Tuesday, March 1, 2011

Mr. Spock and Modern Weather Forecasting

Yesterday we discussed how a major pattern shift was underway that would bring a slug of subtropical moisture and precipitation to the Sierra Nevada.

Amongst the many important products for predicting these events are remotely sensed estimates of Total Precipitable Water from polar orbiting satellites.  When something is remotely sensed, it means it is acquired without direct contact.  Precipitation estimates from radar are remotely sensed.  Satellite images on the nightly news are remotely sensed.  On Star Trek, Mr. Spock uses remote sensing to infer the composition and habitability of new planets being visited by the Starship Enterprise.

Paramount Home Entertainment
Star Trek is science fiction, but scientists do this all the time to understand and predict the Earth's atmosphere.  Instead of the remote sensing instrumentation being aboard the Enterprise, it is aboard satellites.

Even Mr. Spock would envy the remote sensing capabilities provided
by instruments aboard the NASA A-Train satellite constellation (Image: NASA)
Total Precipitable Water (TPW) is one of the variables meteorologists monitor with remote sensing.  TPW is the depth of water you would have if you were to condense out all the water vapor in the atmosphere.  Typical values vary from 50 mm or greater in the tropics to less than 10 mm in the arctic.  Because the amount of water vapor in the atmosphere is strongly related to temperature, most of this water vapor is found within the lowest 1-2 km of the atmosphere.

Atmospheric TPW is estimated using observations of microwave radiation from three polar orbiting satellites, including one from the A-Train above (Aqua).  The gory details are summarized here.  Polar orbiting satellites circle the Earth in a path that nearly passes over the poles, with the satellite scanning a new swath of the Earth each orbit.

Image: NASA
Data from the three satellites is processed and pieced together into a continuous image and loop.  Unfortunately, current processing algorithms do not work effectively over land, but they provide incredible data over water.

Today, thanks to government satellites and the Space Science and Engineering Center at the University of Wisconsin-Madison, you can be Mr. Spock and examine the water vapor loop for the eastern Pacific Ocean, which shows very nicely the large-scale transition that is underway.  Note the low TPW values and inferred large-scale northwesterly flow early in the loop and the slug of moisture moving toward California in southwesterly flow late in the loop.

Image Loop: SSEC, University of Wisconsin-Madison
Indeed change is coming, and we are able to predict it better because of modern science and satellite remote sensing.  Imagine instead a world without satellites in which we relied solely on ship observations taken at the ground and aircraft observations taken mostly at jet-stream level.  Mr. Spock would call such a world quite illogical.

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