Atmospheric parameters that affect radiance (click on image)
Exercise 2: Sea Surface Temperature
Satellites measure sea surface temperature by using the infrared part of the electromagnetic spectrum and capturing the thermal emission. All surfaces emit radiation, whose strength depends on the surface temperature. The higher the temperature, the greater the radiant energy. Skin temperature (SSTskin) is defined as the temperature measured by an infrared radiometer typically operating at wavelengths of 3.7-12 µm that represent the temperature of a surface layer of ~10-20 µm.
The infrared radiance the sensor captures also depends on other variables, such as the surface emissivity (0.98-0.99 over the sea) and the geometry of the viewing. The atmospheric contribution to the signal is small, but since the atmosphere is far cooler than the sea surface, it cannot be ignored. Accurate SST retrieval requires careful consideration of all variables that affect atmospheric absorption and emission.
Satellites measure the brightness temperature of the surface. An equation can be used to derive a set of SST algorithm coefficients that can be applied to the brightness temperature data in order to clean it from the noise caused by the atmosphere. Some multispectral SST retrieval algorithms use the channels that measure radiation in the 11 and 12 µm spectral range during the day. This is called a split-window algorithm, as the atmospheric transmission window where the radiation leaves the surface to space (between 9.8 and 13.5 µm) is split into two channels (11 and 12 µm). At night, an additional 3.7 µm channel can be used, which provides more information on the atmospheric attenuation of the sea surface radiance. This is called a triple-window algorithm. The general formulation of the split-window can be written in this way:
where a, b, c and d are specific to each algorithm because they depend on sensor characteristics and local atmospheric conditions.
In normal conditions, the Eastern Equatorial Pacific is cooler than the Western Equatorial Pacific, although the Eastern area naturally gets more net heat. Trade winds blowing westward along the Equator cause upwelling of the area, bringing up cold water in the East, which is close to the surface, while it brings up warm water in the West, because cold water is much deeper in this area. If the trade winds weaken, the warm water can surge East of the Equator and warm sea-surface temperatures extend far into the Central Equatorial Pacific. In the East, sea surface temperatures are as warm as 3°C higher than normal.
LEOWorks exercise
Make an animation with the series of data provided by the satellite.
There are three series of data, one from 1997-1998, one from 2005-2006, and one from 2007-2008. Compose three animations with the three sets and study them. Describe what happens in every period and compare them.
In the LEOWorks image processing programme, open Tools/Image Animation, which brings up the Select Files for Animation window. Choose the proper animation speed. You can see the number of the slide by activating the Active Slider.
1. Can you tell which of the three series of images corresponds to an El Niño year?
2. What effect does El Niño have on the climate patterns of the Western Pacific and Asia?
3. What happens in the Eastern Pacific and over South America?
4. Using the latest information, evaluate the present situation.
