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Chapter 7: Precipitation Processes Chapter 7: Precipitation Processes

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Page 1: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

Chapter 7: Precipitation ProcessesChapter 7: Precipitation Processes

Page 2: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

From: Introduction to Tropical Meteorology, 1st Edition, VersionFrom: Introduction to Tropical Meteorology, 1st Edition, Version 1.1.2, 1.1.2, Produced by the Produced by the COMETCOMET®® ProgramProgramCopyright 2007Copyright 2007--2008, 2008, University Corporation for Atmospheric ResearchUniversity Corporation for Atmospheric Research. .

Page 3: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

From: Introduction to Tropical Meteorology, 1st Edition, VersionFrom: Introduction to Tropical Meteorology, 1st Edition, Version 1.1.2, 1.1.2, Produced by the Produced by the COMETCOMET®® ProgramProgramCopyright 2007Copyright 2007--2008, 2008, University Corporation for Atmospheric ResearchUniversity Corporation for Atmospheric Research. .

Page 4: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

Growth of Cloud DropletForms of PrecipitationsCloud Seeding

Page 5: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

PrecipitationsPrecipitations

Water Vapor Saturated

Cloud Droplet formed around Cloud Condensation Nuclei

Precipitation

Need Cloud Condensation Nuclei

Need to fall down

Page 6: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

Radius = 100 timesVolume = 1 million times

Page 7: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

Terminal VelocityTerminal Velocity

gravity forcegravity force

drag forcedrag force

VVrr

Terminal velocity is the constant speed that a falling object has when the gravity force and the drag force applied on the subject reach a balance.

Terminal velocity depends on the size of the object: small objects fall slowly and large objectives fall quickly.

Page 8: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

RaindropsRaindrops

Rain droplets have to have large enough falling speed in order to overcome the updraft (that produces the rain) to fall to the ground.

This means the rain droplets have to GROW to large enough sizes to become precipitation.

Page 9: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

Page 10: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

How Raindrop Grows?How Raindrop Grows?

Growth by Condensation (too small)

Growth in Warm Clouds: Collision-Coalescence Process

Growth in Cool and Cold Clouds: Bergeron Process

Page 11: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

Growth by CondensationGrowth by Condensation

Condensation about condensation nuclei initially forms most cloud drops.

Only a valid form of growth until the drop achieves a radius of about 20 μm due to overall low amounts of water vapor available.

Insufficient process to generate precipitation.

Page 12: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

Growth in Warm CloudsGrowth in Warm CloudsMost clouds formed in the Tropics, and many in the middle latitudes, are warm clouds.

Those clouds have temperatures greater than 0ºC throughout.

The Collision-coalescence process generates precipitation.

This process depends on the differing fall speeds of different-sized droplets.

It begins with large collector drops which have high terminal velocities.

Page 13: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

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.

CollisionCollision

Page 14: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

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.

CoalescenceCoalescence

Page 15: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

Page 16: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

Cool and Cold CloudsCool and Cold Clouds

A portion of most mid-latitude clouds have temperatures below the melting point of ice.

Cold clouds are referred to those have temperature below 0ºC throughout and consist entirely of ice crystals, supercooled droplets, or a mixture of two.

Cool clouds are referred to those have temperatures above 0ºC in the lower reaches and subfreezing condition above.

Page 17: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

Cumulonimbus clouds containboth ice (top, fuzzy cloud margins), liquid drops (bottom, sharp margins) and a mix of ice and liquid (middle)

An Example of Cool and Cold CloudAn Example of Cool and Cold Cloud

Page 18: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

Page 19: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

Growth in Cool and Cold CloudsGrowth in Cool and Cold Clouds

Cool month mid-latitude and high latitude clouds are classified as cool clouds as average temperatures are usually below freezing.

Clouds may be composed of (1) Liquid water, (2) Supercooledwater, and/or (3) Ice.

Coexistence of ice and supercooled water is critical to the creation of cool cloud precipitation - the Bergeron Process.

Page 20: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

Bergeron ProcessBergeron Process

Saturation vapor pressure of ice is less than that of supercooled water and water vapor.

During coexistence, water will sublimate directly onto ice.

Ice crystals grow rapidly at the expense of supercooled drops.

Collisions between falling crystals and drops causes growth through riming and aggregation.

Page 21: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

Riming and AggregationRiming and Aggregation

Riming = liquid water freezing onto ice crystals producing rapid growth.

Aggregation = the joining of multiple ice crystals through the bonding of surface water builds ice crystals to the point of overcoming updrafts

Collision combined with riming and aggregation allow formation of precipitation within 1/2 hour of initial formation.

Page 22: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

Page 23: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

Forms of PrecipitationForms of Precipitation

RainSnowGraupel and HailSleetFreezing Rain

Page 24: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

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.

SnowSnow

Page 25: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

– 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

RainRain

Page 26: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

– 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

GraupelGraupel and Hail and Hail

Page 27: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

Hail Formation

Concentric layers of icein hail indicate the cyclicalhailstone formation process

Page 28: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

Hail Frequency in the U.S.Hail Frequency in the U.S.

Page 29: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

– 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

Sleet and Freezing RainSleet and Freezing Rain

Page 30: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

TwomeyTwomey effect, 1974effect, 1974

How CCN (cloud condensation nuclei) from How CCN (cloud condensation nuclei) from anthropogenic pollution may anthropogenic pollution may

increase the cloud lifetime &increase the cloud lifetime &decrease the precipitationdecrease the precipitation

Page 31: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

The objective is to convert some of the supercooleddroplets 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%.

Cloud SeedingCloud Seeding

Page 32: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

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

A raingageMeasuring PrecipitationMeasuring Precipitation

Page 33: Chapter 7: Precipitation Processes - Home |  · Water equivalent of snow, a 10 to 1 ratio is assumed Automated snow pillows are common in many locations Detect snow weight and convert

ESS5Prof. Jin-Yi Yu

Raingages are inadequate for measuring frozen precipitationMeasurements of accumulated snow are used Water equivalent of snow, a 10 to 1 ratio is assumedAutomated snow pillows are common in many locationsDetect snow weight and convert directly to water equivalent

Measuring SnowMeasuring Snow


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