Coastally trapped disturbances caused by the tramontane wind on the northwestern Mediterranean: numerical study and sensitivity to short-wave radiation

摘要

The Tramontane-Cierzo wind system is a recurrent feature of the northwestern Mediterranean basin in front of the Catalan coast (northeast Spain). Associated with this feature, northeast wind surges occasionally affect the coast and become a weather hazard for low-level aircraft operations, affecting for example the Barcelona international airport. This article first reports these surges characterizing them as Coastal-Trapped Disturbances (CTDs). Climatological features are described, showing that CTDs occur frequently during the warm season and between the afternoon and the evening. We classified CTDs into two synoptic patterns related to the location of a mid-level tropospheric geopotential trough and the Iberian Peninsula: pattern A, with the trough crossing eastwards along the north of Spain; and pattern B, with the trough over the Mediterranean, after crossing the Iberian Peninsula. To study the CTDs in detail, numerical simulations were conducted using the non-hydrostatic and convection-permitting numerical weather prediction model HARMONIE-AROME. Two cases, one for each synoptic pattern, were studied showing that CTDs generate in the discontinuity between cool outflows and warmer air progressing southward as a density current, trapped by the mountain ranges parallel to the coastline. Cool outflows may have two different sources: in Pattern A the origin of the cold air is the tramontane itself, while in Pattern B convective outflows associated with storm downdraughts play this role. Both cases show similarities with CTDs studied on the California coast, showing an antitriptic and ageostrophic flow behind the CTD. An additional numerical sensitivity experiment was conducted by varying the short-wave radiation to explore the effects of diabatic warming on CTDs. It is demonstrated that a large warming influences CTDs by enhancing the potential temperature gradient between the density current and the environment, modulating their intensity and speed.