WRF model and ASAR-retrieved 10 m wind field comparison in a case study over Eastern Mediterranean Sea

M M Miglietta,F De Biasio,S Zecchetto

doi:10.5194/asr-4-83-2010

Abstract

Abstract. In Synthetic Aperture Radar (SAR) imagery, signatures of coherent atmospheric structures, due to sea surface roughness modulation by surface winds, are usually well detected. In the present study, the wind field derived from the Envisat Advanced SAR (ASAR) sensor has been analyzed and compared with those simulated with a regional atmospheric model in a case study over an area located in the eastern Mediterranean Sea, extending southward and eastward of Crete island. This is a region subject to complex wind patterns, due to the interaction of the almost steady northerly Etesian wind with the orography of the islands in the region. The ASAR Wide Swath Mode images provide datasets at resolutions exceptionally high compared to model data, appropriate for investigating the mesoscale phenomena on the marine atmospheric boundary layer and to retrieve the surface wind field. The latter has been obtained with a methodology based on the 2-D Continuous Wavelet Transform, suitable to isolate the backscatter patterns on the base of energy and scale considerations. Numerical simulations with the Weather Research and Forecasting (WRF) model have been performed using three 2-way nested domains, the inner one covering the area of interest with a resolution of 1 km. Several simulations, using different diffusion and boundary layer parameterization schemes, have been performed in a case study corresponding to mountain lee waves detected in the ASAR image. The 10 m winds resulting from the numerical experiments have been compared to those retrieved from the ASAR, both quantitatively and qualitatively, in order to analyze the correspondence of observed and simulated wind structures.

Highlights

The spatial and temporal scales typical of the atmosphereocean interaction range from the micro-scale (O(1) m, O(1) s) to the synoptic scale (O(1000) km, O(10) days)
Since the wind directions are retrieved from the backscatter signatures detected by the analysis, which are more structured and dense when the wind speed is high, the spatial resolution of the resulting wind field depends on the wind speed regime
Two different parameterization schemes have been used for the planetary boundary layer (PBL): the Yonsei University (YSU) scheme with a nonlocal turbulent mixing coefficient in the PBL (Hong and Pan, 1996); the Mellor-Yamada-Janjic (MYJ) prognostic scheme that calculates the turbulent kinetic energy (Janjic, 2001)

Summary

Introduction

The spatial and temporal scales typical of the atmosphereocean interaction range from the micro-scale (O(1) m, O(1) s) to the synoptic scale (O(1000) km, O(10) days). Satellite-borne scatterometers are able to measure the wind blowing inside the marine atmospheric boundary layer, providing information about the spatial structure of the wind in the marine surface layer over large areas They have a resolution too coarse for coastal applications, where the wind needs to be determined as close to the coast as possible. The task of simulating the weather fields with better local accuracy is usually pursued by using limited area models, which integrate the model equations only on limited geographical domains, but with finer horizontal resolution The application of these models permits to better represent the small scale forcings, e.g. topographic features, and the internal dynamics and physics, so that a consistent downscaling of the large scale forecasts is produced. Since the wind directions are retrieved from the backscatter signatures detected by the analysis, which are more structured and dense when the wind speed is high, the spatial resolution of the resulting wind field depends on the wind speed regime

WRF model

Model versus ASAR wind

Conclusions