What Is the Raindrop Technique?

Generally, water droplets with a radius smaller than 100 m are called cloud droplets, and water droplets with a radius larger than 100 m are called raindrops. The standard cloud droplet radius is 10m and the standard raindrop radius is 1000m. In terms of volume, a raindrop with a radius of 1mm is equivalent to about 1 million cloud droplets with a radius of 10m. If the conditions for the cloud droplets to increase into raindrops are met in the water-borne cloud, and the raindrops have a certain falling speed, then the rain or drizzle will fall.

Raindrop spectrum refers to the distribution of raindrop size per unit volume.

Precipitation is the result of microphysical processes in clouds, cloud dynamics processes, and the combined effects of various factors. Raindrops are the final product of contradictions between macroscopic and microscopic processes in clouds. The size distribution of raindrops necessarily reflects the precipitation process. Raindrop spectrum, also known as raindrop scale distribution, refers to the distribution of the number of raindrops of various sizes in a unit volume with their diameter. Particles form raindrops in the cloud and drop to the ground. After condensation and coalescence, collision and fragmentation, updraft and evaporation, etc. The raindrop spectrum is a manifestation of the above-mentioned effects. The spectral pattern and related characteristic quantities of the raindrop spectrum can reflect the microphysical characteristics of precipitation. Studying the characteristics of precipitation particles can help explain the mechanism of rain formation in the cloud. It is of great significance to test the conditions and effects, improve the accuracy of radar measurement of precipitation, study the microwave radiation and attenuation process of raindrops, and soil and water conservation during surface processes. Raindrop spectrum research has developed rapidly in the past 30 years, and the observation results of raindrop spectrum are widely used in fields such as meteorology and hydrology. Among them, the raindrop spectrometer provides basic observation data for raindrop spectrum research. Raindrop spectrometer is used to observe rainfall, and then analyze the characteristics of rainfall, which plays an important role in regional rainfall monitoring.

The traditional raindrop spectrum measurement is to manually collect a large amount of raindrop characteristic parameter data, and to classify it according to the size of the raindrop diameter. Due to its tedious process and low accuracy, the traditional raindrop spectrum measurement method has gradually been replaced by new technology. With the photoelectric technology In the development of the technology, people have designed a variety of raindrop spectrometers to realize the automation of raindrop spectrum measurement. The observation data of weather radar contains raindrop spectrum information, which further realizes the large-scale acquisition of raindrop spectrum data.

1 Traditional method for raindrop spectrum measurement

The early technical methods for measuring raindrop spectrum include kinetic method, filter paper stain method, flour dough method, rapid photography method and infiltration method.

Kinetic method

The pressure transducer and voltage sensor are used to measure the drop kinetic energy of the raindrop to obtain the size of the raindrop, which is only suitable for precipitation with uniform spectral band. Because this method has too much error, it is rarely used.

Filter paper stain method

It is based on the size of the raindrops falling on a certain area of filter paper, one by one, and according to the existing test curve, it is converted into the size of raindrops, and after a certain calculation, various characteristic values of the raindrop spectrum are obtained. The error range is 6% ~ 14%, which is equivalent to 0.037 ~ 37mg filter paper speckle method, which can not continuously and continuously observe the ground raindrop spectrum.

Flour ball method

A wide-mouth container with a certain surface area containing flour inside is used to drop the raindrops into the container. After contacting the flour, each raindrop will form a small ball, which is dried and weighed. The particle size of each raindrop is measured and its disadvantages If different flours are used, there will be slight differences between the calibration curves.

Rapid photography

Taking the falling raindrops, placing the photo under a microscope and measuring the photography method directly measures the size and shape of a single raindrop at the same location. It does not measure the spectral distribution of the raindrops. Case

Infiltration

Use a container filled with oil to collect raindrops. Because water and oil do not dissolve, and the density of water is greater than that of oil, raindrops with sufficient final velocity falling into the oil will become spherical water drops due to surface tension. Measure oil The diameter of the water drops can be used to obtain the size of the raindrops.

In the traditional method, the flour ball method and the filter paper stain method are mostly used, but generally have the disadvantages of low accuracy, large workload, poor real-time performance, high cost, and inability to complete the measurement classification automatically. ^[2]

Lightning discharge has a greater effect on the growth of droplets; the magnitude of the effect is closely related to the state of charge of droplets. If some small water droplets can be artificially charged with high charges, this may become a new means for artificially affecting the weather by thunderstorm clouds. ^[4]

As for the initial stage of lightning, people's research focuses on the corona phenomenon on the surface of various water vapor condensates (such as raindrops, ice crystals, etc.). This phenomenon is considered to be the initial stage of lightning and the first step. The interaction between water vapor condensate is the basic physical process of lightning initiation and growth, and it can cause raindrops to grow or break. The interaction of water droplets with different charges in the electric field has proven to be a micro-physical process and basic physical conditions conducive to the initiation and development of lightning. ^[5]