Basin-scale nocturnal regimes in complex terrain

Maria A. Jiménez and Joan Cuxart

Universitat de les Illes BalearsPalma 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 BASINTOPOGRAPHYCALLY INDUCED FLOWS IN THE NORTH SIDE OF THE PYRENEES (BLLAST)

Fog formation in the Ebro river basin: 10-12 December 2009 (Cuxart and Jiménez, 2011; Cuxart et al., 2011)

Horizontal resolution: 2 km x 2 km360x240 gridpoints

Vertical resolution85 gridpoints

Turbulence + Radiation+ICE3 (fog deposition)+Surface schemesTimestep: 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)

WindRassTv(ºC)

MesoNHTv(ºC)

MesoNHwind speed (m/s)

10/12/20091800 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 UTCCloud fraction at 50 m (a.g.l.)

Relative Humidity (%)in Raimat

0300 UTC: the horizontal extension of the fog increases and it is confinedto 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 fogdissapears 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 againduring 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)

11/12/20090300 UTC

11/12/20090900 UTC

11/12/20091500 UTC

12/12/20090000 UTC

12/12/20091200 UTC

10/12/20091800 UTC

Vertical cross-sections of the cloud fraction

The fog is established

Day: the fog dissipatesclose 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 domain2km x 2km (250x240 gridpoints)

Inner domain400m 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

DAY1300 UTC

EVENING2000 UTC

NIGHT0400 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.).

MODEL MODIS

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)

UHFwind speed (m/s)

UHFwind direction(º)

MesoNHwind speed (m/s)

MesoNHwind 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 domainat 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 betweenthe 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 rightbefore 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).