Jordi Mazon

The role of sea-land air thermal difference, shape of the coastline and sea surface temperature in the nocturnal offshore convection

ABSTRACT Nocturnal precipitation cells and lines occur near the coastline in the whole Mediterranean basin in all seasons. The precipitation events are mainly located in areas where coastal mountain ranges and rivers enhance convergence though the interaction of nocturnal mesoscale and local flows (land breeze, katabatic and drainages winds) with prevailing synoptic wind or with other mesoscale and local flows. The methodology used here to study this phenomenon consists of three stages. First, the Tropical Rainfall Measuring Mission (TRMM) radar satellite database is used to detect nocturnal precipitation near the coastline, from 18 to 09 UTC. An event is included in the study if the 3 hours accumulated precipitation detected by TRMM is stationary near the coast, or has moved slightly onshore or offshore, and has lasted no more than six consecutive hours. Second, the NCEP reanalysis database is used to describe the synoptic conditions and to discard precipitation associated with synoptic events (large low pressure areas, dynamic polar fronts, or troughs, for example). In the final step by using the version 3 of the Weather Research Forecast model, we simulate and analyse some of the selected events to determine the role of the land–sea temperature differences, the curvature of the coastline and the sea surface temperature. The simulations confirm that the nocturnal precipitation studied in the Mediterranean basin near the coastline is formed from the interaction between relatively warm and wet sea-air with the cold air mass from drainage winds, as well as from the convergence of several drainage winds offshore. The mechanism is the same that is used to explain nocturnal precipitation in tropical areas.

Nocturnal offshore precipitation near the Mediterranean coast of the Iberian Peninsula

While nocturnal offshore precipitation, which produces rain cells and bands, has been studied in tropical areas, few studies have analyzed the occurrence of this phenomenon at higher latitudes. Using radar reflectivity, nocturnal rainfall in the western Mediterranean area has been detected near the coast of the Iberian Peninsula and North Africa. More than 50 events have been recorded since 2009. MM5 mesoscale simulations of some of the recorded events allow us to establish that the most likely causes for these precipitation events are: (1) the interaction between cold air masses conducted by drainage and katabatic winds, and (2) a wetter and warmer synoptic wind. Two different episodes are presented: one in the northeastern Iberian Peninsula, caused by stratiform clouds, which occurred at the mouths of three rivers; the other case, produced by convective clouds, occurred at the southern Iberian Peninsula and was caused by the drainage winds flowing down from some mountain ranges located close to the coast.

Are the Pyrenees a barrier for the transport of birch (Betula) pollen from Central Europe to the Iberian Peninsula

This work provides a first assessment of the possible barrier effect of the Pyrenees on the atmospheric transport of airborne pollen from Europe to the North of the Iberian Peninsula. Aerobiological data recorded in three Spanish stations located at the eastern, central and western base of the Pyrenees in the period 2004-2014 have been used to identify the possible long range transport episodes of Betula pollen. The atmospheric transport routes and the origin regions have been established by means of trajectory analysis and a source receptor model. Betula pollen outbreaks were associated with the meteorological scenario characterized by the presence of a high-pressure system overm over Morocco and Southern Iberian Peninsula. France and Central Europe have been identified as the probable source areas of Betula pollen that arrives to Northern Spain. However, the specific source areas are mainly determined by the particular prevailing atmospheric circulation of each location. Finally, the Weather Research and Forecasting model highlighted the effect of the orography on the atmospheric transport patterns, showing paths through the western and easternmost lowlands for Vitoria-Gasteiz and Bellaterra respectively, and the direct impact of air flows over Vielha through the Garona valley.

Cold Outbreaks at the Mesoscale in the Western Mediterranean Basin: From Raincells to Rainbands

This paper investigates cold outbreaks that form offshore density currents within the whole mesoscale over the Western Mediterranean basin. Reflectivity radar and satellite images are used to detect clouds and precipitation that are associated with these density currents in the meso-, meso-, and meso- over the Western Mediterranean basin (WMB). Version 3.3 of the WRF-ARW model is used to describe the formation and evolution of these density currents and to estimate their lifetime as well as horizontal and vertical scales. Based on the observations and simulations, this paper suggests that a new perspective could effectively be adopted regarding the WMB region delineated by the Balearic Islands, the northeastern Iberian Peninsula, and the Gulf of Lion, where inland cold outbreaks develop into density currents that move offshore and can produce precipitation ranging from raincells to rainbands at the whole mesoscale.