In the early warning information released by the meteorological observatory, we can often hear the forecast of precipitation in a certain period of time, such as "the precipitation in central and northern Beijing will reach 100 mm in the next two days", "it is predicted that there will be a heavy rainstorm in Washington, with a rainfall of 150 mm or more", and so on. Most of them are calculated by computer using numerical weather prediction model.
Meteorologists can also use a weather radar called "weather radar" to quantitatively estimate precipitation. The weather radar emits electromagnetic waves, which will scatter when encountering raindrops, cloud drops, ice crystals and snowflakes in the air. The returned electromagnetic waves are received by the radar antenna and displayed on the screen. Meteorologists can know the intensity, distribution, movement and evolution of precipitation in the atmosphere according to the echo image, so as to understand the structure and characteristics of the weather system. Weather radar can detect typhoons, strong storms, hail, rainstorms and strong convective clouds in some areas, and monitor the changes of weather.
However, on the radar screen, all we can see is the intensity, distribution, movement and evolution of radar echo. How do meteorological workers quantitatively estimate precipitation? Generally, radar echo intensity and precipitation intensity have the same probability distribution. The meteorological station will collect and count the raindrop spectra of different regions, different precipitation types and different precipitation intensities, that is, the distribution of the number of raindrops of various sizes in unit volume with their diameters, and then find the relationship between the echo intensity of different types of precipitation and its corresponding precipitation intensity, such as stratiform cloud precipitation, convective cloud precipitation, topographic cloud precipitation, dry snow and wet snow, so as to obtain a set of empirical formulas, Used to quantitatively estimate precipitation.
In practical work, in order to measure the total precipitation in a certain period of time in a certain area by using meteorological radar, the area and time can be divided, and then the multiple precipitation measured by radar can be accumulated or averaged. In this way, the influence of random error can be removed, so that the rainfall or average intensity in the area is more accurate than the instantaneous intensity of a single point, so as to ensure the estimation accuracy.
In recent years, the technology of estimating precipitation by meteorological radar is also being renovated. Moreover, by forming a network of radars on the ground and using the radar with satellite as the carrier, we can realize a wide range of precipitation observation, which can make up for the deficiency of single point observation. However, there are also various problems in using radar networking to quantitatively monitor and predict large-scale precipitation. For example, when the radar echo maps of the network are put together, the technical problems of the puzzle itself will make the accuracy of precipitation estimation fail to achieve the expected purpose. Moreover, even for the same type of weather radar, the detection results will be different. For example, the location of each radar is different, the radar echo is affected by different terrain and buildings, and the distribution density of rain gauge stations in the effective coverage area is different. These factors will affect the results, make the data error after networking, and finally affect the prediction quality.