Lesson 02 of 1030 minBeyond SKYWARN

The beam, the curve and the holes

Why the radar sees higher and coarser the further out it looks, what sits directly overhead that it cannot see at all, and how to spot an echo that is not weather.

By the end of this lesson

M9.2.aExplain why beam height increases with range and estimate it from the course table.

M9.2.bExplain beam broadening and the cone of silence.

M9.2.cDescribe where CONUS coverage gaps are worst and why.

M9.2.dIdentify anomalous propagation, ground clutter and beam blockage.

The numbers are in module 1

Module 1 lesson 1 carries the beam-height table and it is the course authority for those figures. Go and read it again if the numbers are not in your head: the short version is that a beam leaving the antenna at half a degree is about 1,900 feet up at 50 km and about 8,600 feet up at 150 km, because the earth curves away underneath a beam that travels almost straight.

A cross-section from a radar site out to long range over a curved earth. The lowest beam leaves the antenna at half a degree and travels almost straight while the ground curves away beneath it, so the gap between beam and ground widens with distance and the beam itself spreads into a widening cone. A storm near the radar is sampled near its base; the same storm at long range is sampled near its middle with its lowest levels entirely beneath the beam. Above the radar a shaded wedge marks the cone of silence, where the highest tilt still cannot look straight up. — Two effects at once, and they compound. The beam rises away from the ground, and it widens, so a distant sample is both higher up and averaged over a much bigger volume.WxAlerts

Beam broadening

The beam is a cone about 0.95 degrees across, not a line. At 25 km that cone is a few hundred metres wide; at the edge of range it is several kilometres. Whatever the radar reports for a point is an average over that whole volume.

So a small feature loses its signature with distance even when the feature itself has not weakened. A tornado a few hundred metres across occupies a sliver of a sample volume kilometres wide, and it is averaged in with the quiet air around it. This is the single most important fact behind lesson 6, and it is why the same circulation produces a dramatic signature near the radar and almost nothing at range.

The hole directly overhead

The holes between radars

Maddox and colleagues mapped this in 2002 and the result is uncomfortable: substantial parts of the contiguous United States have no radar coverage below 1, 2 and even 3 kilometres above ground. The gaps are not evenly distributed, and where you live determines how much of this module applies to you.

WhereWhat the coverage looks like

Intermountain West and Great BasinThe worst in the country. Wide spacing between sites and terrain blocking what does exist. Large areas with nothing sampled below several thousand feet.

Northern and central PlainsGenerally good over the populated corridors, with real gaps between sites in the emptier country.

East of the MississippiDense by comparison, and the limiting factor is more often range from the nearest site than absence of one.

Coastlines and offshoreCoverage falls away over water, and shallow storms coming ashore can be sampled only at their tops until they are almost inland.

Find your nearest site and your distance to it. That single number tells you more about how much to trust the radar picture over your own town than anything else in this module.

Where this bites hardest

Shallow storms at long range are the worst case, and they are not a regional curiosity. Landfalling are small and low-topped, and the beam passes over their circulations well before it reaches them. So do many cool-season and overnight line embedded circulations, which module 8 lesson 5 covered as the environment where both your eyes and the radar fail at once. The unifying rule is that the shallower the feature and the further the range, the less the radar has to say about it.

Four ways the picture lies

Ground clutter: Returns from buildings, terrain and trees near the site, showing as a persistent blotch around the radar that does not move. Filtered out in normal operation, and it reappears when conditions change.
Anomalous propagation: When a temperature inversion bends the beam downward more than usual, it strikes the ground far from the site and returns echoes that look like precipitation. Classic on clear calm nights and mornings: a ragged ring or patch of apparent rain that is not there, often over the same ground each time.
Beam blockage: Terrain or structures between the radar and the target remove part of the beam. Produces permanent radial wedges of missing or reduced data, in fixed directions, and it is a large part of why western coverage is what it is.
Non-meteorological targets: Birds, bats, insects, chaff, smoke plumes and wind turbines all return energy. Lesson 7 gives you the field that separates most of these from weather in one glance.

Knowledge checkNot graded · the exam draws a fresh variant of this item

A tornado-warned supercell is passing almost directly over a WSR-88D site. What happens to the radar picture of it?

The radar has a highest tilt as well as a lowest one.

Sources for this lessonMaddox, Zhang, Gourley & Howard 2002, Wea. Forecasting 17(4), 927–934: weather radar coverage over the contiguous United StatesNOAA JetStream, the online school for weather: radar NWS Warning Decision Training Division, radar and warning guidanceNOAA, gaps in NEXRAD radar coverage, report to Congress

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