Long-term evolution of cold air pools over the Madrid basin

Cold air pools (CAPs) are one of the most severe weather conditions experienced across many basins worldwide, related to episodes of extreme cold temperatures, poor air quality, and disruption of transportation networks. This study offers a basic climatology of CAPs in the southern Spanish Plateau and investigates its evolution since 1961 and their links with local, synoptic, and large-scale climate variability. It is based on the comparison of meteorological records from two stations, one in the Sistema Central Range (Navacerrada, 1,894 m asl) and another at the plain (Madrid-Barajas, 609 m asl). Accuracy and representativeness of both locations to depict the spatial and temporal variability of CAPs was also tested. CAPs days (defined as the simultaneous occurrence of a daily minimum temperature difference above 0.1 degrees C between both stations) were found to occur year-round, but the most frequent and intense occur in winter (NDJ). Some typical features of CAPs, such as local mesoscale processes (katabatic and anabatic flows) in connection with synoptic (advection of mid-troposphere warm air masses during high-pressure regimes) and hemispheric (a positive phase of the North Atlantic Oscillation) variability were also observed, leading to a sheltered boundary layer at the bottom of southern Spanish Plateau, decoupled from the free troposphere. By night, CAPs have maintained both their frequency and intensity, which means that the frequency of extremely cold nights on the plain has remained relatively stable (despite global warming). By day, an enhanced warming of the high-elevation site has increased the temperature difference between the mountains and the plain during CAP days.