What are the error factors in weather radar estimation of precipitation?

Radar measurement of precipitation can obtain real-time precipitation information with a certain degree of accuracy, wide range and high spatial and temporal resolution. Therefore, the application of radar for rainfall monitoring and area rainfall calculation can improve the accuracy and timeliness of flood forecasting. However, it must be clearly recognized that due to the complexity of the technology itself and other reasons, there are certain errors in current radar rain measurement. In particular, the accuracy of large-scale precipitation measurement cannot fully meet the requirements of meteorological business applications. Radar rain measurement errors mainly come from the following aspects:

a) The influence of the wavelength of radar electromagnetic waves on precipitation measurement. In the radar meteorological equation, the average received power Pr is related to the radar wavelength, antenna gain and beam width. When the size of the antenna is fixed, Pr is inversely proportional to the fourth power of the wavelength. That is, the shorter the wavelength, the greater the Pr and the stronger the detection capability. Therefore, a short wavelength is beneficial to detecting precipitation. However, atmospheric gases and precipitation both attenuate electromagnetic waves during their propagation.

b) Uncertainty in raindrop spectrum changes and Z~R relationship. Precipitation intensity is related to the distribution of precipitation particle diameters, that is, the raindrop spectrum, and the Z~R relationship is also closely related to the raindrop spectrum. Under the same precipitation intensity, convective precipitation and warm cloud precipitation have different reflection factors Z due to different raindrop spectra. During the same precipitation process, the raindrop spectrum is also different at different stages of cloud development. Therefore, the Z~R relationship is uncertain. Strictly speaking, the relationship should be calculated based on raindrop spectrum measurements, but it is difficult to achieve in practical applications. Currently, most of the empirical relationships used in business work are fixed empirical relationships, which will inevitably affect the accuracy of calculations.

c) The influence of changes in reflection factors below radar measurement altitude. During the precipitation process, due to the evaporation of water droplets, atmospheric movement and phase changes of water, the radar reflection factor changes greatly in the vertical direction. At the same time, the path of radar electromagnetic waves (even the path of horizontal emission) leaves the ground surface as the distance extends. The farther the horizontal distance, the greater the vertical distance. Therefore, the gap between the precipitation observed by the radar and the actual precipitation is also greater. At the same time, as the distance extends, the spatial expansion of the beam also increases.

d) Ground clutter and obstruction. The main observation object of weather radar is precipitation echo, but at the same time, it is inevitable to observe the echo reflected by mountains and other terrains. Sometimes this terrain clutter is mixed with precipitation echoes and is difficult to distinguish. In addition, because obstacles such as ground objects block the propagation path of electromagnetic waves, subsequent and precipitation echoes cannot be detected. If the elevation angle of the antenna is thus increased, the vertical distance of the beam from the ground surface will be increased, thereby increasing the observation error.

e) Measurement errors related to the performance of the radar itself. Measurement errors related to radar performance are also related to the stability of the radar equipment itself. After a period of operation of radar equipment, the performance of the transmitter and receiver will change.

f) Abnormal propagation (AP). Assuming that the radar beam propagates in a standard atmosphere, the beam height can be calculated. If the atmospheric conditions are not standard, the beam will propagate in a different way, or propagate abnormally. Abnormal propagation generally refers to the fact that the beam is super-refracted and hits the ground far away from the normal ground clutter area. Therefore, AP is truly ground clutter far away from the radar area. These energy from abnormal propagation beams will be included in the Z~R equation, causing overestimation of precipitation just like normal ground clutter.

g) The beam is partially filled. Partial beam filling is generally a problem for meteorological targets that are farther away from the radar. One of the assumptions that must be made in radar measurements is that all targets completely fill the beam, since there is no other way to determine this. Therefore, targets narrower than the beam at this distance will appear larger than they really are. The energy from small targets is averaged over the entire wide beam, resulting in an underestimation of precipitation (overestimation over the entire area).