Driving Mechanisms of Double-Nosed Low-Level Jets during MATERHORN Experiment

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  • 1 Physics and Astronomy Department, University of Bologna, Bologna, Italy
  • | 2 Physics and Astronomy Department, University of Bologna, Bologna, Italy, and Department of Civil, Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, Indiana
  • | 3 Department of Civil, Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, Indiana
  • | 4 Physics and Astronomy Department, University of Bologna, Bologna, Italy
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Abstract

In the realm of boundary-layer flows in complex terrain, low-level jets (LLJs) have received considerable attention, although little literature is available for double-nosed LLJs that remain not well understood. To this end, we use the MATERHORN dataset to demonstrate that double-nosed LLJs developing within the planetary boundary layer (PBL) are common during stable nocturnal conditions and present two possible mechanisms responsible for their formation. It is observed that the onset of a double-nosed LLJ is associated with a temporary shape modification of an already-established LLJ. The characteristics of these double-nosed LLJs are described using a refined version of identification criteria proposed in the literature, and their formation is classified in terms of two driving mechanisms. The wind-driven mechanism encompasses cases where the two noses are associated with different air masses flowing one on top of the other. The wave-driven mechanism involves the vertical momentum transport by an inertial-gravity wave to generate the second nose. The wave-driven mechanism is corroborated by the analysis of nocturnal double-nosed LLJs, where inertial-gravity waves are generated close to the ground by a sudden flow perturbation.

This article is included in the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Special Collection.

Corresponding author: Francesco Barbano, francesco.barbano3@unibo.it

Abstract

In the realm of boundary-layer flows in complex terrain, low-level jets (LLJs) have received considerable attention, although little literature is available for double-nosed LLJs that remain not well understood. To this end, we use the MATERHORN dataset to demonstrate that double-nosed LLJs developing within the planetary boundary layer (PBL) are common during stable nocturnal conditions and present two possible mechanisms responsible for their formation. It is observed that the onset of a double-nosed LLJ is associated with a temporary shape modification of an already-established LLJ. The characteristics of these double-nosed LLJs are described using a refined version of identification criteria proposed in the literature, and their formation is classified in terms of two driving mechanisms. The wind-driven mechanism encompasses cases where the two noses are associated with different air masses flowing one on top of the other. The wave-driven mechanism involves the vertical momentum transport by an inertial-gravity wave to generate the second nose. The wave-driven mechanism is corroborated by the analysis of nocturnal double-nosed LLJs, where inertial-gravity waves are generated close to the ground by a sudden flow perturbation.

This article is included in the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Special Collection.

Corresponding author: Francesco Barbano, francesco.barbano3@unibo.it
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