classes of typical cloud types.fnimmo/eart164...classes of typical cloud types. cloud radiative...
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Classes of typical cloud types.
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Cloud Radiative Properties for Planetary Science:
Longwave (or IR)
• Optically opaque (completely absorbing) for clouds with thicknesses of 10s of meters.
• This occurs because condensed water (liquid or solid) is 1000 times more absorbing (and emitting) than water vapor � intermolecular interactions
• Emission depends on temperature (i.e. altitude) only provided the cloud isn’t super-thin
– details of the cloud properties do NOT matter
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Cloud Radiative Properties for Planetary Science:
Shortwave (or solar or visible)
Cloud optical depth τ depends on:
– typical liquid water content L [kg/m3] � cloud/env interaction
– cloud thickness h [m] � dynamical environment
– cloud particle concentration N [m–3] � aerosol microphysics
– density of cloud particle ρ [kg/m3]
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Cloud microphysics
(control of cloud particle number concentration N)
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Liquid cloud drop formation
• Rising air cools adiabatically with decreasing pressure, increasing RH (more on rising air later under dynamics)
• When air cools to a T where RH > 100%, water vapor condenses onto a subset of existing particles (or cloud condensation nuclei) � controls number concentration of drops N
• Condensation removes water vapor to bring the air parcel to equilibrium (RH = 100%).
<video>
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Solid cloud particle formation (water as example)
• Ice often starts to form below –5°C, but few clouds are entirely glaciated until –40°C
• In between, supercooled liquid co-exists with ice crystals.
• Ice nuclei appear to be relatively rare in the atmosphere, existing in concentrations on the order of 1 to 10 per liter � the structure of the particles matters
• IN or CCN controls number concentration of ice crystals N
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Heterogeneous
deposition
Heterogeneous
freezing
(below –5°C)
Homogeneous
freezing
(below –40°C)
Ice Nucleation Mechanisms
Ice Nucleus (IN)
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Some ice habits
Classification from Magono and Lee, 1966.
These matter to both precipitation formation and radiation.
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Magono and Lee, 1966.
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Lab expts show crystal habit is a function of temperature and supersaturation.
From Pruppacher and Klett, 1997
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Entrainment mixes dry outside air into a cloud
• Entrainment is the process by which a turbulent flow expands into a
non-turbulent flow by “annexation” <see video>
• This causes cloud volume to increase, but dilution and evaporation
of cloud liquid water (decreases L)
• 90% of all clouds dissipate by entrainment-induced evaporation (the
other 10% primarily via precipitation)
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Clouds and dynamics
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MBL Lecture 1, Slide 14
June 9, 1994
GOES-West
Deep convection
Shallow cumulus
Stratocumulus
Stratus
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Clouds require rising air, which can be generated by different
processes
Lift is often generated by surface convergence along air flow boundaries,
such as:
– fronts
– drylines
– sea-breeze convergence lines
– horizontal convective rolls in the boundary layer
– outflow boundaries from previous convection
– thermals (differential surface heating)
Lift can also be generated by upper level disturbances.
More on this topic when we discuss BL and convection.
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Large stratocumulus deck
off of California and
Mexico.
Image courtesy of MODIS.
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Cumulonimbus over Africa – the cirrus shield hides the strong convective
clouds
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June 9, 1994
GOES-West
Deep convection
Shallow cumulus
Stratocumulus
Stratus
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Some examples (what controls cloud thickness h)
• Stratocumulus: thin cloud; strong “lid”; driven by
turbulence.
• Frontal clouds: variety of thicknesses; driven by large-
scale buoyancy contrast
• Tropical deep convection: thick cloud; no “lid”; driven by
large-scale convergence
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Marine stratocumulus structure
(turbulent &
well-mixed)
potential
temperaturetotal moisture
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Marine stratocumulus structure
lifting condensation level
(turbulent &
well-mixed)
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Marine stratocumulus structure
lifting condensation level
(turbulent &
well-mixed)
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Marine stratocumulus processes
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Frontal clouds: cold front
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Frontal clouds: warm front
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Deep convective clouds
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Summary
Cloud effects on radiation (and hence climate) are complicated.
They depend on:
1. droplet-scale processes (condensation; ice formation; L )
2. cloud-scale processes (entrainment; precipitation)
3. large-scale environment (dynamics; “lid”)
These various processes interact in complex ways to give us the
assortment of clouds that we observe.