OC3522Summer 2001 OC Remote Sensing of the Atmosphere and Ocean - Summer 2001 Scattering 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. 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 (  r 2 ) then increases by a factor of 10 4 !

Radius (  m) r2r2 QsQs n(r) d  s /dr =  r 2 Q s n(r) Radius (  m) Cloud Droplets No Color Separation

Why does a cloud appear white in visible imagery? In VIS - cloud droplets are geometric scatterers - Q s ~ 2;  s ~ 0.1m -1 The distribution of cloud droplet sizes with large values of  = 2  r/  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: 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 TYPES When 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 CuCiSt, Cb Climate ZonesSeasonsothers… Synoptic WeatherPersistence Diurnal CyclesSurface Observations Shadows

altostratus Examples from Euromet satellite meteorology pageEuromet stratuscumulonimbus cirrus VIS 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

Hurricane Mitch