Representing Climate Data II Satellite Imagery and Radar

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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 At altitude of 36,000 km, orbit of satellite matches earth’s rotation Satellite is moving at same speed earth is rotating so it appears to stay in one spot and always sees the same place on earth Centered on a particular longitude

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

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 surface 0.31 (31%) Cumulonimbus clouds 0.9 (90%) Stratocumulus clouds 0.6 (60%) Cirrus clouds 0.4 -0.5 (40 – 50%) Fresh snow 0.8 – 0.9 (80 – 90%) Melting snow 0.4 – 0.6 (40 – 60%) Sand 0.3 – 0.35 (30 – 35%) Grain crops 0.18 – 0.25 (18 – 25%) Deciduous forest 0.15 – 0.18 (15 – 18%) Coniferous forest 0.09 – 0.15 (9 – 15%) Tropical rainforest 0.07 – 0.15 (7 – 15%) Water bodies 0.06 – 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

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 6.7- 7.3μ

6.7 – 7.3 µ

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

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

Current water vapor

NOAA website GOES site

REVIEW Geostationary and polar-orbiting satellites Visible satellite imagery: Records albedo Only useful during daylight hours Shows cloud cover, land/water, frontal systems, snow surfaces Infrared satellite imagery: Records emitted IR at 10-12 µ Records day and night Detects temperature Reads cloud height Water vapor satellite imagery: Records infrared at 6.7 – 7.3 µ emitted by water vapor Cannot use for lower atmosphere near surface Tracks moisture Shows storm (low) centers Shows jet streams and sinking air

4. radar (Radio Detection and Ranging) look inside of clouds Now use microwaves instead of radio waves RADAR stands for RAdio Detection And Ranging and was slowly developed over time starting way back in the late 1800s. By the start of World War II, many countries used it to detect enemy ships and aircrafts. When radar operators discovered that precipitation caused 'false' echoes on their screen (masking potential enemy targets) they realized the new found potential of radar. Soon after the war surplus radars were used as precipitation detectors.

Transmitter sends microwave pulses Targets scatter energy back to receiver Amplified and displayed as echo Time between emitting energy and receiving it back from target tells 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) By utilizing the Doppler Effect, Doppler radars provide information regarding the movement and positions of targets. After the radar emits a pulse of radio waves, it tracks the phase shift between the transmitted radio wave and the received echo. This phase shift shows whether the target is moving directly toward or away from the radar, called its radial velocity. A positive phase shift implies motion toward the radar and a negative shift suggests motion away from the radar. The phase shift effect is similar to the "Doppler shift" observed with sound waves. If an object emits sound waves as it approaches a location, the waves are compressed leading to a higher frequency. As the object moves away from a location, the sound waves are stretched leading to a lower frequency. This is often experienced when an emergency vehicle drives past with its siren blaring.

158 Doppler stations in US

The radar dish can rotate 360 degrees in the horizontal and approximately 20 degrees in the vertical. As the radar antenna turns, it emits extremely short bursts of radio waves, called pulses and waits for these pulses to return during the "listening period". Each pulse lasts about 0.00000157 seconds with a "listening period" of 0.00099843 second. The transmitted radio waves move through the atmosphere at around the speed of light. Once it hits a target such as a raindrop or snowflake, the radio waves are scattered with some of the energy returning back to the radar. Radar observes all of this information during the “listening period” with the process repeated up to 1,300 times per second. Observing the time it takes the radio waves to leave the antenna, hit the target, and return to the antenna, the radar can calculate the distance and direction of the target using the “Doppler effect” (hence the title Doppler radar). In addition, the returned energy the radar receives provides information on the target’s characteristics including size, intensity and with the newest Dual Polarized radars, even precipitation type. 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 Insects Birds Turbulence 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

Dual-Polarization radar Conventional radar receives pulses in horizontal directions. Dual-pole : horizontal and vertical Get a better picture of what exactly the targets are: hail, rain, snow, melting snow, insects! The National Weather Service is currently upgrading their radar network to dual-polarization radars. While conventional radars emit and receive pulses in the horizontal direction, dual-polarization radars go a step further and transmit and receive waves in the horizontal and in the vertical direction. This provides a more complete picture of targets in the atmosphere and finally allowing forecasters to differentiate between rain, snow/melting snow, and even hail.

Example of use of dual pole radar: can detect hail

Dual pole

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