The effects of the hygroscopic seeding on the development of precipitation in convective clouds in the Three Rivers' Headstream regions

Abstract:

The Three Rivers' Headstream region is located in southern Qinghai Province, which is the hinterland of Qinghai-Tibet Plateau. It is the Yangtze, Yellow and Lancang River watershed source. The Three Rivers¡¯ Headstream region is called the China's natural wetlands, which has abundant water resources. In recent years, unfortunately, global warming and desertification and other natural and human factors make the region's water gradually depleted. The local ecological environment has been severely broken ring. In field of meteorology, precipitation enhancement is the effective way to alleviate the water shortage.

The aim of this research is to evaluate the role of hygroscopic seeding on the development of clouds and precipitation particles in mixed-phase convective clouds with cold cloud-based.

The effect of the hygroscopic seeding on the development of precipitation is investigated using a 2D bin cloud model with detailed microphysics of both warm and ice phase processes. The model is initialized using statistical sounding measured in the Three Rivers¡¯ Headstream regions. Analyzing the characteristics of development of clouds and precipitation under the three different distributions of CCN (fig. 1), in marine (m), continental (c) and polluted continental (pc) backgrounds, it showed that experiment in m made more rainfall rate than in c and pc. Moreover, initial rainfall time is much earlier also in appearance of the marine CCN due to the more concentration of large particles in marine CCN spectrum. In addition, the effect of increasing natural ice concentration on cloud precipitation is also discussed. The more ice concentration is, the less precipitation is.

The seeding effects were investigated by comparing the development of precipitation particles and rain production between the seeded and unseeded case for clouds with different CCN spectra. It was found that the precipitation increase significantly in pc due to accelerating the coalescence process through some giant particles in seeding materials. The seeding particles broaden the cloud droplets, leading to an earlier formation of raindrops (fig. 2, fig. 3), ice particles and graupel particles. In marine case, reversely, the negative effect has been found. The sensitivity of seeding effects to seeding time, seeding height, seeding domain and seeding amount also was tested. The most significant precipitation enhancement was obtained when seeding was conducted a few minutes after cloud initiation and near cloud base, in the middle domain of cloud. Rain enhancement increased with the increase in the concentration of the seeding material.

These conclusions may be used as guidelines in field experiments. One should keep in mind, however, the limitations of the model, which include the 2D dynamics, limited spatial resolution, simplifications associated with the nucleation process and taken no account of wind shear.

Fig.1 Initial CCN distributions (m: maritime spectrum; c: continental spectrum; pc: pollution continental; s: seeding spectrum)

Fig. 2 Mass distribution functions of drops in unseeded (black line) and seeded (red line) at different spatial points after 24 min (top) and 36 min (bottom) of simulations.