Using the Bin Microphysics Scheme for Estimating the Effect of Seeding on the Precipitation Formation in Stably Stratified Orographic Cloud

A bin microphysical scheme was developed to investigate how artificial increase of ice forming nuclei can affect the precipitation formation in mixed phase orographic clouds. Because there is no any restriction for the shape of size distributions of hydrometeors in bin schemes, they give us a good opportunity to investigate how the precipitation formation can differ in seeded clouds from that of natural clouds.

The model involves five different types of hydrometeors and one particle type for AgI nuclei. The hydrometeor types are water drops, pristine ice, snowflakes and graupel particles. Water drops captured at least one AgI nuclei and drops do not contains them are distinguished. This distinction allows us to track the AgI nuclei inside the water drops and therefore allows the proper simulation of the immersion nucleation. The AgI nuclei were scavenged due to Brownian motion, phoretic forces, turbulence effects and gravitational collection. Beside the immersion freezing, the deposition nucleation, condensation-freezing nucleation and the contact freezing nucleation mechanisms are simulated in the model.

Numerical experiment was performed to investigate how the seeding affects the precipitation formation depending on the location of the seeding in both maritime and continental air masses. The clouds formed over a bell shape mountain with cloud top temperature of about -15 °C. The cloud was in the negative temperature region. The seeding occurred on the surface, at 150 m and at 500 m above the surface during the first three hours of the simulation. The maritime and continental air masses were characterized by CCN concentration of 100 cm-3 and 500 cm-3, respectively. Control calculations were made to simulate the natural precipitation formation for both maritime and continental air masses.

The analysis of the model output data shows:

(i) While in the natural cases the immersion freezing was dominant ice forming mechanism, in the seeded cloud three different ice formations – deposition, condensation – freezing and immersion freezing – produced near the same number of ice particles.

(ii) The role of the contact freezing was negligible in every case.

(iii) About 1 – 2 % of the artificial nuclei were activated, the remaining 98 – 99 % of them did not affect the precipitation formation.

(iv) The amount of the surface precipitation was hardly increased if the seeding has occurred on the surface or near to the surface.

(v) The total amount of surface precipitation increased about 20 – 30 % in the case of maritime CCN when the seeding occurred at 500 m above the surface. The efficiency of seeding was even larger (about 80 % increase of surface precipitation) in the case of continental CCN. The amount of the surface precipitation does not depend on the CCN concentration when the seeding occurred at higher elevation (lower temperature).

(vi) The significant increase of the ice crystal concentration results in smaller mean size and narrower size distribution of the snowflakes. The diffusional growth of ice particles (ice crystals, snowflakes, graupel particles) became more dominant against the riming in case of seeding.

(vii) The evaporation of the water drops has scavenged AgI particles results in regeneration of nuclei are covered with sulphate. Due this process about 10 % of the AgI particles can serve both condensation and ice forming nuclei.