Radiative transfer is the backbone of atmospheric energy exchange. It encompasses emission, absorption, and scattering of electromagnetic radiation as it travels through the atmosphere, shaping our understanding of atmospheric heating and cooling processes.

This topic delves into the electromagnetic spectrum, blackbody radiation, and absorption bands of key atmospheric gases. It explores scattering processes, the radiative transfer equation, and atmospheric radiative balance, providing crucial insights into Earth's climate system and energy distribution.

Fundamentals of radiative transfer

• Radiative transfer forms the basis for understanding energy exchange in the atmosphere through electromagnetic radiation
• Encompasses the processes of emission, absorption, and scattering of radiation as it travels through the atmosphere
• Crucial for studying atmospheric heating, cooling, and overall energy balance in Atmospheric Physics

Electromagnetic spectrum

• Ranges from short-wavelength gamma rays to long-wavelength radio waves
• Visible light occupies a small portion of the spectrum (400-700 nm)
• Infrared radiation (heat) plays a significant role in atmospheric processes
• UV radiation interacts with ozone in the stratosphere, impacting atmospheric chemistry

Blackbody radiation

• Idealized concept of a perfect emitter and absorber of radiation
• Emits radiation according to Planck's law, which relates spectral radiance to temperature
• Stefan-Boltzmann law describes total energy emitted by a blackbody: $E = σT^4$
  • σ represents the Stefan-Boltzmann constant
  • T denotes the absolute temperature in Kelvin
• Wien's displacement law determines the peak wavelength of emission: $λ_max = b/T$
  • b represents Wien's displacement constant

Absorption and emission processes

• Absorption involves the capture of photons by atoms or molecules, increasing their internal energy
• Emission occurs when excited atoms or molecules release photons, decreasing their internal energy
• Kirchhoff's law states that good absorbers are also good emitters at the same wavelength
• Spectral lines result from specific energy transitions in atoms and molecules
  • Absorption lines appear as dark lines in a continuous spectrum
  • Emission lines appear as bright lines against a dark background

Atmospheric absorption bands

• Atmospheric gases selectively absorb radiation at specific wavelengths, creating absorption bands
• These bands significantly influence the Earth's energy balance and atmospheric temperature structure
• Understanding absorption bands helps explain phenomena like the greenhouse effect and ozone depletion

Water vapor absorption

• Absorbs strongly in the infrared region, particularly around 6.3 μm and 2.7 μm
• Rotational absorption bands extend into the microwave region
• Plays a crucial role in the greenhouse effect and atmospheric energy transport
• Absorption varies greatly with altitude due to the concentration of water vapor in the lower atmosphere

Carbon dioxide absorption

• Major absorption bands centered around 15 μm and 4.3 μm in the infrared spectrum
• The 15 μm band significantly contributes to the greenhouse effect
• Absorption in the near-infrared region (1.4 μm, 1.6 μm, 2.0 μm) affects solar radiation
• Increasing atmospheric CO2 concentrations enhance the greenhouse effect, leading to global warming

Ozone absorption

• Strong absorption in the ultraviolet region, particularly in the Hartley band (200-300 nm)
• Weaker absorption in the visible spectrum (Chappuis band, 400-850 nm)
• Infrared absorption around 9.6 μm affects the Earth's energy balance
• Stratospheric ozone layer protects life on Earth from harmful UV radiation
• Tropospheric ozone acts as a greenhouse gas and air pollutant

Scattering processes

• Scattering redirects incoming radiation in various directions without changing its wavelength
• Affects the propagation of solar radiation through the atmosphere
• Influences atmospheric visibility, sky color, and remote sensing measurements

Rayleigh scattering

• Occurs when particles are much smaller than the wavelength of incident radiation (λ/10)
• Intensity of scattered light proportional to 1/λ^4 (inverse fourth power of wavelength)
• Explains the blue color of the sky and red sunsets
• Predominantly affects shorter wavelengths (blue and violet light)

Mie scattering

• Applies to particles with sizes comparable to or larger than the wavelength of incident radiation
• Occurs with aerosols, cloud droplets, and other atmospheric particles
• Scattering intensity less dependent on wavelength compared to Rayleigh scattering
• Produces a forward-peaked scattering pattern, affecting visibility and cloud appearance

Multiple scattering

• Occurs when radiation undergoes repeated scattering events before reaching an observer
• Significant in optically thick media (clouds, dense aerosol layers)
• Complicates radiative transfer calculations and remote sensing interpretations
• Enhances the diffuse component of radiation in the atmosphere
• Affects cloud albedo and the Earth's overall energy balance

Radiative transfer equation

• Describes the change in radiation intensity as it travels through a medium
• Accounts for emission, absorption, and scattering processes
• Fundamental to understanding atmospheric radiation and remote sensing techniques

Beer-Lambert law

• Describes the attenuation of radiation as it passes through an absorbing medium
• Exponential decay of intensity: $I = I_0 e^{-τ}$
  • I represents the transmitted intensity
  • I_0 denotes the initial intensity
  • τ signifies the optical depth of the medium
• Applies to monochromatic radiation and non-scattering media
• Forms the basis for many atmospheric transmission calculations

Schwarzschild equation

• Describes radiative transfer in a non-scattering, plane-parallel atmosphere
• Accounts for both absorption and emission processes
• Expressed as: $dI_ν/dτ_ν = -I_ν + B_ν(T)$
  • I_ν represents the specific intensity at frequency ν
  • τ_ν denotes the optical depth at frequency ν
  • B_ν(T) signifies the Planck function at temperature T
• Crucial for understanding atmospheric temperature profiles and radiative equilibrium

Two-stream approximation

• Simplifies the radiative transfer equation by considering only upward and downward fluxes
• Assumes isotropic scattering and divides the radiation field into two hemispheres
• Provides a computationally efficient method for calculating radiative fluxes
• Widely used in climate models and atmospheric physics simulations
• Accuracy improves with increasing optical depth of the medium

Atmospheric radiative balance

• Describes the equilibrium between incoming solar radiation and outgoing terrestrial radiation
• Crucial for understanding Earth's climate system and energy distribution
• Influences atmospheric circulation patterns and temperature structure

Solar radiation budget

• Incoming solar radiation averages about 1361 W/m² at the top of the atmosphere (solar constant)
• Approximately 30% of incoming radiation reflected back to space (planetary albedo)
• Absorption by atmospheric gases, clouds, and aerosols reduces surface insolation
• Spatial and temporal variations in solar radiation drive atmospheric and oceanic circulation

Terrestrial radiation budget

• Earth emits longwave radiation according to its surface and atmospheric temperatures
• Atmospheric greenhouse gases absorb and re-emit a portion of this radiation
• Outgoing longwave radiation at the top of the atmosphere balances incoming solar radiation
• Surface energy balance includes sensible and latent heat fluxes in addition to radiative components

Greenhouse effect

• Natural process that warms Earth's surface by trapping heat in the atmosphere
• Key greenhouse gases include water vapor, carbon dioxide, methane, and ozone
• Absorption and re-emission of longwave radiation by greenhouse gases increase surface temperature
• Enhanced greenhouse effect due to anthropogenic emissions leads to global warming
• Feedback mechanisms (water vapor, ice-albedo) can amplify or dampen the greenhouse effect

Radiative forcing

• Measures the change in Earth's energy balance due to a perturbation in the climate system
• Expressed in watts per square meter (W/m²)
• Positive forcing leads to warming, while negative forcing results in cooling
• Crucial for understanding climate change and predicting future climate scenarios

Definition and mechanisms

• Defined as the change in net irradiance at the tropopause after allowing stratospheric temperatures to readjust
• Can result from changes in greenhouse gas concentrations, aerosols, solar output, or surface albedo
• Direct forcing affects radiation directly (greenhouse gases)
• Indirect forcing involves feedbacks or secondary effects (aerosol-cloud interactions)

Natural vs anthropogenic forcing

• Natural forcing includes variations in solar output, volcanic eruptions, and orbital changes
• Anthropogenic forcing stems from human activities (greenhouse gas emissions, land-use changes)
• Recent climate change primarily attributed to anthropogenic forcing, particularly greenhouse gas emissions
• Understanding the relative contributions helps in climate change attribution and mitigation strategies

Climate sensitivity

• Measures the response of global mean surface temperature to a doubling of CO2 concentration
• Equilibrium climate sensitivity considers long-term response after reaching equilibrium
• Transient climate response describes short-term warming before equilibrium
• Uncertainty in climate sensitivity affects projections of future warming
• Constraints from paleoclimate data and modern observations help refine sensitivity estimates

Radiative transfer models

• Simulate the propagation of radiation through the atmosphere
• Essential for climate modeling, remote sensing, and understanding atmospheric processes
• Range from simple approximations to highly detailed, computationally intensive models

Line-by-line models

• Calculate absorption and emission for individual spectral lines
• Highest accuracy but computationally expensive
• Use spectroscopic databases (HITRAN) for line parameters
• Employed as references for developing faster, approximate models
• Critical for interpreting high-resolution spectroscopic measurements

Band models

• Simplify calculations by grouping spectral lines into bands
• Balance accuracy and computational efficiency
• Include regular band models, statistical band models, and exponential sum fitting
• Widely used in climate models and radiative transfer applications
• Accuracy depends on the chosen band structure and parameterization

Correlated-k distribution

• Rearranges absorption coefficients within a spectral interval
• Transforms integration over wavelength to integration over a cumulative probability function
• Maintains accuracy of line-by-line calculations with significantly reduced computational cost
• Allows for efficient treatment of overlapping absorption by multiple gases
• Widely adopted in atmospheric radiative transfer calculations and climate models

Remote sensing applications

• Utilizes radiative transfer principles to retrieve atmospheric and surface properties
• Enables global monitoring of atmospheric composition, temperature, and cloud properties
• Crucial for weather forecasting, climate studies, and environmental monitoring

Satellite radiometry

• Measures radiation emitted or reflected by the Earth and atmosphere
• Includes visible, infrared, and microwave wavelengths
• Passive sensors detect naturally emitted or reflected radiation
• Active sensors (radar, lidar) emit radiation and measure the returned signal
• Retrieval algorithms use radiative transfer models to infer atmospheric properties

Lidar measurements

• Uses laser pulses to measure atmospheric properties with high vertical resolution
• Applications include aerosol profiling, cloud detection, and wind measurements
• Differential absorption lidar (DIAL) measures specific gas concentrations
• Raman lidar detects inelastic scattering to measure water vapor and temperature profiles
• Combines principles of radiative transfer and laser physics

Atmospheric sounding techniques

• Retrieve vertical profiles of atmospheric properties (temperature, humidity, trace gases)
• Infrared sounders measure emission from different atmospheric layers
• Microwave sounders penetrate clouds, providing all-weather capabilities
• Radio occultation uses GPS signals to probe the atmosphere
• Inversion techniques apply radiative transfer models to retrieve atmospheric state from measured radiances

Radiative transfer in climate models

• Represents the exchange of radiation between the atmosphere, surface, and space
• Critical for simulating Earth's energy balance and climate response to forcings
• Balances accuracy with computational efficiency to enable long-term climate simulations

Parameterization schemes

• Simplify complex radiative processes for efficient computation in climate models
• Include treatments for gases, aerosols, clouds, and surface properties
• Correlated-k method often used for gas absorption
• Two-stream approximation commonly employed for radiative flux calculations
• Continually updated based on improved understanding and observational constraints

Radiative-convective equilibrium

• Describes the balance between radiative and convective heat transfer in the atmosphere
• Fundamental concept in understanding atmospheric temperature structure
• Simple models of radiative-convective equilibrium provide insights into climate sensitivity
• More complex treatments in climate models account for large-scale dynamics and feedbacks
• Important for studying tropical atmosphere and climate change impacts

Cloud-radiation interactions

• Represent one of the largest sources of uncertainty in climate models
• Include effects on both shortwave (albedo) and longwave (greenhouse) radiation
• Cloud microphysics and macrophysics influence radiative properties
• Feedbacks between clouds and radiation affect climate sensitivity
• Parameterizations must account for subgrid-scale cloud processes and their radiative effects