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Ocean Interactions
Ocean and Atmosphere connecting Scientists, Teachers & Students
2003 Cruise:
Radar
This is a contribution from NOAA Environmental Technology Laboratory Meteorologist Dan Wolfe. Dan has been with NOAA for more than 20 years and is on the Ron Brown to provide expertise with the various radar systems on the ship. I asked Dan to tell us something about weather radar.
Radar (RAdio Detecting And Ranging) technology has experienced almost continuous refinement since it was first developed early in the 1930s. In its simplest form, radar transmits a pulse of electromagnetic energy in a selected direction and then "listens" for a return signal. From the time delay between transmission and reception of the return signal, the distance to the target can be computed. Although the initial applications of radar were for detecting solid objects, such as airplanes, it was soon discovered that radar can also detect hydrometeors (rain, ice, snow, etc.) in the atmosphere, principally those large enough to precipitate. Remote detection of hydrometeors became the foundation for the science of radar meteorology. The conventional weather radar (precipitation radar) consists of a radio transmitter and receiver, a scanning antenna, signal processor, and display device for presenting the received signals. This system can locate a storm and indicate its intensity from the amplitude of the returned signal.
Modern weather radars are able to detect subtle changes in the frequency of the returned signal from which the radial velocity of the target can be measured. This information along with the strength of the reflected signal allows meteorologists to detect such phenomenon as tornados and micro-bursts. Examples of modern weather radar are the ones used by most TV stations. One problem encountered by most radars is interference from unwanted targets. Things such as planes and even birds can produce signals in the radar return that have to be separated out from the desired atmospheric signal. These tasks are part of what the scientists at NOAA's Environmental Technology Laboratory are continually working to solve.
C-Band Doppler Radar
The C-Band Doppler Weather radar is unique as one of only a few weather radars deployed on board a research vessel. It "golf ball" like dome houses a large rotating antenna dish capable of scanning the surrounding skies out to 250 km (~125 miles) from the surface to the top of the tallest thunderstorms. Similar to TV station weather radars, it is also equipped with sophisticated software. This software automatically corrects the antenna dish pointing angles for the ship's motion and can produce products such as winds and rain fall rates from the raw data. The C-Band can also be a valuable tool to the crew of the Ron Brown when navigating the oceans of the world. In the event of a major tropical storm (Hurricane or Cyclone) it could guide the ship to safe waters.
Radar Wind Profiler
The Radar Wind Profiler (RWP) is similar to a weather radar you see on TV in that it transmits electromagnetic waves into the atmosphere and "listens" for a return signal. It differs from these radars in several ways. While the standard weather radar needs clouds or some sort of precipitation particles to reflect off of, the RWP tracks gradients of refractive index in the atmosphere. A refractive index gradient can be related to changes in humidity or temperature caused by turbulence. These gradients move with the wind and are what allow these radars to measure winds in clear sky conditions. Most weather radars also consist of a rotating antenna, while a RWP has a flat plane antenna pointing vertically. The RWP transmits several waves of energy at different angles off zenith that allow winds to be calculated through simple geometry. Advantages of the RWP are its ability to report continuously the winds above the antenna thus aiding in the understanding of the meteorology in complex terrain or highly variable conditions. RWP's are regularly used in the study of air pollution and how the pollution is transported throughout cities. This information combined with sampling of the atmospheric chemistry helps determine what environmental controls might need to be implemented. Data from these radars can be feed into computer models to help better forecast the weather. Currently there is a network of 30 RWP's operated by NOAA. These data and those from many other RWP's around the United States are available to the public on the WWW. During the recent Columbia shuttle disaster, data from the RWP located at Palestine, TX was found to contain information on falling debris.
The RWP on board the Ronald H. Brown is a unique system in that it doesn't need mechanical gyros to stabilize the antenna as the ship pitches and rolls. Instead it uses computers to read in the ship's motion and then electronically steer the antenna.
Weather Balloons
Weather balloons have been used in some fashion for over 100 years to measure the atmospheric conditions above the earth's surface. Initial weather balloons consisted of sending instrument packages [recording temperature (T), relative humidity (RH), and pressure (P) on chart recorders], up in tethered hot air balloons. When brought down the charts could then be read giving the scientists a picture of the vertical structure of the atmosphere. With advances in electronic technology, it became possible to develop electronic packages, called radiosondes or rawinsondes, capable of transmiting the information from a free floating balloon back to a ground receiving station. Winds aloft were calculated by manually tracking the balloon using theodolites and applying simple geometric formulas. Weather balloons rise to tens of thousands of feet. As they rise the balloon, filled with helium, expands and at some point pops. The expendable packages carried aloft falls to the earth. Over land parachutes are used to bring the instrument packages down safely. Today's weather balloons are much more sophisticated in their make-up, but still measure the basic atmospheric parameters (T, RH, P). A significant advancement has been in the use of GPS to calculate winds. In today's highly technical world, the field of "remote sensing" has allowed scientists to use such equipment as radars, lidars and radiometers to measure atmospheric parameters above the earth's surface without sending up a weather balloon. Even with these advances the weather balloon remains a standard by which many of the new sensors are calibrated or compared.
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