ATMOS

• Boundary layer


  Also called "Atmospheric Boundary layer" or "Planetary Boundary Layer", it is the tropospheric layer that is directly influenced by the presence of Earth's surface and responds to surface forcings in an hour or less

  
• Boundary layer


  • Typically 1 km deep during the day and ~100 m deep during the night
  • Above the boundary layer is the free atmosphere

  
• Types of atmospheric boundary layer

  • Convective boundary layer (CBL)
  • Stable boundary layer (SBL)
  • Residual layer (RL)

  
• Convective boundary layer (CBL)


  A BL that is dominated by buoyant turbulence generation and usually forms in the daytime

  
• Stable boundary layer (SBL)

  Forms at nighttime or when warm air moves over a colder surface

  
• Residual layer (RL)


  May occur during the morning or evening when the previous CBL is disconnected from the surface by the SBL
• Diurnal cycle of atmospheric boundary layer

  1. Mixed layer stirred by solar heating and convection

  2. Convective stirring takes 10-20 minutes to go from bottom to top

  3. Entrainment zone where clouds form

  
• Diurnal evolution of atmospheric boundary layer

  1. During sunset, no mixing within convective boundary layer, stays above lower nighttime stable boundary layer (residual layer)

  2. Some mixing still occur during nighttime at the nocturnal boundary layer

  3. Increase of nocturnal layer due to strong shear caused by accelerated wind at residual layer

  4. In the morning, convection and mixing start again

  
• Virtual potential temperature (θV)

  • Temperature of a parcel at a specific pressure level and virtual temperature would have if it were lowered or raised to 1000 mb
  • Important concept in atmospheric boundary meteorology, serves as a stability criterion

  
• • When θV is constant, the atmosphere is statically neutral
  • When it decreases with elevation, the atmosphere is statically unstable
  • When it increases with elevation, the atmosphere is statically stable

  
• Diurnal evolution of atmospheric boundary layer as told by virtual potential temperature profiles

  1. S1 - late morning

  2. S2 - just after sunset

  3. S3 - just before sunrise

  4. S4 - just after sunrise

  5. S5 - mid-morning

  6. S6 - late afternoon

  
• • During morning rush hour, vehicle emissions mix into a shallow boundary layer leading to high pollutant levels
  • During evening rush hour, emissions mix into a larger boundary layer volume so pollutant levels are less severe
• Diurnal variation of planetary boundary layer height influences our daily lives and health as we live, work, and breathe mostly in this layer

  
• Virtual potential temperature


  Found by replacing temperature in the formula for virtual temperature with the potential temperature
• Cumulative heating or cooling (QA)

  The area under the curve of heat flux vs time, more important than instantaneous heat flux
• ABL structure and evolution on temperature

  1. Cumulative heating during daytime

  2. Cumulative cooling during nighttime

  
• Geostrophic wind (G)


  Theoretical equilibrium wind speed for frictionless conditions
• Steady-state winds in the ABL are usually slower than geostrophic (subgeostrophic) due to frictional and turbulent drag against the surface
• Wind profile evolution in the ABL

  1. Shallow mixed layer in the morning

  2. Deeper mixed layer with subgeostrophic winds in the afternoon

  3. Nocturnal jet of supergeostrophic winds at night

  4. Logarithmic wind profile in the surface layer

  5. Linear wind profile in the stable surface layer

  6. Exponential power-law wind profile in the convective radix layer

  
• Drag


  Frictional force between two objects
• Stress


  Force per unit area
• Friction velocity


  Velocity scale related to surface drag
• Roughness length


  Measure of surface roughness
• Surface layer


  • In the bottom 5% of the statically neutral ABL, wind speeds increase roughly logarithmically with height

  
• Statically stable surface layer

  • Logarithmic profile changes to a more linear form
• Winds close to the ground

  • Become slower than logarithmic, or near calm
• Winds just above the surface layer

  • Often not in steady state, and can temporarily increase to faster than geostrophic (supergeostrophic) in a process called an inertial oscillation
• Radix layer (RxL)


  • The bottom 20% of the convective (unstable) ABL
• Winds in the RxL

  • Have an exponential power-law relationship with height
• RxL


  • Faster winds near the surface, but slower winds aloft than the neutral logarithmic profile
• Stress (τ)


  The amount of friction force per unit surface contact area, acts parallel to the surface
• Pressure


  Force per unit area that is perpendicular to the surface
• Stress is felt by both objects that are sliding against each other
• Mean wind


  Relatively constant, but varying slowly over the course of hours
• Waves


  Regular (linear) oscillations of wind, often with periods of ten or longer
• Turbulence


  Irregular, quasi-random, non-linear variations or gusts, with duration of seconds to minutes
• U(t)


  1. direction component of wind at some instant in time, t
• ഥ��


  Mean wind, where the overbar denotes an average
• u'


  Turbulence or gust part, obtained by subtracting the mean wind from the instantaneous wind
• Averaging time


  Typically about 0.5 h