AOPA Pilot Magazine - August 2011 - 96

WXWATCH
at the heart of the matter is a simple fact: What you’re seeing on your cockpit display is something that happened in the past. In real time, that clear path ahead of you may no longer be clear. Nexrad basics To understand some of the traps associated with using datalinked Nexrad, it helps to know a little about their modes of operation. First off, these radars—dubbed WSR-88Ds; the “88” is for the year they debuted, the “D” is for

Doppler—are ground-based and have rotating antennas. The antenna dish can be tilted up or down at several angles to scan the sky, and it takes a few minutes for the antenna dish to make a complete rotation. Here’s where the delays begin to kick in. Nexrad can work in two modes—clear air or precipitation. In clear air mode, the antenna is at its slowest rotation, making up to 14 sweeps at five different tilt angles. In all, it takes about 10 minutes to make a full scan of the atmosphere around the radar site. Clear air mode is used to detect light precipitation and

Clear air mode volume coverage pattern (VCP)
70 Height (thousands of feet) 60 50 40 30 20 10 0 0 20 40 60 80 Range (miles) 100 120 4.5 3.5 2.5 1.5 0.5 Elevation

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Nexrad radar operating in the clear air mode volume coverage pattern makes 360-degree scans of the atmosphere at five different elevation angles. Two full sweeps are made at the 0.5- and 1.5-degree elevations; another one or two sweeps are done at the 2.5-degree elevation. At 3.5 and 4.5 degrees single sweeps are made. Clear air mode is useful for detecting the weakest precipitation returns.

snow because the slow scan rate makes the radar better able to see smaller particulates (birds, bats, and insects, too). In precipitation mode the antenna rotation rate is faster because precipitation is an excellent reflector of radar energy. In this mode, the antenna scans 14 tilt angles in five minutes, looking upwards as high as 19.5 degrees, and as low as 0.5 degrees. The idea here is to gather as much information as quickly as possible, which is what we want when we’re flying around convective weather. This is what the National Weather Service (NWS) uses most in the summer months when convection is occurring. There’s a whole lot more to radar meteorology, but that will do for now. Now we know that there are 14 possible tilt angles, and that scanning them all in precipitation mode takes about five minutes per radar site. But wait, it turns out there are two ways of presenting this radar coverage. Composite reflectivity Composite reflectivity imagery shows an amalgam of all the tilt-angle scans. The goal is to depict the maximum echo intensities from any antenna sweep at any elevation angle. Think of a composite reflectivity image as a stacking of individual scan overlays, with each one showing the highest levels of radar reflectivity. The imagery can extend as far as 248 nm from the radar site. But as you might expect, resolution at that range falls off. In addiAOPA PILOT •

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AUGUST 2011

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AOPA Pilot Magazine - August 2011

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