The rain is the fireworks of the gods, which nourishes the earth with the blessings of the gods to the world, sometimes with love, sometimes with magnificent waves.
Sometimes I forgot to bring an umbrella and fell into the soup chicken, and when I saw the precipitation on my mobile phone, it was moderate rain, and the weather station you came out to explain!
It turns out that in research, we often use the amount of precipitation over a long period of time to characterize the size of precipitation, for example, when the precipitation is less than 10mm/(24h), it is called light rain, and when the precipitation exceeds 200mm/(24h), it is considered a heavy rainstorm. The situation where the instantaneous precipitation is relatively heavy is called short-term heavy precipitation. Therefore, there are also some suggestions, not only to forecast long-term average precipitation, but also to be able to forecast short-term precipitation, so as to provide more convenience for people's lives.
So, where do these quantitative numbers come from, and what modern devices can be used to measure precipitation?
The quickest and easiest way to collect rainwater is to collect rainwater by placing a bucket on the ground, which is called a rain gauge. In order to reduce the loss of human resources, scientists have invented a kind of automatic rain measurement sensor - tipping bucket rain sensor, which collects the received rainfall into a small bucket, and pours it out when the cumulative collection reaches 0.1mm, and at the same time forms a corresponding rainfall record.
Rain gauges and tipping bucket rain sensors
Although the rain gauge is convenient and fast, but can only be observed at a fixed point, for the sparsely populated area rain gauge is sparsely deployed, precipitation measurement is still difficult, at this time the ground-based rain measurement radar comes in handy, it emits electromagnetic waves to the surroundings, when the electromagnetic waves encounter cloud droplets, raindrops and other substances in the air, they will be scattered, and the electromagnetic waves returned by the original way are accepted by the radar, and the precipitation intensity in the surrounding area can be inferred according to the intensity of the radar echo. In practice, with the development of the economy, the number of ground-based radars is increasing, and meteorological observers can form a network of radars set up on the ground to achieve large-scale precipitation observation and make up for the shortcomings of single-point observation.
Huangshan Guangmingding weather radar
However, in cities with tall buildings, the signal transmitted by radar may be blocked by tall buildings, and in mountainous areas, it can also be affected by the terrain, and it is equally difficult to set up ground-based radar in deserts and other areas, so it would be nice to be able to "see" the precipitation from the sky from the perspective of a god. According to this idea, meteorologists have explored a new way to measure precipitation - satellite remote sensing inversion, which is divided into visible light, infrared remote sensing, passive microwave remote sensing and active microwave remote sensing according to the different frequency bands of the received signal and whether the signal is transmitted.
The principle of the visible light method is that due to the higher reflectivity of clouds, in the visible light spectrum, clouds are brighter relative to the surface. Bright clouds tend to be thicker, and thick clouds are more likely to produce precipitation. The principle of the infrared method is that strong convection is likely to develop further when the cloud top temperature of a cloud area falls below a certain threshold and the range of the area continues to expand, or when the cloud area temperature has a downward trend, or when the temperature gradient between the cloud top core area and the surrounding cloud area is quite different. Information such as cloud thickness and cloud top temperature can be used to determine the probability and duration of precipitation, and then estimate the amount of precipitation. The limitation is that both methods can generate large errors.
Infrared cloud image detected by FY4A
Due to the strong penetration of microwave signals in the cloud and rain atmosphere, active and passive microwave detectors can work around the clock under adverse weather conditions and detect precipitation without being affected by surface conditions.
Passive microwave detection of precipitation is mainly done by land and sea with different microwave radiation characteristics, and the passive microwave radiometer itself does not actively emit signals. For the ocean, the ability of the sea surface to transmit signals is low, so the background radiation signal of passive remote sensing is small and close to constant. In this context, the strong emission radiation signal of precipitation, coupled with the difference between the low polarization characteristics of precipitation and the high polarization characteristics of sea surface, can be used to identify and quantitatively invert sea surface precipitation in low frequency bands, such as the microwave precipitation algorithm based on the probability density function of precipitation intensity (PDF) (WILHE-IT).
On land, the terrestrial signal emission capability is strong and variable, so the emission radiation from hydrometeors cannot be well identified and quantified. At the same time, the polarization characteristics of the land surface are not obvious, which increases the difficulty of land precipitation inversion. The scattering effect of ice particles in high frequency bands can weaken the upward radiation intensity of the surface, and this feature can be used for land precipitation inversion, such as the scattering factor algorithm (FERRARO and MARKS). THERE ARE ALSO ALGORITHMS THAT USE ALL RADIATION VECTORS, SUCH AS GPROF (KUMMEROW ET AL.), WHICH IS BASED ON THE CLOUD STRUCTURE DATABASE AND THE SIMULATED UPLINK RADIATION, AND THEN USES THE KNOWLEDGE OF PROBABILITY THEORY TO SELECT THE CLOSEST PROFILE FOR THE REAL OBSERVATION SITUATION AS THE PRECIPITATION INVERSION RESULT.
The difference between active and passive microwave detectors is that they can actively transmit signals and calculate precipitation by receiving radar echoes. The Z-R relation is a commonly used method for inverting precipitation by radar echo intensity (dBZ). The average echo power P of the weather radar is proportional to the reflectivity factor Z, which is related to the precipitation particle spectrum, which in turn is related to the precipitation intensity R, so the reflectivity factor Z has a certain correlation with the precipitation intensity R. Through practical observation, theoretical analysis, mathematical statistics, etc., the Z-R relationship as shown in the following formula can be obtained and verified:
The values of a and b in the Z-R relationship are empirical data, which are related to the distribution of the precipitation particle spectrum and the falling velocity of the precipitation particles. A≈200, B is between 1.5~2. When using the Z-R relationship to measure precipitation intensity, different parameters should be selected according to the actual situation, such as geographical location, precipitation type, etc. Take the logarithm of both sides of the above equation at the same time, and get:
Relationship between radar echo intensity dBZ and reflectivity factor Z:
The two forms will be combined:
After determining the parameters a and b, the reflectivity factor Z or the radar echo intensity can be obtained from the weather radar data, and then the distribution of precipitation intensity R can be calculated by using the Z-R relation.
Satellite remote sensing means have the shortcomings of small sampling frequency and coverage, which can be optimized by multi-satellite networking, such as the GPM (Global Precipitation Measurement) program, which aims to improve the accuracy of global precipitation forecasting, which includes 1 core observation satellite and about 10 other observation satellites, of which the core observation satellite was launched on February 28, 2014. It is equipped with both passive (GMI) and active microwave detector (DPR), and its IMERG products can achieve global coverage, providing precipitation data with high spatial and temporal resolution, with a maximum resolution of 0.1° and 30min.
GPM satellite network
These methods have their own advantages and disadvantages, and the fusion and mutual calibration of different data can obtain a more reliable set of precipitation data with good spatiotemporal continuity, and the generation of these data requires a lot of manpower, material and financial resources.
Dear readers, is it raining in your city today? Don't forget to bring an umbrella before you go out, the rain is a firework of the gods, but when the precipitation reaches 50mm/(24h), people who don't bring an umbrella will be smashed into fools.
END
Written by: Takeshio
Editor: Strange Acc
Source: Stone Science Studio
Edit: Stop clouds
The reprinted content represents the author's views only
It does not represent the position of the Institute of Physics of the Chinese Academy of Sciences
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