Climate Dynamics Lecture Flashcards

Principles of Spatial Scale in Climate Dynamics

• Climate dynamics operates across four primary hierarchical spatial scales, each governed by specific environmental forcing mechanisms:

  • Global Scale: Driven by global atmospheric circulation, oceanic current systems, and the global distribution of incoming solar radiation.

  • Regional Scale: Influenced by regional atmospheric circulation, oceanic currents, spatial solar radiation patterns, and large-scale topography.

  • Local Scale: Dictated by local topography, surface vegetation cover, degree of urbanization, and proximity to large bodies of water.

  • Micro Scale: Governed by micro-topography, localized vegetation canopy structure, surface soil color, and soil moisture availability.

• Microclimatic Vegetation Dynamics:

  • Bare soil exposed to full solar radiation heats to extreme maximum surface temperatures of $48\,^\circ\text{C}$.

  • Low shrub cover provides shading that lowers maximum litter surface temperatures to $29\,^\circ\text{C}$ and soil temperatures beneath to $27\,^\circ\text{C}$.

  • Tall shrub canopy with greater leaf area and numerous twigs intercepts significantly more light, creating the coolest microclimate conditions with litter temperatures of $21\,^\circ\text{C}$ and soil temperatures of $23\,^\circ\text{C}$.

Atmospheric Structure and Vertical Stratification

• The Earth's atmosphere is organized into distinct thermal vertical layers, with the lower two layers exerting the primary control over climate and biological processes:

  • Troposphere ($0\text{--}15\,km$ altitude):

    • Lowest atmospheric layer heated directly from the bottom via longwave re-radiation and sensible/latent heat fluxes from the surface.

    • Serves as the central zone of weather phenomena, surface mass-energy exchanges, and the natural greenhouse effect.

    • Temperature steadily decreases with height up to the tropopause boundary, dropping to approximately $-55\,^\circ\text{C}$ to $-60\,^\circ\text{C}$.

  • Stratosphere ($15\text{--}47\,km$ altitude):

    • Located directly above the tropopause.

    • Heated from the top down due to the absorption of high-energy ultraviolet (UV) radiation by the ozone ($O_3$) layer.

    • Contains the zone of maximum ozone concentration situated between $20\,km$ and $30\,km$ altitude.

  • Mesosphere ($50\text{--}85\,km$ altitude):

    • Extends above the stratopause up to the mesopause.

    • Characterized by declining temperatures with altitude, reaching absolute atmospheric minimums around $-90\,^\circ\text{C}$; zone where meteors burn up upon entry.

  • Thermosphere ($85\,km+$ altitude):

    • Extends beyond the mesopause into outer space, where absorption of extreme solar radiation causes temperatures to rise again; site of atmospheric auroral displays.

Solar Radiation Spectrum and Atmospheric Interactions

• Solar radiation reaching Earth spans distinct spectral wavebands categorized by wavelength ($\lambda$) and photon energy:

  • Ultraviolet (UV) Radiation:

    • Short wavelength (<400\,nm), high photon energy.

    • Destructive to cell structure and biological machinery; largely filtered out by stratospheric ozone ($O_3$).

  • Visible Radiation ($400\text{--}700\,nm$):

    • Spectral range visible to human eyes.

    • Drives terrestrial and aquatic photosynthesis, designated as Photosynthetically Active Radiation (PAR).

  • Infrared (IR) Radiation:

    • Long wavelength (>700\,nm), lower energy; functions as thermal heat radiation.

    • Absorbed by tropospheric greenhouse gases, retaining energy within the Earth system.

• Spectral Irradiance and Atmospheric Absorption Bands:

  • Extraterrestrial solar radiation at the top of the atmosphere follows a $5250\,^\circ\text{C}$ blackbody spectrum.

  • As light penetrates to sea level, atmospheric absorption creates distinct spectral attenuation dips:

    • Ultraviolet absorbed by ozone ($O_3$).

    • Visible light absorbed in part by oxygen ($O_2$).

    • Infrared absorbed across specific bands by water vapor ($H_2O$) and carbon dioxide ($CO_2$).

Global Solar Radiation and Energy Balance

• The Earth operates in a long-term global radiation equilibrium where total incoming shortwave solar radiation equals total outgoing longwave radiation ($235\,W\,m^{-2}$).

• Shortwave Radiation Component:

  • Incoming solar radiation at the top of the atmosphere: $342\,W\,m^{-2}$.

  • Total reflected back into space (planetary albedo): $107\,W\,m^{-2}$ ($77\,W\,m^{-2}$ reflected by clouds, aerosols, and atmosphere + $30\,W\,m^{-2}$ reflected directly by the Earth surface).

  • Absorbed by atmosphere: $67\,W\,m^{-2}$.

  • Absorbed directly by Earth surface: $168\,W\,m^{-2}$.

  • Net absorbed solar shortwave radiation: $342\,W\,m^{-2} - 107\,W\,m^{-2} = 235\,W\,m^{-2}$.

• Longwave Radiation Component and Atmospheric Greenhouse Fluxes:

  • Atmospheric thermal back-radiation emitted down to the surface: $324\,W\,m^{-2}$.

  • Total radiative heat received by Earth surface: $168\,W\,m^{-2} \text{ (shortwave)} + 324\,W\,m^{-2} \text{ (longwave)} = 492\,W\,m^{-2}$.

  • Surface energy dissipation:

    • Sensible heat transfer (Thermals): $24\,W\,m^{-2}$.

    • Latent heat flux (Evapotranspiration): $78\,W\,m^{-2}$.

    • Direct surface longwave radiation: $390\,W\,m^{-2}$ ($350\,W\,m^{-2}$ absorbed by atmospheric gases + $40\,W\,m^{-2}$ passing directly to space via the atmospheric window).

    • Total surface output dissipation: $24 + 78 + 390 = 492\,W\,m^{-2}$.

  • Atmospheric energy balance:

    • Total heat gained by atmosphere: $26\,W\,m^{-2} \text{ (net radiative surface absorption: } 350 - 324) + 102\,W\,m^{-2} \text{ (non-radiative surface heat: } 24 + 78) + 67\,W\,m^{-2} \text{ (direct solar absorption)} = 195\,W\,m^{-2}$.

    • Heat re-emitted by atmosphere to space: $165\,W\,m^{-2}$ (combining with cloud emission to balance internal gains).

    • Total outgoing longwave radiation to space: $195\,W\,m^{-2} \text{ (atmospheric emissions)} + 40\,W\,m^{-2} \text{ (atmospheric window leakage)} = 235\,W\,m^{-2}$.

Seasonality and Latitudinal Solar Insolation

• Earth's rotational axis is tilted at a constant angle of $23.5\,^\circ$ (or approximately $23\,^\circ$) relative to its orbital plane around the Sun.

• This axial tilt alters the solar incident beam angle and day length throughout the year, driving seasonality that intensifies toward poleward latitudes.

• Seasonal Insolation Dynamics Across Latitudes:

  • Equatorial regions ($0\,^\circ$ latitude) receive uniform, high daily solar radiation year-round (~$800\,cal\,cm^{-2}\,day^{-1}$), exhibiting minimal seasonal variability.

  • High-latitude regions ($60\,^\circ\text{--}80\,^\circ$ latitude) undergo extreme seasonal shifts, ranging from $0\,cal\,cm^{-2}\,day^{-1}$ during polar winter darkness to peak summer values exceeding $900\text{--}1000\,cal\,cm^{-2}\,day^{-1}$ due to 24-hour daylight.

• Orbital Positions and Solstices:

  • June Solstice: Northern Hemisphere tilts toward the Sun; direct rays hit the Tropic of Cancer ($23.5\,^\circ\text{N}$).

  • December Solstice: Southern Hemisphere tilts toward the Sun; direct rays hit the Tropic of Capricorn ($23.5\,^\circ\text{S}$).

  • March and September Equinoxes: Neither hemisphere tilts toward the Sun; perpendicular solar beam strikes the Equator ($0\,^\circ$).

Atmospheric Circulation and Global Wind Cells

• Latitudinal gradients in solar heating drive global atmospheric overturning, organized into three primary circulation cells per hemisphere:

  • Hadley Cell: Intense heating at the equator causes warm, moist air to rise (forming the Intertropical Convergence Zone). Air moves poleward in the upper troposphere, cools, and subsides around $30\,^\circ\text{N}$ and $30\,^\circ\text{S}$ latitude, establishing dry subtropical high-pressure zones and driving surface trade winds.

  • Ferrel Cell: Mid-latitude circulation cell operating between $30\,^\circ$ and $60\,^\circ$ latitude, driven by overturning in adjacent cells; governs surface westerly winds.

  • Polar Cell: High-latitude cell where intense polar cooling causes cold, dense air to sink at the poles and flow equatorward toward $60\,^\circ$ latitude.

Global Climate Patterns and Biome Distribution

• Global geographical distributions of mean annual temperature and total annual precipitation dictate the spatial distribution of terrestrial biomes.

• Global Temperature and Precipitation Patterns:

  • Average annual surface temperatures range from above $30\,^\circ\text{C}$ in tropical equatorial regions to below $-30\,^\circ\text{C}$ in polar ice caps.

  • Annual precipitation reaches maxima (>200\text{--}400\,cm) around equatorial zones and drops to severe minima (<10\text{--}20\,cm) in subtropical desert belts ($30\,^\circ\text{N/S}$) and polar extremes.

• Whittaker Biome Classification Parameters:

  • Tropical Rainforest: High temperatures ($20\text{--}30\,^\circ\text{C}$) and high precipitation ($250\text{--}450\,cm$).

  • Tropical Seasonal Forest / Savanna: Warm temperatures ($15\text{--}30\,^\circ\text{C}$) with moderate seasonal precipitation ($50\text{--}250\,cm$).

  • Subtropical Desert: Warm-to-hot temperatures ($15\text{--}30\,^\circ\text{C}$) with minimal precipitation (<50\,cm).

  • Temperate Rainforest: Moderate temperatures ($5\text{--}20\,^\circ\text{C}$) and high annual rainfall (>200\,cm).

  • Temperate Forest: Moderate temperatures ($5\text{--}20\,^\circ\text{C}$) and moderate precipitation ($75\text{--}200\,cm$).

  • Boreal Forest (Taiga): Cold temperatures ($-5\text{--}5\,^\circ\text{C}$) and moderate precipitation ($40\text{--}100\,cm$).

  • Tundra: Severe cold temperatures ($-15\text{--}-5\,^\circ\text{C}$) and low precipitation (<50\,cm).

Topographic Controls on Regional and Microclimates

• Adiabatic Lapse Rates:

  • Dry Adiabatic Lapse Rate: Unsaturated air cools at $10\,^\circ\text{C} / 1000\,m$ ($10\,^\circ\text{C}\,km^{-1}$) as it ascends and expands under lower pressure.

  • Wet Adiabatic Lapse Rate: Condensing, saturated air cools at a lower rate of $5\,^\circ\text{C} / 1000\,m$ ($5\,^\circ\text{C}\,km^{-1}$) because the release of latent heat offsets cooling.

  • Vertical Lapse Rate Profile Example:

    • $0\,m$ elevation: $32\,^\circ\text{C}$

    • $1000\,m$ elevation: $22\,^\circ\text{C}$

    • $2000\,m$ elevation: $12\,^\circ\text{C}$

    • $3000\,m$ elevation (Condensation Level): $2\,^\circ\text{C}$

    • $4000\,m$ elevation: $-3\,^\circ\text{C}$

    • $5000\,m$ elevation: $-8\,^\circ\text{C}$

• Slope Aspect and Gradient:

  • Aspect: Compass direction a slope faces. Northern Hemisphere south-facing slopes receive greater solar radiation, leading to warmer, drier soils than north-facing slopes.

  • Gradient: Slope steepness modifies solar beam incidence angle, concentrating solar flux.

• Cold-Air Drainage and Thermal Inversions:

  • At night, nocturnal radiational cooling cools surface air on high slopes.

  • Dense, cold air drains downhill under gravity into valley bottoms ("cold-air ponding"), producing valley floor temperatures down to $25\,^\circ\text{F}$.

  • Warmer air is displaced upward, establishing mid-slope "thermal belts" ($35\text{--}40\,^\circ\text{F}$) ideal for frost-sensitive agricultural crops like vineyards.

• Orographic Lift and Rain Shadow Effect:

  • Topographic barriers force air masses upward on the windward slope, inducing cooling, cloud condensation, and intense precipitation.

  • Descents on the leeward slope warm air adiabatically, lowering relative humidity and forming an arid rain shadow.

  • Southern California Case Study: Coastal mountains force moist air upward, generating $32\text{--}36\,\text{inches/yr}$ of precipitation on windward slopes, while leeward inland areas (Imperial County / Salton Sea basin) receive under $4\,\text{inches/yr}$.

Natural Drivers of Climate Variability

• Solar Forcing and Orbital Variations:

  • Variations in solar output linked to sunspot activity and solar flares.

  • Long-term orbital periodicity (Milankovitch cycles involving eccentricity, obliquity, and precession) altering Earth's seasonal radiation budget.

• Atmospheric Aerosols and Volcanic Activity:

  • Explosive volcanic eruptions (such as Mt. St. Helens) inject dust and sulfur dioxide into the stratosphere.

  • Stratospheric aerosols reflect incoming shortwave solar radiation back to space, reducing atmospheric clarity and inducing temporary global surface cooling.

• El Niño / Southern Oscillation (ENSO):

  • Normal Pacific Conditions: Strong westward trade winds pile warm surface waters in the western Pacific near Indonesia and Australia, driving deep convective rainfall. Cold, nutrient-rich water wells up along the South American coast.

  • El Niño Conditions: Pacific trade winds weaken or reverse eastward. The warm surface pool shifts east toward South America, suppressing coastal upwelling, causing heavy rainfall and flooding in Peru while driving severe drought in Australia and Indonesia.

Anthropogenic Forcing and Land-Use Impacts

• Natural vs. Human-Enhanced Greenhouse Effect:

  • Natural Greenhouse Effect: Naturally occurring trace gases ($CO_2$, $CH_4$, $N_2O$) retain a portion of re-radiated thermal heat, keeping Earth's surface warm enough to sustain life.

  • Human-Enhanced Greenhouse Effect: Anthropogenic emissions increase atmospheric concentrations of $CO_2$, $CH_4$, and $N_2O$, trapping additional re-emitted thermal energy and restricting heat escape into space.

• Historical Temperature Anomalies (HadCRUT5 & IPCC 2021):

  • For over 1800 years (1–1850 CE), global mean temperature remained stable or experienced a long-term cooling trend.

  • Over the ~170 years since 1850, global temperatures surged by over $1.5\,^\circ\text{C}$ above pre-industrial levels (1850–1900 baseline).

  • The current warming velocity is unprecedented in over 2,000 years, making recent decades the warmest multi-century period in over 100,000 years.

• Radiative Forcing and Attribution (2010–2019 relative to 1850–1900):

  • Observed Total Warming: Approximately $1.06\,^\circ\text{C}$.

  • Total Human Influence: Matches observed warming at ~$1.07\,^\circ\text{C}$.

  • Well-Mixed Greenhouse Gases: Contributed +$1.5\,^\circ\text{C}$ warming ($CO_2$ contributing ~$0.8\,^\circ\text{C}$, $CH_4$ contributing ~$0.5\,^\circ\text{C}$, alongside $N_2O$ and halogenated gases).

  • Anthropogenic Aerosol Forcing: Sulphur dioxide ($SO_2$) and organic carbon exert a net cooling influence of roughly $-0.4\,^\circ\text{C}$ ($SO_2$ driving ~$-0.5\,^\circ\text{C}$ cooling).

  • Natural Drivers (Solar + Volcanic): Net contribution ranges from $-0.02\,^\circ\text{C}$ to $+0.02\,^\circ\text{C}$, indicating negligible influence on modern warming trends.

• Land-Use Dynamics: Deforestation Microclimates:

  • Deforestation (visible in characteristic "fishbone" clearing patterns along tropical roads) dramatically shifts local surface energy and moisture exchange:

  • Rainforest Vegetation: High evapotranspiration, high latent heat loss, low sensible heat loss, low surface temperature, low albedo, and continuous recycling of moisture fueling high precipitation.

  • Pasture Vegetation (Deforested): Higher surface albedo, lower evapotranspiration, reduced latent heat loss, higher sensible heat loss, elevated surface temperatures, and reduced atmospheric humidity, causing regional atmospheric drying and diminished rainfall.