PHYS 622 - Cloudsspring ‘04
S. Platnick (NASA GSFC/UMBC PHYS Adjunct Faculty)
Texts:
Rogers, R.R., and M. K. Yau, A Short Course in Cloud Physics, Pergamon Press, 1989.Twomey, S., Atmospheric Aerosols, Elsevier Publishing, 1977.
PHYS 622 - Clouds, spring ‘04, lect. 1, Platnick
Clouds - The “Wet” Aerosol
• “Wet” aerosol: much of the mathematics (e.g., size distribution, etc.) used for characterizing aerosol microphysics applies to clouds.
• A cloud definition: visible suspension of water and/or ice particles in the atmosphere.
– Key word is visible, but not quantitative. Example, “sub-visual cirrus” (observed through non-visible, non-passive sensors/imagers or lidars).
• Cloud physics: branch of physical meteorology, study of cloud formation (macrophysical & microphysical), lifecycles, precipitation, radiation, etc.
– Macrophysical: larger scale spatial information, total/column water amounts, etc.
– Microphysical: thermodynamic phase, size distribution, ice particle shape (habit), water content, etc.
PHYS 622 - Clouds, spring ‘04, lect. 1, Platnick
Why Clouds?
• Weather – Dynamics: Latent heat and/or radiative effects impacting atmospheric
stability/instability, atmospheric heating/cooling– Radiation (e.g., surface heating)
• Chemical processes
• Climate– General circulation– Hydrological cycle– Radiation budget
Clouds are a critical component of climate models (for reasons cited above) and therefore also to climate change studies
• Not well-represented in climate models• Climate change: cloud-climate feedback, cloud-aerosol
interactions (to be discussed), etc.
PHYS 622 - Clouds, spring ‘04, lect. 1, Platnick
1. Evaporation, transpiration (plants)
2. Atmospheric transport (vapor)
3. Condensation (liquid water, ice)
4. Precipitation
5. Surface transport (continental rivers, aquifers and ocean currents)
Earth’s Hydrological Cycle - Schematic
PHYS 622 - Clouds, spring ‘04, lect. 1, Platnick
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NASA TRMM (Tropical Rainfall Measurement Mission) Tropical cyclone Elita
Precipitation RadarPrecipitation Radar
PHYS 622 - Clouds, spring ‘04, lect. 1, Platnick
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TRMM 16 Feb 2004 weekly global rainfall accumulation
PHYS 622 - Clouds, spring ‘04, lect. 1, Platnick
CERES (Clouds & Earth Radiant Energy System experiment) Radiation Budget Measurements - NASA Terra
(March 2000 - May 2001)
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PHYS 622 - Clouds, spring ‘04, lect. 1, Platnick
Record setting heat wave in CA, NV
Courtesy CERES Science Team, NASA LaRC
PHYS 622 - Clouds, spring ‘04, lect. 1, Platnick
Cloud-aerosol interactions ex.: ship tracks (27 Jan. 2003, N. Atlantic)
MODIS (MODerate resolution Imaging Spectroradiometer)
Cold front - steep frontal slopes
Warm front - shallow frontal slopes
Convective development (mesoscale, local)
Synoptic development
PHYS 622 - Clouds, spring ‘04, lect. 1, Platnick
MODIS (MODerate resolution Imaging Spectroradiometer) true-color composite, NASA Terra, ~1030 LT
PHYS 622 - Clouds, spring ‘04, lect. 1, Platnick
Cloud Classifications (a nomenclature)
PHYS 622 - Clouds, spring ‘04, lect. 1, Platnick
Genera
(main groups - high,
middle, low altitude
clouds, and clouds of
vertical extent)
Species
(shape, structure)
Variety
(special characteristics)
Cirrus uncinus, fibratus, …
Cirrocumulus Stratiformis, lenticularis
Cirrostratus fibratus
Altocumulus
Altostratus
high
leve
l m
id-le
vel
Cloud Classifications, cont.
PHYS 622 - Clouds, spring ‘04, lect. 1, Platnick
Genera Common Species Variety
Nimbostratus
Stratocumulus
Stratus
Cumulus humilus, congestus
Cumulonimbus
low
leve
l (ba
se)
vert
ical
ext
ent
(pot
entia
lly)
cirrus
alto cumulus
cirrostratus: thin cirrus, note 22° halo => hexagonal crystals, parhelia (sundogs) => oriented crystals
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PHYS 622 - Clouds, spring ‘04, lect. 1, Platnick
cumulus (fair weather) mammatus (implies sinking air)
PHYS 622 - Clouds, spring ‘04, lect. 1, Platnick
cumulus congestus (from NASA WB-57, July 13 2002)
Overview of Cloud Climatologies (statistics)- What do we know? How do we know it?
PHYS 622 - Clouds, spring ‘04, lect. 1, Platnick
• Quantities of interest – Cloud frequency (fraction), cloud-top height, cloud phase (ice vs.
liquid water), optical properties (optical thickness), microphysics (particle size), column water amount (aka, “water path”), solar and IR radiative impact
• Historic data sets– Ground observations
• e.g.: Warren, S. G., et al. 1988: Global distribution of total cloud over and cloud type amounts over the ocean. NCAR/TN-317+STR [Available from the National Center for Atmospheric Research, Boulder, CO, 80307]
– Satellite observations• ISCCP (International Satellite Cloud Climatology Project), solar reflective
& IR techniques– amount, temperature, optical thickness, water path
• HIRS (High Resolution Infrared Radiation Sounder - on NOAA polar orbiters), GOES VAS (VISSR [Vis-Ir Spin-Scan Sounder] Atmospheric Sounder)
– amount, effective emissivity, and pressure heights with “CO2 slicing” technique
Overview of Cloud Climatologies (statistics), cont.
PHYS 622 - Clouds, spring ‘04, lect. 1, Platnick
• SSM/I () - microwave passive remote sensing– liquid water path
• ERBE (Earth Radiation Budget Experiment) - mid-80’s, instruments flown on the Earth Radiation Budget Satellite (ERBS), and NOAA -9, -10
• Recent/new data sets– Satellite observations
• Solar IR imager: MODIS, flown on Terra, Aqua• Microwave: AMSR (Japanese ADEOS-II), AMSR-E (Aqua)• Radiation Budget: CERES (Clouds and the Earth’s Radiant Energy
System) - next generation of radiation budget measurements, flown on TRMM, Terra, Aqua
AMSR-E cloud liquid water pathSept 2003
(from F. Wentz, http://www.ssmi.com/)
PHYS 622 - Clouds, spring ‘04, lect. 1, Platnick
Cloud Microphysical Quantities
• Size distribution [n(r)] - droplet size pdf
• Thermodynamic phase (liquid water, ice)
• Number concentration [N] - e.g., cm-3; range: 10’s cm-3 - 100’s cm-3 for liquid water droplets, 10 liter-1 for ice particles
• Water Content [LWC, IWC] - mass density, e.g., g-m-3
• Water Path [W] - vertical integration of water content, e.g., g-m-2
• Particle Size Moments
– Mean
– Effective radius [re] - radiatively relevant moment
PHYS 622 - Clouds, spring ‘04, lect. 1, Platnick
Cloud Microphysical Quantitiesschematic of vertically inhomogeneous cloud (horizontally homogeneous)
N(z) n0
(r,z)dr
LWC(z) l4
3 r3(z) N(z)
liquid water path : W LWCzbase
ztop
(z)dz
re (z) r3(z)
r2(z)one def. for nonspherical ice particles
3
4
V (z)
Ac (z)
size distribution, n(r,z)
z, p
surface
zztop, pptop
zzbase, ppbase
MODIS monthly cloud fraction - Sept. 2003
Ice cloud fraction
Liquid water cloud fraction
Sc regimes
ITCZ (climatological feature)
PHYS 622 - Clouds, spring ‘04, lect. 1, Platnick
MODIS monthly cloud particle size retrievals - Sept. 2003
Ice cloud particle effective radius
Liquid water cloud particle effective radius (QA)
Sc regimes
PHYS 622 - Clouds, spring ‘04, lect. 1, Platnick
PHYS 622 - Clouds, spring ‘04, lect. 1, Platnick
Clouds are difficult, in part, by the natureof the relevant spatial scales and interdisciplinary fields
Scale Relevant Physics
synoptic ~1000s km
(large scale dynamics/thermodynamics, vapor fields)
mesoscale ~100s km
local (cloud scale) <1-10 km
(dynamics/thermodynamics, turbulence, mixing)
particle µm - mm
(nucleation, surface effects, coagulation,
turbulence, stat-mech)
molecular
PHYS 622 - Clouds, spring ‘04, lect. 1, Platnick
Co
ld C
lou
d P
roce
sses
War
m C
lou
d
Pro
cess
es
PHYS 622 Clouds
Emphasis on cloud microphysics: cloud particle nucleation, growth
• Water Clouds – Formation concepts– Water path for adiabatic cloud parcel– Nucleation theory for water droplets
• Ice Clouds
• Precipitation mechanisms
PHYS 622 - Clouds, spring ‘04, lect. 1, Platnick
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