PHYS 622 - Clouds spring ‘04 S. Platnick (NASA GSFC/UMBC PHYS Adjunct Faculty)

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.


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.


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


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NASA TRMM (Tropical Rainfall Measurement Mission) Tropical cyclone Elita

Precipitation RadarPrecipitation Radar




TRMM 16 Feb 2004 weekly global rainfall accumulation



Earth’s Radiation Budget - Schematic

CERES (Clouds & Earth Radiant Energy System experiment) Radiation Budget Measurements - NASA Terra

(March 2000 - May 2001)


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Record setting heat wave in CA, NV

Courtesy CERES Science Team, NASA LaRC


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


MODIS (MODerate resolution Imaging Spectroradiometer) true-color composite, NASA Terra, ~1030 LT


Cloud Classifications (a nomenclature)


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.


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




cumulus (fair weather) mammatus (implies sinking air)


cumulus congestus (from NASA WB-57, July 13 2002)

PHYS 622 - Clouds, spring ‘04, lect. 1, PlatnickKelvin-Helmholtz waves

Cumulonimbus (from NASA ER-2, July 19 2002)


Overview of Cloud Climatologies (statistics)- What do we know? How do we know it?


• 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.


• 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

HIRS cloud frequency vs. month[Wylie et al., Journal of Climate, Vol. 7, No. 12, December, 1994]

HIRS cloud frequency vs. altitude (September)

HIRS cloud frequency vs. month for “thicker” clouds (opt thickness >6)

ISCCP climatology exampleslow-level cloud amount (%)

ISCCP climatology examplesmid-level cloud amount (%)

ISCCP climatology exampleshigh-level cloud amount (%)

AMSR-E cloud liquid water pathSept 2003

(from F. Wentz, http://www.ssmi.com/)


Cloud Microphysics



Clouds Particle Scales

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


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)


MODIS monthly cloud particle size retrievals - Sept. 2003

Ice cloud particle effective radius

Liquid water cloud particle effective radius (QA)

Sc regimes



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


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


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PHYS 622 - Clouds spring ‘04 S. Platnick (NASA GSFC/UMBC PHYS Adjunct Faculty)

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