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 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 1 1

- ACCURATE PREDICTION OF WIND FIELD OVER THE SEA 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 2

THEORETICAL BACKGROUND (1) G.W. are associated with a variable wind at the sea surface that modulates the short-scale sea surface roughness 3

THEORETICAL BACKGROUND (2) l2(z) = N2(z)/U2(z) - Uzz(z)/U(z) wind curvature term stability term For trapped lee waves, the Scorer parameter l2 must decrease sufficiently rapidly with height. Temperature inversion and vertical wind shear in the lower atmosphere favor the generation of trapped lee waves. 4

THEORETICAL BACKGROUND (3) H=hN/U H < 1 H > 1 5 Smith (1989)

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 scheme MYJ: TKE closure scheme 6

SYNOPTIC CONDITIONS 7

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 with sea 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 signatures 8

OBSERVED SOUNDINGS IN HERAKLION Only at 12 We can calculate Scorer parameter and H at 12 UTC 9

SCORER PARAMETER IN HERAKLION AT 12 UTC l2(z) = N2(z)/U2(z) - Uzz(z)/U(z) H >> 1 :for the highest peaks of Crete and Rhodes H =< 1: for Karpathos, Kassos, the lower part of Crete and Rhodes 10

YSU MYJ 10m wind 900 hPa wind Appreciable Differences Due to larger N and smaller U → higher H in MYJ 10m wind 900 hPa wind 11

W at 800 hPa (YSU vs MYJ) 12

CROSS SECTION ACROSS KARPATHOS 13

ASAR (0820Z) vs QuikSCAT 14

WIND PROFILES AND FOURIER POWER SPECTRA (AA') 15

WAVELENGTHS (km) SECTION ASAR YSU run MYJ run AA' 9.8 8.5 11.8 BB' 9.0 10.8 CC' 27.1 22.5 DD' 18.3 20.1 16

TAYLOR DIAGRAM YSU run MYJ run BIAS -0.65 -0.12 CORR 0.65 0.37 RMSD 1.60 2.06 17

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 18

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. 19

MODEL SOUNDINGS UPSTREAM OF KARPATHOS (08 UTC) 20

ASAR IMAGE (0818 UTC, 10/03/09) 21

SYNOPTIC CONDITIONS 22

OBSERVED SOUNDINGS IN HERAKLION 23

SCORER PARAMETER l2(z) = N2(z)/U2(z) - Uzz(z)/U(z) 24

ASAR VS WRF MODEL WIND ASAR WRF YSU WRF MYJ 25

W at 800 hPa (YSU vs MYJ) 26

CROSS SECTION ACROSS KARPATHOS 27

WIND SPEED FOURIER POWER SPECTRA 28

TAYLOR DIAGRAM YSU run MYJ run BIAS -0.70 -0.08 CORR 0.60 0.59 RMSD 2.98 3.01 29

(courtesy: Morris Weisman) 30