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Considerations in the Interpretation of NEXRAD Radar Products
Critical in the interpretation of NEXRAD products is the distance between the radar and the weather phenomenon being analyzed. NASP is 52 nautical miles from the Mobile AL radar. By the time the radar beam reaches the NASP region, even at its lowest scan elevation, the beam is at about 5000 feet in elevation. What this means is that NASP is at the edge of subrefraction where curvature of the Earth effectively bends the radar beam upwards, so the information one can obtain about low-level atmospheric events at the NSAP regions is reduced. On the other hand, at night due to cooling, super-refraction can occur, causing the beam to bend down toward the Earth. The forecaster must always be aware of the overall level of refractivity in interpreting NEXRAD products.
Also critical in the interpretation of NEXRAD products is storm-relative motion. The NEXRAD reflectivity product shows the highest returns within the volume that is being scanned. This relates to way the dBZ return is color-coded in the NEXRAD display. If a storm (or a tornado) is moving away from the radar, its apparent velocity can be reduced, but a reduction in apparent velocity can still represent an increase in actual velocity. This can be critical in determining whether mesocyclonic activity will result in severe weather (mesocyclones do not always result in severe weather), and in forecasting tornados. Velocity may apparently decrease as a storm cell moves away from the radar, but that can be because of the movement, not because of any change in storm intensity. Cell size may apparently decrease as a storm cell moves away from the radar, but the size of storm cells at low levels may increase as the cell moves away from the radar because at low levels the cell spreads out.
Another bias related to distance is "range folding." The NEXRAD computer may have difficulty distinguishing a return from a signal sent at time-1 from a signal sent at time-2 if the return from the time-1 signal arrives at the antenna after the return from the time-2 signal. The computer may not be able to compute reflectivity, and in such circumstances will portary regions filled by a purple color. Even within such range-folded regions there can appear information concerning important weather events, however, including subtle indications of tornado signatures when they are a mid or upper levels and hence are not as subject to range folding as events at lower levels, again due to distance from the radar site. |
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