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

Abstract

The Tramontane-Cierzo wind system is a recurrent feature of the north-western Mediterranean basin in front of Catalan coast (NE Spain). Associated with this feature, northeast wind surges affect occasionally the coast and become a weather hazard for low-level aircraft operations, affecting for example the Barcelona international airport. This paper 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 evening. We classified CTDs related to 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 north 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 NWP 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 downdrafts 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 on CTDs by enhancing the potential temperature gradient between the density current and the environment modulating its intensity and speed.