...wisps of personality passing on the zephyrs of time...©
Ephemerata Weather Radar Project
Text Discussion
Disclaimer - I am providing this information for my personal use, and making it available for friends and interested observers. It should not be relied on as your sole source of information in regard to dangerous weather (nor should any single source). When in doubt, review all available information sources and take personal precautions as is necessary in the circumstances. Look out and up, rather than rely strictly on technological aids!
Introduction:The Ephemerata Weather Radar Project is a personal project that focuses on monitoring the weather in the region in which I live, for general interest and emergency preparedness, and to also monitor weather in selected areas for interested parties, or for general interest and study. Additionally, the project involves a collaboration on developing the capabilities of the software to enhance its overall utility.
This web presentation is but one facet of the software and the project, offering a means by which others may both learn about active weather, and if they live in the areas targeted, to learn and perhaps be advised about current active weather in their own area. Scans A and B will normally concentrate on my home turf weather, while other scans will typically feature weather events remote from me which I am also observing, or which may be of general interest to others, e.g. Alaska's Redoubt Volcano, which has been featured here and is still being monitored in the EWR Blog.
Scan A typically presents the most recent radar updates from Ephemerata Weather Radar's alert system processor, powered by Intelliweather's Storm Predator software. Scan B images are generated by multiple iterations of Gibson Ridge's GRLevel3 software.
For scans A and B, data is downloaded from the US National Weather Service NEXRAD Doppler systems for radar centres in Buffalo, N.Y., Cleveland, Oh., Watertown, N.Y., or Detroit, Mi., depending on the anticipated weather direction or service status of the various radar stations. The radar system builds a model of the weather over the area it scans by sending out radio pulses and listens to hear if they are reflected back off water droplets in the atmosphere. The strength of the "echos" give a measure of the amount of water in the atmosphere, which relates directly to the precipitation that may fall.
The NEXRAD Doppler ("NEXt generation RADar") system does a complex radar scan over a 5-10 minute sampling period (see VCP discussion below for details), collecting radar data that is then processed and made available to the NWS servers for download. Because of the complexity of the scan and the processing involved, NEXRAD is not quite a "real time" display and there will always be a time-lag of 10-20 minutes to the web update. Nonetheless, the analysis provided by NEXRAD is especially useful for forecasting the significance of impending weather, due to the detailed information the scans provide.
The web map will generally report either the "long range base reflectivity (268 mi)", short range base reflectivity (143 mi)" or "short range composite base reflectivity" for moisture in the air. The radar scan comprises a more or less circular sweep around the radar site (the scan appears oval on the map due to a distortion inherent in the mapping). Since the radar is less able to discriminate soft targets at distance, the radar image will appear to show less "weather" out at the edges. This is not necessarily the case.
If there is no significant active weather in the Great Lakes/Northeast region or approaching within the next 12-24 hours, the display will normally be in long range scan mode, on reduced update frequency or offline, to reduce unnecessary network demand and NWS server load. A recent time stamp on the map may therefore be an indication that active weather is anticipated within a few hours, even if no active weather is presently displayed.
Radar loops are available of the active weather passing through the radar site, and will be presented based on network and traffic resource availability. The loop, when available, will be accessible by clicking on the link below the image. The loop consists of 10-12 previous scan frames spaced either 6 or 10 minutes apart in time, depending on the mode of the radar site.
Other radar product maps which are available and may appear from time to time are: Base velocity, Storm relative motion, One-hour precipitation, Storm total precipitation, Vertically Integrated Liquid, and Echo Tops.
My interest centres around weather conditions for the western end of Lake Ontario, especially during times of severe weather. However, all other areas serviced by the U.S. National Weather Service can be called up on special request. As a matter of general interest and for interpretation practice, I may switch to areas of severe weather in other areas if the local weather is quiet.
How to use the Ephemerata Weather Radar site:The Ephemerata Weather Radar project web presentation comprises several facets: the main radar scan image and associated documentation, the smaller US national maps above it (expandable and with available animation), and the severe weather email alert system (see Scan Zone, below). Additionally, I may report other notes concerning the page on the EWRadar blog (accessible at links top and bottom of this page.
Using the page starts with reviewing the main Scan A image above for weather activity in the territory covered by the radar station currently displayed, and especially in and around the scan circle if you are in the GTA (see below), or in southwestern Ontario, if the SW Ontario alert zone feature is activated (see immediately under the Scan A image). Following Scan A are regional scans, Scan B and Scan B Supplemental(#). These scans are varied according to location and severity of local weather, and are based on level 3 NWS doppler radar data, which includes some automated analysis features such as storm track, mesocyclone, hail and tornado signatures. See the Scan B Notes accompanying these scans for more discussion about the automated icon popups. More extensive interpretive notes follow to assist you in understanding what is conveyed by the radar scan.
If no active weather is shown, or, if it appears that current weather may soon end, check the small national maps above, centre and right, to see what the composite frontal, NEXRAD and satellite picture for the broader region looks like. The small prognostication chart, far left, will take you to a matrix of 4 12-hr forecast displays. These roll forward with time and come from the US National Center for Atmospheric Research (NCAR). These maps are clickable and some can be animated. They will quickly tell you whether there is weather pending that is outside the range of the local station. The animation feature will show you how the larger system is moving, and whether it will move into range of the station. Use your browser "back" button to return to the EW Radar page when done. When attended, I may switch radar stations and scan update rates to reflect the current or anticipated weather pattern, returning to focus on the scan zone coverage (see below) when active weather occurs at the western end of Lake Ontario.
The small map, above left, indicates the status of the NWS NEXRAD grid (our experience is that it may not be entirely accurate; - its based on whether the station polled responds in a timely fashion, not necessarily whether the station is active). Stations do go offline occasionally for maintenance. Infrequently, the main scan page may report an alert indicating the data feed is offline- this is usually due to traffic congestion at the NWS server and our processor is waiting for an update. The web scan will usually update on the next updater cycle. [return]
Scan Zone Warnings:The 35 mile(52 km) radius yellow scan circle around the Golden Horseshoe (western end of Lake Ontario) indicates a warning zone which will trigger an automated email response to qualified subscribers (email for this, if you wish) when severe weather (>=60 dBZ,>4-8"/hr rain, 12cm/hr snow - third red box) enters the zone. Once an email alert has been sent, subsequent alerts are suppressed for 60 minutes, to avoid slow moving cells from sending too many multiple alerts. The email alert will include a mini version of the most recent scan issued, and may be more recent than what appears here on the website, as it issues immediately upon update from the NWS data. Therefore, refer to it to determine if you may be in the path of severe weather.
A comparable scan zone alert system has been added for southwestern Ontario, and is implemented as indicated by weather patterns. If this alert zone is active, there is a clickable option under the Scan A image that will take you to the southwestern Ontario scan map. This map shows the scan zone in effect for SW Ontario and the current weather over the region, monitoring KDTX Detroit or KCLE Cleveland according to the flow pattern of the incoming weather.
The Sw Ontario alert zone feature is normally only active when weather conditions indicate, as it requires that a second computer be brought online to the system to manage this zone alert system. The SW Ontario alert system maintains a separate email notification list, so you may receive alerts for either SW Ontario or the GH-GTA, or both as you wish, depending on your subscription preference. Due to Ontario's geography, this SW Ontario alert zone is somewhat larger than the GH-GTA, approximately 65 mile radius. This means you may receive alerts that are not that close to your location, but this is unfortunately a result of Ontario's geography and the limits of the scan zone shape currently available. When this scan zone triggers an alert, it is the SW Ontario scan map that is sent out. Both alert systems will send out alerts to the EWR Wx Alert Twitter feed.
Due to the variability between the NWS updates and our web updater, the web map updates approximately every 6-15 minutes when the updater is active (An automatic web updater has been added to the program so that when it is running, the web image is updated immediately after the NWS data has come through. During active weather, that means the web images should be about 6-7 minutes apart and within 7-10 minutes of the current time). In clear air where no active weather is expected for some time, the web update may run on hourly updates or be off to reduce network traffic.
The 35 mile radius (GH-GTA) was chosen to reduce the number of false alarms to the target area and to relate to the speed at which weather systems typically move. A storm front comes through the zone at an average speed of 25-50 miles per hour. Therefore, the lead time for weather bearing down on your location within the zone varies from imminent, to about an hour, depending upon the speed of the storm and your location within the zone. A storm cell or front moving at 35 mph(52 kph) will take 2 hours to cross the zone if it moves through the centre. Always check the time and date on the map to be sure of the interval between your present time and when the image was run. One hour will generally be the MAXIMUM advance warning, and usually less - about a half hour, if you are in the path.
While local weather events may come from any direction into the scan zone due to circulation around highs (clockwise) and lows (counter-clockwise), the predominant air mass flow is easterly (i.e. trending west to east) from within the SW<-->NW quadrant. Check outside the zone in this quadrant to see if there is weather about to enter it. This is especially true for signatures tagged red on the displays, as they signal heavy precipitation, including thunderstorms or potential blizzards in season. [return]
How to Interpret the Radar Scan:The radar display will be presented in either contoured or raw mode. The raw mode shows the pixelated radar returns as they are obtained from the scan, i.e. they are represented by point-in-time discrete return "blocks" of scan data as provided by the NEXRAD radar system. This mode is better at displaying peak levels, and will be used frequently during active weather. It is this data that the alert system uses to send out warnings.
The contoured view is an averaging of the return elements to produce a smoother visual display. This display is more representative of what the atmosphere looks like, but the averaging process means that returns with a low pixel count will get lost in the averaging and may not display. For example, a passing thunderstorm may have core elements of 70dBZ(pink) or higher, but on averaging, contour may only show dark red(60dBZ). The alert system will detect the higher return value and report it. Map details are easier to read in contour, and this mode will be the default mode for low impact weather. The map legends will indicate contoured display in most cases. Once you are familiar with the display, the two styles are easy to distinguish.
Trace amounts of precipitation may fall for dBZ levels +05 through +20. Active rain starts to fall at dBZ levels of +20 (trace) or greater. As snow is less radar reflective than liquid water, snowfall may be heavier than radar returns indicate. The scale is logarithmic, meaning that each level potentially means twice as much precipitation as the previous one. Local geography may influence what you experience compared to what the radar "sees", as does distance from the radar site (effects of scan angle).
Seasonal Colour Tables:
RAIN mode: When RAIN mode is indicated in the map legend,
Green areas denote light to moderate active rain,
progressing to yellow and red with intensity. Dark
red(s)(>4 in/hr) will trip the scan alarm as possibly severe
weather. In Short Range Composite mode (not "decluttered", i.e.
not adjusted to remove echos unrelated to weather), beige,
purple and gray shades (minus dBZ values) in clear
air generally represent ground clutter, haze, pollution, heavy
cloud and inversions in the atmosphere (In clear air mode (VCPs
31 & 32), display not contoured, you may also see aircraft in and
around Toronto, Hamilton and Buffalo, and out over the lakes.
Look for the small beige "blips" - you can easily pick out the
landing patterns into Hamiton and Toronto(Pearson) airport (and
occasional goose flocks). For Buffalo, ground clutter tends to
obscure the planes). When the radar system switches to a
precipitation mode (VCPs 11,12,121,21,212), the minus dBZ scan
values are suppressed automatically ("decluttered").
The display does not show clouds specifically (but may), but rather, is a measure of the density (i.e. amount) of moisture in the air that approaches a level that may result in discernable precipitation. Yellow, gold, red and darker areas are areas of heavy precipitation (or strong reflectivity - hail in the upper levels will return strong echos even though rain may only be moderate). Red areas containing purple, pink and white are considered dangerous areas of severe precipitation. While tornadic activity can occur in any colour area from yellow up, its more likely to be present in the interface areas with reds and beyond.
SNOW mode: When SNOW mode is indicated in the map legend, white through blue areas denote light to moderately heavy snowfall. Moving through purples to the reds and beyond indicate increasingly heavy and wet snowfall, or may signal the potential for freezing rain or sleet. Red and beyond will generally denote very dangerous winter condition conditions. From purples on up, the water content of the cold atmosphere in increasing and may lead to dangerous ice storm conditions. Its important to note that due to the decreased reflectivity of atmospheric snow, snowfalls may be more intense than the radar return indicates.
Rate of snowfall is a bit difficult to estimate from radar traces due to the variable reflectivity of snow, but the following is a general guide: DBZ reflections in light purple to blue (5-35dBZ) represent flurries to moderately heavy snowfall. 25dBZ is about 1/2cm/hr fall. Colours between 30-35dBZ generally represent about 1cm/hr or more snowfall. Reflections above 35 dBZ (deep purple zones, and above) are uncommon for snow in persistent cold climates due the lower density of snow compared to ice or rain. Therefore, winter storm traces with deep purple and red cores indicate conditions which may be associated with either freezing rain or sleet - the higher intensity reflections indicate melting of snow up in the atmosphere, which may or may not refreeze depending on other atmospheric conditions, especially if ground temperatures have been cold for some time. Driving conditions will generally begin to become difficult at 30dBZ and above if persistent, and is somewhat dependent on temperature and local snow removal practices.
Lake Effect Snow: The Great Lakes are known, of course, for their "lake effect snows". These range from intense snow squalls to light flurries and they result from strong cold winds, typically from the west around to the north, blowing across the relative warm lakes in the early to middle winter period (December and January), picking up moisture from the lakes, and dropping it as snow when the moisture laden winds rise and cool over the lee shore of the lakes. On radar, they are characterised by patches of echo return on the lee shore of the lakes, or as long thin streamers of returns aligned with the direction of wind. In both circumstances, the squall depth is shallow, usually less than 10,000 feet. This low altitude means that radar beams will overshoot the squall fairly quickly, therefore you can expect there to be snowfall at a distance from the radar sites in line with the streamers that is not being detected by the radar beam. Once the lakes freeze in late January or early February, the lake effect squalls diminish.
MIXED mode: In keeping with the standard colour scheme for mixed precipitation, a colour array based on purple is now displayed when the precipitation type is of mixed character. As intensity increases, the MIXED mode progresses through purples to blues to reds as severe weather is approached. MIXED mode presentation is an artificial construct as the NEXRAD radar is unable to determine the type of precipitation. MIXED mode presentation therefore must be interpreted as meaning precipitation may be rain, snow, or a mix of both depending on the local circumstances. Intensity rules outlined above for rain and snow are equally valid in this presentation, e.g. if its raining, then the dbZ values for rain are in effect. In typical televised radar presentations, precipitation is typically coloured blue, purple and green/yellow in relation to the type and air temperature. It should be recognised that this is an interpreted artifact created by the weather service providing the graphics, and is not data direct from the radar system. This mode may operate with the cities overlay turned off so as not to interfere with the interpretation display.
I have added a history loop of radar scan frames which is accessible from a link below the current scan image, when it is operational. The loop consists of the most recent 10-12 scan frames spaced either 6 or 10 minutes apart in time, depending on the precipitation mode of the radar site, and will therefore trace the previous 40-70 minutes of active weather. This will allow the viewer to see the directional trend and speed of the weather patterns without having to wait for a series of successive scans to be displayed. This is not on all the time due to the bandwidth and cpu requirements needed to run it in the current software configurations.
The radar cannot see directly overhead, so during active weather, on occasion there may be an apparent "hole" in the weather over the radar site. This is not the case, only that the radar cannot see it. During periods of heavy weather the minus dBZ radar returns ("clutter") in the display are usually turned off, in order to more clearly show the active weather present.
NEW! I have implemented mosaic radar views comprised of four-panel scan arrays that are designed to fill the screen with associated radar products from a given NEXRAD site. Additionaly, selectable national views of basic current atmospheric conditions are available, courtesy Intelliweather Inc. Formats have been prepared for small monitors (1024x768 resolution, as well as large monitors (20" or greater with 1600x1000 resolution) The Mosaics Selector page is click-selectable from the general link bar accompanying each scan. I am also working on a independent-window version of the entire page for multi-display users.
Level III Doppler Radar (Scan B, currently) includes automated algorithms to detect potentially severe weather, and report it on the radar image as icon overlays. Four basic circumstances are reported: estimated storm cell track, potential hail , "mesocyclonic" rotation in a storm cell, and Tornadic Vortex Signatures(TVS and ETVS-Elevated TVS). 
Once a storm cell is identified and characterized, its estimated track is placed on the display as a velocity vector, showing direction of movement and apparent speed (length of line - each "+" is an estimated 15 minute position from the current position). Coloured triangles (points up) on storm cells are hail indicators (small open/solid triangle, 30%/50% chance of hail; large open/solid triangle, 30%/50% chance of severe hail). Arrows in a circle are "mesocyclones" (literally, "small cyclones") - localized storm rotation conditions from which tornados may develop. Inverted triangles (points down) indicate tornadic signatures (open/solid, ETVS/TVS) have been detected by the radar system. The colour of the icon is an indication of intensity similar to that of the radar trace itself. All of these indicators are re-evaluated with each sweep cycle of the radar. It is important to understand these are reports of potential conditions for severe weather, not an actual occurrence. Prudence demands that when these indicators appear, precautions should be taken to confirm weather conditions and seek appropriate shelter.
| dBZ | Rain Rate (in/hr) |
|---|---|
| 65 | 16+ |
| 60 | 8.00 |
| 55 | 4.00 |
| 52 | 2.50 |
| 47 | 1.25 |
| 41 | 0.50 |
| 36 | 0.25 |
| 30 | 0.10 |
| 20 | Trace |
| < 20 | No rain |
Time, Date, Radar Location, Scan Type: VCP:31,32 - Clear air modes - the radar system shifts to precipitation modes VCP:11,12,21,121,212 in heavy weather to adjust for increased reflectivity due to amount of water in the atmosphere. RAIN, SNOW and MIXED indicate display colour set and not necessarily precipitation type. A detailed discussion of the VCP modes and how they influence the scan display can be found below.
Warning Boxes:The NWS overlays NEXRAD radar maps with warning boxes colour-coded to specific concerns. The list below will guide the interpretation of the warning overlays. For Canadian terrritory, the warnings are not displayed; however, if radar signatures for your area of interest resemble those for adjacent areas which the NWS boxes cover, then you can reasonably assume similar severity may occur in your area.
You may freqently notice warning boxes at the edge or beyond the current radar scan. These boxes may appear in areas where the radar does not appear to report any significant weather. At the limit of the radar site's coverage, the radar beam, even at its lowest scan angle, is overshooting the tops of cloud systems. Therefore, the presence of warning boxes (usually generated by the adjacent station)is evidence that significant weather extends beyond the local radar site and may affect your local area in time depending on the direction of the weather system.
Red - Tornado Warning. Issued when a tornado is imminent or occurring. A Tornado Warning implies an immediate threat to life and property.
Yellow - Severe Thunderstorm Warning. Issued when a severe thunderstorm is imminent or occurring. A severe thunderstorm is defined as hail ¾"(20mm) or greater and/or a wind speed of 58mph(90kmph) or greater.
Green - Flash Flood Warning. Issued when flash flooding is imminent or occurring in the marked area.
Orange - Marine - Special Marine Warning. Issued for hazardous weather conditions (thunderstorms over water, thunderstorms that will move over water, cold air funnels over water, or waterspouts) usually of short duration (2 hours or less) and producing sustained winds or frequent gusts of 34 knots (68kmph) or more that is not covered by existing marine warnings.
Pink - Special Wind Warning. [return]
Recommended Learning Resources
The US National Weather Service and the National Oceans and Aeronautics Administration have an excellent online learning program called JETSTREAM. Designed especially for web use, it provides interested novices with a reasonably detailed, yet beginner-friendly overview of what makes weather, and how the NEXRAD/DOPPLER Radar System interprets it, and how you can make sense of the radar images. For a quick guide to the Jetstream topics look over the topic matrix. [return]
Environment Canada has an online Severe Weather Watcher's Handbook which is quite detailed and abundantly illustrated with actual storm and cloud images. Photos could be bigger, but the presentation is nicely done. For information about common radar "anomalies" leading to interpretation problems, EC has a section illustrating the most common things that interfere with the radar scan image.
and What They Tell You About the Radar Scan
from Jetstream
The radar continuously scans the atmosphere by completing volume coverage patterns (VCP). A VCP consists of the radar making multiple 360° scans of the atmosphere, sampling a set of increasing elevation angles.
There are two main operating states of the WSR-88D; Clear Air Mode and Precipitation Mode. Within these two operating states there are several VCPs the NWS forecasters can utilize to help analyze the atmosphere around the radar. These different VCPs have varying numbers of elevation tilts and rotation speeds of the radar itself. Each VCP therefore can provide a different perspective of the atmosphere.
Unlike its predecessors, the WSR-88D antenna is not directly controllable by the user. Instead, the radar system continually refreshes its three-dimensional database via one of several predetermined scan patterns. Since the system samples the atmosphere in three dimensions, there are many variables that can be changed, depending on the desired output. There are currently nine Volume Coverage Patterns (VCP) available to NWS meteorologists. Each VCP is a predefined set of instructions given to the antenna that control the rotation speed, transmit/receive mode, and elevation angles. The radar operator chooses from the VCPs based on the type of weather occurring:
Common among all VCPs (except for VCP 12) is the tilt elevation of the lowest five elevation angles. The scanning begins with 0.5° elevation, meaning the centerline the radar beam antenna is angled 0.5° above the ground. Since the the beam itself is 1° wide, it returns information about what it "sees" between 0° and 1° above the horizon. As it completes that elevation scan the radar is tilted another degree with the center line of the beam now at 1.5°, and the process of observing the atmosphere begins again then continues through the 2.4°, 3.4° and 4.3° elevation angles.
Clear Air Mode
Clear Air mode is used when there is
no rain within the range of the radar. In this mode, the radar is
in its most sensitive operation state. 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 typical radar image in clear air mode will not reveal much.
Generally, the only returned energy to the radar will be very
close to the radar's location. A lot of what is seen will be
airborne dust, bugs, and particulate matter (image at right).
However, snow does not reflect energy sent from the radar very well. So clear air mode will occasionally be used for the detection of light snow as well. Also, this mode is helpful in detecting discontinuities in the air mass, such as a frontal boundary, and in monitoring the onset of precipitation.
There are two clear mode VCPs; VCP 31 and VCP 32. Both VCPs complete a volume scan using five elevation angles in 10 minutes. For both VCP's, the radar makes two 360° scans of the atmosphere at both the 0.5° and 1.5° elevation angles. During the first scan at each elevation, the radar is in surveillance mode and is looking for objects. During the second sweep at each of these two lowest elevation angles, the radar is determining the velocity of the wind. In the remaining three elevation angles, the radar conducts both surveillance and velocity operations together.
The difference between VCP 31 and VCP 32 is defined by the pulse mode. VCP 31 uses a "long pulse" mode meaning the time the radar is transmitting each pulse is 4.7x10-6 seconds. This is repeated 314 times a second (the pulse repetition frequency is at its lowest). Therefore, the wavelength is much longer in this mode, increasing the radar's sensitivity. This comes at a cost which is a decrease in the range of the winds the velocity radar can determine.
VCP 32 has a higher PRF (more pulses per second) so it is not quite as sensitive as VCP 31, but it can now detect a wider range of the wind's velocity. For this reason, most NWS doppler radars will be in VCP 32 during the Clear Air Mode.
Precipitation ModeWhen precipitation is occurring, the radar does not need to be as sensitive as in clear air mode, since 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 meteorologists switch the radar to precipitation mode.
Currently, there are four precipitation mode VCPs. VCP 11 has 14 elevations slices and completes 16 360° scans in 5 minutes, up to 19.5°, to provide better sampling of the vertical structure of storm clouds and to produce images at a much quicker pace. For several years, VCP 11 was the most common operating mode during severe weather. This mode provides rapid updates as well as the ability to see high into the atmosphere.
VCP 21, while it also tilts up to 19.5° to see high into the atmosphere, operates at a slower rotation speed and eliminates some of the upper elevation tilts. In this mode, the radar takes 6 minutes to move though these 9 elevation tilts. This is used primarily for "strato-form" precipitation where vertical features of rain clouds are not as important as they may be during the convective, thunderstorm-type of rain.
The two newest VCP's available to the NWS forecasters are VCP 12 and 121. VCP 12 also has 14 elevations slices, like VCP 11, but performs 17 360° scans in a very fast 4 minutes 6 seconds. Instead of 1° elevation tilt increments seen in all other VCP's, the elevation tilt increase in VCP 12 range from 0.4° to 0.9° up to 4°. In other words, the radar beams overlap each over.
This provides a denser vertical sampling at lower elevation angles, which means better vertical definition of storms, improved detection capability of radars impacted by terrain blockage, better rainfall and snowfall estimates, and results in more storms being identified, in addition to the quicker update cycle.
VCP 121 addresses velocity aliasing, or, the ability of the radar to determine wind velocity and problems caused by "second trip echoes". With the same nine elevation tilts as VCP 21, VCP 121 completes 20 rotations in five minutes. The difference is the radar makes several elevation scans at the same elevation tilt, but at different pulse durations (called "pulse repetition frequency" or PRF).
This gives the radar the ability to minimize "range folding" The radar normally determines the range to an object based on the time it transmits a pulse, until the time it receives a returned signal. However, depending upon how fast the radar is transmitting pulses, the returned signal may be associated with one of the previous pulses, known as second (or third) trip echoes.
If the PRF is low (longer time between transmission of pules) the signal can travel farther to the more distant objects and reduces second trip echoes. However, the ability to determine velocity is greatly reduced. High PRF's (less listening time between pulses) greatly improve the radar's ability to determine velocity. Yet, it also increases the number of second or third trip echoes. This tradeoff between distance and velocity is known as the doppler dilemma.
VCP 121 combines varying PRF's and different antenna dish rotation speeds to help decrease range folding. [return]
How to Use EWR Scan Zone Mosaic Display Interpretation Map Legend VCPs Top EWR Blog
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