Radar Discussion – Velocity Azimuth Display / Rose Marie Kern
Date: Tuesday, 20 October 2009 @ 03:02:12 EDT
Topic: Air Traffic Control
The Velocity Azimuth Display (VAD), is a National Weather Service (NWS) radar product little used in fixed wing aviation, but familiar to balloonists or anyone flying low. It is a tool that pilots and flight service specialists can use to provide critical information concerning low level wind shear and provides data about changes in wind flow patterns during a climb to higher altitudes.
Like normal weather radar, the VAD takes readings from precipitation, but the sensitivity of this instrument also reads tiny dust particles floating aloft. It can sense planes, birds, and butterflies as well, though its purpose is to reveal patterns over a given point. This means that any life form or aircraft picked up for just a moment in time does not normally affect the display. One exception to this is the seasonal or diurnal migration of large flocks of birds, which can alter the VAD readings at lower altitudes.
A VAD wind readout is measured in AGL altitudes at thousand foot increments. What an amateur will see the first time is what looks like an excel spreadsheet full of checkmarks with extra ticks. Those familiar with the NWS wind flow charts will see that each indicator, or flag, is actually pointing in the direction the wind is flowing towards. It is aligned with standard compass headings so that if it points up, wind flow is from south to north. It does not provide a specific direction in degrees.
The VAD display shows the winds at the MSL levels. For instance, ABQ VAD display does not even begin until 6,000 feet - because the surface there is 5,340. The Amarillo site starts with 1,000 - but the first 4,000 is always ND (no data) for the same reason.
The tick marks on the tail are speed measurements. A full tick is 10 knots, a half tick is five knots. A triangular sail at the end of the tick indicates fifty knots. At the bottom of the page are times listed in Greenwich Mean Time or GMT. Each reading is six minutes apart. The wind/speed indicators above indicate what was observed at that moment in time by the radar.
As long as there is something floating in the air, the VAD will read it. Sometimes the VAD has no readings. At the surface up to about 12,000 feet no readings indicate that winds are calm, but if there are winds — even strong winds above 15,000 feet, the VAD may have no return simply because of a lack of particulate matter floating about.
The VAD is used extensively for ballooning. During the Balloon Fiesta in Albuquerque each year, there is a competition wherein the participants must launch their balloons at least one mile away from the balloon field, then attempt to catch the correct winds to float over the field. On top of a 20 foot pole is the key to a brand new car. Whoever gets the key wins the car!
Early that morning, dozens of balloonists call Flight Service and request the VAD wind readouts. A minor difference between altitudes can mean the difference between missing the field entirely or winning the prize.
During the cross country gas balloon race one year, about four of the helium balloons were on the north side of the Texas panhandle when an isolated afternoon thunderstorm built up quickly. Two of the balloonists chose to call Flight Service to get the VAD winds. By altering their altitudes, they were able to skirt well away from the storm. The other two were forced to land.
Fixed wing aircraft can benefit in areas where a high windshear potential exists. In southern California during a Santa Ana windflow, the surface winds can be almost dead calm, but within 500 feet the wind from the northeast can be ripping along at 30 to 40 knots. Checking the VAD wind readings in the area can provide the data needed to anticipate the sudden onset.
VAD winds can also point out potential problems in mountain passes. Pilots know to watch out for and avoid roll clouds and altocumulus standing lenticulars (ACSL) when they are trying to keep away from shearing, but when there is not enough moisture present, these visual indicators may not be available. This is a good time to call Flight Watch and request any VAD wind readouts in the area.
To access VAD wind readouts at home there are a couple options. One is www.weatherunderground.com. At the top, look for NEXRAD. Click on that, it will show a U.S. map with crosses wherever there is a radar site. Click on the one where you want to see the radar. When the radar comes up, on the left edge of the picture a long thin bar reads “advanced radar types.” Click there, then at the bottom of the list click on Velocity Azimuth Display to see a current readout.
A simpler method is available at www.pilotsandweather.com – on the top of the first page after logging in there is a box labeled 2D-WX. The symbol on the far right takes you straight to a VAD wind display. You do have to sign up for the service.
If you need help interpreting the VAD, call Flight Service and ask for New Mexico or West Texas. These briefers talk to balloonists more than the others in the country so they use VAD the most and will be able to discuss how VAD operates. Remember that if you want to call Flight Service for extended discussions on weather, call in the evening when there are not so many other pilots competing for briefings on imminent flights.
Rose Marie Kern has worked in ATC for over 25 years. If you have a question you can contact her email: author@rosemariekern.com
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