RECOGNITION OF HAIL DANGEROUS CUMULONIMBUS CLOUDS BY LASER SENSING IN THE SHORT-WAVE VISIBLE OPTICAL RANGE

Abstract

The protection of crops dependent on the prompt and reliable determination of hail dan- gerous clouds is the original task of agriculture. The main way of recognizing hail danger are the radar methods at present based on the interaction of radio waves in the centimeter range (3 and 10 cm) with the clouds. The method of recognition of hail dangerous cumulonimbus C_b clouds based on laser sensing in the short-wave visible wavelength range is considered. The reflected signal from the sensing radiation of hail-containing cloud is formed by the ice and water reflectivity difference. In the short-wave visible optical range (400…500 nm), there is a resulting reflected signal caused by the large difference in the reflectivity of ice and water re- gardless of the ratio of the wavelength and the scattering particle (hailstones) size. The reflect- ed signal will be a background signal if the cloud consists only of water droplets. The calcula- tion of the intensity of the reflected probe radiation on the CdS detector is given. It is shown that the threshold sensitivity on the detector in the daytime is determined by background radia- tion formed from the Earth's surface reflected solar radiation and it is 8.7 • 10^-13 W/cm². A comparative analysis of cloud sensing by pulsed and continuous laser is presented. On the basis of calculations, it is shown that in the direct detection mode, a solid-state pulsed YAG:Nd⁺³ laser with a second harmonic (503 nm) mode and a CdS photoconductive device (detector sig- nal is 2.5 •10^-8 W/cm²) based system is more effective in comparison with a continuous laser. The following acceptable data have been used for definiteness in calculations: laser beam di- vergence - α ~ 10^-3 rad; diameter of hailstones in the cloud - 3 cm and probed distance - 3000

1. m. The solved problem on the recognition of hail dangerous clouds presented in this paper is local, and the proposed system can be successfully applied to regions with rapid convection, for example, Ararat Valley (Armenia). A sudden appearance with a short (up to 1 hour) lifespan of hail dangerous C_b clouds is typical for these regions. Radar methods, which are the main way to determine the hail dangerous C_b clouds, in this case are not effective.