Anthropogenic aerosols invigorating hail storms

Here we report the main results of the ANTISTORM (Anthropogenic Aerosols Triggering and Invigorating Severe Storms) EU FP6 project.

According to the ANTISTORM conceptual model, a main cause for the lack of hail in pristine warm base clouds is the depletion of cloud water to rain before reaching the freezing level. The water that does ascend to the supercooled levels produce many ice precipitation embryos that compete on the remaining cloud water, and hence cannot grow to large hailstones. Suppressing the warm rain up to the supercooled levels in tropical clouds requires large amounts of aerosols, which, according to the explicit microphysics model simulation, produce more than 1500 drops cm-3 at cloud base. Higher amounts of aerosols can suppress the warm rain to the extent of creating too few ice hydrometeors that would therefore produce less hail, unless significant recirculation of precipitation occurs. Recirculation should produce the largest hailstones, especially in the case of very high aerosol amounts that suppress much of the precipitation in the intense updrafts except when containing re-circulated precipitation particles. In such case all the cloud water would be available for the efficiently growing large hailstones. Therefore, the "optimum" amount of aerosols for warm base clouds should be very high, or even non-existent in certain dynamic circumstances that are prone to significant recirculation.

A shorter distance between cloud base and the freezing level exists in convective clouds with cooler bases, as was observed and simulated. Therefore smaller concentration of aerosols is required for suppressing the early warm rain that would prevent the formation of large hail. On the other hand, large concentrations of aerosols would more easily suppress the warm rain to the extent of scarcity in hail embryos.

Clouds with very cold base, near 0°C, already form as supercooled clouds with little room for rainout. In such clouds only quite pristine conditions would produce excess of precipitation embryos that would compete on the available cloud water and prevent the formation of hail. Already moderate concentrations of aerosols can suppress the formation of ice precipitation embryos to the extent that hail is substantially reduced.