Wintertime Cloud Microphysics Review
This training was designed to give an overview of how ice and liquid particles both are created and interact in the atmosphere and more importantly how this influences operational forecasting.
Although many forecasters do not consider cloud microphysics in the operational forecast setting, it has very many applications.
This particular review is designed for wintertime forecasting in particular and discusses microphysics' relationship to precipitation type expected at the surface.
The module begins with how clouds initiate ice from liquid drops via cloud condensation nuclei and works into critical temperatures for certain precipitation types.
It then progresses through hydrometeor growth, operational aids, and finally case examples.
The operational forecaster should come away with a better grasp of precipitation type expected and possible heavy snow microphysics to look for in both observed and model data.
First and foremost, cloud condensation nuclei (CCN) are particles suspended in the air which support the growth of cloud droplets or ice on their surface.
Of all of the CCN floating in the air, a low percentage act as ice forming nuclei (IN) that have the ability to act as a surface for ice growth to initiate (from water in the vapor or liquid phase).
Lets take a simple case of a solo cloud in the sky with no others around it. Also, we assume the cloud has a temperature of <0c>activated.
Since not all CCN particles are IN, or said another way - not all CCNs promote the growth of ice themselves - there must be something special about them. This "something special" is their chemical makeup.
Water changing phase to grow as ice in a cloud is very particular as to the chemical composition of the particle on which it would like to grow initially. (image to left is ice molecule, from Edinformatics.com)
It also depends on the relative humidity and temperature of the cloud. CCN particles have a better chance of being an IN as the temperature decreases and the relative humidity increases.
In fact, no IN's can be activated (or have ice begin to grow on them) above the temperature of -4C even if the cloud is supersaturated (relative humidity > 100%).
A look at the larger CCN family shows that CCN are simply particles floating in the air and consist of soils, sand, sea salt, volcanic debris, and also particles emitted from urban factories.
The number of CCN vary greatly from place to place dependent on urbanization and continental versus sea locations.
CCN also generally decrease rapidly from the surface of the earth and then maximize again in the lower stratosphere.
Work done by various cloud physicists suggests that about 80-90% of all IN over the upper Midwest consist of some type of clay material with vermiculite leading the charge.
In these studies, it was found that melted snow crystals typically contained one solid IN particle in the center of the snow crystal. IN activation, or the initiation of ice on a particle in a supercooled liquid cloud, occurs at different temperatures for IN particles of different material.
Here are some of the common IN and their activation temperatures:
- Silver Iodide (used for cloud seeding) -4C
- Kaolinite (clay family) -9C
- Volcanic Ash -13C
- Vermiculite (most prevalent clay) -15C