5.- FILTERED NOCTURNAL EVOLUTION

1. 1.- OBJECTIVE Climatology of the stably stratified nights in the Ebro Valley F. Molinos *, D. Martínez and J. Cuxart Universitat de les Illes Balears, Spain *e-mail address: felipemolinos@hotmail.com 8thAnnual Meeting of the EMS / 7th ECAC Amsterdam , The Netherlands, 29 September - 3 October 2008 The stably stratified boundary layer is extremely dependent on the topography of the area of interest. The terrain variability inside a drainage valley may cause important differences among the measurements of the climatological stations within. In this study, a set of Automatic Weather Stations (AWS) separated a typical distance of 10 km are used to perform a simultaneous analysis of the time series of wind, temperature and humidity of the area surrounding the city of Lleida, in western Catalonia and inside the Ebro Valley. For this purpose, a filter is built to select stable nights (defined as those ones with very weak synoptic winds and clear skies) for the period 1998-2007. 80 km 2.- DATASET Cantabric Sea FRANCE • The data correspond to 17 AWS belonging to the official Catalan Met. Service and located in the oriental Ebro Valley (see figure). • The 10 year dataset (1998-2007) consists in hourly data • and includes the following variables: • Air temperature at 1.5 m AGL (above ground level). • Relative humidity at 1.5 m AGL. • Wind speed and wind direction at 2.0 m or 10 m AGL. • Precipitation. Lleida Lleida Z (m AGL) 50 km Z (m AGL) SPAIN Mediterranean Sea The Ebro Valley Selected AWS 3.- AVERAGED DIURNAL EVOLUTION 5.- FILTERED NOCTURNAL EVOLUTION Cooling, cooling rate and wind speed evolution for the stable nights have been analyzed in all AWS. A representative example is shown for the Gimenells (VH) AWS: I II III Nocturnal evolution of the mean cooling (left), cooling rate (center) and wind velocity (right) Cooling is very similar for all seasons and cooling rate can be divided into three different zones according to its intensity, being the first zone (I) the one with the highest cooling rate. Wind speed decreases with time for all seasons and the weakest winds coincide with the lowest cooling rate (III). Diurnal variation of the mean relative humidity (left) and mean temperature (right) for the Raimat (VK) AWS computed for the whole dataset. The diurnal cycle present in Raimat is important in all seasons, like in the rest of the AWS selected (not shown). 4.- SELECTION OF THE STABLE NIGHTS Three parameters are defined to select the nights with clear skies and very weak synoptic wind: Excellent correlation for temperature but wind speed is badly correlated due to the influence of the local topography Night cooling: Humidity index: Nocturnal mean wind: RHd: Daily-mean relative humidity. RHs: Average of the relative humidity during the sunlight hours. Tset: T at sunset Tset+8: T 8 hours after sunset. Correlation for temperature for the different AWS with the distance (left) and for the wind speed (right) 6.- FILTERED NOCTURNAL COOLING The filtered dataset contains 1417 stable nights from a total of 3608 nights (39%). Winter Spring 7.- NOCTURNAL WIND DIRECTION VARIABILITY The analysis of the cooling for the whole night at all AWS allows to divide the studied zone into two sectors: low plain (sector I) and plateau (sector II). The wind direction analysis for the filtered nights shows two main behaviors: weak westerly winds and local circulation. Each wind rose shows the two most frequent regimes conditioned by the topography: weak westerlies and drainage flows in case of local circulation. These two regimes are adapted to the local topography around the AWS. Calm wind is strongly variable depending on the AWS. Summer Autumn The cooling and cooling rate has similar behavior in both sectors but the intensity of the temperature drop is larger in the plain. Cooling rate during the first hours after the sunset is higher in sector I than in sector II. Wind roses for five selected AWS. Calm wind is defined as wind with mean velocity less than 0.2ms-1 . Most frequent nocturnal wind direction and general winds rose 8.- CONCLUSIONS According to the wind direction analysis, in many AWS, the most frequent direction corresponds to the local slope winds • A conditioned climatology for stable stratified nights has been constructed from a set of seventeen AWS datasets and for a complex terrain like the eastern Ebro Valley. • Diurnal evolution of the mean temperature and relative humidity is well defined in all selected AWS and are well correlated. • The filtered nights (39%) show a very similar cooling pattern in all AWS. The total cooling depends on its localization but the intensity is similar during all seasons. Thus, the zone of interest can be divided into two different sectors. • The cooling rate is higher in the first two hours of the night in both sectors. • The wind direction pattern is very different for each AWS due to the influence of the local topography. • The most frequent wind direction for the majority of the AWS during the stable nights corresponds to the presence of drainage flows. The composite wind rose for the whole area shows that the most frequent directions are from the East and Southwest (15%), followed by Southeast, South and Northeast (10%). These results reflect, as well, the predominance of the two main regimes mentioned above: weak westerlies and local circulations following the terrain. Winds rose computed from all the AWS with wind measured at 2 m AGL. References The Meteorological Met. Service (Servei Meteorològic de Catalunya) is acknowledged for providing the data that has made possible the present study. Martínez et al. (2008). Conditioned climatology of the stably stratified nights at the area of Segrià. Tethys,5 (In press).