alborada fireworks, there is also widespread fireworks use during New Year’s Eve (NYE) celebrations, with significantly less rejection by the community.…”

Lidar Observations in South America. Part II - Troposphere

“…Nevertheless, in most regions, the radiosondes are launched only twice a day, which does not provide a detailed observation of the ABLH behavior. In this scenario, due to the lidar systems' high temporal and spatial resolution, the utilization of this kind of equipment to estimate the ABLH and other ABL properties had increased significantly in the last decade, mainly in South America [20][21][22][23][24][25][26][27][28][29][30].…”