An Overview of NEXRAD Products

Available via UCAR's Unidata Program

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AN INTRODUCTION

The History of Weather Radar

Access to radar imagery began in the mid 1960's when there were only 37 radars situated across the United States. During this time, all radar images contained three VIP levels (Video Integrator Processor) which displayed the intensity of storms using three shades of gray.

During the early 1970's demand for color radar images increased, and by the late 1970's and early 1980's color radar imagery could be accessed from National Weather Service radars (WSR-57 and WSR-74) and the Federal Aviation Administration (FAA) radars situated across the country. It was at this time that the original three VIP levels in the gray imagery were replaced by six DVIP levels (Digital Video Integrator Processor) which display all six intensity levels in color.

The Next Generation Weather Radar

In 1980, the National Weather Service's Next Generation Weather Radar (NEXRAD) Program was established to replace the 128 aging radars across the country. Their goal is to deploy, and bring on-line, 137 new NEXRAD radars (called WSR-88D) throughout the country and parts of the Caribbean by 1997. These new Doppler-type radars offer significant improvements over conventional (WSR-57/74) radars.

Special algorithms, within the NEXRAD radar's computer facility, process large volumes of data and simplify it into

valuable products which give you a better operational understanding of the atmosphere.

The new WSR-88D radars encompass many new features:

1. The ability to see motion using the Doppler effect. This can give early warning to potential severe weather.

2. Their increased sensitivity allows you to view atmospheric conditions, such as cold fronts, dry lines, and thunderstorm gust fronts, never before visible within storms.

3. Their improved resolution allows you to view weather events in greater detail and at greater distances.

4. Their volume scanning function gives you a three-dimensional view of the weather, enabling you to better identify areas of potential severe weather; analyze the vertical storm structure; and receive more frequent and detailed wind profile updates.

WSI Corporation Selected as Unidata NEXRAD Provider

WSI Corporation was selected in 1990 by the National Weather Service to provide the public with access to NEXRAD data as new NEXRAD sites are commissioned through the NEXRAD Information Dissemination Service (NIDS) program.

WSI was recently selected by UCAR to provide NEXRAD information over the Internet to all LDM software users. This allows all participating universities to take advantage of the highest quality data available at very affordable rates.

Conventional Radar vs. NEXRAD System Comparison

The following table summarizes some of the differences between conventional radars (WSR-57/74) and the NEXRAD system (WSR-88D), for the continental United States:

       Description                  WSR-57/74          WSR-88D                    
Data Resolution               1 km to 1 (to 111 km)    1 km to 1 (to 230 km)      







Number of Products   *Update frequency dependent on product mode:      5           
minute = Severe Weather, 6 minute = Precipitation, and 10 minute = Clear Air

Frequently Used Terminology

1. NEXRAD

The entire system of WSR-88D radars and associated processing equipment.

2. WSR-88D

A single radar from the national NEXRAD network.

3. Historical or Archived Data

All data or products that are 24 hours old.

4. Unaltered WSR-88D Products

The NEXRAD products as delivered to the NIDS ports or the same products when retransmitted unchanged.

5. Enhanced Products

Any NEXRAD products which are significant modifications of unaltered products. This excludes any color changes, rescaled presentations, minor changes to map backgrounds, and annotations which do not affect the content of the product.

6. Delivery

The date on which the WSR-88D is delivered to the NWS site and installation begins.

7. Acceptance

The date on which the NWS accepts the radar from the manufacturer, and operational testing begins (approximately two months following delivery).

8. Commissioning

The date on which the NWS has verified operation of the radar and all of the associated tasks have been completed (training, acquisition of spare parts, documentation, etc.). This is approximately seven months after acceptance (nine months after delivery). Following commissioning, the NWS will rely on the WSR-88D radars for 24-hour operational surveillance of the weather.

NEXRAD Products Available from WSI

WSI gives you access to all NWS unaltered products from each NEXRAD radar as soon as it comes on-line. Unlike the conventional (WSR-57/74) radars which produce a continuous, two-minute update of reflectivity, the new NEXRAD radars provide you with a suite of unaltered products which are determined by the type of volume coverage pattern (VCP) the NEXRAD radar is in. Depending on the type of weather in the area of the NEXRAD radar, the operator at the site will put the radar into one of the following three VCP modes:

1. Clear Air Mode

Selected by the NWS radar meteorologist when the radar does not detect any precipitation. This mode increases the sensitivity of the radar, allowing it to detect clear air phenomena, such as dry lines, dry microbursts, and wind shift lines. Products released under this mode are updated every 10 minutes. During this time, some precipitation-based products may not be available.

2. Precipitation Mode

Selected by the NWS radar meteorologist when the radar detects precipitation. Under this mode, all NWS unaltered products are available and updated every six minutes.

3. Severe Weather Mode

Selected by the NWS radar meteorologist when the radar detects severe weather. This mode increases the delivery frequency of products which aid in severe weather forecasting. Products released under this mode are updated every five minutes.

12 Unaltered NWS Products