Monday, February 8, 2010

Taking A Look At Raw UAH Data

Now that we can read Level 1B AMSU files, we can look at the raw data used to build UAH temperature anomalies. First, lets review how the AMSU scans temperatures.

Each level 1B data file contains 45 scan lines. These scan lines are along the path of the satellite as it orbits the Earth. Each scan line has 30 footprints. These footprints are perpendicular to the path of the satellite. Finally, each footprint has 15 channels. These channels measure brightness temperatures at different heights of the atmosphere.

A graph of a single scan line containing all 30 foot prints for channel 5 as it was read from an actual Level 1B data file is shown above.

The footprints scan positions further and further away from the satellite. Footprints 15 and 16 and basically scanning directly below the satellite, whereas footprints 1 and 30 are far away from the satellite. The further away from the satellite a footprints are, the larger the error in its temperature measurements.

In the journals, the error estimate for footprints 1 and 30 for channel 5 is said to be about -10 K. In practice, it seems the actual error can be a bit larger, as the error was as much as -14 K in the file I tested, a difference of about 40%.

Moreover, it seems the errors are different for different temperatures. The graph above shows the readings for footprints 1 through 15 for channel 5 for all 45 scan lines in the file. When the temperatures are higher, the differences between footprint 1 and 15 are greater than when the temperatures are lower.

On the left side of the graph, where overall temperatures are higher, the difference between footprint 1 and footprint 15 is about 14 K. On the right side of the graph, where overall temperatures are lower, the difference between footprint 1 and footprint 15 is about 10 K. So in this example, higher temperatures produce an error amount that is about 40% higher than the cooler temperatures.

Recalling that each Level 1B AMSU data file contains six minutes of data, the chart above is saying that in those six minutes a temperature drop of between 3 K and 7 K was detected.

So that's our first look at raw UAH temperature data. Next, we'll take a look at the raw AMSR-E data.

References And Previous Posts In This Series:
HDF Reader C++ Code Is Written!
Some Useful Climate Code
Summary Of Aqua Satellite Data, Computer Code, And Broken Equipment
Aqua Satellite Raw AMSR-E Data
Aqua Satellite Raw UAH Data, Part 2
Aqua Satellite Raw UAH Data, Part 1
Satellite Summary
Aqua Satellite Data Processing
A Note On The UAH And RSS Raw Data
How UAH And RSS Temperatures Are Measured
Overview Of The Aqua Satellite
Looking At The Aqua Satellite Data
UAH Satellite Data
Dangit! More Climate Stuff. UAH and RSS Raw Data

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