Forecast improvements

After many years of planning and procurement, the Nation Weather Service (NWS) is in the process of a major technological leap forward. For all mariners, it should mean improved forecasts. And for coastal mariners the possibility exists for obtaining nearly real-time, sophisticated radar imagery with the help of some onboard electronics.

The modernization of NWS involves four elements: powerful doppler weather radars, sophisticated ground data collection stations, a new communications and data processing system, and a new generation of geosynchronous weather satellites. The first authorizations for this upgrade came in the mid-1980s, with the initial equipment being delivered in the first years of this decade. Forecasts are for the system to be largely completed by late 1990s.

A critical element in improving forecasts is upgrading the sensors that gather weather data. One of these is the WSR-88D doppler radar, also known as “next generation weather radar” or NEXRAD. These units will use powerful microwave pulses (peak transmitter power of 475 kilowatts on frequencies between 2.7 to 3.0 GHz) to probe the atmosphere out to 248 nautical miles from the antenna site and up to 70,000 feet in altitude. Since many of these sites will be located close to the coast, NEXRAD radars will provide substantial coverage over water.

Rather than transmitting a wide vertical beam that attempts to cover as big a slice as possible, NEXRAD units use a narrow, one-degree beam. This beam is moved through 360° of azimuth and various elevation angles to develop a three-dimensional model of the atmosphere called a volume coverage pattern (VCP). “We use different VCPs depending on the parameter we’re interested in,” says Randy Racer, Coordinator for the NEXRAD program at NWS headquarters. “For example, one VCP is used for precipitation and another for clear air.”

In a typical VCP, the unit begins by raising the antenna to an elevation angle of 0.5° and then sweeping it through 360°. The next step is to pitch up to 1.5° degrees and do another 360° sweep. This increase in elevation angle continues in steps until the radar does its last sweep at 19.5°. VCPs take anywhere from six to 10 minutes, both to complete the scan and do the data processing involved in giving the 3-D image. Forecasters can observe the 2-D data from each elevation sweep, but must wait for the complete 3-D picture. While the low-end sensitivity of the previous generation of weather radars was roughly 18 db, NEXRAD units can pick up returns down to five db (or roughly four orders of magnitude). This added sensivity means NEXRAD radars can detect not only falling raindrops, but also suspended water droplets and even particulates, such as ash and dust. Amazingly enough, the radars can track the speed and direction of clear air winds by picking up the water vapor and dust they carry.

NEXRAD processes returns in three ways: reflectivity (returns from water droplets, dust, etc.), rainfall accumulation, and Doppler analysis. (The Doppler effect is so omnipresent that we rarely think much about it: When a automobile passes us on the street, we hear it as a high-pitched noise on approach and then as a low-pitched tone as it departs.)

Doppler radar differs from a marine radar in the way returning echoes are processed. Just like standard radars, a doppler unit notes the time elapsed between transmission of a pulse and the return of an echo. There is, however, an extra step involved: the frequency of the returning pulse is compared with that of the original frequency. If there is no difference, then the return is coming from a stationary target. If the frequency is shifted higher, the target is moving toward the antenna; lower and the object is moving away. The amount of frequency shift can be directly related to the target’s speed.

This effect can be clearly seen when looking at a NEXRAD radar display that is in doppler mode. To aid quick recognition, colors are used to indicate various parameters. Different shades of red, for example, indicate targets moving toward the radar, while green is used for targets moving away.

The ability to take precise measurements of clear air allows NEXRAD units to produce some startling images. When I visited the Portland, Maine, NWS office, I saw NEXRAD printouts showing the leading edge of the daily sea breeze as it works its way inland. While most mariners might expect the sea breeze effect to penetrate 10 or 20 miles, Fred Ronco of NWS showed me NEXRAD data that had the sea breeze reaching all the way to Rumford, Maine70 miles from the coast.

Plans call for 137 NEXRAD installations in the 48 continental states, plus seven in Alaska, four in Hawaii, four in the West Indies, and one at Guam. The first operating radar went on line at Sterling, Va., in January 1992, and the last units should be in place by September 1996.

Measurements of conditions on the ground will be improved by the new Automated Surface Observation System (ASOS). These data collection platforms, which will take a set of measurements every minute, will replace less sophisticated and slower automatic sensors, and will also automate data collection at 250 NWS sites where observations are now performed manually.

Having fixed problems with the satellite’s faulty mirrors, NWS is also putting the next generation of geosynchronous operational environmental satellites (GOES) in place. The first of the new weather satellites was launched on April 13, 1994. Dubbed GOES 8, it has been put through six months of testing and evaluation and should be declared operational in Oct. 1994. This generation of GOES has more sophisticated visible light imaging systems. “The clarity and detail is stunning,” says Jamie Hawkins, GOES product manager. “The images are aesthetically beautiful as well as scientifically valuable.” New GOES also carry better infrared imagers as well. “The infrared sensors can penetrate into the lower atmosphere and see what is going on in thunderstorms.” says Hawkins. GOES 8 will be capable of taking four pictures every 30 minutes compared with the earlier generation’s best of one picture every 30 minutes.

Tying all these systems together will be an improved nationwide communications systemAdvanced Weather Interactive Processing System (AWIPS)plus more powerful supercomputers at the National Meteorological Center in Maryland. The AWIPS upgrades are necessary to handle the transmission and processing of huge volumes of data being produced by all these new systems.

One interesting way that mariners will be able to take advantage of this new technology is being offered on a computer bulletin board service (BBS) called BBSea (BBS: 610-328-9405; voice: 610-328-5669). Mike Pollum, the system operator (or “sysop” in BBS lingo) and founder of BBSea, decided that it would be a great idea if mariners could obtain NEXRAD images on an almost real time basis.

Pollum set up and configured his BBS to allow coastal mariners who have a laptop, a modem, and cellular phone capable of receiving data (or a satellite link such as Inmarsat M) on their boat to dial in and download NEXRAD data. These radar images will be roughly 15 minutes old and will provide valuable information about weather conditions. Pollum plans to provide the images from each of the 44 coastal and Great Lakes NEXRAD sites.

By Ocean Navigator