Climatology of the Iberia coastal low-level wind jet: weather research forecasting model high-resolution results

Abstract

A B S T R A C T Coastal low-level jets (CLLJ) are a low-tropospheric wind feature driven by the pressure gradient produced by a sharp contrast between high temperatures over land and lower temperatures over the sea. This contrast between the cold ocean and the warm land in the summer is intensified by the impact of the coastal parallel winds on the ocean generating upwelling currents, sharpening the temperature gradient close to the coast and giving rise to strong baroclinic structures at the coast. During summertime, the Iberian Peninsula is often under the effect of the Azores High and of a thermal low pressure system inland, leading to a seasonal wind, in the west coast, called the Nortada (northerly wind). This study presents a regional climatology of the CLLJ off the west coast of the Iberian Peninsula, based on a 9 km resolution downscaling dataset, produced using the Weather Research and Forecasting (WRF) mesoscale model, forced by 19 years of ERA-Interim reanalysis (1989Á2007). The simulation results show that the jet hourly frequency of occurrence in the summer is above 30% and decreases to about 10% during spring and autumn. The monthly frequencies of occurrence can reach higher values, around 40% in summer months, and reveal large inter-annual variability in all three seasons. In the summer, at a daily base, the CLLJ is present in almost 70% of the days. The CLLJ wind direction is mostly from north-northeasterly and occurs more persistently in three areas where the interaction of the jet flow with local capes and headlands is more pronounced. The coastal jets in this area occur at heights between 300 and 400 m, and its speed has a mean around 15 m/s, reaching maximum speeds of 25 m/s.

Mechanisms underlying temperature extremes in Iberia: a Lagrangian perspective

“…The heat low generates low-level wind jets along the coastal areas of the IP (Soares et al, 2014), which underlie the nearly circular shape of the trajectory densities. In contrast to winter, the association with teleconnection patterns, such as the NAO and EA, is less clear in summer, when the North Atlantic large-scale atmospheric circulation is generally much weaker ( Barnston and Livezey, 1987).…”

Structure and variability of the Oman coastal low-level jet

“…Upwelling alters the seasurface temperature (SST) pattern along the coast by lowering the ocean temperature, and consequently, intensifies the landÁsea temperature, and hence pressure gradient, strengthening the wind speed and CLLJ occurrences through positive feedback along the coast (Rijo et al, 2014;Soares et al, 2014). This decrease of SST along the coast also lowers the ocean surface evaporation, which combined with the strong coast-parallel winds, limits the onshore moisture advection and contributes to the aridity and dryness of some of the mid-latitude western coastal regions (Warner, 2004).…”

Adjustment of the summertime marine atmospheric boundary layer to the western Iberia coastal morphology

“…To further investigate the causes of these wind features, three events representative of each location and strength categories were selected. It was demonstrated that, for the three events, a LLCJ was present consistently with studies conducted by other authors for the region [ Ranjha et al ., ; Soares et al , ], establishing the background conditions in which MABL response can occur. In the three events studied, a distinct low atmosphere temperature inversion capping the MABL was found south of 42°N.…”