Ch 7: Precipitation Processes:

Why does it rain on us???

• 4 different lifting mechanisms

• 3 types of stability

• Two factors limiting the height of clouds 3 types of inversions

• 3 cloud properties. Types of clouds

Review of last lecture

Environment

ParcelEnvironment

ParcelEnvironment

Parcel

The three types of stability

AbsolutelyUnstable

ConditionallyUnstable

AbsolutelyStable

Create classification scheme based on height and form:• high clouds – cirrus, cirrostratus, cirrocumulus• middle clouds – altostratus, altocumulus• low clouds – stratus, stratocumulus, nimbostratus• extensive vertical development – cumulus, cumulonimbus

Cloud classification in the textbook

NASA’s International Satellite Cloud Climatology Project (ISCCP) NASA’s International Satellite Cloud Climatology Project (ISCCP) Cloud Classification - commonly used in climate researchCloud Classification - commonly used in climate research

Satellite observation of clouds - ProjectsSatellite observation of clouds - Projects

• NASA’s International Satellite Cloud Climatology Project (ISCCP) Combine the measurements of 5 geostationary and 1-2 polar orbiting satellites. 1983-Now, cloud top height and optical depth.

• NASA’s Earth Observation System including a set of polar orbiting satellites (A-Train), especially CloudSat (with a cloud radar) and CALIPSO (with a cloud lidar). Ongoing, cloud particle information, detailed vertical structure.

The 3rd in-class assignmentThe 3rd in-class assignment

Satellite observation of precipitationSatellite observation of precipitation

• Infrared-derived or visible-derived (GPI)• Microwave-derived (MSU, SSM/I, TMI)• Radar: Tropical Rainfall Measurement Mission (TRMM) • Merged with surface gauge measurements and model

forecast

Global distribution of precipitationGlobal distribution of precipitation

Precipitation formation - cloud drop growthPrecipitation formation - cloud drop growth

• Not all clouds precipitate due to their small sizes and slow fall rates• Balance between gravity and frictional

drag eventually become equal to achieve terminal velocity VT, which is proportional to the square root of cloud drop radius VT=c r0.5 ,where r is drop radius and c is a constant.

• For a cloud drop to fall, its terminal velocity must exceed the vertical velocity of the upward-moving air parcel. Otherwise it will be carried up.

• Cloud drop growth is required for precipitation to form

Fgravity

Fdrag

1. Collision Coalescence (warm clouds, T > 0 C, form rain)

2. Bergeron Process (cool/cold clouds, T < 0 C, form snow)

Mechanisms for cloud drops to grow larger

Cold Clouds Cool Clouds

• Process begins with large collector drops which have high terminal velocities

• Collector drops collide with smaller drops

• If drops too small: compressed air beneath falling drop forces small drops aside

• If drop is too big (same size as collector) it will fall at same speed and no collision will occur

• So, collection efficiency is greatest for drop sizes that are intermediate with respect to the size of the collector

• When collisions occur, drops either bounce apart or coalesce into one larger drop. Coalescence efficiency is generally very high, indicating that most collisions result in the two drops joining.

1. Collision Coalescence: Growth in Warm Clouds

• spherical initially – as frictional drag increases parachute shape

• eventually flatten out – bar shape

• split when frictional drag > surface tension of water

• maximum drop size of about 5 mm

Raindrop Shape

• Clouds are usually composed of: liquid water, super-cooled water, and/or ice (supercooled water exists down to T= -40C !!)

• Supercooled water can exist at T<0C because ice formation requires ice nuclei, which, unlike condensation nuclei, are rare unless the temp. is very cold

• Coexistence of ice and super-cooled water is critical to the creation of cool/cold cloud precipitation - the Bergeron Process

• Saturation vapor pressure of ice < super-cooled water. When ice and water are present, water will be deposited directly onto ice. Ice crystals grow rapidly at the expense of super-cooled drops

http://www.uwsp.edu

2. Bergeron Process: Growth in Cool/Cold Clouds

• Bergeron Process usually not enough to produce large enough crystals for preciptation

• Further growth is due to collisions between falling crystals and drops riming and aggregation

• Riming = liquid water freezing onto ice crystals

• Aggregation = the joining of ice crystals through the bonding of surface water builds ice crystals

• Collision combined with riming and aggregation allow formation of crystals large enough to precipitate within 1/2 hour of initial formation

Further growth: Riming and Aggregation

Variable shapes/sizes depends on moisture content and temperature

Shape of snowflakes

Dendrite ice crystals

Plate ice crystal

Wilson Bentley, a Vermont farmer, took photographs of snowflakes under a microscope as a hobby. These photographs were published in the "Monthly Weather Review" in 1902.

• Graupel – 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 – concentric layers of ice build around graupel

• graupel carried aloft in updrafts high altitudes freezing temperatures

• water accreting to graupel freezes, forming a layer

• Hail begins to fall, carried aloft again by updrafts, process repeats

• Hailstones are very heavy – high density

• Capable of tremendous amounts of damage

• Great Plains = highest frequency of hail events

Concentric layers of ice in hail indicate the cyclicalhailstone formation process

• 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

• Induce precipitation injection of dry ice or silver iodide into clouds

• convert super-cooled droplets to ice initiate Bergeron process

• Dry ice (frozen CO2) – lowers temperature to -40C, • no ice nuclei are required for water droplets to freeze

• Silver iodide – acts as ice nuclei at warmer temp (-5C)

• Effectiveness is debated

Cloud Seeding

Summary of Precipitation processes:

Condensation

Collision-coalescence

Bergeron Process

Warm

clouds

Cool/cold clouds

Rain Snow

(can change to rain, sleet, or any other type of precipitation depending on underlying atmosphere

Riming/Aggregation

• Forces acting on a cloud/rain droplet. Terminal velocity. How does it change with cloud drop radius?

• Growth mechanisms for rain and snow

Concepts

Next week we will discuss about different cyclones Next week we will discuss about different cyclones and storms (Part Four in the textbook)and storms (Part Four in the textbook)

• Monday: Tropical storms and hurricanes (Ch 12)