What can we see?

SAR imagery is very suitable for describing mesoscale meteorological phenomena such as island and mountain wakes, gap flows, and synoptic fronts. Three examples are presented below. A more extensive gallery of images and interpretations can be found in the publication High Resolution Wind Monitoring With Wide-Swath SAR: A User's Guide by Beal et al. (2004).

For the three examples presented here the wind retrieval was initiated using wind directions from the Navy Operational Global Atmospheric Prediction System (NOGAPS) interpolated in time and space to match the satellite data. NOGAPS data are available at 6-hour intervals mapped to a 1° latitude/longitude grid. The NOGAPS wind vectors are shown with the same color coding as the SAR wind speed.

Example 1: Wakes of the Canaries in Spain

 

A high-resolution ENVISAT ASAR image has been used to generate this wind field over three of the Canaries in Spain. The image is from 30 March 2009 at 11:17 UTC. The model wind vectors show that the wind direction is from the northeast. The wind speed is generally low (5-7 m/s). Downwind from all three islands a region of very low wind speed is found (< 5 m/s) as a result of island shadowing. This wake effect remains up to 50 km downwind of each island. Higher wind speeds are observed towards the sides of the islands where compression of the flow occurs.

Click here to view the image in Google Earth.

Example 2: Flow through the Strait of Gibraltar

The wind field shown here is generated from an ENVISAT ASAR image acquired over the Straight of Gibraltar on 20 February 2009 at 22:22 UTC. According to the model wind vectors, winds are from easterly directions and generally weak (5-7 m/s). A significant increase of the wind speed is seen around the narrow gap between the European and the African continents. In this region, the wind speed is 15-20 m/s due to speed-up effects. The strong increase of the wind speed as the wind is funneled through the Straight of Gibraltar is not predicted by the model.

Click here to view the image in Google Earth.

Example 3: A weather front over the North Sea

This image shows a wind field generated from an ENVISAT ASAR scene acquired over the North Sea on 30 October 2006 at 21:14 UTC. The model data capture very well the complex wind pattern associated with a weather front. To the north of the front, strong winds are coming from the northeast. To the south of the front, winds are from the southwest and weaker. On the lee side of Norway, wind speeds are reduced significantly. Wind speeds around 5 m s^-1 are found near the coastline. The wind speed increases to more than 15 m s^-1 over a distance of ~100 km offshore. There is an excellent overall agreement between the satellite and model wind speeds.

Click here to view the image in Google Earth.