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High Level Weather Reflectivity Table
Notice: The level of reflectivity of the echo received from each type of weather is not shown in specific units as this will vary between transmission frequency and will most likely not be the same for any two radar sites!
Each weather radar station will hold a „look-up” table of reflections intensities which it can use to help to interpret and distinguish between different types of weather and objects based on the back-scattered echoes received at the antenna.
| Reflectivity | Weather / Object Type |
|---|---|
Most Reflective
|
Ground clutter |
Table 1: comparison of the reflectivity against the possible weather
All objects carry some degree of reflectivity; for weather, this varies by its intensity. Therefore, the level and power of echoes received back at the antenna can be interpreted to make an intelligent guess at the type and severity of a weather formation.
Tuning: It is stressed that the levels of reflectivity are rough estimates. Tuning of systems would often be carried out using other sensors (such as rain gauges or any other independent accurate source of weather data and parameters) to ensure optimum performance (this is done on a site by site basis!).
All types of weather and object provide some level of echoed returns. Clearly solid formations such as terrain in the line of sight or flying mechanical objects will provide the highest degree of returns.
This table breaks down comparative return types for precipitation only (clearly, there are other types of weather but these are not considered in this table).
Rainfall is most often measured using a relationship between the reflectivity factor (Z in mm6/m-3) and its precipitation intensity (R in mm / hr-1). Pre-defined reflectivity tables are held at a station and used as the basis for the initial interpretation and calculation.
The rainfall rate R can be empirically related to the reflectivity factor Z by the expression:
Z = aRb
where a and b are constants and R is the rainfall rate in mm/hr. The reflectivity factor Z
is dependent on the size and number of rain drops per unit volume of space and has the units of
mm6/m-3. Many researchers have produced a large variety
of values a and b. The value of a and b will be specific to each radar station configuration
(i.e. stored in a table). A Number of ambiguities are known and possible, depending on the radar
configuration and the particular meteorological situation.
| stratiform rain | Z = 200·R1.6 |
| orographic rain | Z = 31·R1.71 |
| thunder storms | Z = 286·R1.37 |
| snow | Z = 2000·R2 |
Table 2: Typical expressions for types of precipitation
There are other forms of identifiable weather which could be interpolated by weather radar
systems (although, depending on the frequency spectrum of the system, the type of equipment used,
its operational goals and the software of the station, not all of these might be available.
The types of information might cover;
- Rainfall (or storm intensity), further classified in terms of its precipiation intensity against echo intensity. Rainfall is commonly classified into 6 bands (light, moderate, heavy, very heavy, intense and extreme). Rainfall rates will be quite different between stratiform cloud and convective cloud formations. The FAA, for example, have issued a classification scale as an information circular.
- Non-precipiating clouds (liquid or ice)
- Fog and feezing fog - sometimes also providing a figure for the estimated visibility.
- Clear-air-turbulence (only with certain spectrums - most effective systems whitch can identify clear-air-turbulence operate in the L-band!)
In the absence of further information it is generally acceptance, in mid-latitude temperature climates, to use a default value of Z = 200R1.6. Usually Z is specified in a logarithmic scale where ZdB = 10 logZ.
