The importance of topography in the formation of cold-air pooling in urban spaces. The example of the city of Coimbra (Portugal)

Abstract

The aim of this paper is to analyze the existence of cold-air pools on anticyclonic nights in a narrow urban valley where three hospital units and a road with intense traffic are located and where the slopes associated with proximity to a medium- size mountain introduce different factors from that observed in other valleys where this phenomenon forms on cold, calm nights. On these nights, it is observed that cold-air pools develop in the valley just after sunset, in opposition to the ridge that dominates the valley (thermal belts), and it is found that their formation, development, and disappearance after the first hours of the day is a daily phenomenon. Using fixed data loggers and portable data loggers on the surface and at altitude coupled to a UAV (drone), it was possible to calculate the variation of the air temperature (as well as to locate the inversion layer) in different sectors defined along the valley. The finding of temperature oscillation along the longitudinal profile and at altitude, reflects a double source of feeding: in-situ irradiation, and the air draining down the slopes of the massif.