Class #4: Stability, cloud development, and precipitation

Class #4: Stability, cloud development, and precipitation

Chapters 6 and 7

Class #4 Monday, July 12, 2010 1

Chapter 6

Stability & Cloud development


Fig. 6-CO, p. 140

Fig. 6-1, p. 142

Importance of Clouds

• Release heat to atmosphere

• Help regulate energy balance

• Indicate physical processes


Atmospheric Stability

• Clouds from as air rises and cools

• Adiabatic processes: change in temperature without giving or removing

– Dry rate = 10°C/1000m

– Moist rate = 6°C/1000m

• Stability is a state of equilibrium in terms atmospheric movement; no vertical movement occurs


Determining Stability

• Warm air rises or is unstable

• Cool air sinks or is stable

• Compare air parcel lapse rate to environmental lapse rate



Table 6-1, p. 143


• Stable environment

– Environmental lapse rate less than moist lapse rate

– If an air parcel is forced it will spread horizontally and form stratus clouds

– Usually a cool surface (radiation, advection)

– Inversion: warm over cool.





Fig. 6-6, p. 145


• Special Topic: Subsidence Inversions

– Strong subsidence exacerbates air pollution due to the lack of vertical motion.

– Pollution is not diluted.



• An Unstable Atmosphere

– Environmental lapse rate greater than the dry adiabatic lapse rate

– As air parcel rises it forms a vertical cloud

– Convection, thunderstorms, severe weather




• A Conditionally Unstable Atmosphere

– Moist adiabatic lapse rate is less than the environmental lapse rate which is less than the dry adiabatic lapse rate

– Stable below cloud unstable above cloud base

– Atmosphere usually in this state







Fig. 6-13, p. 149

Fig. 6-14, p. 149

Fig. 1, p. 150


• Causes of Instability

– Cool air aloft (advection, radiation cooling in clouds)

– Warming of surface (insolation, advection, warm surface)


Cloud Development

• Clouds develop as an air parcel rises and cools below the dew point.

• Usually a trigger or process is need to initiate the rise of an air parcel.



Cloud Development

• Convection

– Differential land surface heating creates areas of high surface temperature.

– Air above warm land surface heats, forming a

‘bubble’ of warm air that rises or convection.

– Cloud base forms at level of free convection.



Class #4 Monday, July 12, 2010

Stepped Art

32

Fig. 6-16, p. 152




Fig. 2, p. 155

Cloud Development

• Topography

– Orographic uplift

– Orographic clouds

– Windward, leeward, rain shadow

– Lenticular clouds






Cloud Development

• Topic: Adiabatic charts

– Adiabatic charts show how various atmospheric variables change with height: pressure, temperature, humidity.


Fig. 3, p. 158

Fig. 4, p. 158

Fig. 5, p. 158

Fig. 6, p. 159

Fig. 7, p. 159

Cloud Development

• Changing cloud forms

– Stratus clouds can change to cumulus clouds if the top of the cloud cools and the bottom of the cloud warms.

– Alto cumulus castellanus: towers on alto stratus

– If moist stable air without clouds is mixed or stirred it can form stratocumulus clouds.







Chapter 7

Precipitation


Fig. 7-1, p. 166

Fig. 7-2, p. 166

Fig. 7-3, p. 167

Table 7-1, p. 168

Precipitation Processes

• Precipitation is any form of water that falls from a cloud and reaches the ground.

• How do cloud drops grow?

– When air is saturated with respect to a flat surface it is unsaturated with respect to a curved droplet of water.

• Super saturated



• Collision & Coalescence

– Droplets of different sizes collide and coalesce into larger droplets; warm cloud process

– Ice-Crystal Process

• Cold clouds a mixture of ice & water

• Ice crystals grow at expense of surrounding water droplets

• Saturation vapor pressure greater over water as compared to ice.





Stepped Art

63

Fig. 7-5, p. 169


Fig. 1, p. 171




Fig. 7-10, p. 173

Fig. 7-10, p. 173

Fig. 7-10, p. 173

Fig. 7-10, p. 173


• Topic: Freezing of Cloud Droplets

– Spontaneous or homogeneous freezing

– Ice embryo



• Cloud Seeding

– Inject cloud with small particles that act as condensation nuclei, starting the precipitation process.

– NEED CLOUDS: seeding does not generate clouds

– Cold clouds with a low seed ration best

– Dry ice, silver iodide





Stepped Art

77

Fig. 7-12, p. 174

Precipitation in Clouds

• Starts quickly

• Most Precipitation formed through accretion

• Many times rain starts as ice


Fig. 7-12, p. 174

Fig. 7-12, p. 174

Stepped Art

Fig. 7-12, p. 174


Precipitation Types

• Rain: falling drop of liquid water

– Drizzle less than 0.5 mm

– Virga

– Cloudburst

• Snow: frozen water falling from sky (crystal or flake)

– Most precipitation starts as snow

– Freezing level, snow & cloud appearance, fall streaks, drifting snow, blizzard

– A blanket of snow is a good insulator



Table 7-2, p. 176

Fig. 2, p. 177

Fig. 7-16, p. 178

Fig. 7-17, p. 178



Table 7-3, p. 178

Fig. 3, p. 179

Fig. 7-18, p. 180

Table 7-4, p. 180




Precipitation Types

• Topic: Tear Drops

– Raindrops not tear shaped

– Shape is size dependent

• Less than 2 mm = sphere

• Greater than 2 mm = flattened sphere


Precipitation Types

• Topics: Sounds and snow

– A blanket of snow will act like an acoustic tile and absorb sound waves.

• Topics: Snow with Temperature above

Freezing

– Unsaturated wet bulb temperature below or equal to 0°C, rain cooled by evaporation forms snow despite environmental temperature above freezing.


Precipitation Types

• Sleet: air below freezing, then travels through a layer of air above freezing, begins to melt and then falls through a layer of air below freezing just above the ground surface.

• Freezing Rain: ground surface is freezing as rain hits the surface it freezes.



Fig. 7-21, p. 182

Fig. 7-22, p. 182

Fig. 7-23, p. 182


• Precipitation is any form of water that falls from a cloud and reaches the ground.

• How do cloud drops grow?

– When air is saturated with respect to a flat surface it is unsaturated with respect to a curved droplet of water.

• Super saturated






Precipitation Types

• Observation: Aircraft Icing

– Aviation hazard is created by the increase in weight as ice forms on the body of the airplane.

– Spray plane with anti-freeze.


Precipitation Types

• Snow Grains: solid equivalent of drizzle, no bounce or shatter

• Snow Pellets: larger than grains, bounce, break, crunch underfoot

• Graupel: ice particle accumulation with rime

• Hail: graupel act as embryo in intense thunderstorm, grow through aggregation as pushed up by updraft.








Stepped Art

117

Fig. 7-29, p. 185

Measuring Precipitation

• Instruments

– Rain gauge: standard, tipping bucket, weighing

• Snow: average depth at 3 locations, 10:1 water equivalent

• Doppler Radar

– Transmitter generates energy toward target, returned energy measured and displayed

• Brightness of echo = amount/intensity of rain

– Doppler: measures speed of horizontal rain






Stepped Art

122

Fig. 7-33, p. 188



Measuring Precipitation

• Measuring from space

– Specific satellites designed to assess clouds, atmospheric moisture, and rain

• TRMM

• CloudSat



Class #4: Stability, cloud development, and precipitation

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