OC3522OC3522 - Remote Sensing of the Atmosphere and Ocean - Summer 2001Summer 2001

Scattering by Clouds & ApplicationsScattering by Clouds & Applications

Clouds droplets form when air parcels reach critical supersaturation (RH>100%) and water vapor condenses on aerosol particles, growing them to large enough sizes where vapor continues to condense.

http://atmos.es.mq.edu.au/AMOS/weatherwatch/photos/1995/0129mb08.jpg

This allows droplets to grow to sizes that are on the order of 100 times larger than aerosol particles.

The change in particle cross-section (r2) then increases by a factor of 104 !

10-2

10-1

100

101

102

103

103

4

3

2

1

101

10-1

10-3

0.02

0.06

0.04

Radius (m)0.1 1.0 10.0

r2

Qs

n(r)

ds/dr = r2 Qs n(r)

10-2

10-1

100

101

102

103

103

4

3

2

1

101

10-1

10-3

2

6

4

Radius (m)0.1 1.0 10.0

Cloud Droplets

No Color Separation

Why does a cloud appear white in visible imagery?

In VIS - cloud droplets are geometric scatterers - Qs ~ 2; s ~ 0.1m-1

The distribution of cloud droplet sizes with large values of = 2r/s, all are scattered equally

This increase in s results in cloud optical depths between about 10 and 80.

Therefore the forward RT problem relies on a very complicated path radiance term:

€

Lt(λ,Θ,Φ) = L0(λ,Θ,Φ)e-δ(λ)/μ +γs(r,r',λ,X)L(r',λ,X)dΩ

4π∫

σ e(λ,z)e-δ(λ,z)/μ

0

δ(λ)

∫ dδμ

Since the radiance from all directions is required.

Path radiance

The composition of clouds (liquid water or ice crystals) controls the potential for absorption as solar photons transfer through a cloudy atmosphere. The single scatter albedo (0) describes this potential and is determined by the imaginary index of refraction:

The physical attributes of a cloud that control reflectance then includes:

CLOUD TYPESWhen clouds are present: indicator of presence of moisture and cooling mechanisms

cause of cooling: - upward motion, radiative cooling, advection

Categories of clouds: 1) Low level (< 3 km)

2) Middle Level (3-7km) 3) High Level (7-12km)

4) Precipitating cloud: Nimbus

Current Western US - 1kmCurrent GOES-10 Full Disk

Information available in Visible Imagery:

Brightness

DARK BRIGHT

Ocean

Land

Snow/Ice

Cumulonimbus

Cumulus

Stratus/StratocumulusCirrus

Locale

Texture

Cu Ci St, Cb

Climate Zones Seasons others… Synoptic Weather PersistenceDiurnal Cycles Surface Observations

Shadows

altostratus

Examples from Euromet satellite meteorology page

stratus cumulonimbus cirrus

VIS

IR

IR

VIS

stratocumulus

Threshold value is chosen at which anything brighter or colder - pixel is a cloud used in SST analysis

Problems with this method

Histogram shows clusters of values that represent surface or clouds

Pattern Recognition use of area means and standard

Cloud detection & Classification

Spatial coherence

http://atmos.es.mq.edu.au/AMOS/weatherwatch/photos/1991/0120mb04.jpghttp://atmos.es.mq.edu.au/AMOS/weatherwatch/photos/1995/0205mb12.jpg

http://atmos.es.mq.edu.au/AMOS/weatherwatch/photos/1996/0731jd24.jpg http://www.cira.colostate.edu/

http://atmos.es.mq.edu.au/AMOS/weatherwatch/photos/1991/0120mb01.jpghttp://atmos.es.mq.edu.au/AMOS/weatherwatch/photos/1995/0129mb08.jpg

http://atmos.es.mq.edu.au/AMOS/weatherwatch/photos/1996/0731jd09.jpghttp://www.orbimage.com/main/imgweek/highsnow01_99_2ns.jpg

http://atmos.es.mq.edu.au/AMOS/weatherwatch/photos/1995/0415jd01.jpg

http://earth.jsc.nasa.gov/lores.cgi?PHOTO=STS043-076-061

http://atmos.es.mq.edu.au/AMOS/weatherwatch/photos/1996/0731jd23.jpghttp://atmos.es.mq.edu.au/AMOS/weatherwatch/photos/1996/0731jd38.jpg

Hurricane MitchHurricane Mitch

http://rsd.gsfc.nasa.gov/rsd/images/Mitch/mitch3_lg.jpg