9.1 Structure of the planetary boundary layer

The planetary boundary layer (PBL) is the lowest part of the atmosphere directly influenced by Earth's surface. It plays a crucial role in atmospheric physics, mediating exchanges of heat, moisture, and momentum between the surface and free atmosphere.

The PBL's structure varies diurnally and consists of multiple sublayers with distinct characteristics. Understanding its vertical structure, turbulence patterns, and interactions with different surfaces is essential for weather forecasting, air quality modeling, and climate predictions.

Planetary boundary layer definition

• Lowest part of the troposphere directly influenced by Earth's surface
• Responds to surface forcings on timescales of about an hour or less
• Plays crucial role in atmospheric physics by mediating exchanges of heat, moisture, and momentum between surface and free atmosphere

Characteristics of PBL

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• Turbulent flow dominates air motion within the layer
• Strong diurnal variations in temperature, humidity, and wind speed
• Depth varies from hundreds of meters to a few kilometers
• Contains most atmospheric aerosols and water vapor
• Experiences rapid mixing of air parcels due to surface-induced turbulence

PBL vs free atmosphere

• PBL exhibits stronger vertical mixing compared to the free atmosphere
• Turbulence intensity decreases rapidly above PBL in the free atmosphere
• Wind in free atmosphere approaches geostrophic balance, while PBL winds are influenced by surface friction
• Temperature lapse rate in PBL often differs from the standard atmospheric lapse rate
• Moisture content typically higher in PBL than in the free atmosphere above

Vertical structure of PBL

• Consists of multiple sublayers with distinct characteristics
• Structure evolves throughout the day in response to surface heating and cooling
• Understanding vertical structure crucial for weather forecasting and air quality modeling

Surface layer

• Lowest 10% of the PBL where vertical fluxes are nearly constant with height
• Characterized by strong vertical gradients in temperature, humidity, and wind speed
• Monin-Obukhov similarity theory applies to describe turbulent fluxes
• Roughness length determines the effect of surface elements on wind flow
• Logarithmic wind profile typically observed in this layer

Mixed layer

• Occupies bulk of the daytime PBL, extending from surface layer to entrainment zone
• Well-mixed due to strong convective turbulence
• Potential temperature, specific humidity, and wind speed nearly constant with height
• Capped by a temperature inversion that limits vertical mixing
• Depth varies diurnally and spatially, typically 1-2 km in mid-latitudes

Entrainment zone

• Transition region between mixed layer and free atmosphere
• Characterized by strong gradients in temperature, humidity, and wind
• Site of entrainment processes where free atmosphere air mixes into PBL
• Thickness varies but typically 10-20% of the PBL depth
• Plays crucial role in PBL growth and evolution throughout the day

Diurnal cycle of PBL

• PBL structure undergoes significant changes over a 24-hour period
• Driven primarily by diurnal variations in solar heating and surface cooling
• Understanding diurnal cycle essential for accurate weather and air quality predictions

Daytime convective boundary layer

• Develops after sunrise as surface heating generates buoyant thermals
• Characterized by strong vertical mixing and turbulent eddies
• Grows in depth throughout the day, reaching maximum height in late afternoon
• Convective mixing results in nearly uniform potential temperature profile
• Cumulus clouds often form at the top of the convective boundary layer

Nocturnal stable boundary layer

• Forms after sunset as surface cools radiatively, creating temperature inversion
• Characterized by weak, sporadic turbulence and suppressed vertical mixing
• Typically shallow, ranging from tens to a few hundred meters in depth
• Wind speeds often increase with height, forming nocturnal low-level jet
• Fog and dew formation common in this layer due to radiative cooling

Residual layer

• Remnant of the daytime mixed layer that persists above nocturnal stable layer
• Neutrally stratified with weak turbulence and minimal vertical mixing
• Retains characteristics of the previous day's mixed layer (temperature, humidity)
• Can influence the development of the following day's convective boundary layer
• Often contains elevated pollution layers from previous day's emissions

Turbulence in PBL

• Dominant mechanism for transport and mixing within the planetary boundary layer
• Crucial for understanding and predicting weather patterns and air quality
• Characterized by irregular fluctuations in wind velocity, temperature, and humidity

Mechanical turbulence

• Generated by wind shear and surface roughness
• Dominant in neutral and stable atmospheric conditions
• Intensity increases with wind speed and surface roughness
• Creates eddies that mix air vertically and horizontally
• Important for dispersing pollutants near the surface in urban areas

Thermal turbulence

• Driven by buoyancy forces due to surface heating or cooling
• Dominates in unstable atmospheric conditions (daytime convective boundary layer)
• Creates larger-scale eddies that efficiently mix the entire boundary layer
• Responsible for formation of thermals and cumulus clouds
• Enhances vertical transport of heat, moisture, and pollutants

Turbulent kinetic energy

• Measure of the intensity of turbulence in the PBL
• Defined as the mean kinetic energy per unit mass associated with eddies
• TKE budget equation describes production, transport, and dissipation of turbulence
• Key parameter in many PBL parameterization schemes used in numerical models
• Can be measured directly using sonic anemometers or estimated from wind variance

PBL height determination

• Critical parameter for understanding atmospheric processes and air quality
• Defines the volume of air directly influenced by surface processes
• Varies diurnally, seasonally, and with different synoptic conditions

Methods of measurement

• Radiosonde profiles analyze vertical gradients of temperature, humidity, and wind
• Lidar systems detect aerosol gradients at the top of the PBL
• Sodar instruments use acoustic backscatter to identify the PBL top
• Radar wind profilers detect changes in refractive index at the PBL top
• Ceilometers measure cloud base height, often correlated with PBL height

Factors affecting PBL height

• Surface heat flux drives convective growth during the day
• Atmospheric stability influences the rate of PBL growth or decay
• Large-scale subsidence can suppress PBL growth
• Surface characteristics (albedo, roughness) affect energy balance and turbulence
• Synoptic conditions (high/low pressure systems) impact PBL development

Temperature profiles in PBL

• Reflect the balance between surface heating/cooling and atmospheric mixing
• Critical for understanding atmospheric stability and potential for convection
• Vary significantly between day and night and with different surface types

Potential temperature gradients

• Indicate atmospheric stability and mixing potential
• Negative gradient (decreasing with height) indicates unstable conditions
• Positive gradient (increasing with height) indicates stable conditions
• Near-zero gradient indicates neutral conditions typical of well-mixed layers
• Superadiabatic layer often observed near surface during strong daytime heating

Inversion layers

• Layers where temperature increases with height, suppressing vertical mixing
• Surface-based inversions form at night due to radiative cooling
• Elevated inversions often mark the top of the mixed layer
• Strength and depth of inversions affect pollutant dispersion and fog formation
• Persistent inversions can lead to air quality problems in urban areas

Wind profiles in PBL

• Reflect the balance between pressure gradient force, Coriolis force, and surface friction
• Critical for understanding transport of heat, moisture, and pollutants
• Vary significantly with height and atmospheric stability conditions

Ekman spiral

• Describes wind direction change with height due to balance of forces in PBL
• Wind veers (turns clockwise) with height in Northern Hemisphere
• Magnitude of turning typically 20-40 degrees through the PBL depth
• Results from decreasing influence of surface friction with height
• Geostrophic wind approximation valid above the PBL

Logarithmic wind profile

• Describes wind speed increase with height in the surface layer
• Based on Monin-Obukhov similarity theory for neutral conditions
• Wind speed proportional to natural log of height above surface
• Roughness length parameter characterizes surface drag effects
• Deviations from log profile occur in unstable or stable conditions

Moisture in PBL

• Plays crucial role in energy balance, cloud formation, and precipitation processes
• Distribution strongly influenced by surface evaporation and atmospheric mixing
• Understanding moisture dynamics essential for weather prediction and climate modeling

Water vapor distribution

• Typically decreases with height in the PBL due to surface moisture source
• Well-mixed in convective boundary layer during daytime
• Strong gradients often observed in nocturnal stable boundary layer
• Entrainment of dry air from free atmosphere affects PBL moisture content
• Horizontal advection can lead to significant moisture variability

Cloud formation processes

• Cumulus clouds often form at top of convective boundary layer
• Lifting condensation level determines cloud base height
• Entrainment of dry air can lead to cloud evaporation (cumulus humilis)
• Stratocumulus clouds common in marine boundary layers
• Fog formation in stable boundary layer due to radiative cooling or advection

PBL over different surfaces

• Surface characteristics strongly influence PBL structure and dynamics
• Understanding these variations crucial for accurate weather and climate predictions
• Different surface types lead to distinct energy balances and turbulence regimes

Land surface interactions

• Heterogeneous surface properties create complex PBL structures
• Soil moisture affects partitioning of surface energy into sensible and latent heat fluxes
• Vegetation influences surface roughness, albedo, and evapotranspiration
• Topography creates local circulations (mountain/valley breezes) affecting PBL
• Urban heat island effect modifies PBL structure over cities

Marine boundary layer

• Generally more uniform and stable than over land
• Strong influence of sea surface temperature on PBL structure
• Often capped by strong temperature inversion due to large-scale subsidence
• Stratocumulus clouds common, especially in eastern ocean basins
• Coastal zones experience complex interactions between marine and continental air masses

Urban boundary layer

• Modified by anthropogenic heat sources and urban surface properties
• Typically warmer and drier than surrounding rural areas (urban heat island)
• Increased surface roughness leads to stronger mechanical turbulence
• Air pollution often trapped within urban boundary layer
• Complex flow patterns around buildings affect local dispersion of pollutants

Atmospheric stability in PBL

• Determines the tendency for vertical motions and mixing in the atmosphere
• Crucial for understanding pollutant dispersion, cloud formation, and severe weather potential
• Varies diurnally and with different synoptic conditions

Static stability

• Based on vertical temperature profile compared to dry adiabatic lapse rate
• Unstable conditions promote vertical mixing (superadiabatic lapse rate)
• Stable conditions suppress vertical motions (temperature inversion)
• Neutral conditions neither enhance nor suppress vertical mixing
• Potential temperature used to account for pressure changes with height

Dynamic stability

• Considers both temperature profile and wind shear
• Richardson number quantifies ratio of buoyancy to shear production of turbulence
• Critical Richardson number determines onset of turbulence in stable conditions
• Low-level wind shear can destabilize otherwise stable layers
• Important for predicting formation and intensity of convective storms

Modeling the PBL

• Essential component of numerical weather prediction and climate models
• Challenges arise from small-scale processes occurring within the PBL
• Accurate representation of PBL processes crucial for model performance

Parameterization schemes

• Represent sub-grid scale processes that cannot be explicitly resolved
• K-theory approaches model turbulent fluxes using eddy diffusivity coefficients
• Non-local schemes account for large-eddy transport in convective conditions
• Mellor-Yamada scheme popular for operational weather forecasting models
• Pleim-Xiu scheme designed for air quality modeling applications

Numerical weather prediction

• PBL schemes crucial for accurate surface temperature and moisture forecasts
• Vertical resolution near surface important for capturing PBL structure
• Land surface models coupled with PBL schemes to represent surface-atmosphere interactions
• Data assimilation techniques incorporate observations to improve PBL representation
• Ensemble forecasting accounts for uncertainties in PBL processes

PBL and air quality

• PBL structure and dynamics strongly influence pollutant concentrations
• Understanding PBL processes crucial for effective air quality management
• Diurnal variations in PBL height impact pollution levels near the surface

Pollutant dispersion

• Turbulent mixing in PBL primary mechanism for dispersing pollutants
• Stable conditions (temperature inversions) trap pollutants near the surface
• Convective mixing during daytime generally improves air quality
• Horizontal wind speed and direction affect transport of pollutants
• Fumigation process brings elevated plumes to surface as PBL grows

Mixing height impact

• Defines volume of air available for pollutant dilution
• Low mixing heights lead to higher pollutant concentrations
• Diurnal variations in mixing height cause fluctuations in air quality
• Seasonal changes in PBL depth affect long-term air quality patterns
• Urban heat island effect can increase mixing height over cities

Climate change effects on PBL

• Alterations in PBL structure and dynamics expected with global warming
• Changes in PBL processes can feedback on larger-scale climate system
• Understanding these effects crucial for accurate climate projections

Temperature inversion changes

• Frequency and strength of inversions may change with warming climate
• Potential for more persistent inversions in some regions (Arctic)
• Changes in land-sea temperature contrasts may affect coastal inversions
• Altered inversion patterns impact air quality and fog formation
• Feedback effects on low-level cloud formation and radiation balance

Altered PBL dynamics

• Increased surface heating may lead to deeper convective boundary layers
• Changes in soil moisture affect partitioning of surface energy fluxes
• Potential for more intense but less frequent turbulent mixing
• Shifts in global circulation patterns may affect large-scale subsidence
• Changes in PBL depth and structure impact cloud formation and precipitation patterns