Revealed frontogenesis of Galernas in the Bay of Biscay: frontal identification system (FIS) wind thresholds tests
Creators: name: Gangoiti, Gotzon
name: Rodríguez-García, Ana
name: Sáez-de-Cámara, Estíbaliz
name: Torre-Pascual, Eduardo
name: Gómez, María Carmen
name: De Blas, Maite
name: García, José Antonio
name: García-Ruiz, Estíbaliz
name: Zuazo, Iñaki
name: Valdenebro, Verónica
name: Iza, Jon
Publisher: name: Zenodo
Year: 2022
Subjects: Galernas
Bay of Biscay
Frontogenesis
Mesofront

description: Mapping of ERA-5 hourly surface wind shifts in the Bay of Biscay during 3 galerna events (04jun2015, 31aug2009 and 04mar2007). The Front Identification Scheme (FIS), developed to track cold fronts over the sea (https://doi.org/10.5281/zenodo.6832274) is tested using different values for the threshold increase of the meridional and zonal wind component to detect wind shifts (cold fronts) from the southwest to the northwest quadrant and from the east to the west sector respectively. DDMMYYYY_1 stands for a 1 ms-1 wind velocity threshold, DDMMYYYY_2 for 2 ms-1 and DDMMYYYY_4 for 4 ms-1 thresholds.


(g1) stands for Grid 1, which includes a large part of Europe and North Africa, while


(g2) stands for Grid 2, an area centered on the Bay of Biscay, southern France, and northern Iberia.


Surface pressure charts with fronts on a selection of days are included for comparison. They can also be download from https://www2.wetter3.de/


For the lower velocity thresholds (1-2 ms-1), most of the oceanic fronts keep their integrity, while the performance of the largest threshold (4 ms-1) shows an excessive filtration of the fronts. Some of them look segmented and others simply disappear. For instance, on 4 March 2007, at 00 UTC the westernmost Atlantic front is more fragmented when using the 4 ms-1 threshold value, while for the lowest velocity thresholds it shows a more continuous wind shift region, extending north-to-south at longitude 20 ⁰ W. At the same time, in the Mediterranean Sea using the lower thresholds (1-2 ms-1), the FIS represents a Tramontana front (north of the Balearic Islands), which can be tracked for the following 4 hours. This front is not represented in the synoptic surface charts and it can be removed by using the largest (4 ms-1) threshold. Consequently, the FIS applied to the ERA5 wind fields with low velocity thresholds (1-2 ms-1), seems to be more appropriate to detect mesoscale fronts, also keeping the integrity of the large synoptic fronts. This is also the case of the non-frontal galernas on 31 August 2009 and 4 June 2015, where the lower thresholds show a better representation of the galerna mesofronts inside the Bay of Biscay region than the largest one. These types of fronts are absent on the synoptics scale surface pressure charts. Small wind shift regions without a coherent frontal structure, which disappear in consecutive FIS hourly wind field maps, are most frequently located on land and in regions of weak winds at sea, as shown in the temporal sequence of the fronts in the three case studies presented here. The largest threshold (4 ms-1) can filter out these non-coherent regions better that using the smaller threshold values, but at the expense of removing real fronts.