Image Interpretation for Weather Analysis

Image Interpretation for Weather Analysis Part 2 26 October 2010 Dr. Steve Decker

Severe Thunderstorm Detection • Severe thunderstorms often have notable overshooting tops • Vis: Shadow effects • IR: “Enhanced-V” signature • Example: VisIR

Boundary Detection • Boundary: Subtle separation between two air masses • Region of enhanced lifting • Clouds • Thunderstorms • Best seen in Vis • Lake Breeze example

Blowing Snow • Can produce whiteout conditions, even with no precipitation • Vis example

Common Channels • Visible • 0.65 μm (red) • Infrared (IR) • 10.7 μm • Water Vapor • 6.7 μm • Shortwave IR • 3.9 μm

Atmospheric Absorptivity

Shortwave IR • An infrared window channel • Just like “longwave” IR • Also sees solar radiation (blackbody curve overlap) • Works best for warmer temps • > -30°C • Cold clouds (e.g., cirrus) look mottled • Good for fire detection • Fog detection • Supercooled vs. ice clouds • Snow vs. cloud

Fire Detection with Shortwave IR • Fires show up as “hot spots” • SoCal fire example

Fog Detection • Emissivity of liquid water cloud at 3.9 μm is less than at longer wavelengths. • Fog shows up as lower temperatures • Appears brighter • Opposite true for ice crystals (cirrus)

Fog Detection • Emissivity of liquid water cloud at 3.9 μm is less than at longer wavelengths. • Fog shows up at lower temperatures • Appears brighter • Differences can be maximized by taking the difference between the longwave and shortwave IR images

Supercooled Cloud Detection • Supercooled cloud droplets frequently occur for -20°C < T < 0°C • Detection method • Identify cloud-top temperatures conducive for supercooled droplets using longwave IR • Just like fog/stratus droplets, supercooled droplets emit less radiation in shortwave IR

Supercooled Example http://weather.msfc.nasa.gov/sport/goes_imager/goes_imager.html

Snow vs. Cloud • During the day, low clouds will reflect more solar radiation than snow at 3.9 μm, so low clouds appear darker (more signal) than snow.

Urban Heat Islands • Shortwave IR is more sensitive to emissions from warmer temperatures • Urban heat islands show up better

Water Vapor Channel • Not an IR window • Does not see the ground (Exception) • Absorbed/emitted by water vapor • Colder temperatures imply: • More moisture in the mid and upper troposphere • Possible regions of ascent • Temperature differences important; not their magnitudes • Example

Identifying Jet Streams • Jet Streams • Ribbons of quickly moving air near the tropopause • Separate air masses • Support active weather • Vis: Band of cirrus clouds on equatorward side • Vapor: Strong moisture gradient • Dry air poleward • Moist air equatorward

Locating Ridges and Troughs • Upper tropospheric flow often contains a ridge/trough pattern • Clouds often occur downstream of troughs, but upstream of ridges • If ridge has small amplitude, clouds may “spill over” ridge • Cloud band ahead of trough often indicates “warm conveyor belt” immediately ahead of a surface cold front • Southern extent of solid band marks trough axis

Water Vapor Examples • Eddies • Cyclone development • Occlusion stage 12 • Mountain waves • Java example • Current weather

Many More Examples • CIMSS Satellite Blog

Image Interpretation for Weather Analysis