National radar weather difficulties
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national radar weather
offices employ is an optical
which has problems similar to other optical tools. Issues such as unwanted reflections and refractions (bending).
For instance, fibre-optics work by sending light down a duct of solid glass. None of the energy gets out because the sharp changes in density, index of refraction, and the curvature at the boundary result in total internal reflection. We end up with a very efficient medium.
In other cases, though, light escapes where we don't want it to and efficiency is compromised. Stray light may enter or re-enter the system and give false signals as well.
In fact, quite a variety of limitations and artifacts can interfere with accurate radar imagery.
National radar weather and technical problems
In the first three examples that follow, the same principles can affect the national radar weather data. They then result in effects such as anomalous propagation, AP.
These show up as large bright areas on the animated radar weather display which are capable of dancing around on an animated screen at times. The instrument can sometimes show weather pictures from very far away. Why? Because of reflecting boundaries caused by:
- Nighttime radiation/Nocturnal inversions - temperature on clear nights often increases with height. Internal reflection arising from inversions can cause radar depictions and distant radio stations to come in more clearly, leaving a ghost-like figure on the picture.
I remember as a kid, listening to Vancouver radio on summer nights in Medicine Hat, a thousand kilometres of mountains away. It worked late at night because of internal reflection from an upper horizontal boundary of this sort.
- Cool lakes and seas - the extra moisture near the surface gives the same result as the longwave inversions described above.
- Sharp temperature and humidity changes such as those near a thunderstorm. Unfortunately this can distort the storm's image, just when the local weather radar is most needed.
- Simultaneous increase of temperature and humidity with height. This leads to Sub-refraction. The opposite consequence to item #1 can happen and the national radar weather signals gets lost in space. Therefore we may never see them in this case.
- Precipitation freezing or melting while falling. This forms a glossy boundary and looks very strong on the radar display. Meteorologists call this the bright band.
- Radiation leaks from a radar - in several directions, all the time. This can make objects behind the radar, for example, look like they are in front on the image. It picks up a reflected signal without correctly determining its direction.
- Two or more distant objects - close to each other, will often look like a single object.
- Objects near the edge of the beam - a "still photograph" will be on the edge of the next photograph as well. When two are spliced together, the object will appear much larger than it really is.
- The same storm would naturally appear more intense if closer to the radar. Radar systems compensate for this by installing circuitry which amplifies the signal from more distant objects.
- The earth is not flat - its curvature requires the radar to deal with viewing storms at incorrect angles. The radar can miss part or all of a storm hiding in the earth's shadow. Fortunately larger countries have weather radar with multiple stations to help clarify the picture.
What may seem ambiguous from the point of view of one radar station will usually show more accurately when viewed from another on the opposite side.
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- If the shape of a storm is changed by wind, it looks to a radar like it covers more area than it actually does. Wind speeds and directions change at different heights, an effect known as "wind shear", and can stretch a storm significantly. Again, a network of national radar weather sites clarifies this somewhat.
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