Representing Climate Data II Satellite Imagery and Radar

Weather Satellites original used black and white cameras TIROS, 1960

Radiometers electronic sensors Detect radiation from atmosphere, clouds, surface Can sense specific wavelengths of radiation “spectral signatures” of gases Scans surface Scans continuously adjacent squares arranged in scan lines Sweep: length of scan line

1. Geostationary Moves with earth rotation Fixed point Altitude 36,000 km (22,240 mi) Advantage: Real time data Disadvantage: distorts polar regions

GOES

Polar-orbitting Follows parallel meridian lines Altitude 850 km (540 mi) Passes poles on every revolution Earth rotates eastward and satellite scans successive passes Advantage: Better coverage of high latitudes

polar-orbitting

Types of weather images: 1. Visible 2. Infrared satellite 3. Water Vapor 4. Radar

1. visible Detects visible wavelengths. Reading shortwave reflected by earth, ocean, clouds (albedo) Daytime only

Albedos of various surfaces: Earth’s surface0.31 (31%) Cumulonimbus clouds0.9 (90%) Stratocumulus clouds0.6 (60%) Cirrus clouds (40 – 50%) Fresh snow0.8 – 0.9 (80 – 90%) Melting snow0.4 – 0.6 (40 – 60%) Sand0.3 – 0.35 (30 – 35%) Grain crops0.18 – 0.25 (18 – 25%) Deciduous forest 0.15 – 0.18 (15 – 18%) Coniferous forest0.09 – 0.15 (9 – 15%) Tropical rainforest0.07 – 0.15 (7 – 15%) Water bodies0.06 – 0.10 (6 – 10%) increases at low sun angles

Visible imagery High vs. low albedo : High albedo: lighter Cloud tops, snow, ice Low albedo: darker Land, ocean Cloud thickness Thicker cloud cover is more reflective: brighter Cloud height (IR better) Cumulonimbus : very bright white Low : bright High (cirrus) : not- bright white

Notice that visible imagery records radiation that passes through atmospheric window.

2. Infrared Detects IR at 10 – 12 μ (thermal) Clouds, land, ocean, snow/ice reflect visible but emit IR (visible imagery records reflected shortwave; IR records emitted IR)

Why 10 – 12 µ?

Infrared imagery Detects temperature Low temperatures: lighter shades of gray High temperatures: darker shades of gray Cloud Heights: Low clouds warmer than high clouds Low clouds: dark High clouds: light Cumulonimbus clouds: bright white Can record at night IR images are often color-enhanced to highlight temperature differences

visible

Infrared (IR)

Enhanced IR

1. What is a disadvantage of geostationary Satellite imagery? 2. What is “stationary” about a geostationary satellite? 3. Which type of satellite would be better for gathering information about Artic sea ice changes: polar-orbiting or geostationary? 4. What is a distinct advantage of infrared imagery compared to visible? 5. Visible satellite imagery records: a)Reflected visible (0.4 – 0.7 µ) b)Emitted infrared (10 – 12 µ) 6. Infrared satellite imagery records: a)Reflected visible (0.4 – 0.7 µ) b)Emitted infrared (10 – 12µ)

Types of weather images: 1. Visible 2. Infrared satellite 3. Water Vapor 4. Radar BRING YOUR LAPTOPS ON WEDNESDAY!!!!!

3. water vapor Visible and IR images: Record radiation transmitted through atmospheric windows Water vapor images: record IR emitted by water vapor in the atmosphere Water vapor absorbs and emits IR at μ

6.7 – 7.3 µ

a)Does NOT detect water vapor in LOWER troposphere Because it will be absorbed by water vapor at higher altitudes and therefore will not go out to space to be recorded by satellite

b) If upper troposphere is dry, any radiation detected will be coming from MIDDLE troposphere lower=warmer=relatively darker gray c) If upper troposphere is wet, radiation detected is from HIGH (cold) water vapor higher = colder = relatively brighter gray/white (movement of water vapor indicates upper and mid tropospheric winds)

Enhanced IR of same time period

Water vapor images can also be color enhanced

Water Vapor Images are useful for: Tracking moisture (at mid and upper levels) Locating Low pressure / storm centers Identifying the jet stream location Can see rising and sinking air regions

Water vapor Current water vapor

NOAA website GOES site

4. radar (Radio Detection and Ranging) look inside of clouds Now use microwaves instead of radio waves

Transmitter sends microwave pulses Targets scatter energy back to receiver Amplified and displayed as echo Emission to receipt time shows distance to target

Shorter microwave wavelengths (~1 cm) detect small targets (e.g., tiny droplets of water in clouds) Longer micro-wavelengths (3 -10 cm) detect larger targets (e.g.,precipitation) Brightness of echo Amount of precipitation

Doppler Radar Based on principle of Doppler shift: Waves moving towards observer have different frequencies than waves moving away from observer. e.g., sound from approaching vs. leaving ambulance Doppler radar can measure direction Knowing wind speeds and directions within clouds gives info about vorticity (spin)

158 Doppler stations in US

Tour the radome!

Makes repeated 360˚scans of atmosphere at increasing elevation angles. 2 modes: Clear Air mode No rain Dust, light snow VCP 31, 32 (volume coverage pattern) Precipitation mode rain

Clear Air Mode

Precipitation Mode

Reflectivity units dBZ : decibels of Z “Z” is energy reflected back to radar Values increase with strength of signal Clear air Precipitation dBZ equate to approximate rainfall rates

Ground clutter Ground, buildings, trees, cars Within 25 km of radar Not moving with respect to radar, so can be detected by radial velocity Insects Birds Turbulence Effects density

Base reflectivity Reflectivity in lowest elevation “slice” Used to survey area close to radar

Composite reflectivity Combines all elevation scans Shows highest reflectivity

Velocity units Radial velocities in knots Red: wind moving AWAY Green: TOWARD radar Need to know where radar is!

Vorticity signature, tornado

Hook echo

Arkansas

RIDGE (Radar Integrated Display with Geospatial Elements) Combine radar with topography, roads, county lines, rivers, warnings Overlay maps as layers Toggle layers on or off GIS compatible NWS Duluth