Satellite and Radar Imagery Meteo 3: Chapter 5 Satellite and Radar Imagery

Remote Sensing In-situ measurements expensive and lack spatial coverage Need instruments and platforms to observe large portions of the atmosphere quickly Passive remote sensors: Radiometers Active remote sensors: Radar

Satellite Basics Used to see clouds (and other phenomena) Passive remote sensors observes without being in direct contact collects energy emitted or scattered by object Two relevant types (others we won’t worry about) Geostationary- orbits Earth 22,500 mi above ground over equator at same rate Earth rotates GOES- Geostationary Operational Environmental Satellite Polar-orbiting- orbits pass over the poles Solves GOES problem of being unable to accurately “see” pole Can only see small swaths of Earth on each pass

Satellite Orbits Current Satellite Orbits

Visible Satellite Imagery Radiometer- measures visible light reflected off clouds/objects (albedo)…very sensitive to differences in reflected visible light Useful only during day

Thin vs. Thick Clouds Thin cirrus duller than thick cumulonimbus because cirrus has a much lower albedo Distinguishes between thick (bright) & thin (dull) clouds

Clouds vs Snow Animations help (show motion & evolution of weather systems) Rivers visible?

Infrared Satellite Imagery Senses infrared radiation emitted by clouds and other objects between 10-12μm Useful at all times

High vs Low Clouds Low clouds appear gray, high clouds bright Temperature decreases with height => E=σT4 => more infrared radiation emitted from low clouds than high clouds Measures emitted radiation, or temperature, differences Cold objects are bright, warm are grey

Water Vapor Imagery Measures infrared radiation emitted at 6.7 μm…water vapor emits strongly here Can only sense water vapor in mid-upper troposphere…water vapor emission below will be absorbed Dark spots dry in upper troposphere, bright are moist

Color Enhanced

Radar Imagery Radio Detection and Ranging Active remote sensor emits pulses of electromagnetic energy (microwaves), then measures how much scattered back off targets (precipitation, insects, mountains, etc.) Distance of echo from radar determined by considering elapsed time between radar emitting radiation and it returning, as radiation travels at speed of light Detects precipitation in reflectivity mode intensity of returned energy depends on number, size, and composition of targets the more & the larger the targets, the higher the reflectivity…if targets are rain, the rainfall rate is greater

WSR-57 (Weather Surveillance Radar, 1957)

WSR-88D or NEXRAD Next Generation Radar D stands for Doppler 175 mi range

Single image vs composite

Displays including precipitation type

Derive rainfall totals from reflectivity

Wet hail has high reflectivity

Snowflakes have smaller reflectivities than similar-sized raindrops

Radar and severe weather: hook echoes

Bright-banding

Doppler Radar Johann Christian Doppler noticed change in pitch (frequency) of sound as source moved away or toward stationary observer frequency of radiation off target is changed if target is moving faster the speed toward or away, the greater the frequency change for radar, these frequency changes translate into wind speeds and directions toward or away from radar site

Doppler Schematic www.colorado.edu/physics/2000/applets/doppler2.html

Tornado Vortex Signature Present as early as 20-30 minutes before tornado