Hail causes considerably damage to crops and property. In many areas of the world the cloud seeding with the goal of suppressing hail is common practice. The seeding agent is injected into the target cloud from aircraft, ground-based generators or the agent is injected into the cold peripheral parts of a cloud by rockets. The success of hail suppression activity is influenced by careful selection of seeding time, seeding dynamics, seeding agent amount and location of initial seeding zone. In the last decade, the cloud-resolving mesoscale models become widely used in testing the seeding criterions with respect to above parameters. The
simulation of seeding effects can be done by either explicit microphysics or bulk microphysics schemes. Bulk microphysics scheme is frequently used in the cloud-resolving mesoscale models due to lower computational cost. This scheme assumes a distribution function for the cloud and precipitation size particles. The variation in accumulated convective precipitation due to the uncertainties inherent in the selection of distribution functions and their parameters must be assessed. Until now the cloud-resolving mesoscale models are used in some studies that quantify considerable sensitivity of the amount of accumulated precipitation from a hailstorm on variations of cloud drop size distribution. Main consequence of the hail suppression activity is the accumulated convective precipitationchange. The selection of cloud drop size distribution is therefore critical for an adequatetreatment of seeding effects.
We use the numerical model of cloud with two microphysical schemes involving the unified Khrgian-Mazin size distribution of cloud drops and a scheme involving monodisperse cloud droplet spectrum and the Marshall-Palmer size distribution for raindrops, respectively. The unified Khrgian-Mazin size distribution approximates the entire drop spectrum that splits into cloud droplets and raindrops at diameter of 100 μm. This drop size distribution is a function of two parameters: total liquid water mixing ratio and mean cloud drop spectrum radius. Sensitivity tests with respect to the amounts of seeding agent, location, time and dynamics of seeding are performed in order to investigate accumulated precipitation change in comparison with an unseeded case using both microphysical schemes. Silver-iodide agent is used in all experiments. Three mean cloud drop radii of 10, 30 and 50 μm are used in sensitivity tests with the unified Khrgian-Mazin size distribution.
Our principal findings are as follows:
For an unseeded hail cloud, the unified Khrgian-Mazin size distribution with a mean cloud drop spectrum radius of 10 μm leads to the huge increase of accumulated rain precipitation (up to 275%) and decrease in hail precipitation (-71%) compared to the counterpart with the Marshall-Palmer size distribution of raindrops and the monodisperse cloud droplet spectrum. Comparison of seeded cases with an unseeded one show the maximum increase of rain precipitation (13.7%) and decrease of hail precipitation (50.2%) if the Khrgian-Mazin size distribution is used. In general, this precipitation changes are greater than those simulated using the alternative approach. Analysis of above results leads to the conclusion that the radar reflectivity criterion alone is insufficient for decision making about hail suppression. The drop spectrum must be also known just before the agent injection due to the optimal seeding agent consumption.