Notes

Outline

Chapter 7: Precipitation Processes

Precipitations

Terminal Velocity

Slide 4

Raindrops

How Raindrop Grows?

Growth by Condensation

Growth in Warm Clouds

Collision Collector drops collide with smaller drops. Due to compressed air beneath falling drop, there is an inverse relationship between collector drop size and collision efficiency. Collisions typically occur between a collector and fairly large cloud drops. Smaller drops are pushed aside. Collision is more effective for the droplets that are not very much smaller than the collect droplet.

Coalescence When collisions occur, drops either bounce apart or coalesce into one larger drop.  Coalescence efficiency is very high indicating that most collisions result in coalescence.  Collision and coalescence together form the primary mechanism for precipitation in the tropics, where warm clouds dominate.

Cool and Cold Clouds

An Example of  Cool and Cold Cloud

Growth in Cool and Cold Clouds

Bergeron Process

Riming and Aggregation

Forms of Precipitation

Snow Snowflakes have a wide assortment of shapes and sizes depending on moisture content and temperature of the air. Snowfall distribution in North America is related to north-south alignment of mountain ranges and the presence of the Great Lakes. Lake effect: snows develop as the warm lake waters evaporate into cold air.

Rain Rain is associated with warm clouds exclusively and cool clouds when surface temperatures are above freezing Rainshowers are episodic precipitation events associated with convective activity and cumulus clouds Drops tend to be large and widely spaced to begin, then smaller drops become more prolific Raindrop Shape begins as spherical As frictional drag increases, changes to a mushroom shape Drops eventually flatten Drops split when frictional drag overcomes the surface tension of water Splitting ensures a maximum drop size of about 5 mm and the continuation of the collision-coalescence process

Graupel and Hail Graupel are ice crystals that undergo extensive riming Lose six sided shape and smooth out Either falls to the ground or provides a nucleus for hail Hail forms as concentric layers of ice build around graupel Formed as graupel is carried aloft in updrafts At high altitudes, water accreting to graupel freezes, forming a layer Hail falls but is eventually carried aloft again by an updraft where the process repeats The ultimate size of the hailstone is determined by the intensity of the updraft. Great Plains = highest frequency of hail events

Slide 20

Hail Frequency in the U.S.

Sleet and Freezing Rain Sleet begins as ice crystals which melt into rain through a mid-level inversion before solidifying in colder near surface air Freezing Rain forms similarly to sleet, however, the drop does not completely solidify before striking the surface

Cloud Seeding The objective is to convert some of the supercooled droplets in a cool clouds to ice and cause precipitation by the Bergeron process. Two primary methods are used to trigger the precipitation process. Dry ice is used to lower cloud temperature to a freezing point in order to stimulate ice crystal production leading to the Bergeron process. Silver iodide initiates the Bergeron process by directly acting as freezing nuclei. Under ideal conditions, seeding may enhance precipitation by about 10%.

Measuring Precipitation Standard raingages, with a 20.3 cm (8”) collected surface and 1/10 area collector are used to measure liquid precipitation Depth of water level conveys a tenfold increase in total precipitation Automated devices provide a record of precipitation amount and time of the event

Measuring Snow Raingages are inadequate for measuring frozen precipitation Measurements of accumulated snow are used Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations  Detect snow weight and convert directly to water equivalent