Which Is Effective in Enhancing Rainfall from Mixed-Phase Convective Clouds: Hygroscopic or Glaciogenic Seeding?

Hygroscopic seeding was originally designed to produce large droplets and accelerate a collision-coalescence process in warm rain clouds. However, these days this technique has also been used for mixed-phase convective clouds. In order to check if hygroscopic seeding is really effective for mixed-phase convective clouds and to check which is more effective, hygroscopic or glaciogenic seeding, we investigate possible maximum seeding effects by hygroscopic and glaciogenic seeding of diurnal convective mixed-phase clouds over the Oman Mountains and the UAE desert areas through sensitivity tests of total precipitation to CCN and INP concentrations using 3D non-hydrostatic model, CReSS.

For CCN sensitivity tests, CCN number concentration is set 500 cm^-3 for control (CNTL) case, 100 cm^-3 for low CCN (LCCN) case, 1000 cm^-3 for high CCN (HCCN) case, and 2500 cm^-3 for extremely high CCN (ECCN) case. For INP sensitivity tests, we use default heterogeneous ice nucleation formula for control (CNTL) case, one tenths of the default values for low INP (LIN) case, and ten times the default values for high INP (HIN) case.

The results of CCN and INP sensitivity tests are analyzed for domain 1 (desert area), domain 2 (foothill area), and domain 3 (mountain area). Diurnal convective clouds over mountain area include cumulonimbi and are more vigorous than those over desert and foothill areas.

Condensation/deposition amount for HCCN and ECCN cases shows almost 30 - 50 % increase in all the areas compared with CNTL case due to dynamic seeding effect; high concentrations of CCN Increase droplet number concentrations and reduce mean droplet sizes, resulting in weakening warm rain process and carrying up cloud water beyond freezing level. Latent heat release increases due to droplet freezing (riming and freezing) at higher altitudes and strengthens convection, leading to an increase in condensation/deposition amounts.

On the other hand, condensation/deposition amounts for HIN case show almost 10 - 20 % increase over foothill and mountain areas compared with CNTL case due to static seeding effect; high concentrations of INPs increase solid hydrometer number concentrations and enhance deposition growth, leading to an Increase in condensation/deposition amounts.

HCCN, ECCN, and HIN cases produce larger accumulated rainfall amounts (seeding effects) over foothill area according to the increase in condensate/deposition amounts. However, over mountain area, ECCN (HIN) case shows the reduction of seeding effect due to a kind of over seeding effect; too many ice particles from homogeneous/heterogeneous freezing (heterogeneous freezing) of cloud droplets, leading to smaller ice particle sizes and their easy evaporation/sublimation.

Over desert area, cloud top height is the lowest among the three areas, and the layer where cold rain processes are dominant is the shallowest. Therefore, HCCN and ECCN cases reduce mean droplet sizes and suppress warm rain process, resulting in negative seeding effects.

Since the results mentioned above are based on limited number of case studies, dependency of seeding effects on vertical shear of horizontal wind and microphysics parameterization, which are thought to have a large influence on precipitation processes, needs to be studied more in detail.