Toulouse, 23 Avril 2007 Atmophere-Lake Interactions at a reservoir in the South of Portugal Rui Salgado Centro de Geofísica de Évora / Universidade de Évora Pedro M. A. Miranda (Universidade de Lisboa),

Toulouse, 23 Avril 2007 Impact of Alqueva on Fog In February 2002, the lock gates of the new Alqueva dam in the Guadiana River, located in a semi-arid region of South Portugal, were closed, beginning a water reservoir that, when completed filled, will cover an area of 250 km2. It is the largest European artificial lake. May this artificial lake have any impact on the formation and development of winter fog in the region? It is not easy to find examples of scientific studies about the question. However, some technical studies reported in ICOLD (1996) suggest that generally fog occurs more frequently after a reservoir is built. As shown in those studies the effects of water reservoirs vary not only with its dimension but also with the altitude, local orography and background regional climate.

Toulouse, 23 Avril 2007 Localisation du lac artificiel Barrage d'Alqueva Lac numérique Simulation méso-NH

Toulouse, 23 Avril 2007 Meso-NH (4.5) Simulations Design 90 Case studies All 2003 January, February and Mars days. Simulation Period 30 hours for each simulation, beginning at 18 TU Domain and resolution 450  450 km 2 ; 5 km  5 km Vertical levels 40 stretched levels; 1º level at 10 m Physical parameterizations: Stratiform Clouds and explicit precipitation Kessler (1969) type warm microphysical scheme Boundary Layer TKE equation Bougeault and Lacarrére (1989) Convection Kain and Fritsch type (Bechtold et al., 2001) Surface processes ISBA (Noilhan and Planton, 1989) TEB ( Masson, 2000) ; Charnock (1955) over water Data bases for surface parameters Ecoclimap (Masson, 2003) FAO global soil map and GTOPO 30 Radiation ECMWF radiative scheme Lateral Boundary conditions Open, radiative. Initial and forcing fields Arpege analysis updated every six hours Water surface temperature Constant in each experiment, values derived from data collected in reservoirs existent in the region. For each numerical experiment a set of two simulations was done: one control simulation with the pre Alqueva landscape and an impact simulation with the presence of the reservoir filled up to its maximum level (152 m above sea level).

Toulouse, 23 Avril 2007 Surface Water Temperature Constant in each experiment, values derived from data collected in others reservoirs existent in the region Constant in each experiment, values derived from data collected in others reservoirs existent in the region “Monte Novo” 3m depth water temperature (shallow lake) “Monte Novo” 3m depth water temperature (shallow lake)

Toulouse, 23 Avril 2007 Validation against observations CorrelationBiasRMSETSBS 2m air temperature K 2.75 K 2m air relative humidity % % 2m air specific humidity gkg gkg-1 10m wind speed ms ms-1 10 m wind direction º º Daily precipitation mm 4.65 mm 0.35 (for a threshold value of 1mm/6h) 1.14 Days with fog episodes (CLDFR (1) > 0.1) Fog over Portalegre Meso-NH obs

Toulouse, 23 Avril 2007 Impact on the fog field at the regional scale (Winter) -15% Almost no effect on the global fog field, at the considered scale The effect of the lake on particular days may be considerable Significant reduction of the number of hours with fog over the lake area

Toulouse, 23 Avril 2007 Sensitivity to the water surface temperature (1) 1 case study (4/1/2003) Different water temperature The impact of the lake on fog depends strongly on the difference between the surface water temperature and the daily minimum air temperature : When the water is warmer relatively to air, the impact is negative on fog; When the water is colder, the lake induces more fog ControloTS = 15ºCTS = 5ºC Número total de horas com nevoeiro e respectiva anomalia.

Toulouse, 23 Avril 2007 Sensitivity to the water surface temperature (2) Integrated daily Fog anomaly (in km 2  H) versus (daily minimum air temperature minus water surface temperature) The effect of cooling the air is more important than the injection of water vapor in the atmosphere. Fog anomaly T air_min – T s_lake (ºC)

Toulouse, 23 Avril 2007 Local effects In order to investigate the local effects, a high resolution (1km) set of 2 simulations (control and lake) of 1 case study (04/01) was performed, with grid nesting. In order to investigate the local effects, a high resolution (1km) set of 2 simulations (control and lake) of 1 case study (04/01) was performed, with grid nesting. Local effects may be more important than those shown by the coarser resolution model The reservoir increases fog downstream The reservoir increases fog downstream over the lake the fog is less persistent, as its dissipation begins earlier over the lake the fog is less persistent, as its dissipation begins earlier

Toulouse, 23 Avril 2007 Conclusions The results of Meso-NH simulations indicate: At the regional scale, global effect of the reservoir on fog is marginal At the regional scale, global effect of the reservoir on fog is marginal On concrete days the effect may be considerable, in some cases contributing for the existence of more fog, on others for its reduction On concrete days the effect may be considerable, in some cases contributing for the existence of more fog, on others for its reduction Over the lake there are less fog. Over the lake there are less fog. On another hand, the results suggest an marginal increase of the occurrence of periods of fog On another hand, the results suggest an marginal increase of the occurrence of periods of fog The effect of the cooling of air predominate on the increase of the evaporation The effect of the cooling of air predominate on the increase of the evaporation The simulation results depend critically on the water temperature The simulation results depend critically on the water temperature

Toulouse, 23 Avril 2007 Offline tests with a lake model FLAKE (Mironov, 2005), used at German Weather Service (DWD) lake model capable of predicting the surface temperature in lakes of various depth on the time scales from a few hours to a year. The model is based on a two layer parametric representation of the temperature profile, where the structure of the stratified layer between the upper mixed layer and the basin bottom, the lake thermocline, is described using the concept of self-similarity of the evolving temperature profile. The FLAKE model were forced using Meso-NH output over the Alqueva lagoon. Daily surface water temperature amplitude may be >1ºC ~ + 5% on surface flux of latent heat

Toulouse, 23 Avril 2007 Summer Conditions Summer Conditions FLAKE forced with Meso-NH output (24 and 25 July 1998) FLAKE forced with Meso-NH output (24 and 25 July 1998) Amplitude of Surface water temperature > 3ºC Amplitude of Surface water temperature > 3ºC More LE on day time hours More LE on day time hours Less LE at night Less LE at night Offline tests with a lake model