| ARES - Linn County, Iowa |
This document provides instructions on how to gain access to the radar lab, activate the radar and ATV transmitter, and to operate and interpret radar displays. We suggest operators print this page in case you need it in a hurry (which is usually the case).
| Set Ham Callsign | H xxxxxx | Set Amateur Radio operator callsign |
| Basic Commands | B 1 | 15 Mile Range (Precip and Doppler) |
| B 2 | 25 Mile Range (Precip and Doppler) | |
| B 3 | 40 Mile Range (Precip and Doppler) | |
| B 4 | 150 Mile Range (Precip only) | |
| B 5 | 225 Mile Range (Precip only) | |
| B 6 | Close Range Vertical Scan to 50k feet (currently inoperative) | |
| B 7 | Close Range Vertical Scan to 75k feet | |
| B 8 | 50 Mile Range Vertical Scan to 50k feet | |
| B 9 | 50 mile range Vertical Scan to 75k feet | |
| Operating Modes | O P | Precipitation Only |
| O B | Both Precipitation and Doppler VAriance | |
| O D | Doppler Variance | |
| O V | Velocity | |
| O T | Test Pattern | |
| O G | Gradient | |
| Gain Selection | G 0 | Normal Calibrated Gain |
| G +-xx | Set Manual Gain | |
| Ground Clutter | C | Toggle Ground Clutter Suppression (Enable/Disable) |
| Doppler Variance | V x | Set minimum Doppler Variance Threshold (from 5 to 12 m/s) |
| Manual Azimuth | A xxx | Enter Degrees from True North |
| Watch/Warning | B x 1 | Severe Thunderstorm Watch |
| B x 2 | Severe Thunderstorm Warning | |
| B x 3 | Tornado Watch | |
| B x 4 | Tornado Warning |
Velocity indications at low antenna elevation angles (tilt) may be used to determine surface wind direction and speed. Examination of the full sweep (exclusive of individual storm cells) will indicate the general wind direction.
Velocity indications when set to higher antenna elevation angles may be used to determine the variation of wind direction and speed vs. altitude. Since the antenna is elevated, velocities at increasing ranges will be sampling velocities at increasing altitudes.
A Microburst is a rapid descent of a pool of rain cooled air. As the dropping pool of air spreads out as it hits the ground, it can cause damaging winds at the surface. These are difficult to detect, as the events are short lived. The radar will show a microburst as increased negative Doppler at the surface on the near side of the cell (closer to the radar), and positive Doppler on the far side. This effect will decrease or disappear at higher elevations. Remember that the Doppler radar can only measure winds toward or away from the radar site. It cannot measure winds moving at right angles to the radar beam.
The signature of a tornado is the result of rotation of winds in a cell. The characteristics to watch for is winds approaching the radar site (negative Doppler) to the left or right of the cell, and winds away from the radar site (positive Doppler) on the opposite side of the cell. The strength of the winds toward or away from the radar will increase closer to the center of the cell. Unlike a microburst, this effect will first be noticed at higher elevations of 10000-15000 feet (a mesocyclone) before it extends to ground level (a tornado). Due to the relatively wide beam of the radar antenna, this effect will be less pronounced at greater distances from the radar site. At long range, a weak tornado may only show up as turbulence. Unfortunately, turbulence may also be produced by non-tornadic conditions. For further examples of radar limitations, see http://www.srh.noaa.gov/abq/feature/presentations/TornadoDetectionCapabilitiesandLimitations.pdf . Note that our radar antenna is much smaller than the NWS radars, and will lose resolution at a much closer distance.
If Doppler indications (Magenta) are present in areas of little or no precipitation, then aliasing may be occurring. A high pulse repetition rate is necessary to detect Doppler. This may result in more distant echoes aliasing into the displayed range. To confirm this phenomenon, select the next greater range. If there is a major area of precipitation in the same direction, and at a range of greater than 50 miles, then the Doppler indication may be due to this distant cell.
Distant, and often very strong, reflections may be indicated due to thermal inversions. These inversions bend the radar beam, causing it to reflect off the earth's surface and to produce a false reflection. These false indications often occur behind a cold front, and may be recognized by the tendency to produce widely varying indications between successive radar sweeps of the area.
Velocity indications are limited in display range on this system. If the velocity exceeds TDB m/s, then it will be displayed as - TBD m/s. Close attention should be paid to the velocity gradients to determine if opposing velocities are due to rotation, or just due to velocities that have overflowed the display capability and now indicate the opposing velocity.
The 400 Hz AC power required to power some of the radar equipment will need to be manually restarted if commercial power is interrupted (due to a nearby lightning strike). The computer and display will come up, but the antenna will not rotate. Call maintenance at X55595, and request that 400 Hz power be restored to the 106 radar lab as soon as possible.
If very little velocity variation is indicated when selecting VELO25 or VELO50, verify that the LRU above the radar cabinet has a red sticker on it indicating that velocity is available. If not, then these selections will not display valid information. However, turbulence displays (V5 through V9) will still be valid.