☁️Meteorology Unit 3 – Solar Radiation and Earth's Energy Balance

Key Concepts and Terminology

• Solar radiation the primary source of energy for Earth's climate system
• Electromagnetic spectrum range of all possible frequencies of electromagnetic radiation
  • Includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays
• Insolation amount of solar radiation received on a given surface area during a specific time interval
• Albedo fraction of solar radiation reflected by a surface or object
  • Ranges from 0 (completely black) to 1 (completely reflective)
• Greenhouse effect process by which atmospheric gases trap heat radiating from Earth's surface
• Blackbody an idealized physical object that absorbs all incident electromagnetic radiation
• Stefan-Boltzmann law states that the total energy radiated per unit surface area of a blackbody is directly proportional to the fourth power of its absolute temperature

The Sun as Earth's Energy Source

• Sun is the central star of our solar system and the primary energy source for Earth
• Powered by nuclear fusion reactions in its core, converting hydrogen into helium
• Emits electromagnetic radiation across a wide spectrum, with peak emission in the visible light range
• Solar energy drives Earth's climate system, including atmospheric and oceanic circulation patterns
• Variations in solar output can influence Earth's climate over long timescales
  • Examples include the 11-year solar cycle and longer-term changes in solar activity
• Distance between Earth and the Sun (1 astronomical unit or AU) affects the amount of solar radiation received
• Earth's axial tilt and orbital eccentricity contribute to seasonal variations in solar insolation

Solar Radiation Basics

• Solar radiation consists of electromagnetic waves emitted by the Sun
• Intensity of solar radiation decreases with distance from the Sun following an inverse square law
• Solar radiation spectrum spans from short-wavelength gamma rays to long-wavelength radio waves
  • Visible light makes up a small portion of this spectrum
• Peak wavelength of solar emission is around 500 nanometers (green light)
• Solar constant average amount of solar radiation received at the top of Earth's atmosphere (approximately 1,360 W/m²)
• Atmospheric gases, aerosols, and clouds can absorb, scatter, or reflect incoming solar radiation
• Shorter wavelengths (ultraviolet) are more easily scattered by atmospheric particles (Rayleigh scattering)
  • This is why the sky appears blue

Earth's Energy Budget

• Earth's energy budget balance between incoming solar radiation and outgoing terrestrial radiation
• Incoming solar radiation (insolation) is mostly in the visible and near-infrared wavelengths
• Approximately 30% of incoming solar radiation is reflected back to space by clouds, aerosols, and Earth's surface
• Remaining 70% is absorbed by the atmosphere, oceans, and land surfaces
• Earth emits longwave (infrared) radiation back to space to maintain energy balance
• Greenhouse gases (water vapor, carbon dioxide, methane) absorb and re-emit some of this outgoing infrared radiation
  • This process warms the lower atmosphere and surface
• Global energy balance determines Earth's average temperature and climate patterns

Atmospheric Interactions with Solar Radiation

• Earth's atmosphere interacts with incoming solar radiation through absorption, scattering, and reflection
• Atmospheric gases absorb specific wavelengths of solar radiation
  • Ozone absorbs harmful ultraviolet radiation in the stratosphere
  • Water vapor and carbon dioxide absorb infrared radiation
• Scattering of solar radiation by atmospheric particles (aerosols) and molecules affects the amount reaching the surface
  • Rayleigh scattering by air molecules is more effective at shorter (blue) wavelengths
  • Mie scattering by larger particles (dust, pollutants) is less wavelength-dependent
• Clouds play a significant role in reflecting solar radiation back to space (cloud albedo)
  • Low, thick clouds (stratus) have a higher albedo than high, thin clouds (cirrus)
• Atmospheric absorption and scattering can lead to diffuse solar radiation (skylight) in addition to direct sunlight

Albedo and Surface Effects

• Albedo the fraction of incoming solar radiation reflected by a surface
• Different surfaces have varying albedos, affecting the amount of solar energy absorbed
  • Fresh snow has a high albedo (0.8-0.9), while dark forests have a low albedo (0.1-0.2)
• Changes in land cover (deforestation, urbanization) can alter surface albedo and impact local climate
• Ocean albedo varies with surface conditions and sun angle
  • Calm, smooth water has a lower albedo than choppy, rough seas
• Ice-albedo feedback a positive feedback mechanism where melting ice reduces surface albedo, leading to increased absorption of solar radiation and further warming
• Urban heat island effect cities tend to have lower albedos and higher temperatures compared to surrounding rural areas
  • Due to dark surfaces (asphalt, rooftops) and reduced vegetation

Climate Implications and Feedback Loops

• Changes in Earth's energy budget can have significant impacts on global climate
• Positive feedback loops amplify initial changes, while negative feedback loops dampen them
• Examples of climate feedback loops related to solar radiation:
  • Ice-albedo feedback (positive): Melting ice reduces surface albedo, leading to more absorption of solar radiation and further warming
  • Water vapor feedback (positive): Warmer air can hold more water vapor, a potent greenhouse gas, leading to additional warming
  • Cloud feedback (uncertain): Changes in cloud cover and properties can either amplify or dampen warming, depending on cloud type and altitude
• Long-term variations in Earth's orbit (Milankovitch cycles) affect the distribution of solar radiation and contribute to glacial-interglacial cycles
• Anthropogenic activities (greenhouse gas emissions, land-use changes) can alter Earth's energy balance and drive climate change

Measuring and Modeling Solar Radiation

• Accurate measurements of solar radiation are crucial for understanding Earth's energy budget and climate system
• Instruments used to measure solar radiation:
  • Pyranometers measure global (direct + diffuse) solar radiation on a horizontal surface
  • Pyrheliometers measure direct solar radiation at normal incidence
  • Pyrgeometers measure longwave (infrared) radiation emitted by the atmosphere and surface
• Satellite observations provide global coverage of solar radiation and Earth's energy budget
  • Examples: NASA's CERES (Clouds and the Earth's Radiant Energy System) and SORCE (Solar Radiation and Climate Experiment) missions
• Climate models incorporate solar radiation and Earth's energy budget to simulate past, present, and future climate conditions
  • Models consider factors such as atmospheric composition, surface properties, and cloud cover
• Comparing model simulations with observations helps improve our understanding of the climate system and the role of solar radiation