3. Locally induced flows and fog formation in the Ebro river valley

Several mesoscale runs have been performed over the Ebro river basin, in the north-eastern part of the Iberian Peninsula (Figure 3.1), to further study climatologically weak-winds and clear-sky nights conditions close to the city of Lleida. Three cases are take: thermal within-basin flows (5-6 November 2005, Figure 3.2); weak westerly flows (3-4February 2005, Figure 3.3); fog case (10-12 December 2009, Figures 3.4, 3.5 and 3.6).

The fog is extensively analysed in Cuxart and Jiménez (2012) since the radiation fog episodes are very common in winter and they can last for several days. The impact on human activities is high, especially on road and air transportation. The installation in July 2009 of a WindRASS in the area, which is able to work in the presence of fog, allowing the inspection of the vertical structure of the temperature and wind profiles across the roughly 300-m-thick fog layer.



Figure 3.1: Simulated domain covering the Ebro river basin at 2km horizontal resolution



3.2a 3.2b

Figure 3.2: 10m streamlines at (left) 1500 UTC on 5th November 2005 and (right) 0300 UTC on 6th November 2005



3.3a 3.3b

Figure 3.3: 10m streamlines at (left) 1500 UTC on 3rd February 2005 and (right) 0300 UTC on 4th February 2005





Figure 3.4:

(a) Modelled relative humidity in a location close to Lleida (from 10th to 12th December 2009).

(b) and (c) Time series of the vertical profiles of the wind speed obtained from the model and the WindRass, respectively.

(d) and (e) The same as in (b) and (c) but for the virtual temperature.

3.4b 3.4c
(b) (c)
3.4d 3.4 e
(d) (e)






(a) (b)



(c) (d)

Figure 3.5: Evolution of three selected AWSs between 9 and 13 December: Raimat (at the WindRASS site), El Poal (at a low part of the basin) and Cervera (at the slopes, in the limit of the fog deck). a Wind speed, b temperature, c relative humidity, d solar radiation. Shaded areas indicate night-time.



3.6a 3.6b
(a) (b)

Figure 3.6: (a) modelled cloud-cover at 0900 UTC on 11 December 2009. The cross indicates the city of Lleida. (b) Land-surface temperature fields derived from MODIS (white areas indicate the extension of fog) at 1030 UTC on 11 December 2009.