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Exercise 1: Spiraling Hurricane Katrina from cloud tops to ocean waves - continued 
Radio Detection And Ranging (Radar) systems are active sensors which provide their own source of electromagnetic energy. Active radar sensors emit radio wave radiation in a series of pulses from an antenna. When the energy reaches the target, some of the energy is reflected back towards the sensor. This backscattered microwave radiation is detected, measured and timed. The time required for the energy to travel to the target and return back to the sensor determines the distance or range to the target. By recording the range and magnitude of the energy reflected from all targets as the system passes by, a two-dimensional image of the surface can be produced. Because radar provides its own energy source, images can be acquired day or night. Microwave energy is also able to penetrate through clouds and most rain, making it an all-weather sensor. This makes it capable of piercing through Hurricane Katrina’s cloud tops.
Open the ASAR image of Hurricane Katrina. Inspect its histogram first and see if you can understand why LEOWorks is not capable of displaying the image correctly.
Your next task consists in projecting this radar image onto the 8-bit scale by applying interactive stretching.
Inspect the radar image now.
1. Compare the image with a map in an atlas. Can you locate the coastline at the top of the image?
2. Describe how the different features of Katrina are visualised in the radar image.
3. Using the Measure tool, measure the distance from the eye of Katrina to the coast. How large is the diameter of the eye of Katrina?
Unlike in the MERIS image, you will not identify clouds because, as mentioned above, the microwave radiation used by radar sensors can pierce through clouds. Now it is time to examine the ocean surface below the hurricane. Once more, try to draw conclusions from the direction of the spiraling ocean to the direction of the spiraling hurricane itself. As radar imagery (e.g. acquired by ASAR) is created in a very different way to optical imagery (e.g. acquired by MERIS), it also needs to be interpreted differently. Instead of measuring intensity of reflected and emitted radiation within a spectral band, the backscattered portion of the microwave pulse emitted by the active sensor is measured. High values, meaning bright pixels, indicate that a comparatively big portion is scattered back, and vice versa.
4. What does ocean brightness in the radar image reveal about the roughness of the sea surface?
The following picture may help to answer this question.
Once more, inspect the greyscale values of the hurricane’s eye and the edges of the eye with the help of this information.
5. Can you derive any information on the wind speed in the eye of Katrina?
Last update: 17 April 2013
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