Nocturnal low-level jets over complex terrain: driving mechanisms and analytical modeling

Several studies in the Atmospheric Physics have focused on the analysis of the atmospheric circulation phenomena at the small scale and turbulent processes within the planetary boundary layer (PBL), whose formation and evolution is complicated by the topography. The Low-Level Jet (LLJ) is one of the most studied phenomena and it is not yet completely understood. It consists in the occurrence of strong winds within a thin layer of the PBL. A particular case is the formation of double-nose LLJs (or multiple LLJs), i.e. the simultaneous occurrence of two (or multiple) noses within the PBL. Analysing data retrieved in complex terrain during the MATERHORN field campaigns, the objectives of this thesis are to understand the mechanisms which drive the formation of the double-nose LLJs identified using a newly-proposed criterion that upgrade the already existing ones. The new identification criterion is used to analyse quiescent Intensive Observing Periods (IOPs), in which nocturnal stable boundary-layer conditions and local thermal circulation are not altered or driven by synoptic forcing. During these IOPs, the formation of a LLJ due to inertial oscillations is typical few hours after the sunset. However, the LLJ structure can be temporary perturbed generating a double-nose LLJ. Two double-nose LLJ driving mechanisms are identified. The wind-driven mechanism is when the two noses are associated with different air masses flowing one on top of the other. The wave-driven mechanism is when a surface perturbation generates an inertial-gravity wave which transports momentum during its propagation. Momentum is lost by the LLJ nose and transported upward where a second one is generated. Furthermore, the evolution of the LLJs driven by the inertial oscillations is investigated by means of two models retrieved from the literature. The models provides similar representation of the observed LLJs, concluding that the LLJs are well simulated when they are fully developed.