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How does the radar work?
Is every thing I see on the images an accurate picture of my weather?
What are the different types of radar images?
How often are the images updated?
What is Clear Air Mode?
What is Precipitation Mode?
What do the colors mean in the reflectivity products?
What is the difference between base and composite reflectivity?
What is UTC Time?
Introduction to the WSR-88D The WSR-88D is the most powerful and advanced Weather Surveillance Doppler Radar in the world. Since first being built and tested in 1988, it has been installed and used operationally at over 160 locations across the United States, including Alaska and Hawaii. The WSR-88D has also been installed in Puerto Rico and several islands in the Pacific. The NWS Northern Indiana radar began warning operations on March 17th, 1998. The WSR-88D is considered by many to be the most powerful radar in the world, transmitting at 750,000 watts (an average light bulb is only 75 watts)! This power enables a beam of energy generated by the radar to travel long distances, and detect many kinds of weather phenomena. It also allows energy to continue past an initial shower or thunderstorm near the radar, thus seeing additional storms farther away. Many other radar systems do not have this kind of power, nor can they look at more than one "slice" of the atmosphere. During severe weather, the NWS WSR-88D is looking at 14 different elevations every 5 minutes, generating a radar image of each elevation. That's about 3 elevations per minute, or one radar image every 20 seconds! What other operational weather radar can do that??
How does the radar work?
How often are the images updated?Image updates are based upon the operation mode of the radar at the time the image is generated. The WSR-88D Doppler radar is operated in one of two modes -- clear air mode or precipitation mode. In clear air mode, images you see are updated every 10 minutes. In precipitation mode, images you see are updated every five or six minutes. The collection of radar data, repeated at regular time intervals, is referred to as a volume scan. Meteorologists at the NWS have access to many more products than those available on the internet. Our warning meteorologists are looking at new products continuously, and at several different levels in the atmosphere.Clear Air ModeIn this mode, the radar is actually in its most sensitive operation. This mode has the slowest antenna rotation rate which permits the radar to sample a given volume of the atmosphere longer. This increased sampling increases the radar's sensitivity and ability to detect smaller objects in the atmosphere than in precipitation mode. A lot of what you will see in clear air mode will be airborne dust and particulate matter. Also, snow does not reflect energy sent from the radar very well. Therefore, clear air mode will often be used for the detection of light snow.The radar continuously scans the atmosphere by completing volume coverage patterns (VCP). A VCP consists of the radar making several 360� scans of the atmosphere, sampling a set of increasing elevation angles. There are two clear mode VCPs. In clear air mode, the radar begins a volume scan at the 0.5� elevation angle (i.e., the radar antenna is angled 0.5� above the ground). Once it makes two full sweeps (a surveillance/reflectivity sweep and a Doppler/velocity sweep) at the 0.5� elevation angle, it increases to 1.5� and makes two more 360� rotations. For one of the clear air mode VCPs, two full sweeps are also made at 2.5�. Otherwise, at the higher elevations (2.5�, 3.5�, and 4.5�) a single sweep is made (reflectivity and velocity data are collected together). This process is repeated at 2.5�, 3.5�, and 4.5�. Then the radar returns to the 0.5� elevation angle to begin the next volume scan which will repeat the same sequence of elevation angles. In clear air mode, the complete scan of the atmosphere takes about 10 minutes at 5 different elevation angles. Precipitation ModeWhen precipitation is occurring, the radar does not need to be as sensitive as in clear air mode as rain provides plenty of returning signals. At the same time, meteorologists want to see higher in the atmosphere when precipitation is occurring to analyze the vertical structure of the storms. This is when the meteorologists switch the radar to precipitation mode using one of two volume coverage patterns.Both precipitation VCP's begin like the clear air mode mentioned above with the same evaluations scans as in the clear air mode. The difference is the radar continues looking higher in the atmosphere, up to 19.5� to complete the volume scan. The time it takes to complete the entire volume scan is also less. In the slower VCP, the radar completes the volume scan of nine different elevations in six minutes. In the faster VCP, the radar completes 14 different elevation scans in five minutes. Differences in the quality of radar images between the two precipitation mode VCPs are relatively minor. Therefore, during severe weather, the faster VCP is almost always used as it provides the meteorologists with the quickest updates and most elevation slices through the storms. In summary, when the radar is in clear air mode, radar images on the internet will be updated approximately every ten minutes. In precipitation mode, the updates will occur around five to six minutes apart. What do the colors mean in the reflectivity products? |
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The scale of dBZ values is also related to the intensity of rainfall. Typically, light rain is occurring when the dBZ value reaches 20. The higher the dBZ, the stronger the rainrate. Depending on the type of weather occurring and the area of the U.S., forecasters use a set of rainrates which are associated to the dBZ values. These values are estimates of the rainfall per hour, updated each volume scan, with rainfall accumulated over time. Hail is a good reflector of energy and will return very high dBZ values. Since hail can cause the rainfall estimates to be higher than what is actually occurring, steps are taken to prevent these high dBZ values from being converted to rainfall. |
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What is the difference between
base and composite reflectivity? The main difference is composite reflectivity shows the highest dBZ (strongest reflected energy) at all elevation scans, not just the reflected energy at a single elevation scan. This can be seen in the images below from the Salt Lake City radar.
Why the difference? Base reflectivity only shows reflected energy at a single elevation scan of the radar. Composite reflectivity displays the highest reflectivity of ALL elevations scans. So, if heavier precipitation is higher in the atmosphere over an area of lighter precipitation (the heavier rain that has yet to reach the ground), the composite reflectivity image will display the stronger dBZ level. This occurs often with severe thunderstorms. The updraft, which feeds the thunderstorm with moist air, is strong enough to keep a large amount of water aloft. Once the updraft can no longer support the weight of suspended water then the rain intensity at the surface increases as the rain falls from the cloud. What is UTC Time?Weather observations around the world (including radar observations) are always taken with respect to a standard time. By convention, the world's weather communities use a twenty four hour clock, similar to "military" time based on the 0� longitude meridian, also known as the Greenwich meridian. Prior to 1972, this time was called Greenwich Mean Time (GMT) but is now referred to as Coordinated Universal Time or Universal Time Coordinated (UTC). It is a coordinated time scale, maintained by the Bureau International des Poids et Mesures (BIPM). It is also known a "Z time" or "Zulu Time".To obtain your local time here in the United States, you need to subtract a certain number of hours from UTC depending on how many time zones you are away from Greenwich (England). The table below shows the standard difference from UTC time to local time.
The switch to daylight savings time does not affect UTC. It refers to time on the zero or Greenwich meridian, which is not adjusted to reflect either changes either to or from Daylight Saving Time. However, you need to know what happens during daylight savings time in the United States. In short, the local time is advanced one hour during daylight savings time. As an example, the Eastern Time zone difference from UTC is a -4 hours during daylight savings time rather than -5 hours as it is during standard time.
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