Weather Radar

What is the WSR-88D or NEXRAD?

NEXRAD or Nexrad (Next-Generation Radar) is a network of 159 high-resolution Doppler weather radars operated by the National Weather Service, an agency of the National Oceanic and Atmospheric Administration (NOAA) within the United States Department of Commerce. Its technical name is WSR-88D, which stands for Weather Surveillance Radar, 1988, Doppler. NEXRAD detects precipitation and atmospheric movement or wind. It returns data which when processed can be displayed in a mosaic map which shows patterns of precipitation and its movement. The radar system operates in two basic modes, selectable by the operator – a slow-scanning clear-air mode for analyzing air movements when there is little or no activity in the area, and a precipitation mode, with a faster scan for tracking active weather. NEXRAD has an increased emphasis on automation, including the use of algorithms and automated volume scans.

Dual polarization

The next major upgrade is polarimetric radar, which adds vertical polarization to the current horizontal radar waves, in order to more accurately discern what is reflecting the signal. This so-called dual polarization allows the radar to distinguish between rain, hail and snow, something the horizontally polarized radars cannot accurately do. Early trials have shown that rain, ice pellets, snow, hail, birds, insects, and ground clutter all have different signatures with dual-polarization, which could mark a significant improvement in forecasting winter storms and severe thunderstorms.[8] The deployment of the dual polarization capability (Build 12) to NEXRAD sites will begin in 2010 and last until 2012. The Vance AFB radar is the first operational WSR-88D to be modified to Dual Polarization. The modified radar went operational on 3 March 2011.[9]

Usage

NEXRAD data are used in multiple ways. It is used by National Weather Service meteorologists and is freely available to users outside of the NWS, including researchers, media, and private citizens. The primary goal of NEXRAD data is to aid NWS meteorologists in operational forecasting. The data allows them to accurately track precipitation and anticipate its development and track. More importantly, it allows the meteorologists to track and anticipate severe weather and tornadoes. Combined with ground reports, tornado and severe thunderstorm warnings can be issued to alert the public about dangerous storms. NEXRAD data also provides information about rainfall and aids in hydrology forecasting. Data is provided to the public in several different forms. The most basic form is graphics published to the NWS website. Data is also available in two similar, but different, raw formats. Available directly from the NWS is Level III data. Level III data consists of reduced resolution, low-bandwidth, base products as well as many derived, post-processed products. Level II data consists of only the base products, but at their original resolution. Because of the higher bandwidth costs, Level II data is not available directly from the NWS. The NWS distributes this data freely to several top-tier universities who in turn distribute the data to private organizations.

Weather Radar displays.

The above image is the Weather Radar image supplied by Google Earth when you enable the display of Weather Radar in the "Layers" section of the sidebar. Note this image is displaying areas of precipitation only, greens and yellows indicating various levels of rain, and out west there is some pink and grey indicating forms of frozen precipitation.

The above image is the same area, but instead of using the Google Earth supplied weather radar information, this is live weather radar data through a network link downloaded from the National Weather Service as a Google Earth Overlay. I have it set to refresh the image every two minutes. Note the difference in the image displays. The raw radar data includes all radar return information including those with the "negative dB", which creates the "ring around" the individual radar sites. Each of the NOAA/NWS radar sites are depicted using a Green radar antenna placemark. RING AROUND: A condition in which a repeater jammer's total gain, from receiver antenna to transmitter antenna, exceeds the antenna isolation resulting in the repeater amplifying it's own internal noise. Akin to positive feedback in an amplifier that causes unwanted oscillations.

RING AROUND (RADAR-TO-RADAR): The condition where radio frequency interference signals from a transmitter of one radar enter the receiving circuits of another radar.

Radar returns are usually described by colour or level. The colours in a radar image normally range from blue or green for weak returns, to red or magenta for very strong returns. The numbers in a verbal report increase with the severity of the returns. For example, the U.S. National Doppler Radar sites use the following scale for different levels of reflectivity:^[14]

• magenta: 65 dBZ (extremely heavy precipitation, possible hail)



• red: 52 dBZ



• yellow: 36 dBZ



• green: 20 dBZ (light precipitation)

Doppler interpretation

Radial component of real winds when scanning on 360 degrees.

If one thinks of an autumn rain uniformly filling the radar area coverage and moving from west to east, one notes that a radar beam pointing west will "see" the raindrops moving toward itself, while a beam pointing east will "see" the drops moving away. On the other hand, looking north or south, since there is no motion toward the radar in those directions, the radial velocity is null.^[8]

As the beam is scanning 360 degrees around the radar, data will come from all those angles and be the radial projection of the actual wind on the individual angle. The intensity pattern formed by this scan can be represented by a cosine curve, as seen on the right. One can then calculate the direction and the strength of the motion of particles as long as there is enough coverage on the radar screen.

However, the rain drops are falling. As the radar only sees the radial component and has a certain elevation from ground, the radial velocities are contaminated by some fraction of the falling speed. This component is negligible in small elevation angles, but must be taken into account for higher scanning angles.

Now lets look closely at the KJAX (Jacksonville, Florida WSR-88D) area.

This image is from Google Earth, using Google Earth's Weather Radar Layer in the sidebar. Note, no precipitation returns anywhere. Depicted is the KJAX WSR-88D just north of the I-95 Bypass around Jacksonville. The actual radar antenna is located in the southeast corner of the Florida Air National Guard property on the Jacksonville International Airport. Southwest of the I-95 loop is the former NAS Cecil Field, and further south-southwest is the Florida National Guard facility at Camp Blanding.

Due east of Jacksonville on the coastline is Naval Station Mayport. Presently NS Mayport is home port to 12 US Navy Frigates, 4 US Navy Cruisers, and 4 US Navy Destroyers. It was the former homeport of the aircraft carrier USS John F. Kennedy.

Near the tip of the point for the entrance into NS Mayport the Navy operates an AN/ULM-4 Shipboard Electronics Systems Evaluation Range. The orange pie shaped wedge off of the coast between 100 and 130 degrees true from five to fifteen nautical miles is the location of this range area.

To the northeast of NS Mayport are two yellow wedges, these are used as Shipboard Laser Training Ranges. One used April through November, the other used December through March, depending upon the Northern Right Whale breeding season. Also in this area are the three blue outlined and shaded boxes which are used for Shipboard Gunnery Practice.

The three orange outlined almost "V-shape" areas are submarine transit corridors: San Simmon, St. Marys, and Mayport; and the longer red-outlined is also a submarine transit corridor. All essentially are associated with the US Navy's Fleet Ballistic Missile Submarine Base at Kings Bay, Georgia.

Now note the raw radar data from the National Weather Service Radar Overlay. Negative dB (decibel) returns surround the antenna, as well as four areas of what I'll call strobing, one through the Laser Training Range, one through the Shipboard Electronics System Evaluation Range, and two through the vicinity of Camp Blanding National Guard Training Site.

This is an example of a radar control panel. It allows the user to select the various levels of return that they would like to see on their display. The raw radar information seen in the second and fourth pictures above are just that raw radar data, that has not been filtered through a control panel.