On 22th August 2011, the tropical storm Irene was classified as stage 1 Hurricane while it was passing over Puerto Rico Island. With time, it gained power, reaching stage 2 on 23th in the morning and stage 3 as it was approaching the coast of the Bahamas, before heading north aside the US East Coast (about 400km off the shore). Its intensity then diminished down to stage 1 when it passed Cap Hatteras. Still, the forecast trajectory announced it would reach New York City which, unlike the Caribbean’s, is not very used to this type of natural events (Gloria, the last hurricane that hit the city, dates back to 1985).
As soon as the news came out, ASAR (Advanced Synthetic Aperture Radar onboard ENVISAT) acquisition plans were updated by ESA (the European Space Agency) to get a radar snapshot of the hurricane. Roughness products show the two radar images, acquired over Cap Hatteras before the cyclone reached the American East Coast. The eye of the hurricane is well depicted on the first image (dark disk). The displacement of the hurricane is shown by the new location of the eye, passing over the Sound of Pamlico 12 hours later.
Radar observation presents many advantages: the capacity to see through rain and clouds, at night and days and with a high resolution to assess geophysical parameters such as surface wind, swell and currents. Besides, its wide swath of 400km gives a synoptic view of such meso-scale phenomenon.
Information such as the wind distribution or the eye diameter happen to be of critical importance to foresee the evolution of the hurricane. L2 wind products display the two wind maps associated with the radar images acquired by ENVISAT. These winds reached values above 35m/s. The analysis of the Doppler signal gives information about the radial speed of the sea surface (which is the speed projected in the range direction in the image geometry). L2 Doppler products show a strong displacement eastward in the southern part of the hurricane with a symmetrical westward motion in the northern part. This is consistent with the winds rotating counterclockwise around the eye.
Such storms also generate powerful swell that can be imaged by the radar. The analysis brought by CLS enables the forecast of swell field evolution through the ocean basins on a global scale. In Irene case, as observed on the Fireworks animation of the North Atlantic Basin, the waves generated by the hurricane will reach the coasts of France during the night of 2nd September. Their height should not exceed 1.5m.
The fireworks also reveal a large area of cross sea, which is a region with waves coming from significantly different directions. Navigation in such areas can be very risky. The present case is a quite complex situation since three different swells encounter: one of them comes from Hurricane Irene while the others come from two successive extra-tropical storms occurring on 20th August in the South-East Atlantic and the 21th in the South-West Atlantic Ocean. Despite the distance they have travelled, the generated swells still present a significant height of 1.5m as they meet the waves generated by Irene.