Meteo 3: Chapter 5Meteo 3: Chapter 5

Satellite and Radar ImagerySatellite and Radar Imagery

Remote SensingRemote Sensing

In-situ measurements expensive and lack In-situ measurements expensive and lack spatial coveragespatial coverage

Need instruments and platforms to observe Need instruments and platforms to observe large portions of the atmosphere quicklylarge portions of the atmosphere quickly

Passive remote sensors: RadiometersPassive remote sensors: Radiometers Active remote sensors: RadarActive remote sensors: Radar

Satellite BasicsSatellite Basics

Used to see clouds (and other phenomena)Used to see clouds (and other phenomena) Passive remote sensorsPassive remote sensors

– observes without being in direct contact observes without being in direct contact – collects energy emitted or scattered by objectcollects energy emitted or scattered by object

Two relevant types (others we won’t worry about)Two relevant types (others we won’t worry about)– Geostationary- orbits Earth 22,500 mi above ground over Geostationary- orbits Earth 22,500 mi above ground over

equator at same rate Earth rotatesequator at same rate Earth rotates GOES- Geostationary Operational Environmental Satellite GOES- Geostationary Operational Environmental Satellite

– Polar-orbiting- orbits pass over the polesPolar-orbiting- orbits pass over the poles Solves GOES problem of being unable to accurately “see” poleSolves GOES problem of being unable to accurately “see” pole Can only see small swaths of Earth on each passCan only see small swaths of Earth on each pass

Satellite OrbitsSatellite Orbits

Current Satellite Orbits

Visible Satellite ImageryVisible Satellite Imagery

RadiometerRadiometer- measures - measures visible light reflected off visible light reflected off clouds/objects (albedo)…clouds/objects (albedo)…very sensitive to very sensitive to differences in reflected differences in reflected visible lightvisible light

Useful only during dayUseful only during day

Thin vs. Thick CloudsThin vs. Thick Clouds

Thin cirrus duller than Thin cirrus duller than thick cumulonimbus thick cumulonimbus because cirrus has a because cirrus has a much lower albedomuch lower albedo

Distinguishes between Distinguishes between thick (bright) & thin thick (bright) & thin (dull) clouds(dull) clouds

Clouds vs SnowClouds vs Snow

Animations help Animations help (show motion & (show motion & evolution of weather evolution of weather systems)systems)

Rivers visible?Rivers visible?

Infrared Satellite ImageryInfrared Satellite Imagery

Senses infrared Senses infrared radiation emitted by radiation emitted by clouds and other clouds and other objects between 10-objects between 10-12μm12μm

Useful at all timesUseful at all times

High vs Low CloudsHigh vs Low Clouds

Low clouds appear gray, Low clouds appear gray, high clouds brighthigh clouds bright

Temperature decreases Temperature decreases with height => E=σTwith height => E=σT44 => => more infrared radiation more infrared radiation emitted from low clouds emitted from low clouds than high cloudsthan high clouds

Measures emitted radiation, Measures emitted radiation, or temperature, differencesor temperature, differences

Cold objects are bright, Cold objects are bright, warm are greywarm are grey

Water Vapor ImageryWater Vapor Imagery

Measures infrared radiation Measures infrared radiation emitted at 6.7 μm…water emitted at 6.7 μm…water vapor emits strongly herevapor emits strongly here

Can only sense water Can only sense water vapor in mid-upper vapor in mid-upper troposphere…water vapor troposphere…water vapor emission below will be emission below will be absorbedabsorbed

Dark spots dry in upper Dark spots dry in upper troposphere, bright are troposphere, bright are moist moist

Color EnhancedColor Enhanced

Radar ImageryRadar Imagery

RaRadio dio DDetection etection aand nd RRanginganging Active remote sensorActive remote sensor

– emits pulses of electromagnetic energy (microwaves), then emits pulses of electromagnetic energy (microwaves), then measures how much scattered back off targets (precipitation, insects, measures how much scattered back off targets (precipitation, insects, mountains, etc.)mountains, etc.)

– Distance of echo from radar determined by considering elapsed time Distance of echo from radar determined by considering elapsed time between radar emitting radiation and it returning, as radiation travels between radar emitting radiation and it returning, as radiation travels at speed of lightat speed of light

Detects precipitation in reflectivity modeDetects precipitation in reflectivity mode– intensity of returned energy depends on number, size, and intensity of returned energy depends on number, size, and

composition of targetscomposition of targets– the more & the larger the targets, the higher the reflectivity…if targets the more & the larger the targets, the higher the reflectivity…if targets

are rain, the rainfall rate is greaterare rain, the rainfall rate is greater

WSR-57 (Weather Surveillance Radar, 1957)WSR-57 (Weather Surveillance Radar, 1957)

WSR-88D or NEXRADWSR-88D or NEXRAD

NexNext Generation t Generation RadRadarar D stands for DopplerD stands for Doppler 175 mi range175 mi range

Single image vs compositeSingle image vs composite

Displays including precipitation typeDisplays including precipitation type

Derive rainfall totals from reflectivityDerive rainfall totals from reflectivity

Wet hail has high reflectivityWet hail has high reflectivity

Snowflakes have smaller reflectivities than similar-sized Snowflakes have smaller reflectivities than similar-sized raindropsraindrops

Radar and severe weather: hook echoesRadar and severe weather: hook echoes

Bright-bandingBright-banding

Doppler RadarDoppler Radar

Johann Christian Doppler noticed change in pitch Johann Christian Doppler noticed change in pitch (frequency) of sound as source moved away or (frequency) of sound as source moved away or toward stationary observertoward stationary observer– frequency of radiation off target is changed if target is frequency of radiation off target is changed if target is

movingmoving– faster the speed toward or away, the greater the faster the speed toward or away, the greater the

frequency changefrequency change– for radar, these frequency changes translate into wind for radar, these frequency changes translate into wind

speeds and directions toward or away from radar sitespeeds and directions toward or away from radar site

Doppler SchematicDoppler Schematic

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

Tornado Vortex SignatureTornado Vortex Signature

Present as early as 20-30 minutes before tornadoPresent as early as 20-30 minutes before tornado