Hail dangerous analysis in Bulgaria

Hail, floods, fogs, frosts, draughts and other unsuitable weather phenomena cause huge damage to the economies of many countries worldwide. Only natural science since early in the twentieth century has strived to forecast as well as suppress such phenomena.

The Bulgarian climate is of a continental type. Geographical position and relief variety are playing the major role for Bulgaria being one of the European countries with very frequent hail events. Hail occurrence averages from 20 to 60 days per year and has the highest intensity from May to August.

Existing clouds and hail formation research work made better their forecasting and reduce hail damage by hail suppression means. Nevertheless, hail is one of the most dangerous meteorological phenomena to agricultural crops in Bulgaria. According to the Agency Civil defense data, the hail damage is 29% of all natural calamity damage in Bulgaria.

Hailstorm structure and hail stone formation process

The convective clouds are compose of liquid water droplets /if the temperature is below 0 C liquid water droplets may be in the thermodynamically unstable supercooled state/ and ice crystals. In Bulgaria these cloud's altitude reaches to 15-16 km. The clouds may be divided in three sections: warm - the region with a temperature under 0 C, when there are only liquid water drops, supercooled, mixed - the region with a temperature below 0C, where there are liquid water droplets and ice crystals. °t a temperature below -20C the quantity of the liquid water droplets decreases greatly and at the temperature of about -40C there are only ice crystals - cloud anvil. Hail stones formation and growth are realized in cloud supercooled areas. The hail stones growth in strong updraft through collision-coalescence /as the cloud particles move around they collide with each other to form larger particles/ and Bergeron processes /it is easier for water vapor to condense onto an ice surface than a water surface, so ice particles within the cloud will grow faster than nearby liquid droplets/. Cloud seeding is designed to take advantage of this process. Before reaching the bottom of the cloud some of the ice pellets are carried by the updraft back to the top of the cloud. When the updraft is stronger, this cycle recurs and another layers of ice are added and hailstones grow. Once the hailstones become too heavy to be supported by the updraft, they fall out the cloud. The hailstones reach the ground before melting.

Hail suppression

The mechanism for the reduction of hailfall is based on the microphysics concept of "beneficial competition". Beneficial competition assumes many numbers of artificial ice nucleus /by injection of AgI/ and natural ice nucleus in clouds "compete" for the available supercooled liquid water within the cloud. The hailstones, that are formed within the seeded cloud will be smaller and will melt completely before reaching the ground or produce less damage if they aren't be melted.

The strong updraft in thunderstorm cloud continuously provides it with new quantities of supercooled water and carries the ice crystal to the anvil. Therefore the supply of artificial ice nucleus must be at determined intervals of time during the hail-danger lifetime of the cloud.

Differences between seeding to achieve hail suppression and rain enhancement are subtle. The ice crystals grow rapidly in the cloud supercooled region, drawing moisture from the surrounding cloud droplets, until their weight causes them to fall. These falling ice crystals may melt and join small liquid cloud droplets, growing into the raindrops. Seeding with artificial ice nucleus (such as silver iodide) to supply naturally deficient clouds with the proper concentration of ice crystals will increase rainfall through the rain process.

Bulgarian hail suppression technology

The Bulgarian Hail Suppression System uses rockets to deliver the seeding agent (AgI).
The cloud parameters are determined using meteorological radars working on two wavelengths 3.2 and 10cm and having automated systems for data processing. The radar data for the different reflectivity levels and the aerological radiosondes data allow elaborating a discriminant function used to estimate the before-hail transition of the cloud. Vertical and horizontal cross-sections data is stored in a computer. The seeding area and its dose is determined in real time using developed software.

Horizontal cross-sections of the storm

Vertical cross-sections of the thunderstorm cloud

Real-time graphical visualization of the digital radar data is made in synchrony with the antenna rotation. Using a specific data processing, the information from a single radar monitoring could be used simultaneously for meteorological screening and for operational tasks of hail suppression. The developed software permits to examine in real time the vertical and horizontal cross-sections of the cloud in arbitrary levels of reflectivity gradation, to monitor the dynamics of the isocontours on each cross-section for localizing the most active part of the cloud. The radial resolution is 250 m and the angular resolution depends on the rotational velocity of the antenna.

The results of hail suppression operations show a 75% decrease in hail damage in the protected area.