book05soundings - Weather Observation and Analysis John...

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Weather Observation and Analysis John Nielsen-Gammon Course Notes These course notes are copyrighted. If you are presently registered for ATMO 251 at Texas A&M University, permission is hereby granted to download and print these course notes for your personal use. If you are not registered for ATMO 251, you may view these course notes, but you may not download or print them without the permission of the author. Redistribution of these course notes, whether done freely or for profit, is explicitly prohibited without the written permission of the author. Chapter 5. SOUNDINGS 5.1 Sounding Observations While airplanes are helpful, the primary tool for probing the detailed vertical atmosphere has long been the rawinsonde. After World War II, the coordinated launch of soundings became a worldwide effort. The timing of rawinsonde launches, at 0000 UTC and 1200 UTC, is the main reason for twelve hours being a common interval for forecast model runs. It is at those times that the greatest amount of upper air data are available and the most accurate analyses are likely. There exist four basic types of sounding observations. Radiosondes are instrument packages that include sensors for temperature, humidity, and pressure, all attached to a balloon filled with a lighter-than- air gas such as helium. Because pressure decreases upward at a rate that depends on the temperature and humidity, the combination of sensors makes it possible to determine the altitude of the radiosonde and construct vertical profiles of temperature and humidity with respect to both height and pressure. Pibals, short for ‘pilot balloons’, have no instrument package at all. Their purpose is to provide a vertical profile of the wind speed and direction. To accomplish this, their position is tracked from a ground station using a direction-finding system known as a theodolite. The direction (relative to north) of the balloon and the elevation angle (relative to horizontal) provide two of the three independent bits of information needed to pin down the balloon’s location. The third bit is obtained by ensuring that the balloon is inflated precisely so that it will ascend at a ATMO 251 Chapter 5 page 1 of 28
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known speed. This is done by attaching weights to the bottom of the balloon and then inflating the balloon until the weights just barely lift off the ground. By keeping track of the elapsed time after release, the height of the balloon can be computed. Finally, wind is determined from the change in horizontal position of the balloon from one time to the next as it is carried by the wind. Rawinsondes combine the thermodynamic information of radiosondes with the wind information of pibals. The height of the balloon need not be estimated from elapsed time because it can be computed from the measured vertical profiles of temperature, humidity, and pressure. Modern rawinsondes are equipped with GPS transmitters that detect and transmit the balloon’s location using GPS navigation, making tracking with a ground station unnecessary.
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