FILTERED NOCTURNAL EVOLUTION

1. OBJECTIVE Conditioned climatology of the stably stratified nights in the Ebro basin J. Cuxart(1), F. Molinos(1), D. Martínez(1), M. A. Jiménez(1) and J. Cunillera(2) (1)Universitat de les Illes Balears, Palma de Mallorca (2) Servei Meteorològic de Catalunya, Barcelona 30th International Conference on Alpine Meteorology Rastatt, Germany. 11 – 15 May 2009 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 statistical 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 DATASET Cantabric Sea FRANCE • Data from 23 AWS, 22 of them from the official Catalan Met. Service (see black dots in figure) and one from the Spanish Met. Service (see red cross in figure) and located in the oriental Ebro Valley. The 10 year dataset (1998-2007) consists of 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. Lleida x Z (m ASL) 50 km Z (m ASL) SPAIN Mediterranean Sea The Ebro Valley Selected AWS SELECTION OF THE STABLE NIGHTS TWO REPRESENTATIVE CASES OF STABLE NIGHTS Three parameters are defined to select the nights with clear skies and very weak synoptic wind: LOCAL CIRCULATION WEAK WESTERLIES 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. dT and HUM have been adjusted to discard cloudy cases or days with persistent fog since they are related with the daily cycle of temperature and relative humidity, respectively. V is used to discard windy nights. The filter has been adjusted and applied to the Gimenells (VH) AWS (Martínez et al., 2008), obtaining 1417 stable nights from a total of 3608 (39%). This classification is assumed to apply for all the area of interest. day night day day night day FILTERED NOCTURNAL EVOLUTION Hours after the sun set Hours after the sun set Cooling, cooling rate and wind speed evolution for the stable nights have been analyzed for all AWS. A representative example is shown for Gimenells (VH): I II III Hours after the sun set Hours after the sun set Hours after the sun set Nocturnal evolution of the mean cooling (left), cooling rate (center) and wind velocity (right) Hours after the sun set Hours after the sun set Cooling is very similar for all seasons and cooling rate can be divided in three different periods according to its intensity, being the first period (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). Winter Spring The analysis of the cooling for the whole night at all AWS allows to divide the studied zone into two sectors: low plain and mountain slope. The magnitude of the total cooling during night is similar in all seasons and it is higher in the plain than in the mountain slope. Summer Autumn Low plain Hours after the sun set Hours after the sun set Mountain slope Time evolution of temperature (top), wind speed (middle) and wind direction (bottom) for seven AWS during two representative cases of the most frequent patterns: weak westerlies (left) and local circulations (right). Temperature and wind speed follow the same patterns that are found in the 10 year statistics, showing that the later may describe real average circulations. Temperature is more homogeneous in the westerly case due to the presence of the well defined general wind that follows the axis of the Ebro valley. For the local circulation case, the temperature drop is lower over the mountain slope than for the plain, where the wind speed is near zero. The wind direction depends strongly on the topography around each AWS, blowing from the west before the sunset and after the sunrise in most of the locations. Low plain Slope The cooling and cooling rate have similar behaviour in both regions but the intensity of the temperature drop is larger in the plain probably due to the presence of drainage flows in the mountain slope area. Cooling rate during the first hours after the sunset is higher in the plain because the air is calmer in this sector, leading to a thermal amplitude also larger there. Hours after the sun set Hours after the sun set Low plain Slope CONCLUSIONS Hours after the sun set Hours after the sun set Each wind rose shows the two most frequent regimes conditioned by the topography around the AWS. (i) Weak westerlies are channeled by the valley axis (Raimat, Gimenells), the river direction (Lleida-Bordeta) and are less frequent at the eastern edge of the valley (El Poal, Tàrrega) due to the blocking role of the mountains at this side. (ii) Local circulations are driven by drainage flows that mainly come from the East/Southeast directions, except in the points channeled by the river (Lleida-Bordeta). The wind roses for the filtered nights show two main wind patterns: weak westerly winds and local circulations. • The statistical data analysis applied to the 23 AWS from the western Ebro basin is in agreement with the results previously found for a single AWS (Martínez et al., 2008). • The filtered nights (39%) show a very similar cooling pattern in all AWS. The total cooling is important in all the basin and its intensity depends on its localization, being similar in all seasons. Temperature drop is higher in the plain probably related to very low wind speeds. • The wind speed decreases during the night in all the area of interest and the wind direction pattern is different for each AWS due to the influence of the local topography. References Martínez, D., Cuxart, J. and Cunillera, J. (2008). Conditioned climatology for stably stratified nights in the Lleida area. Tethys, 5, 13-24 . Wind roses for six selected AWS. Calm wind is defined as wind with mean velocity less than 0.2 m·s-1 .