2. Organization of the flow under strongly stratified conditions in the Duero river basin
The Duero river basin is studied through mesoscale modelling, satellite radiative surface temperatures, data from a meteorological network and a 100m tower. To inspect the heterogeneous surface cooling in the basin a strongly stratified night (14-15 January 2003) is taken. During that night the skies were clear and the synoptic wind was weak. The setup is similar to the one taken previously to study the organization of the flow in Mallorca (Cuxart et al., 2007). The same setup was taken with MesoNH and MM5 to check the performance of both models under strongly-stratified conditions (Bravo et al., 2008). The chosen domain is shown in Figure 2.1 and the modelled local nocturnal winds at 0000 UTC are shown in Figure 2.2.
Although the basin cools almost as a unit, there exists a large diversity of local regimes. Katabatic winds are generated at the slopes (Figure 2.3, further analysed in Martínez and Cuxart, 2009) and the coldest air is concentrated in the lowest parts of the basin, specially in the closed valleys (Figure 2.4). To quantify the variety of local regimes, the 3D fields of the temperature at wind speed at 0000 UTC are classified as it is seen in Table 2.1 and in Figure 2.5. The moderate jets dominate the airflow over the foothills and the weak jets dominate over gentle slopes, with no-jet weak winds occurring along the river valleys and over the plateaux. For the cases with jets, the stratification is weakly stable and the turbulence is the dominant cooling mechanism.
Figure 2.1: Studied domain (2km x 2km resolution in the horizontal).
Figure 2.2: Topography (in m) of the centre of the Duero basin and simulated streamlines at 50-m height at 0200 UTC. The locations of four automatic weather stations from AEMET are displayed in dots. The 100-m tower in the CIBA site is located in the Torozos Plateau, an elevated area in the centre of the basin.
Figure 2.3: (a) The blue line indicates the vertical cross-section of the following plots.
Vertical cross-sections of the (b) wind speed (in m/s) and (c) temperature (in K) at 0200 UTC.
Figure 2.4: a Mean nocturnal temperature T b(x) for the Duero basin derived from Meteosat-7 (without the snow area). b Total cooling from the beginning of the night (1730 UTC) to just before sunrise (0730 UTC). Only points with a total cooling equal or greater than 4 K are represented. In both plots, surface elevation contours are plotted every 100 m within the range 500–2,100 m a.s.l. The locations of the automatic weather stations from AEMET are displayed with dots.
Figure 2.5: Location of the points corresponding to the categories in Table 2.1. a Classification according to the wind maxima up to 100 m a.g.l.: (red) weak jet, (green) moderate jet, (blue) strong jet and (yellow) no jet (when the wind speed up to 100 m is smaller than 2m s−1). The topography is plotted in a grey scale for the range of 600–1,000 m a.s.l. b The same as in (a), but for the classification according to the strength of stratification in the surface layer: (red) weakly stratified, (green) moderately stratified and (blue) strongly stratified. In both plots, data classified as no jet-moderate in Table 2.1 are excluded.