☁️Atmospheric Physics Unit 5 – Atmospheric Dynamics and Circulation

Fundamentals of Atmospheric Motion

• Atmospheric motion is driven by differences in pressure, temperature, and density
• Wind is the horizontal movement of air from high pressure to low pressure areas
• Vertical motion in the atmosphere is caused by buoyancy forces and convection
• Coriolis effect deflects wind to the right in the Northern Hemisphere and to the left in the Southern Hemisphere due to Earth's rotation
• Geostrophic wind is a theoretical wind that results from the balance between the pressure gradient force and the Coriolis force
  • Flows parallel to isobars (lines of constant pressure) at a constant speed
• Gradient wind is a more realistic approximation of wind flow that includes the effects of friction and centrifugal force
• Thermal wind is the difference in geostrophic wind between two pressure levels, caused by horizontal temperature gradients

Thermodynamics and Stability

• Atmospheric thermodynamics deals with the transfer of heat and energy in the atmosphere
• First law of thermodynamics states that energy cannot be created or destroyed, only converted from one form to another
• Second law of thermodynamics states that entropy always increases in a closed system
• Adiabatic processes occur without heat exchange between a system and its surroundings (e.g., rising or sinking air parcels)
  • Dry adiabatic lapse rate (DALR) is the rate at which unsaturated air cools as it rises (~9.8°C/km)
  • Moist adiabatic lapse rate (MALR) is the rate at which saturated air cools as it rises (varies with temperature and pressure)
• Atmospheric stability refers to the atmosphere's resistance to vertical motion
  • Stable atmosphere: air parcel displaced vertically will return to its original position
  • Unstable atmosphere: air parcel displaced vertically will continue to rise or sink
  • Conditionally unstable atmosphere: stability depends on the saturation state of the air parcel

Atmospheric Forces and Equations

• Pressure gradient force (PGF) is the force that moves air from high to low pressure areas
• Coriolis force is an apparent force caused by Earth's rotation that deflects moving objects to the right in the Northern Hemisphere and to the left in the Southern Hemisphere
• Friction is a force that opposes motion and is most significant near the Earth's surface
• Centrifugal force is an apparent outward force experienced by an object moving in a curved path
• Hydrostatic balance is the balance between the vertical pressure gradient force and gravity in a stationary atmosphere
  • Hydrostatic equation: $\frac{\partial p}{\partial z} = -\rho g$, where $p$ is pressure, $z$ is height, $\rho$ is density, and $g$ is gravitational acceleration
• Equation of state relates pressure, density, and temperature in a gas: $p = \rho R_d T$, where $R_d$ is the gas constant for dry air and $T$ is temperature
• Equations of motion describe the acceleration of air due to various forces (e.g., PGF, Coriolis, friction)

Global Circulation Patterns

• Global circulation is driven by uneven heating of the Earth's surface and the rotation of the planet
• Hadley cells are large-scale atmospheric circulations that transport heat from the equator to about 30° latitude
  • Rising motion near the equator, poleward flow aloft, descending motion around 30°, and equatorward flow near the surface
• Ferrel cells are mid-latitude circulations that transport heat from about 30° to 60° latitude
  • Rising motion around 60°, equatorward flow aloft, descending motion around 30°, and poleward flow near the surface
• Polar cells are small-scale circulations that transport cold air from the poles to about 60° latitude
• Jet streams are fast-moving, narrow bands of strong winds in the upper troposphere that flow from west to east
  • Polar jet stream is located around 60° latitude and is stronger in the winter
  • Subtropical jet stream is located around 30° latitude and is stronger in the summer
• Intertropical Convergence Zone (ITCZ) is a region near the equator where the trade winds converge, leading to rising motion, cloudiness, and precipitation

Synoptic-Scale Weather Systems

• Synoptic-scale weather systems are large-scale systems (1,000-5,000 km) that can be identified on weather maps
• Extratropical cyclones (mid-latitude cyclones) are low-pressure systems that form along frontal boundaries in the mid-latitudes
  • Characterized by counterclockwise rotation in the Northern Hemisphere and clockwise rotation in the Southern Hemisphere
  • Associated with fronts, precipitation, and strong winds
• Anticyclones are high-pressure systems with clockwise rotation in the Northern Hemisphere and counterclockwise rotation in the Southern Hemisphere
  • Associated with descending motion, clear skies, and light winds
• Fronts are boundaries between air masses with different temperatures and densities
  • Cold fronts occur when cold air advances and displaces warmer air, often associated with strong convection and severe weather
  • Warm fronts occur when warm air advances over colder air, often associated with steady precipitation and stratiform clouds
  • Occluded fronts form when a cold front overtakes a warm front, lifting the warm air off the surface
• Upper-level troughs and ridges are disturbances in the upper-level flow that can influence surface weather patterns
  • Troughs are associated with lower pressure and rising motion
  • Ridges are associated with higher pressure and sinking motion

Mesoscale and Local Circulations

• Mesoscale circulations are smaller-scale systems (10-1,000 km) that are influenced by local factors such as topography, land-sea contrasts, and urban heat islands
• Sea and land breezes are diurnal circulations driven by temperature differences between land and water surfaces
  • Sea breeze: onshore flow during the day as land heats up faster than water
  • Land breeze: offshore flow at night as land cools down faster than water
• Mountain and valley breezes are diurnal circulations driven by temperature differences between mountain slopes and valleys
  • Valley breeze: upslope flow during the day as mountain slopes heat up
  • Mountain breeze: downslope flow at night as mountain slopes cool down
• Urban heat islands are areas of elevated temperature in cities compared to surrounding rural areas due to human activities and modified land surfaces
• Thunderstorms are convective systems that develop in unstable environments with sufficient moisture and lifting mechanisms
  • Characterized by strong updrafts, downdrafts, lightning, and heavy precipitation
• Tornadoes are violently rotating columns of air that extend from a thunderstorm to the ground
  • Associated with strong wind shear and specific types of thunderstorms (supercells)

Climate Dynamics and Variability

• Climate is the long-term average of weather conditions in a given area
• Climate variability refers to variations in climate on timescales ranging from months to decades
• El Niño-Southern Oscillation (ENSO) is a coupled ocean-atmosphere phenomenon that affects global weather patterns
  • El Niño: warming of the eastern tropical Pacific Ocean, associated with increased precipitation in some regions and droughts in others
  • La Niña: cooling of the eastern tropical Pacific Ocean, often associated with opposite impacts to El Niño
• North Atlantic Oscillation (NAO) is a fluctuation in atmospheric pressure differences between the Icelandic Low and the Azores High
  • Positive NAO: stronger pressure gradient, more intense and frequent storms in the North Atlantic
  • Negative NAO: weaker pressure gradient, fewer and weaker storms in the North Atlantic
• Arctic Oscillation (AO) is a variation in atmospheric pressure patterns over the Arctic and mid-latitudes
  • Positive AO: lower pressure over the Arctic, higher pressure in mid-latitudes, and stronger westerly winds
  • Negative AO: higher pressure over the Arctic, lower pressure in mid-latitudes, and weaker westerly winds
• Climate change refers to long-term changes in climate due to natural factors and human activities
  • Greenhouse gases (e.g., carbon dioxide, methane) trap heat in the atmosphere, leading to global warming
  • Impacts include rising sea levels, changes in precipitation patterns, more frequent and intense extreme weather events, and shifts in ecosystems

Atmospheric Modeling and Forecasting

• Atmospheric models are mathematical representations of the atmosphere used for weather forecasting and climate simulations
• Numerical weather prediction (NWP) models simulate the evolution of the atmosphere using the equations of motion and thermodynamics
  • Global models cover the entire Earth and have lower spatial resolution (e.g., GFS, ECMWF)
  • Regional models cover a limited area and have higher spatial resolution (e.g., WRF, NAM)
• Data assimilation is the process of incorporating observations into atmospheric models to improve initial conditions and forecasts
  • Methods include 3D-Var, 4D-Var, and ensemble Kalman filtering
• Ensemble forecasting involves running multiple model simulations with slightly different initial conditions or model physics to account for uncertainty
  • Ensemble mean and spread provide information about the most likely outcome and the range of possible outcomes
• Model output statistics (MOS) are statistical techniques used to correct systematic biases in model forecasts based on past performance
• Forecast verification is the process of evaluating the accuracy and skill of weather forecasts using various metrics
  • Measures include bias, root mean square error (RMSE), and anomaly correlation coefficient (ACC)
• Climate models are used to simulate long-term changes in the Earth's climate system
  • Coupled atmosphere-ocean general circulation models (AOGCMs) include interactions between the atmosphere, oceans, land surface, and sea ice
  • Earth system models (ESMs) incorporate additional components such as the carbon cycle and dynamic vegetation