Rainfall Enhancement and Fog Dissipation Experiments in Wuling Mountain in 2020 Using Artificial Strong Sound Wave

Abstract

Low-frequency sound wave is a new type of operational approach that has the potential for enhancing rainfall and dissipating fog. To investigate the impact of this type of equipment, field operations and observational experiments are conducted in Wuling Mountain from August to September 2020. Wuling Mountain is located at Chengde of Hebei outside the northeastern boundary of Beijing. The main peak of the Yanshan Mountains is renowned for its foggy summers with an altitude of 2118 m. In the experiment, a prototype of an electronic acoustic low-frequency strong sound wave device is used. This device has a maximum sound pressure level of 155 dB. Meanwhile, observation instruments such as a disdrometer, visibility meter, fog droplet spectrometer, and automatic weather station with an ultrasonic anemometer are deployed. These instruments are used to obtain the background conditions and to monitor macro and micro changes during rainfall enhancement and fog dissipation operations for evaluating the effectiveness.In two typical cases with an obvious defogging effect, within 2 to 3 minutes after the start of the operation, the number of droplets smaller than 10 μm decreased, while the number of droplets larger than 10 μm increased. Subsequently, the size of the droplets on most scales decreased significantly, resulting in improved visibility. Within a span of 10 minutes, visibility could increase from less than 100 m to a maximum of 1000 m. The relationship between wind speed, wind direction, and the dissipation effect of fog shows that cases with a noticeable defogging effect occur when the average wind speed is less than 1.5 m·s-1 and the wind direction causes the fog to pass through the near side of the influence range of the sound wave device, while cases with an average wind speed greater than 2 m·s-1 hardly show any change in visibility trends. Results, which align with the experimental expectations, are observed during an operation on a convective cloud precipitation when the surface mean wind speed is 1.4 m·s-1. In this case, the rainfall intensity increases rapidly from 0.3 mm·h-1 to more than 7 mm·h-1 within 3 min of operation, and large raindrops with rapid occurrence but short duration are observed. In other rainfall enhancement experimental cases, the average wind speed exceeded 3 m·s-1 during the operation period, and no clear and consistent evidence of increased rainfall is observed, which may be affected by the high wind speeds and only one single observation point.

Keywords

• artificial strong sound wave
• rainfall enhancement
• fog dissipation