Summary

  1. Weather Radars are very useful in the detection, tracking and intensity assessments of many meteorological phenomena such as rainfall, boundaries (sea breezes, fronts, gust fronts), and storms
  2. Weather radars mainly receive energy backscattered from water droplets, ice crystals and hail. This returned energy is commonly displayed as a radar reflectivity factor Z.
  3. A connection exists between the radar reflectivity Z and the rainfall rate R, but it is highly variable in space and time. Nevertheless, fixed Z-R relationships are in widespread use and yield some useful information on quantitative rainfall.
It is very important to keep in mind the target characteristics which determine the effective reflectivity by radar for a particular target. The table below is a general summary of target types and their associated effective reflectivity factors. Exceptions to these general descriptions may occur for a wide variety of reasons.
 
TARGET TYPE TYPICAL Ze RAINFALL ESTIMATE
Cold snow Low Underestimate
Stratiform rain Low/Moderate Accurate
Melting snow Moderate/High Overestimate
Thunderstorm High Accurate/Overestimate

Comparative effective reflectivities and rainfall accuracies (using standard Z/R relationships) for different types of precipitation.

The table below is a similar summary of range, and refraction characteristics which influence estimates of effective reflectivity factors.
 
CONDITION
EFFECT ON Ze ESTIMATE
Long range (> 120 km) Underestimate
Subrefraction Underestimate
Superrefraction

(Not consistent)*
(*depends upon degree of superrefraction but often
results in target detection at abnormally long range)

Range and refraction characteristics as they influence estimates of effective reflectivity for precipitation targets.

Always keep in mind, when using radar data, that a volume of atmosphere is averaged to produce the effective reflectivity indicated at any particular location. The size and height of the volume which is sampled to produce reflectivity patterns varies dramatically with antenna elevation angle and distance from the radar site. 

IN ORDER TO EFFECTIVELY INTERPRET AND USE RADAR DATA, IT IS ESSENTIAL TO KNOW AND CONTINUOUSLY APPLY THE CHARACTERISTICS OF RADAR, TARGET AND RANGE WHICH INFLUENCE ESTIMATES OF EFFECTIVE REFLECTIVITY. ONLY THEN CAN RADAR DATA PLAY THE MOST COMPLETE POSSIBLE ROLE IN FORECASTING AND WARNING.

 

iDevice icon Exercise

Any unwanted electrical disturbance or spurious signal is referred to as ; unwanted signal generated outside the radar is said to be , while unwanted signal generated within the radar is said to be .

The weakest signal which the radar can detect is referred to as the

If the internal noise level is increased, radar sensitivity is and the mds must be at a wattage.

  

iDevice icon Exercise 2

The decibel (dB) is the standard unit for expressing relative (work, power, frequency) levels.

Returns due to reflection are from targets with diameters (larger, smaller) than the wavelength of the radar.

Returns due to scattering are from targets with diameters (larger, smaller) than the wavelength of the radar.

In general, returns from precipitation on weather radars are due to (reflection, scattering).

Radar reflectivity is a measure of the of a target in intercepting and returning radar energy.

How much backscatter a precipitation particle generates depends primarily on the particle’s and (shape, size, state, charge).

As the radar beam propagates outward from the radar, power density at any point in the pulse volume (decreases, increases).

The radar directly measures (dB, returned power, particle size distribution, precipitation rate).

  

Exercise 3


The radar can determine particle size distribution in a precipitation target

True False


Typically, mds is expressed in dBm, since it is a very small power

True False
IDevice Question Icon Exercise 4
If the transmitted power is doubled, the average echo (return) power would be
  
halved
doubled

If the antenna gain is doubled, the average echo (return) power would be how many times as great?
  
2
4
8
16

Spreading of the beam width with increased range may lead to inaccurate reflectivity estimates because precipitation targets may not do what to the beam?
  
occupy
fill

If the pulse length is doubled, the average echo power is increased by a factor of
  
2
4
8

Target returns from ice particles are about how many times less than from liquid particles, when both have equivalent water content?
  
5
10
20

Although C-band radars receive four times as much target return as S-band radars, attenuation by what reduces the signal enough so that C-band radars are not suitable network radars?
  
oxygen
precipitation
lithometeors

The reflectivity factor Z is determined by the sum of the which power of all drop diameters in the volume being sampled?
  
first
second
sixth
tenth