Nature and Characteristics of Weak and Intense Katabatic Flows

We perform a detailed analysis of the katabatic flows taking place during Summer 2017 in the surroundings of the Guadarrama mountain range (Spain). The interest of this area lies in the proximity to the large city of Madrid (~50 km) and the significant aridity over summer. Data from the 10-m high meteorological tower of La Herrería site are employed in this study: temperature and wind-speed measurements at 3 vertical levels (3, 6 and 10 m), and turbulent fluxes and CO₂ and water-vapour concentrations at 2 vertical levels (4 and 8 m). Firstly, a systematic and objective algorithm is applied to filter out the katabatic events not affected by strong synoptic winds, cold fronts or convective showers. We find that the katabatic occurrences show contrasting characteristics depending on the maximum wind speed (MWS) after the onset. We distinguish two outermost cases: weak katabatics (MWS < 1 m s^-1) and intense katabatics (MWS > 3.5 m s^-1). Weak katabatics give rise to a well formed surface-based thermal inversion, sporadic decoupling between the vertical levels and a relative strong accumulation of CO₂ close to the surface. In some cases, a shallow low-level jet is observed at the 3-m level. In contrast, intense katabatics break the surface-based thermal inversion down, and produce considerable mixing associated with a great increment of turbulence (u*~1 m s^-1). In fact, the intense katabatics share a common characteristic: the maximum of turbulence after their onset is greater than the one measured during central hours of the day, which is usually the maximum of the daily cycle.The main cause of having the two distinct types of katabatics seems to be the direction of the synoptic flow relative to the axis of the mountain range. We quantify the consequences of the weak and intense katabatic flows by analyzing the CO2 and water-vapor budget, and by calculating the shear capacity to characterize the nocturnal stable boundary layer.