Effects of Urbanization on the Breakup of Temperature Inversion in an Idealized Mountain Valley

Urbanization has become one of the main drivers of global change. Currently, more than 50 % of the global population live in cities. In general, urbanization affects the interactions between land surface and the atmosphere at multiple spatial and temporal scales, by altering the surface energy and water balances. Among other factors, urbanization modifies the energy fluxes from the surface, the wind and temperature fields, and the emission and storage of heat in the surface, as well as in the atmospheric boundary layer. One important atmospheric phenomenon occuring in the low atmosphere, and particularly in mountain valleys and/or urban environments, is temperature inversion. The dynamics (formation and breakup) of temperature inversion can exert strong effects on the environmental conditions of an urbanized mountain valley, as a result of its influence on processes such as the vertical transport of pollutants. Previous works have studied temperature inversion in both mountain valleys and cities. However, the effects of urbanization on the breakup of temperature inversion have escaped wide attention. Here we study these effects by using a numerical model (Eulag) to simulate the breakup of temperature inversion in an idealized moutain valley for different percentages of urban and non-urban land cover. Besides, we simulate the dynamics of a passive tracer released from the urban surface, to have insights on the air quality implications of temperature inversion under the studied conditions. We consider the three patterns of inversion breakup described by C.D. Whiteman depending on the relative magnitude of two driving processes: the rise of the bottom of the inversion layer resulting from the growth of the convective boundary layer, and the descent of the top of the inversion layer caused by the removal of air from the bottom of the valley by the slope winds. We found that the level of urbanization can change the inversion breakup pattern as well as its time period, resulting from the increased roughness and sensible heat flux in the urban land, whereby the dynamics of the slope winds and the convective boundary layer become altered. Urbanization introduces some contrasting effects with respect to its implications on temperature inversion breakup and air quality. The net effect of urbanization depends on the nonlinear aggregation of three main effects: (i) higher (lower) sensible heat flux from urban (non-urban) land tend to enhance (reduce) slope winds owing to the increase (decrease) of the temperature and pressure gradients between the city and its surrounding areas, thus augmenting (diminishing) the removal of mass from the bottom of the valley. (ii) Higher (lower) roughness of urban (non-urban) land tend to reduce (enhance) slope winds because of the increased (decreased) friction in the surface, thus diminishing (augmenting) the removal of mass from the bottom of the valley. (iii) Higher (lower) roughness of urban (non-urban) land tend to enhance (reduce) the growth of the convective boundary layer, as a result of the enhancement (reduction) of the turbulence in the surface and, consequently, the vertical exchange of mass, energy, and momentum, between the surface and the atmosphere. Our results indicate that the net effect of urbanization is an enhancement of the temperature inversion breakup: the more urban land, the faster the inversion breakup. Besides, the higher the level of urbanization, the higher the maximum instantaneous value of pollutant concentration inside the valley, which always occurs during the morning before the inversion breakup. However, the more urban land does not necessarily imply higher concentration of pollutants throughout the day owing to the faster inversion breakup. This points out a fundamental difference between urbanized and non-urbanized valleys, regarding the environmental implications of temperature inversion.