Monitoring atmospheric gravity waves by means of SAR, MODIS imagery and high-resolution ETA atmospheric model: a case study
Maria Adamo(1) , Giacomo De Carolis(1) , Sandra Morelli(2) , and Flavio Parmiggiani(3)
CNR, Institute of Intelligent Systems for Automati,
Via Amendola 122/D-I,
(2) University of Modena and Reggio Emilia, via Campi 213/A, 41100 Modena, Italy
(3) CNR, Institute of Atmospheric Sciences and Climat , Via Gobetti 101, 40129 Bologna, Italy
Synergy between SAR imagery of the sea surface and predictions from a numerical mesoscale weather meteorological model can be accomplished to correctly explain atmospheric phenomena occurring in the boundary planetary layer.
The aim of this paper is to discuss an experiment for surface wind field retrieval in presence of an atmospheric gravity wave (AGW). It is known that AGWs modulate the main air flow at spatial scales of about 1-10 Km. Actual inversion procedures of the NRCS measured on SAR imagery into the wind vector are based either upon the a priori knowledge of the wind direction (e.g. ) or upon the predictions of an atmospheric weather model . In the latter case, the atmospheric model could not be able to predict the occurrence of atmospheric phenomena, such as AGWs, which require specialized modeling to be forecasted. Besides, weather models could be suitably exploited by considering those atmospheric parameters, which trigger the occurrence of AGWs. Coupled to dedicated AGW propagation model, remotely sensed observations of the phenomenon could be assessed.
In this paper, atmospheric parameters predicted by the weather meteorological model ETA was successfully exploited to capture the major AGW features detected by ERS-2 SAR imagery, which occurred in an area located in the North-West Mediterranean Sea, close to the Italian coasts and Corsica Island. ETA is a three-dimensional, primitive equation, grid-point operational model running at the National Centers for Environmental Prediction of the U.S. National Weather Service, which was adapted to predict atmospheric physical parameters with very high, up to about 4.0 Km, horizontal resolution.
First, ETA model was used to assess the wind vector field at 10 m above the sea level as observed by SAR imaging at 4.0 Km horizontal spatial resolution. SAR wind vector estimation was performed using a Bayesian inversion scheme based on NRCS predictions of the semi-empirical backscatter models CMOD4 and CMOD-IFREMER, respectively.
At a spatial scale lower than the mesoscale airflow circulation provided by ETA, a periodic modulation of SAR NRCS was detected on an expanse of sea between the peninsula North of Corsica and the Italian coast. Furthermore, a co-located cloud band pattern was revealed on a MODIS image acquired about 30 minutes after SAR passage. Both observations were explained as the effects of a quasi-stationary AGW generated by the disturbing effect of the northern Corsica peninsula on the westerly air flow. Periodic cloud band formation frequently occurs as a result of the spatially periodic updraughts imposed by an AGW, which conveys humid air forming the cloud. In addition, the AGW affects the spatial distribution of the surface wind speed in the lee side of the terrain disturbance, thereby allowing its detection on SAR image. The temporal evolution of the cloud pattern from the very beginning was documented by supplementary NOAA/AVHRR imagery. Satellite observations revealed that the cloud pattern lasted for at least 7 hours. SAR and MODIS acquisitions documented the phenomenon at its last stage.
ETA did not predict any AGW phenomena in that area, but was able to accurately forecast atmospheric conditions upstream Corsica peninsula able to trigger the generation of AGWs. A simple lee wave propagation model was indeed used as theoretical support to satellite observations . Although the formulation of the lee wave model did not exhaustively account for the actual atmospheric conditions, the main features of the observed SAR wind speed modulation were nevertheless reproduced. As no surface wind information can be retrieved from near-infrared and visible satellite images, only estimates of cloud band wavelengths were carried out and successfully compared with lee wave model predictions. These results were accomplished using as input to lee wave model the atmospheric ETA parameters extracted at the closest time of every satellite acquisition.
 C.C. Wackerman, C.L. Rufenach, R.A. Shuchman, J.A. Johannessen and K.L. Davidson, “Wind vector retrieval using ERS-1 synthetic aperture radar imagery”, IEEE Trans. Trans. Geosci. and Remote Sens., 34, 6, 1343-1352, 1996
 M. Portabella, A. Stoffelen and J.A. Johannessen, Toward an optimal inversion method for synthetic aperture radar wind retrieval, J. Geophys. Res., 107, 10.1029/2001JC000925, 2002
 E. Palm and A. Foldvik, “Contribution to the theory of two-dimensional mountain waves”, Geophysica Norvegica, XXI, 6, 1-30, 1959