Basin-scale nocturnal regimes in complex terrain Maria A. Jiménez and Joan Cuxart Universitat de les Illes Balears Palma de Mallorca, Spain 6th MesoNH user´s meeting, Toulouse, 13th October 2011

Objective: to study the local nocturnal circulations at the basin scale under stably stratified conditions and weak pressure gradient conditions (very weak synoptic winds) Meso-NH (MASDEV4_8) at ECMWF Lateral conditions: analysis from ECMWF Verification against observations: AWS, satellite images (NOAA, MODIS and MSG), WindRass and UAV Two cases: FOG FORMATION IN THE EBRO RIVER BASIN TOPOGRAPHYCALLY INDUCED FLOWS IN THE NORTH SIDE OF THE PYRENEES (BLLAST)

Fog formation in the Ebro river basin: December 2009 (Cuxart and Jiménez, 2011; Cuxart et al., 2011) Horizontal resolution: 2 km x 2 km 360x240 gridpoints Vertical resolution 85 gridpoints Turbulence + Radiation+ICE3 (fog deposition)+Surface schemes Timestep: 2s (CFL condition close to the surface) x

relative humidity (%) x wind speed (m/s) temperature (ºC) Simulated time series of the vertical profiles in Raimat

WindRass, wind speed (m/s) WindRass Tv(ºC) MesoNH Tv(ºC) MesoNH wind speed (m/s)

10/12/ UTC Cloud fraction at 50 m (a.g.l.) Evolution of the fog in the Ebro basin (1/6) Relative Humidity (%) in Raimat x 1800 UTC: starting of the fog

MODIS 11/12/2009 a les 0300 UTC Cloud fraction at 50 m (a.g.l.) Relative Humidity (%) in Raimat 0300 UTC: the horizontal extension of the fog increases and it is confined to the bottom parts of the basin Evolution of the fog in the Ebro basin (2/6)

Cloud fraction at 50 m (a.g.l.) Relative Humidity (%) in Raimat 0900 UTC: the fog occupies the whole basin Evolution of the fog in the Ebro basin (3/6) MODIS 11/12/2009 a les 1030 UTC

Relative Humidity (%) in Raimat 1500 UTC: during the day the fog dissapears in most of the places (radiation at the top of the fog layer) Evolution of the fog in the Ebro basin (4/6)

Cloud fraction at 50 m (a.g.l.) Relative Humidity (%) in Raimat 0000 UTC: the fog is formed again during the 2nd night and occupy the whole basin again Evolution of the fog in the Ebro basin (5/6) MODIS 12/12/2009 a les 0200 UTC

Cloud fraction at 50 m (a.g.l.) Relative Humidity (%) in Raimat 1200 UTC: the fog dissapears Evolution of the fog in the Ebro basin (6/6)

11/12/ UTC 11/12/ UTC 11/12/ UTC 12/12/ UTC 12/12/ UTC 10/12/ UTC Vertical cross-sections of the cloud fraction The fog is established Day: the fog dissipates close to the surface Night: the fog is formed again

Summary * A locally-generated fog in the Ebro river valley is studied. The fog is formed in the bottom of the valley and extends (horizontally and vertically) during the night. However, during the day the fog dissipates at levels close to the surface. * The topography plays an important role since the fog is confined to the bottom part of the basin (700 m, a.s.l.). * The model is able to properly capture the evolution of this fog event. The WindRass profiler is used to verify the fog deck and the horizontal extension is verified using satellite images. * The fog layer is practically isothermal, with very weak winds and it is dominated by the processes at its top (growing = entrainment, dissipation = radiation).

Outer domain 2km x 2km (250x240 gridpoints) Inner domain 400m x 400m (200x300 gridpoints) Topographycally induced flows in the north side of the Pyrenees (Jiménez et al., 2011; Jonassen et al. 2011) Pre-BLLAST Start: 0600 UTC, 30th June 2010 Finish: 1000 UTC, 1st July 2010

DAY 1300 UTC EVENING 2000 UTC NIGHT 0400 UTC Streamlines at 50 m (a.g.l.) Day: upslope flow (originated near the mountains) Night: downslope flow (converges over the center of the basin). It is largest right before the sunrise and reaches about 120 km. The wind maximum of about 7 m/s is located at around 50 m (a.g.l.).

MODELMODIS Land-Surface Temperature (LST) at 0200 UTC Both temperatures are not comparables in les Landes because the LST of MODIS corresponds to the temperature at the top of the trees whereas for the model this is the surface temperature at the ground ºC

Bordeaux (COAST)Pau (SLOPE)Toulouse (PLAIN)

UHF wind speed (m/s) UHF wind direction(º) MesoNH wind speed (m/s) MesoNH wind direction(º) Time series of the vertical profiles in Lannemezan (UHF and MesoNH)

0400 UTC at 50 m (a.g.l.) downslope 1000 UTC at 50 m (a.g.l.) upslope From the inner outputs the direction is better captured due to a better representation of the topography. The soil moisture also plays an important role. outer domain at 0400 UTC

Wind direction (º)Potential temperature (K)Wind speed (m/s) 1300 UTC 0400 UTC SE N E NW SE

The reverse of the up and down slope flow coincides with the change in sign of the temperature contrast between the plain and slope. plain foothill slope

Summary * The model is able to reproduce the main observed patterns. The spatial resolution and the soil moisture are key factors to properly describe the upslope/downslope winds by the model * The horizontal extension of the downslope flow is largest right before the sunrise and reaches about 120 km. The wind maximum of about 7 m/s is located at around 50 m (a.g.l.). * Further work is needed to properly characterize the physical mechanisms that explain the upslope and downslope winds and its transition in this area (summer 2011 field campaign in Lannemezan).