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Remote Sensing Notes: GOES Satellites, Imagery, and Radar

GOES Satellites and Geostationary Orbits

  • Geostationary satellites are located directly over the equator and have an orbital period equal to the Earth's rotation, making them stationary with respect to a point on Earth.
  • Benefits: continuous monitoring of the same area, especially over oceans where in-situ observations are sparse.
  • Primary GOES satellites for the USA: GOES West and GOES East.
  • Altitude: approximately h \approx 22{,}300\ \text{miles} \approx 35{,}900\ \text{km} (geostationary orbit).

GOES Imagery Channels and Data Products

  • GOES-East provides several channels, including the Clean IR LW window (ABI Channel 13) for infrared imagery.
  • GOES-East Band 2 refers to Red Visible imagery.
  • Rayleigh Corrected Reflectance and Red/Veggie Pseudo Green/Blue color mappings are used to create enhanced true/false color composites.
  • Example products shown: GOES-East Rayleigh Corrected Reflectance; GOES-East Red Visible; GOES East Clean IR LW window (ABI ch 13).
  • Imagery pages often display timestamps (e.g., 2025 Aug 20 18:56:25 GMT) and institutional attributions (e.g., Atmospheric and Oceanic Sciences).

GOES-16 Imagery Variants

  • GOES-16 Red Visible (ABI ch 2): natural color, enhanced color, and Rayleigh-corrected reflectance variants.
  • Timelines illustrate multiple GOES-16 products on the same date (e.g., 2023 Jan 23).
  • Common color composites: Natural color visible; Enhanced color visible; Rayleigh Corrected Reflectance (Red/Veggie, Green/Blue).

Electromagnetic Spectrum and Windows (Overview)

  • The spectrum ranges from radio to gamma; different wavelengths probe different phenomena.
  • Visual, infrared, and water vapor channels correspond to different atmospheric windows and physical processes.
  • Visual imagery measures reflected solar radiation (albedo) in the visible band; IR measures emitted longwave radiation; Water Vapor channels measure radiation related to atmospheric moisture.
  • Visual-IR-WV relationships enable weather analysis, cloud properties, and moisture transport.

Planck's Law, Wien's Law, and Kirchhoff's Law (Foundational Concepts)

  • Stefan-Boltzmann law: Energy radiated by a black body is proportional to the fourth power of its temperature: E = \sigma T^4
  • Wien's Law: The wavelength of peak emission is inversely proportional to temperature: \lambda_{\max} \propto \frac{1}{T}
  • Kirchhoff's Law: A good absorber is a good emitter; emissivity (and absorptivity) determine radiative behavior.
  • Small temperature changes produce large changes in emitted radiation due to the T^4 relationship.
  • Higher temperatures emit peak radiation at shorter wavelengths.

Planck Spectral Distribution (Figure Concept)

  • Spectral intensity distribution shows how radiated energy varies with wavelength for different temperatures.
  • As temperature increases, the peak shifts to shorter wavelengths.
  • For warm objects (e.g., Earth-atmosphere system), peak emission falls in the infrared range.
  • Typical reference temperatures: around 3000–6000 K for illustrative plots; actual Earth-atmosphere temperatures are much lower, yielding IR-dominated emission.

Atmospheric Windows and Radiative Transport

  • Atmospheric windows are wavelength ranges where radiation passes through the atmosphere with minimal absorption.
  • Visible window: approximately 0.55-0.75\ \mu\text{m} (per transcript; some slides list 0.55–0.75 mm, which appears to be a units typo; common value is μm).
  • Infrared window: approximately 10.2-11.2\ \mu\text{m} (per transcript).
  • These windows enable satellite sensing of surface and cloud properties with minimal atmospheric interference.

Visible Imagery (What It Measures and How It Is Used)

  • Visible imagery detects solar radiation reflected by the surface and clouds; effectively measures albedo.
  • True color images are synthesized by combining discrete red, green, and blue bands.
  • Major limitation: daytime only; clouds and features are visible only when illumination is available.

GOES-16 and Visible Imaging Examples

  • GOES-16 Red Visible (ABI ch 2) demonstrates the use of high-resolution visible imagery.
  • Natural color visible and Enhanced color visible products provide more interpretive detail for weather analysis.

Infrared (IR) Imagery (Longwave) – Key Concepts

  • IR radiometers measure outgoing longwave radiation emitted by the Earth and atmosphere; this is related to temperature via the Stefan–Boltzmann law: E = \sigma T^4
  • Higher radiation implies higher temperature; lower radiation implies cooler regions (e.g., higher cloud tops).
  • In the atmosphere, temperature generally decreases with height; hence, higher clouds appear cooler (darker in IR imagery) than lower, warmer clouds.
  • IR imagery is valuable for nighttime observation and for assessing convection strength, cyclone development, and hurricane intensity.
  • Many IR images include color-enhanced schemes to aid interpretation of temperature differences.

Uses of Infrared Imagery

  • Nighttime satellite imagery; convection strength (including mesoscale convective systems, MCCs, and MCSs); cyclone development; hurricane analysis; and more.

Visible vs Infrared vs Water Vapor – How to Tell Them Apart

  • Visual: true-to-color appearance, high detail in daylight; emphasizes albedo features.
  • Infrared: temperature-fired; color-enhanced temps indicate cloud height and surface features.
  • Water Vapor: measures atmospheric moisture; different channel wavelength (~6.7 μm) and sensitivity to water vapor content.

Atmospheric Absorption/Emission Windows (Detailed)

  • Visible window: 0.55-0.75 μm (note: transcript lists 0.55-0.75 mm; likely a unit error in the source slides).
  • IR window: 10.2-11.2 μm.
  • Water vapor radiometer channel: ~6.7 μm (with reported alternate ranges in transcript, e.g., 6.5-7 μm or 6.5-7 mm; the correct spectral location is ~6.5-7 μm).

Water Vapor Imagery (Channel, Use, and Interpretation)

  • Water Vapor radiometer measures radiation at ~6.7 μm; more water vapor in the air leads to less radiation received at this wavelength (colder appearance due to higher emissivity attenuation).
  • Uses include identifying large-scale features (troughs, ridges, lows), dry intrusions in mid-latitude cyclones or hurricanes, large-scale moisture transport, and vertical motion (areas of subsidence and rising air).

Water Vapor Channel Examples and Resources

  • Notable reference: GOES16 Mid-level Water Vapor (ABI ch 9) imagery (valid 2023 Jan 23).
  • Water vapor animation helps identify vertical motion patterns (ascending vs subsiding air), shortwave troughs/ridges, and convective development.

Polar-Orbiting Satellites and MODIS

  • Polar-orbiting spacecraft have a low Earth orbit (~880 km altitude) compared to geostationary satellites (>36,000 km).
  • Benefits: high spatial resolution imagery; excellent for polar regions.
  • AQUA and TERRA satellites carry MODIS (Moderate Resolution Imaging Spectroradiometer) with 36 spectral bands and resolutions down to \approx 250\ \text{m}.
  • MODIS enables a wide range of applications and derived products across multiple spectral bands.

Real-Time Satellite Products and Resources

  • RAP (Radar and Atmospheric Public) site: simple, clean interface for satellite products. URL: http://weather.rap.ucar.edu/satellite/
  • RAMMB/CIRA SLIDER: interactive satellite imagery platform. URL: https://rammb-slider.cira.colostate.edu/
  • SSEC: offers a variety of non-standard products for specific applications. URL: http://www.ssec.wisc.edu/data/
  • Other sources exist for additional products and data access.

Radar Fundamentals (WSR-88D/NEXRAD)

  • WSR-88D stands for Weather Surveillance Radar, 1988, with Doppler capability and dual polarization enhancements in many systems.
  • Provides precipitation estimates and wind information via reflectivity and velocity fields.
  • Coverage: rivers from ~4,000 to ~10,000 feet AGL in typical maps; radar networks include many stations (e.g., KBHX, KTLX, KEAX, KLOT, etc.).
  • Range and beam geometry: radar beams originate at the radar, propagate outward, and sample a volume of atmosphere; beam width increases with distance due to geometry and the curvature of the Earth.

Radar Basics: Transmit, Receive, and Backscatter

  • The radar transmits a short pulse of radiation at a fixed wavelength (WSR-88D uses ~10 cm). It then receives the backscattered energy from precipitation particles (raindrops, snowflakes, hail).
  • The returned energy provides information about precipitation amount and particle size.
  • The measured quantity is radar reflectivity factor, Z, which is often expressed on a logarithmic scale as dBZ.

Radar Reflectivity and dBZ

  • The returned energy is quantified as the radar reflectivity factor Z.
  • Because Z values span a wide range, they are commonly reported as dBZ: \text{dBZ} = 10\log{10}\left(\frac{Z}{Z0}\right) where Z0 is a reference reflectivity.
  • Reflectivity is highly sensitive to particle size, roughly following the relation Z \propto D^{6} where D is the characteristic particle diameter.

Radar Resolution and Pixel Detail

  • Radar resolution depends on distance from the radar; closer targets yield finer resolution (smaller pixel size) than distant targets.
  • Beams widen with distance, reducing detail at greater ranges.
  • Pixel size increases with range, affecting the clarity of features on reflectivity or velocity plots.

Radar Scans and Scanning Geometry

  • Scans start near the surface (0.5° above ground) and complete 360° sweeps; then tilts up (e.g., to 19.5°) to sample higher altitudes.
  • Beam height increases with distance, and beam geometry is affected by the curvature of the Earth.
  • Multiple tilt angles are used to build a 3D picture of the atmosphere (slice by slice).

Real-Time Radar Interfaces and Tools

  • NWS provides real-time radar products at radar.weather.gov.
  • Tools and options include reflectivity, velocity, dual-polarization products, and clutter filters (various VCPs and product suites).

Radar Volume Coverage Pattern (VCP) Modes

  • VCP 212: Precipitation mode – optimized for detecting precipitation structures.
  • VCP 32: Clear Air mode – optimized for signals in clear air, often used for boundary layer features and infrequent events.
  • Tilt and elevation settings (e.g., Tilt 1 at Elevation = 0.5°) affect the observed cross-section of the atmosphere.
  • Examples show Super-Res Reflectivity and Super-Res Velocity with Elevation = 0.5°.

Radar Visualization and Scout Tools

  • RadarScope Pro features: real-time radar data visualization, tilt views, and velocity/reflectivity overlays.
  • Maps show station coverage, VCPs, and beam paths across regions (e.g., Topeka, Kansas City, Lawrence, etc.).

Doppler Velocity and Rotation Signatures

  • Doppler radar measures radial velocity (velocity along the line of sight). Red indicates motion away from the radar; green indicates motion toward the radar; lighter colors indicate faster motion.
  • Interpretations can be complex due to projection effects and multiple wind components; detailed study recommended.
  • Doppler data helps identify rotation in storms and potential tornado-producing supercells.
  • Rotational signatures include enhancement in velocity couplets and is often associated with a “hook echo” in reflectivity imagery.

Rotation Signatures and Supercells

  • Velocity data are crucial for identifying rotation in supercells.
  • Classic signs include a hook echo in base reflectivity, and a strong gate-to-gate velocity gradient (shear) indicating rotation.
  • A “gate-to-gate shear” of 110+ knots is indicative of strong rotation potential.
  • These patterns help meteorologists issue timely tornado warnings and study storm dynamics.

Case Studies and Real-World Examples

  • Notable historical event: April 25–28, 2011 – The Super Outbreak with 358 confirmed tornadoes; multiple EF-rated tornadoes (EF5, EF4, EF3, etc.) across several states.
  • Radar imagery from this event showed classic tornadic signatures (hook echoes, debris signatures in some cases) across complex storm systems.
  • Case displays often integrate radar reflectivity, velocity, and MODIS/MODIS-like imagery to analyze damage tracks and meteorology.

RadarScope and Public Tools

  • RadarScope provides an accessible interface for viewing radar products, including high-resolution reflectivity and velocity; often used by hobbyists and professionals for quick-look analysis.

Anomalies and Miscellaneous Notes

  • Sometimes radar data visualizations include non-weather phenomena for educational or demonstration purposes (e.g., Mayflies producing notable radar returns). These illustrate that radar is sensitive to non-precipitation targets and the importance of interpretation to avoid misidentification.

Quick Reference – Key Equations and Concepts

  • Stefan-Boltzmann law: E = \sigma T^4
  • Wien’s law: \lambda_{\max} \propto \frac{1}{T}
  • Kirchhoff’s law: good absorber ⇄ good emitter (emissivity governs radiative behavior)
  • Radar reflectivity: \text{dBZ} = 10\log{10}\left(\frac{Z}{Z0}\right) and Z \propto D^6 for particle size D
  • Visible window wavelengths (as stated): 0.55-0.75\ \mu\text{m} (note: transcript lists μm vs mm inconsistently; standard value is μm)
  • Infrared window wavelengths (as stated): 10.2-11.2\ \mu\text{m}
  • Water vapor channel: \lambda \approx 6.7\ \mu\text{m} (transcript also references 6.5–7 μm; some ranges shown as 6.5–7 mm in slides, which is inconsistent with standard practice)
  • MODIS resolution: down to \approx 250\ \text{m} for moderate-resolution bands
  • GOES height: h \approx 22{,}300\ \text{miles} \approx 35{,}900\ \text{km}

Connections and Real-World Relevance

  • Geostationary satellites enable continuous weather monitoring over fixed geographies (critical for tropical cyclone tracking and maritime weather).
  • Visible and IR imagery provide complementary views: structural cloud patterns (VIS) and temperature/height information (IR).
  • Water vapor imagery reveals moisture transport and vertical motions, aiding in understanding mid-latitude cyclone dynamics and tropical systems.
  • Polar-orbiting satellites like MODIS provide high-resolution data for detailed analysis and calibration/validation of geostationary observations.
  • Radar (NEXRAD/WSR-88D) complements satellite data with higher temporal resolution and detailed precipitation structure, including detection of rotation and severe storm signatures; essential for short-term forecasting and warnings.

Practical and Ethical Implications

  • Accurate interpretation of satellite and radar data is essential for public safety (weather warnings, aviation, disaster response).
  • Misinterpretation (e.g., mistaking insect returns for meteorological targets) underscores the need for training and context in radar interpretation.
  • Data accessibility (RAP, RAMMB Slider, SSEC, RadarScope) supports education, research, and operational decision-making; attention to data provenance and update cadence matters for reliability.
  • Visualization choices (color scales, enhancements) affect perception; standardization of color maps aids cross-agency communication and reduces misinterpretation.