Image Interpretation for
Weather Analysis
Part 2
3 November 2009
Dr. Steve Decker
Improvement Example
 Registration GOES-12 vs. GOES-13
 Battery power
Severe Thunderstorm Detection
 Severe thunderstorms
often have notable
overshooting tops
 Vis: Shadow effects
 IR: “Enhanced-V”
signature
 Example: Vis IR
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 1 2
 Mountain waves
– Java example
 Current weather
Many More Examples
 CIMSS Satellite Blog