Transcript Class #4: Stability, cloud development, and precipitation

Class #4: Stability, cloud
development, and precipitation
Chapters 6 and 7
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Chapter 6
Stability & Cloud development
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Fig. 6-CO, p. 140
Fig. 6-1, p. 142
Importance of Clouds
• Release heat to atmosphere
• Help regulate energy balance
• Indicate physical processes
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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
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Determining Stability
• Warm air rises or is unstable
• Cool air sinks or is stable
• Compare air parcel lapse rate to
environmental lapse rate
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Table 6-1, p. 143
Determining Stability
• 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.
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Fig. 6-6, p. 145
Determining Stability
• Special Topic: Subsidence Inversions
– Strong subsidence exacerbates air pollution due to
the lack of vertical motion.
– Pollution is not diluted.
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Determining Stability
• 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
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Determining Stability
• 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
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Fig. 6-13, p. 149
Fig. 6-14, p. 149
Fig. 1, p. 150
Determining Stability
• Causes of Instability
– Cool air aloft (advection, radiation cooling in
clouds)
– Warming of surface (insolation, advection, warm
surface)
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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.
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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.
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Stepped Art
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Fig. 6-16, p. 152
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Fig. 2, p. 155
Cloud Development
• Topography
– Orographic uplift
– Orographic clouds
– Windward, leeward, rain shadow
– Lenticular clouds
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Cloud Development
• Topic: Adiabatic charts
– Adiabatic charts show how various atmospheric
variables change with height: pressure,
temperature, humidity.
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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.
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Chapter 7
Precipitation
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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
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Precipitation Processes
• 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.
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Fig. 7-5, p. 169
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Fig. 1, p. 171
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Fig. 7-10, p. 173
Fig. 7-10, p. 173
Fig. 7-10, p. 173
Fig. 7-10, p. 173
Precipitation Processes
• Topic: Freezing of Cloud Droplets
– Spontaneous or homogeneous freezing
– Ice embryo
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Precipitation Processes
• 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
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Fig. 7-12, p. 174
Precipitation in Clouds
• Starts quickly
• Most Precipitation formed through accretion
• Many times rain starts as ice
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Fig. 7-12, p. 174
Fig. 7-12, p. 174
Stepped Art
Fig. 7-12, p. 174
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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
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Table 7-2, p. 176
Fig. 2, p. 177
Fig. 7-16, p. 178
Fig. 7-17, p. 178
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Table 7-3, p. 178
Fig. 3, p. 179
Fig. 7-18, p. 180
Table 7-4, p. 180
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Precipitation Types
• Topic: Tear Drops
– Raindrops not tear shaped
– Shape is size dependent
• Less than 2 mm = sphere
• Greater than 2 mm = flattened sphere
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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.
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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.
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Fig. 7-21, p. 182
Fig. 7-22, p. 182
Fig. 7-23, p. 182
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
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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.
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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.
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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
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Fig. 7-33, p. 188
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Measuring Precipitation
• Measuring from space
– Specific satellites designed to assess clouds,
atmospheric moisture, and rain
• TRMM
• CloudSat
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