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Exercise 1: Spiraling Hurricane Katrina from cloud tops to ocean waves - continued
 
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a) Cloud tops - MERIS
 
A spectrometer is an optical instrument used to measure properties of light – in this case the intensity – over a specific portion of the electromagnetic spectrum. Having applied some calibration corrections, each pixel is assigned a certain value, from 0 to 255, representing the intensity of incident light within a distinct wavelength range. A value of 0 stands for lowest, a value of 255 for highest intensity. The intensity of the incident light is dependent on the reflectivity and emissivity of a certain ground area in the considered spectral range. In the case of MERIS the measured intensity is related to the reflection of solar light, because the sensor does not emit radiation itself. This is what you call a passive sensor. The ground area represented by one pixel is defined by the spatial resolution of the sensor.  
 
Open some channels of the MERIS image.

1. Examine and compare the images and their respective histograms.

A histogram displays the number of pixels in the whole image for every intensity value. High values (maximum 255, if you have an 8-bit image) indicate high reflectivity and vice versa. Recommended channels: 1, 5, 10, 14.
 
 


 
 
Histograms for channels 1, 5, 10 and 14 of the MERIS image
 
 
Of course, you are well aware that visible light cannot pierce through clouds. Due to their complete reflection of visible light, clouds appear white. But MERIS can also detect reflected radiation in the NIR wavelength range.

2. Visually examine some spatial subsets of NIR channels showing only the hurricane itself by using the crop feature when opening each image and compare them with channel 8 which indicates the reflectance of visible light (red).

3. Inspect the histograms and comment on the NIR transmittance of a hurricane, i.e. the transmittance of clouds in this spectral region.
 
 


 
 

 
 
Hurricane Katrina as seen by Envisat’s MERIS channels 5 (upper image) and 13 (lower image) with their respective histograms
 
 
4. Compare the images and their histograms. Can you verify that it is impossible for electromagnetic radiation to pass through clouds?

Now it is time to visualise Hurricane Katrina. In the previous task you figured out that it is impossible for electromagnetic radiation in MERIS’ operating range to pierce through clouds. That’s why visualising means displaying the cloud tops of Hurricane Katrina in this case.

Give some RGB combinations a try.

5. Which ones do you regard best for hurricane monitoring?

6. Try to create a near true colour combination in order to make it look like it was seen by a human eye from above.

7. Can you detect the direction of rotation by examining the true colour composite? Is it clockwise or anti-clockwise?

Spend some time inspecting the hurricane’s eye and compare it to the edges of the eye.

8. Can you spot the reason for the different appearances?

Reread the introductory paragraph “Hurricane Katrina” about Katrina’s formation.

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Storm surges
Introduction
Hurricane Katrina
Introduction
Exercises
Exercise 1: Spiraling Hurricane Katrina from cloud tops to ocean wavesExercise 2: Visualising and examining flooded areas in New Orleans caused by Hurricane Katrina
Eduspace - Software
LEOWorks 3
Eduspace - Download
Envisat and SPOT data (zip)
Related links
Hurricane Katrina - NOAAHurricane Katrina - WikipediaNational Hurricane Center - NOAAStorm surge overview - NOAAGlobal Monitoring for Environment and Security - ESAInternational Charter Space and Major DisastersTropical weather and hurricanes - Physicalgeography
Related news
Envisat sees whirling Hurricane Katrina from ocean waves to cloud tops
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