Transcript Understanding Weather and Climate Ch 5

Effects of different surface types
and human activities
Review of last lecture
• Reynolds averaging: Separation of mean and turbulent
components u = U + u’, < u’ > = 0
• Intensity of turbulence: turbulent kinetic energy (TKE)
TKE = ‹ u’ 2 + v’ 2 + w’ 2 › /2
• Eddy fluxes
Fx = - <u’w’>/z
• The turbulent closure problem: Number of unknowns >
Number of equations
• Surface layer: related to gradient
• Mixed layer:
Local theories (K-theory): < w’a’ >= - Ka dA/dz
always
down-gradient
Non-local theories: organized eddies filling the entire BL,
could be counter-gradient
Effects of different surface types
BL depth decreases
Convective instability increases
Deeper heat reservoir (smaller T change)
Deeper water reservoir (Wetter surface)
Bowen ratio decreases (More LH contribution)
Effects of vegetation
• Makes water/heat reservoir deeper (transport deep water
out of soil)
• Enhances evaporation (leafs increase evaporation area)
• Dependent on vegetation type
Global distribution of different
vegetation types
(From Bonon et al. 2002)
Vegetation feedback
Vegetation in turn is affected by environmental
conditions (e.g. seasons, droughts, global warming)
Effects of human activities
Human beings are living in the BL and affect the BL in three
different ways:
• Change land cover (deforestation and afforestation)
• Release or cleanse pollutants (aerosols)
• Release or cleanse greenhouse gases
Local effect I: The heat island effect
• Nighttime: City warmer than surrounding rural area
• Daytime: City has same air temperature as rural area
Causes of the heat island effect
• Increased SW absorption caused
by canyon geometry (increased
area and multiple reflection)
• Decreased LW loss caused by
canyon geometry
• Increased greenhouse effect
caused by air pollution
• Anthropogenic heat source
• Increased sensible heat storage
caused by construction materials
• Decreased latent heat flux caused
by change of surface type
• Decrease sensible and latent heat
fluxes caused by canyon geometry
(reduction of wind speed)
“Canyons” between buildings
Mitigation of heat island effect
• Greening the city
(streets and roof top)
• Change construction
materials
• Reduce anthropogenic
heat sources
Local effect II: Dispersion of air pollution
• Dispersion depends on stability of BL
Unstable
Neutral
Stable
• Dispersion also affected by inversions
Low level inversion
Upper level inversion
Global effect:
Anything released by human beings will be
transported globally by atmospheric circulations
and ocean circulations
The Global Carbon Cycle
Summary
• Effects of different surface types: desert, city, grassland,
forest, sea. Deeper heat/water reservoir, decreased
Bowen ratio, thinner BL and enhanced convective
instability.
• Effects of vegetation: (1) makes heat/water reservoir
deeper, (2) enhance evaporation, (3) grows and dies in
response to environmental conditions
• Heat island effect. 7 causes
• Dispersion of air pollution. Dependence on stability
(name of 3 types) and inversion (name of 2 types)
• Global carbon cycle: linking the world together.
Therefore we need to protect the environment.