Mistral and its Siblings in RCMsANIKA OBERMANN, BODO AHRENS

IAU, Goethe University Frankfurt

Presented at COSMO/CLM/ART User Seminar 2014 in Offenbach

DAILY MEAN WIND SPEED DATA

• Buoy data daily mean wind speed from hourly measurementsat Côte d’Azur and Gulf of Lion [1].

• QuikSCAT gridded 0.25◦ scatterometer data of wind speedover sea (measurements ≈ 6 a.m and 6 p.m. local time) [2] fittedto buoy measurements at Gulf of Lion [3].

• SAFRAN gridded daily mean wind speed over France usingoptimal interpolation method [4].

• ERA-Interim daily mean windspeed [5].

MODELS

ERA-Interim driven runs on Med-CORDEX domain:

• COSMO-CLM [6] with 0.44◦ and 0.088◦ resolution.• ALADIN-climate [7] with 0.44◦ and 0.11◦ resolution.

ESTIMATING FETCHDistance the wind travelled overwater before reaching grid cellestimated on QuikSCAT grid:

1. Distance to coastal cells ineach direction (black cir-cles).

2. Line of Sight: Shading offar away cells by closercells (red line).

3. Avaraging due to uncer-tainties in wind direction:Fetch in φ ± 90◦ intervalweighted with a cos2 func-tion (blue line).

Fetch at Gulf of Lion buoy.

MISTRAL & SIBLINGS

• Mistral: Regional cold anddry north to northwest-erly wind following thelower Rhône Valley. Itcauses deep water forma-tion in the MediterraneanSea. Therefore it is im-portant for modeling of thecirculation in the Mediter-ranean Sea.

• Tramontane: In valley be-tween Pyrenees and MassifCentral.

• Cierzo: Along Ebro Valleysouth of Pyrenees.

Terrain height (m) of COSMO-CLM0.088◦ Med-CORDEX run. Circles:buoys, triangles: surface stations.

WIND TIME SERIESGusts ≥ 16m

s and gust directionat 13 stations in France [8].

• Mistral (≈ 19%): At leastone station in the valley,one at its exit, and oneclose to sea show gusts.

• Tramontane (≈ 43%): Atleast one inland stationand one station close to seashow gusts.

FETCH DEPENDENCE OF ERRORS

Fetch (0.25◦ pixels) on 09.02.2000.

Mistral Days Tramontane Days Other Days

Rel

.Bia

sR

MSE

Red: COSMO-CLM 0.44◦, orange: COSMO-CLM 0.088◦, blue: ALADIN0.44◦, light blue: ALADIN 0.11◦, yellow: ERA-Interim.

Relative bias and RMSE 2000-2008 in Mistral and Tramontanearea (black square).

• Negative relative bias de-creases with increasingfetch (except for COSMO-CLM 0.088◦).

• RMSE decreases with in-creasing fetch and shows adrop at about 12 pixels onwindy days.

WIND SPEED

Mean wind speed 2000-2008 fol-lowing Mistral and Tramontanetracks (black lines in right fig-ure).

• Rhône Valley (blue ar-rows): Increase in windspeed during Mistral days.

• Coast (black arrows): Lo-cation, shape and amountof wind speed gain in mod-els differ from observationsfor Mistral and Tramon-tane days.

Observed wind speed (m/s) on09.02.2000.

Mistral Days Tramontane Days Other Days

Mis

tral

Tram

onta

ne

Black: Observations, red: COSMO-CLM 0.44◦, orange: COSMO-CLM 0.088◦,blue: ALADIN 0.44◦, light blue: ALADIN 0.11◦, yellow: ERA-Interim.

OUTLOOKModels work fine for non-windy days but underestimatewind speed in the Gulf of Lionon windy days.

• Possible modifications ofCharnock formula forhigh winds and fetch?

• Error propagation fromland to sea?

– Funneling in valley?– Deceleration at valley

exit?– Other effects?

• Effects in other winds:Bora, Cierzo, ...

REFERENCES[1] provided by S. Belamari, Meteo France

[2] Physical Oceanography DAAC (2001): SeaWinds onQuikSCAT Level 3 Daily, Gridded Ocean Wind Vectors (JPLSeaWinds Project): Guide Document

[3] P. M. Ruti et Al. (2008): Comparison of analyzed and mea-sured wind speeds in the perspective of oceanic simulationsover the Mediterranean basin: Analyses, QuikSCAT andbuoy data, Journal of Marine Systems 70: 33-48

[4] J. P. Vidal et Al. (2010): A 50-Year High-Resolution Atmo-spheric Reanalysis Over France with the Safran System, INTJ CLIMATOL 30 (11): 1627-1644

[5] D. P. Dee et Al. (2011): The ERA-Interim reanalysis: config-uration and performance of the data assimilation system, QJ ROY METEOR SOC 137 (656): 553-597, April 2011 Part A

[6] G. Doms et Al. (2011): A Description of the NonhydrostaticRegional COSMO Model, Part II: Physical Parameterization

[7] M. Herrmann et Al. (2011): Representation of daily windspeed spatial and temporal variability and intense windevents over the Mediterranean Sea using dynamical down-scaling: impact of the regional climate model configuration,Nat. Hazards Earth Syst. Sci. 11: 1983-2001

[8] V. Jacq (2011): personal communication