Why can weather radar quantitatively estimate precipitation?

Meteorological radar emits electromagnetic waves, which will be scattered when it meets 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 understand the intensity, distribution, movement and evolution of precipitation in the atmosphere according to the echo images, so as to understand the structure and characteristics of the weather system.

In the early warning information released by the Meteorological Observatory, we can often hear the precipitation forecast for 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 rainstorm in Washington, and the rainfall will reach 150 mm or more", and so on. Most of them are calculated by computer with numerical weather forecast model.

However, on the radar screen, all we can see is the intensity, distribution, movement and evolution of radar echo. How do meteorologists quantitatively estimate precipitation? Generally, radar echo intensity and precipitation intensity have the same probability distribution. The Meteorological Observatory will collect and count the raindrop spectra of different regions, different precipitation types and different precipitation intensities, that is, the distribution of raindrops of various sizes with their diameters in unit volume, and then find out the relationship between the echo intensities of different types of precipitation and their corresponding precipitation intensities, such as stratiform cloud precipitation, convective cloud precipitation, topographic cloud precipitation, dry snow and wet snow, and thus obtain a set of empirical formulas for quantitative estimation of precipitation.

In practical work, in order to measure the total amount of precipitation in a certain area in a certain period of time by using meteorological radar, we can divide the area and time, and then accumulate or average the multiple precipitation measured by radar, so as to eliminate the influence of random error and make the rainfall or average intensity in this area more accurate than the instantaneous intensity of a single point, thus ensuring the estimation accuracy.

In recent years, the technology of weather radar to estimate precipitation has been constantly updated. Moreover, the radar set on the ground will form a network, and the radar with satellite as the carrier can realize large-scale precipitation observation, which can make up for the shortage of single-point observation. However, there are also various problems in quantitative monitoring and forecasting of large-scale precipitation by using radar networking. For example, if the radar echo images 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 with 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 rainfall stations in the effective coverage area is different, which will affect the results, make the data after networking appear errors, and finally affect the forecast quality.