Barrier winds in the Italian region and effects of moist processes

Abstract

Barrier winds form when a sufficiently high mountain range causes a stratified airflow directed towards the orographic barrier to slow down at the low levels, producing a pressure imbalance, which in turn generates a mountain-parallel wind. The present study analyses barrier wind occurrence in the Italian region and surrounding seas from both a climatological and dynamical perspective. The main goals are to assess the applicability of the linear theory of upstream blocking to real-world cases and to investigate the role of atmospheric moist processes in promoting the formation and influencing the evolution of barrier winds. A 2-year climatology of barrier winds reveals that they occur more frequently than the opposite regime of flow over the orography, especially in the cold season. Barrier wind events are found to be associated with an upstream flow having a significantly higher inverse Froude number (or non-dimensional mountain height) values than flow-over events on average, consistently with the theory. An Alpine case study is simulated using numerical models and sensitivity tests are performed to investigate the dependence of barrier wind development and properties on the non-dimensional mountain height. Simulations reveal the strong influence of precipitation-related diabatic processes on the development of an upstream cold pool, which in turn supports barrier wind maintenance and strengthens its intensity. Idealized numerical experiments with varying upstream flow moisture shed light on processes leading to significant strengthening of barrier wind intensity, in particular on the crucial role of latent heat exchanges associated with precipitation.