Bergeron Process - Required Conditions

Required Conditions

It is often assumed that the Bergeron process is the dominant process in all mixed phase clouds, but this is not necessarily the case. At subfreezing temperatures, is always greater than, but the ambient vapor pressure is not bounded to a particular range. This results in the three possible scenarios:

Of these three scenarios, only the second describes the Bergeron process. It is worth noting that in the absence of evolving supersaturation, a population of ice and liquid particles in region 1 will eventually transition into region 2 before reaching equilibrium. Both ice and water particles will grow until the ambient vapor pressure falls into equilibrium with respect to liquid water, at which point droplets will cease to grow. During this process, both liquid water and ice are competing for vapor, limiting the growth rate of both species. With the liquid water in equilibrium, the environment is still supersaturated with respect to ice, which will allow ice crystals to continue to grow, moving the population into region 2. The ice crystals will continue to grow under the Bergeron process until all liquid water has evaporated and they come into equilibrium with the vapor field. During this phase of growth, the role of liquid water is reversed; instead of competing with ice for vapor, it serves as an additional source, enhancing the growth rate. Once in equilibrium, the ice crystals will remain in this state until the equilibrium is externally perturbed.

In an adiabatic updraft, expansion of the parcel results in a direct decrease in the vapor pressure as well as a decrease in temperature which in turn decreases the saturation vapor pressure. The saturation vapor pressure decreases more rapidly than the vapor pressure, resulting in a supersaturated condition. The strength of the supersaturation is a function of the rate of production of excess vapor (a function of updraft speed) and the rate of vapor depletion (a function of particle phase, size and number density).

Using these relations, Korolev and Mazin derived expressions for critical updraft speeds that represent the boundaries between regions one, two and three:

(1)

(2)

where,

• is the critical updraft speed separating region 1 and 2
• is the critical downdraft speed separating region 2 and 3
• η and χ are coefficients dependent on temperature and pressure
• and are the number densities of ice and liquid particles (respectively)
• and are the mean radius of ice and liquid particles (respectively)

For values of typical of clouds, ranges from a few cm/s to a few m/s. These velocities can be easily produced by convection, waves or turbulence, indicating that it is not uncommon for both liquid water and ice to grow simultaneously. In comparison, for typical values of, downdraft velocities in excess of a few are required for both liquid and ice to shrink simultaneously. These velocities are common in convective downdrafts, but are not typical for stratus clouds.