A comparison of WRF model simulations with SAR wind data in case studies of orographic lee waves

over the Eastern Mediterranean Sea

M. M. Miglietta1,2, S. Zecchetto3, and F. De Biasio3

(1) Institute of Atmospheric Sciences and Climate (ISAC–CNR), Lecce, Italy(2) Institute of Ecosystem Study (ISE-CNR), Verbania Pallanza, Italy(3) Institute of Atmospheric Sciences and Climate (ISAC–CNR), Padova, Italy

E-mail: m.miglietta@isac.cnr.it

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Miglietta M.M., Zecchetto S. and De Biasio F., A comparison of WRF model simulations with SAR wind data in two case studies of orographic lee waves over the Eastern Mediterranean Sea, Atmospheric Research, in press

MOTIVATION:

- ACCURATE PREDICTION OF WIND FIELD OVER THE SEA

-- PROBLEMATIC ISSUE FOR LIMITED AREA MODELS ESPECIALLY OVER THE MEDITERRANEAN SEA

– SCARCITY OF SURFACE DATA …

– BUT FORTUNATELY WE HAVE SCATTEROMETERS AND SYNTHETIC APERTURE RADAR (SAR) DATA!

- ANALYSIS OF MOUNTAIN TRAPPED LEE WAVES

GOAL:

- COMPARE WRF MODEL SIMULATIONS WITH ENVISAT ASAR WIND DATA USING TWO DIFFERENT PBL SCHEMES IN THE AEGEAN SEA

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THEORETICAL BACKGROUND (1)

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G.W. are associated with a variable wind at the sea surface that modulates the short-scale sea surface roughness

THEORETICAL BACKGROUND (2)

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l2(z) = N2(z)/U2(z) - Uzz

(z)/U(z)

stabilityterm

windcurvature

term

Temperature inversion and vertical wind shear in Temperature inversion and vertical wind shear in the lower atmosphere favor the generation of the lower atmosphere favor the generation of trapped lee waves.trapped lee waves.

•For trapped lee waves, the Scorer parameter l2 must decrease sufficiently rapidly with height.

THEORETICAL BACKGROUND (3)

5 Smith (1989)

H=hN/U

H < 1

H > 1

WRF MODEL DOMAINS

- version 3.0- 3 grids (2-way nesting): 16 km (109x109 points), 4 (161x161),1 km (321 x 321)- 24 h runs in operational mode (ECMWF an/fc -TL799 as i./b. conditions)- Thompson microphysics, Kain cumulus (grid 1), RRTM (lw), Dudhia (sw)-PBL parameterization schemes:YSU: non-local first order closure schemeMYJ: TKE closure scheme6

SYNOPTIC CONDITIONS

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ENVISAT ASAR IMAGE (0820 UTC, 15 MAY 2008)

SAR is an active microwave instrument which emits radar pulses and maps their echoes reconstructing the radar sketch of the geographical scene. On the sea surface, radar echoes are primarily produced by the interaction of electromagnetic waves withsea waves produced by surface wind (resonance scattering)

Wind extraction based on a 2D wavelet decomposition technique (Zecchetto and De Biasio, 2002, 2008)- spatial resolution of O(1) km, depending on the spatial density of wind speed signatures8

OBSERVED SOUNDINGS IN HERAKLION

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Only at 12

We can calculate Scorer parameter and H at 12 UTC

SCORER PARAMETER IN HERAKLION AT 12 UTC

10H >> 1 :for the highest peaks of Crete and RhodesH =< 1: for Karpathos, Kassos, the lower part of Crete and Rhodes

l2(z) = N2(z)/U2(z) - Uzz

(z)/U(z)

YSU MYJ

10m wind

900 hPa wind

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Appreciable Differences

Due to largerN and smaller U→ higher H in MYJ

W at 800 hPa (YSU vs MYJ)

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CROSS SECTION ACROSS KARPATHOS

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ASAR (0820Z) vs QuikSCAT

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WIND PROFILES AND FOURIER POWER SPECTRA (AA')

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WAVELENGTHS (km)

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SECTION ASAR YSU run MYJ run

AA' 9.8 8.5 11.8

BB' 9.0 10.8 10.8

CC' 27.1 27.1 22.5

DD' 18.3 18.3 20.1

TAYLOR DIAGRAM

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YSU run MYJ run

BIAS -0.65 -0.12

CORR 0.65 0.37

RMSD 1.60 2.06

SUMMARY- Agreement between the wind from ASAR and WRF runs is qualitatively good

- Wavelengths along different transects are well reproduced

- The YSU scheme reproduces better the spectrum of energy and the wavelengths along selected transects. Global statistical parameters confirm that the YSU run performs better than the MYJ run in this case.

- ASAR helps to selcct the best run, while model fields can be useful for the determination of wind directions in areas with low wind speed

- Importance of PBL schemes, which affect mesoscale flow as well as environmental stability

- Added value of multiple runs

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FUTURE WORK

- ASAR-retrieved winds do not represent correctly the directions in the low level lee-side vortices downstream of Crete. The method used here is problematic where the wind speed is low and the wind direction is variable in space. The present study suggests that in regions with complex morphology, such a procedure could be improved, taking advantage of the limited area model fields.

- The analysis of the present study is limited to two case studies, thus the generality of the previous considerations should be further tested in different environmental conditions, such as frontal systems, barrier jets, sea breeze, or convection.

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MODEL SOUNDINGS UPSTREAM OF KARPATHOS (08 UTC)

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ASAR IMAGE (0818 UTC, 10/03/09)

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SYNOPTIC CONDITIONS

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OBSERVED SOUNDINGS IN HERAKLION

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SCORER PARAMETER

l2(z) = N2(z)/U2(z) - Uzz

(z)/U(z)

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ASAR WRF YSU WRF MYJ

ASAR VS WRF MODEL WIND

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W at 800 hPa (YSU vs MYJ)

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CROSS SECTION ACROSS KARPATHOS

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WIND SPEED FOURIER POWER SPECTRA

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TAYLOR DIAGRAM

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YSU run MYJ run

BIAS -0.70 -0.08

CORR 0.60 0.59

RMSD 2.98 3.01

(courtesy: Morris Weisman)

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