NATS 101

Lecture 13Precipitation Processes

Supplemental References for Supplemental References for Today’s LectureToday’s Lecture

Danielson, E. W., J. Levin and E. Abrams, 1998: Meteorology. 462 pp. McGraw-Hill. (ISBN 0-697-21711-6)

Gedzelman, S. D., 1980: The Science and Wonders of the Atmosphere. 535 pp. John-Wiley & Sons. (ISBN 0-471-02972-6)

Review: Vertical Stability

Rising unsaturated air, and all sinking air

Temp changes at DAR of 10oC/km

Dew Point (DP) changes at rate of 2oC/km

Rising saturated air

Latent Heating Mitigates Adia. Cooling

Temp and DP cool at MAR of 6oC/km

Water Vapor Condenses into Liquid

Review: Vertical Stability

Vertical Stability Determined by ELR

Conditionally Unstable

(MAR < ELR < DAR)

Temp Difference between Environmental Air and Air Parcel, and the Depth of Conditionally Instability Controls

Vertical Extent and Severity of Cumulus

Conditionally Unstable: Lower Rock

Ahrens, Fig 5.7

Environmental Lapse Rate (ELR)

ELR is the Temp change with height that is recorded by a weather balloon

ELR is absolutely unstable in a thin layer just above the ground on hot, sunny days

Ahrens, Meteorology Today 5th Ed.

ELR is 6.5o C/km, on average, and thus is conditionally unstable!

6.06.0oo C/km C/km

10.010.0oo C/km C/km

6.56.5oo C/km C/km

Cloud Droplets to Raindrops

A raindrop is 106 bigger than a cloud droplet

Several days are needed for condensation alone to grow raindrops

Yet, raindrops can form from cloud droplets in a less than one hour

What processes account for such rapid growth?

106 bigger

106 bigger

Ahrens, Fig. 5.15

Terminal Fall Speeds (upward suspension velocity)

1.E-061.E-051.E-041.E-031.E-021.E-011.E+001.E+011.E+021.E+03

0.0002 0.02 0.1 0.2 1 2 5Diameter (millimeters)

Ter

min

al F

all S

peed

(cm

/s)

Small-Large RaindropsCloud Droplets-DrizzleCCN

small raindrop

Area swept issmaller than

area of drop

Collision-CoalescenceBig water drops fall faster than

small drops

As big drops fall, they collide with smaller drops

Some of the smaller drops stick to the big drops

Collision-Coalescence

Drops can grow by this process in warm clouds with no ice

Occurs in warm tropical cloudsCollection Efficiency 10-50%

Warm Cloud Precipitation

As cloud droplet ascends, it grows larger by collision-coalescence

Cloud droplet reaches the height where the updraft speed equals terminal fall speed

As drop falls, it grows by collision-coalescence to size of a large raindrop

Ahrens, Fig. 5.16

Updraft (5 m/s)

Mixed Water-Ice Clouds

Clouds that rise above freezing level contain mixture of water-ice

Mixed region exists where Temps > -40oC

Only ice crystals exist where Temps < -40oC

Mid-latitude clouds are generally mixed

Ahrens, Fig. 5.17

glaciated regionglaciated region

SVP over Liquid and Ice

SVP over ice is less than over water because sublimation takes more energy than evaporation

If water surface is not flat, but instead curves like a cloud drop, then the SVP difference is even larger

So at equilibrium, more vapor resides over cloud droplets than ice crystals

Ahrens, Meteorology Today 5th Ed.

SVP near Droplets and Ice

SVP is higher over supercooled water drops than ice

Ahrens, Fig. 5.18

Ice Crystal Process

Since SVP for a water droplet is higher than for ice crystal, vapor next to droplet will diffuse towards ice

Ice crystals grow at the expense of water drops, which freeze on contact

As the ice crystals grow, they begin to fall

Ahrens, Fig. 5.19

Effect maximized around -15oC

Accretion-Aggregation Process

Accretion (Riming)

Aggregation

Supercooled water droplets will freeze on contact with ice

ice crystalice crystal

Small ice particles will adhere to ice

crystals

snowflakesnowflake

Splintering

Ahrens, Fig. 5.17

Also known as the Bergeron Process after the meteorologist who first recognized the importance of ice in the precipitation process

Summary: Key Concepts

Condensation acts too slow to produce rain

Several days required for condensationClouds produce rain in less than 1 hour

Warm clouds (no ice)

Collision-Coalescence Process

Cold clouds (with ice)

Ice Crystal ProcessAccretion-Splintering-Aggregation

Examples of Precipitation Types Type Size Description

Drizzle < 0.5 mm Small uniform drops that fall from stratus clouds

Rain 0.5 - 5 mm Size of drops generally vary from one place to another

Freezing Rain 0.5 - 5 mm Rain that freezes on contact with object

Sleet 0.5 - 5 mm Ice particles from raindrops that freeze during descent

Snow 1 - 2 mm Aggregated ice crystals that remain frozen during descent

Hail 5 to 10 cm or larger

Hard pellets of ice from cumulonimbus clouds

Temp Profiles for Precipitation

Snow - Temp colder than 0oC everywhere (generally speaking!)

Sleet - Melting aloft, deep freezing layer near ground

Freezing Rain - Melting aloft, shallow freezing layer at ground

Rain - Deep layer of warmer than 0oC near ground

Ahrens, Meteorology Today 5th Ed.

Summary: Key Concepts

Precipitation can take many forms

Drizzle-Rain-Glazing-Sleet-Snow-HailDepending on specific weather conditions

Radar used to sense precipitation remotely

Location-Rate-Type (liquid v. frozen)Cloud drops with short wavelength pulseWind component toward and from radar

Assignment for Next Lecture

• Topic – Atmospheric Pressure

• Reading - Ahrens pg 141-148

• Problems - 6.1, 6.7, 6.8