Studies on the Microphysical Characteristics of an Aircraft Seeding in a Complex System of the Stratiform and Embedded Convective Cloud

Seeding effect evaluation is focused on whether the clouds and precipitation produced the expected significant changes after cloud seeding, which manifested in the process of macro and micro changes, so it is necessary to learn about the change of cloud microphysical characteristics after cloud seeding. The microphysical characteristics is analyzed on the basis of the PMS data of an aircraft seeding experiment in a complex system of the stratiform and embedded convective cloud in Hebei Province on April 18, 2009. It is indicated that a significant reduction of the liquid water content occurred, the average diameter of cloud drops increased from 20.4 µm to 23.9 µm, and the spectrums of cloud particles changed a lot after seeding. Simultaneously, the number of ice crystals reduced and the size of precipitation particles increased significantly. It is found that the accumulated rainfall reached the maximum in the seeding-affected area one hour after seeding.

In this case, it was the vertical thermal and microphysical structure of the precipitation process that the relative humidity was high at 650 hPa and 850 hPa, the absolute humidity was high at 750 hPa where more water vapor concentrated, and it was the temperature inversion layer from 850 hPa to 750 hPa. This vertical structure of temperature and humidity made it available to accumulate water vapor at the lower layer to form stratiform cloud. At the same time of disappearance of the temperature inversion layer, the embedded convection occurred, which formed a complex system of the stratiform and embedded convective cloud. The upward movement of air reached the region over 650 hPa with a high relative humidity, and the particles of hydrometeors uplifted to the height where it was more impactful for them to grow, so that the average diameter of precipitation particles increased.

The possible growth mechanism of precipitation particles is as following. The temperature at the catalyzing layer was from -8 to -3 °C. Before seeding, supercooled water content was rich and it increased with height increase, whereas the number of ice particles decreased with height increase. After sowing the catalyst, the number of ice crystals increased, and they grew to be precipitation particles supported by the rich supercooled water, which resulted in an enhancement of rainfall on the ground. It seemed mainly to be a desublimation process for the increase of precipitation particles.