ATM OCN 100 Summer 2002 1 ATM OCN 100 - Fall 2001 LECTURE 13 (Con’t.) PRECIPITATION & the PRECIPITATION FORMATION PROCESS A. Introduction B. Background.

MADISON’S CURRENT WEATHERMADISON’S CURRENT WEATHER

Madison Weather at 1000 AM CDT 17 JUL 2002 Updated twice an hour at :05 and :25 Sky/Weather: PTSUNNY Temperature: 79 F (26 C) Dew Point: 65 F (18 C) Relative Humidity: 62% Wind: SW5 MPH Barometer: 30.07F (1018.3 mb)

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 2727

Last 24 hrs in MadisonLast 24 hrs in Madison

FOGFOG

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 2828

CURRENT CURRENT VISIBLEVISIBLE

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 2929

CURRENT IRCURRENT IR

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 3131

Current Surface Weather Map Current Surface Weather Map with Isobars (“iso” = equal & “bar” = weight), Fronts and Radarwith Isobars (“iso” = equal & “bar” = weight), Fronts and Radar

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 3535

Current Temperatures (Current Temperatures (°°F) & IsothermsF) & Isotherms(“iso” = equal +”therm” = temperature)(“iso” = equal +”therm” = temperature)

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 3636

Current Dewpoints Current Dewpoints ((ooF) F)

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 3737

Sample Radiosonde SitesSample Radiosonde Sites

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 3838

RAPID CITYRAPID CITY

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 3939

GREEN BAYGREEN BAY

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 4343

Tomorrow AM Forecast MapTomorrow AM Forecast Map

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 4444

AnnouncementsAnnouncements

Homework #3 is due today Homework #3 is due today If you have ??, please see me. If you have ??, please see me.

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 8585

D. BUILDING A RAINDROPD. BUILDING A RAINDROP (con’t.) (con’t.)

RequirementsRequirements– Need to build Rain Drop from Cloud DropletNeed to build Rain Drop from Cloud Droplet– But… amount of water needed is But… amount of water needed is

proportional to the cube of the radius!proportional to the cube of the radius! Factors opposing PrecipitationFactors opposing Precipitation

– Cloud-producing updrafts greater than Cloud-producing updrafts greater than droplet’s droplet’s terminal velocityterminal velocity;;

– Dry atmosphere below cloud base may Dry atmosphere below cloud base may evaporate droplet to form evaporate droplet to form VIRGAVIRGA.. where ... where ...

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 8787

VirgaVirga

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 8888

A Convective RainshowerA Convective Rainshower

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 8989

E. POSSIBLE PRECIPITATION MECHANISMS E. POSSIBLE PRECIPITATION MECHANISMS (Growth of Liquid rain drops)(Growth of Liquid rain drops)

The current proposed mechanisms:The current proposed mechanisms:

– Simple Condensation by Diffusion Simple Condensation by Diffusion ProcessProcess

– The Collision - Coalescence ProcessThe Collision - Coalescence Process

– The Ice Crystal The Ice Crystal oror Bergeron-Findeisen Process Bergeron-Findeisen Process

or specifically, ... or specifically, ...

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 9090

PRECIPITATION MECHANISM:PRECIPITATION MECHANISM: CONDENSATION-DIFFUSION PROCESSCONDENSATION-DIFFUSION PROCESS

Based upon HBased upon H220 vapor molecules condensing 0 vapor molecules condensing upon cloud droplets;upon cloud droplets;

Competing EffectsCompeting Effects– Curvature EffectCurvature Effect– Solute EffectSolute Effect

Condensation needs to overcome surface tension Condensation needs to overcome surface tension of droplet;of droplet;

Process would take 24 to 48 hours;Process would take 24 to 48 hours; Hence, process is too slow!Hence, process is too slow!

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 9191

PRECIPITATION MECHANISM:PRECIPITATION MECHANISM: COLLISION-COALESCENCE PROCESSCOLLISION-COALESCENCE PROCESS

Requires droplets of various sizes, with some Requires droplets of various sizes, with some greater than 20 greater than 20 m diameter;m diameter;

Large droplets have greater fall speed;Large droplets have greater fall speed;

Figure 8.2 Moran & Morgan (1997)Figure 8.2 Moran & Morgan (1997)

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 9292

COLLISION-COALESCENCE THEORYCOLLISION-COALESCENCE THEORY (con’t.)(con’t.)

Different sized droplets have different Different sized droplets have different surface electrostatic charges;surface electrostatic charges;

Large droplet has a wake “low”;Large droplet has a wake “low”; Thus, large droplets collect smaller Thus, large droplets collect smaller

droplets and form a rain drop.droplets and form a rain drop.

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 9393

PRECIPITATION MECHANISM:PRECIPITATION MECHANISM:ICE CRYSTAL ICE CRYSTAL

((or or Bergeron-Findeisen) PROCESSBergeron-Findeisen) PROCESS

Requires a mixture of ice crystals and Requires a mixture of ice crystals and supercooled supercooled liquid droplets;liquid droplets;

– supercooled droplet remains a liquid supercooled droplet remains a liquid atat -40 -40ooC (-40C (-40ooF) < T < 0F) < T < 0ooC (32C (32ooF);F);

– smaller droplets are more frequently smaller droplets are more frequently supercooled.supercooled.

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 9494

Saturation Vapor Pressure as a Function of Temperature Saturation Vapor Pressure as a Function of Temperature See Fig. 6.3 Moran & Morgan, 1997See Fig. 6.3 Moran & Morgan, 1997

[Source: Smithsonian Meteorological Tables, 1971]

0

2

4

6

8

10

-15 -10 -5 0 5 10

TEMPERATURE [deg CELSIUS]

SATU

RATI

ON V

APOR

PR

ESSU

RE [m

b]

OVER LIQUID WATER Series3

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 9595

DIFFERENCE BETWEEN VAPOR PRESSURE OVER

LIQUID & ICE [es , liquid - es, ice] [Source: Smithsonian Meteorological Tables, 1971]

0.00

0.10

0.20

0.30

-60 -50 -40 -30 -20 -10 0

TEMPERATURE [deg CELSIUS]

SATU

RATIO

N VA

POR

PRES

SURE

[m

b]

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 9696

ICE CRYSTAL THEORY ICE CRYSTAL THEORY (con’t.)(con’t.)

Saturation vapor pressure over Saturation vapor pressure over supercooled liquid >> over ice at same T; supercooled liquid >> over ice at same T;

Therefore, ice crystal grows at expense of Therefore, ice crystal grows at expense of supercooled water droplet.supercooled water droplet.

Figure 8.3 Moran & Morgan (1997)Figure 8.3 Moran & Morgan (1997)

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 9797

F. PRECIPITATION TYPESF. PRECIPITATION TYPES

Distinguishing Features of Distinguishing Features of Precipitation Types Precipitation Types– Liquid Liquid (Rain, Drizzle)(Rain, Drizzle)

– Frozen Frozen (Snow, Ice Pellets, Hail)(Snow, Ice Pellets, Hail)

– Freezing Freezing (Freezing Rain, Freezing Drizzle) (Freezing Rain, Freezing Drizzle)

where --where --

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 9898

LIQUID PRECIPITATIONLIQUID PRECIPITATIONNational Weather ServiceNational Weather Service

Rain Liquid water drops usuallygreater than 0.5 mm (0.02 in)diameter.

Drizzle Numerous tiny water dropletswith diameters between 0.2 and0.5 mm (0.008–0.020 in.), thatappear to float on air currents.Usually of fairly uniformintensity.

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 9999

FROZEN PRECIPITATIONFROZEN PRECIPITATION National Weather ServiceNational Weather Service

Snow Loose white or translucent icecrystals that are branched.Crystals may agglomerate intosnow flakes at temperaturesless than -5°C (23°F).

Ice Pellets[also known as Sleet]

Transparent or translucentspherical beads of ice, whichhave a diameter of 5 mm or less.

Hail Small balls of ice with adiameter ranging from 5 to 50mm (0.2 to 2 in.) falling from Cbclouds.

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 100100

SnowSnow

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 101101

RecallRecallPHYSICAL PHASES of HPHYSICAL PHASES of H22OO

Solid (Ice) Solid (Ice) -- Note Hexagonal (6 sided) Structure-- Note Hexagonal (6 sided) Structure

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 102102

Varieties of SnowflakesVarieties of Snowflakes

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 104104

Varieties of SnowflakesVarieties of SnowflakesSee Fig. 8.6, Moran & Morgan, 1997See Fig. 8.6, Moran & Morgan, 1997

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 105105

FREEZING PRECIPITATIONFREEZING PRECIPITATION National Weather ServiceNational Weather Service

Freezing Rain Rain where drops freezeupon impact with the objectson the earth’s surface or withaircraft in flight.

Freezing Drizzle Drizzle where droplets freezeon impact with objects on theearth's surface or withaircraft in flight.

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 106106

Freezing RainFreezing Rain

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 107107

Freezing Rain/Ice StormFreezing Rain/Ice Storm

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 108108

Hail StonesHail Stones

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 109109

Hail StonesHail Stones

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 110110

Hail StormHail Storm

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 111111

Snow EventSnow Event

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 112112

Rain EventRain Event

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 113113

Freezing Rain EventFreezing Rain Event

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 114114

Ice Pellet (Sleet) FormationIce Pellet (Sleet) Formation

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 115115

Summary of Precipitation Type Formation Summary of Precipitation Type Formation

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 116116

Freezing Rain ClimatologyFreezing Rain Climatology

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 117117

G. PRECIPITATION CLIMATOLOGY - G. PRECIPITATION CLIMATOLOGY - AnnualAnnual

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 118118

G. PRECIPITATION CLIMATOLOGY - JulyG. PRECIPITATION CLIMATOLOGY - July

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 119119

G. PRECIPITATION CLIMATOLOGY - G. PRECIPITATION CLIMATOLOGY - JanuaryJanuary

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 120120

H. WEATHER MODIFICATION H. WEATHER MODIFICATION BACKGROUNDBACKGROUND

Statement of ProblemStatement of Problem– Why no precipitation?Why no precipitation?– Can humans cause rain?Can humans cause rain?

For precipitation, need:For precipitation, need:– sufficient atmospheric humidity;sufficient atmospheric humidity;– a cloud;a cloud;– sufficiently large raindrops/snowflakessufficiently large raindrops/snowflakes

to fall & reach ground as rain or snow. to fall & reach ground as rain or snow.

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 121121

INTENTIONAL ARTIFICIAL WEATHER INTENTIONAL ARTIFICIAL WEATHER MODIFICATIONMODIFICATION

GoalsGoals

– Stimulate PrecipitationStimulate Precipitation

– Dissipate Clouds & FogDissipate Clouds & Fog

– Suppress HailSuppress Hail

– Modify HurricanesModify Hurricanes

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 122122

C. WEATHER MODIFICATION C. WEATHER MODIFICATION (con’t.)(con’t.)

Precipitation Stimulation Techniques - Precipitation Stimulation Techniques - "Cloud seeding""Cloud seeding"– Early Weather Modification TechniquesEarly Weather Modification Techniques– Modern Precipitation Stimulation Modern Precipitation Stimulation

StrategiesStrategies– ProblemsProblems

ScientificScientific LegalLegal

ATM OCN 100 Summer 2002ATM OCN 100 Summer 2002 123123

TYPICAL STRATEGIESTYPICAL STRATEGIES

WARM CLOUDSWARM CLOUDS– Add large hygroscopic nuclei. Add large hygroscopic nuclei.

COLD CLOUDSCOLD CLOUDS– Freeze all supercooled dropletsFreeze all supercooled droplets

with Dry Ice (Solid CO with Dry Ice (Solid CO22););– Add freezing nuclei with ice-like Add freezing nuclei with ice-like

crystal structurecrystal structure with Silver Iodide (AgI). with Silver Iodide (AgI).