Effects of different surface types Effects of different surface types and human activitiesand human activities

Review of last lectureReview of last lecture• Reynolds averaging: Separation of mean and turbulent

components u = U + u’, < u’ > = 0• Intensity of turbulence: turbulent kinetic energy (TKE)

• 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

TKE = ‹ u’ 2 + v’ 2 + w’ 2 › /2

Effects of different surface typesEffects of different surface types

Deeper heat reservoir (smaller T change) Deeper water reservoir (Wetter surface) Bowen ratio decreases (More LH contribution)

BL depth decreases Convective instability increases

Effects of vegetationEffects 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 Global distribution of different vegetation typesvegetation types

(From Bonon et al. 2002)

Vegetation feedbackVegetation feedbackVegetation in turn is affected by environmental Vegetation in turn is affected by environmental

conditions (e.g. seasons, droughts, global warming)conditions (e.g. seasons, droughts, global warming)

Effects of human activitiesEffects of human activitiesHuman 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 effectLocal 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 effectCauses 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 effectMitigation of heat island effect

• Greening the city (streets and roof top)

• Change construction materials

• Reduce anthropogenic heat sources

Local effect II: Dispersion of air pollutionLocal effect II: Dispersion of air pollution

• Dispersion depends on stability of BL

Unstable StableNeutral

• Dispersion also affected by inversions

Low level inversion Upper level inversion

Global effect: Global effect: Anything released by human beings will be Anything released by human beings will be

transported globally by atmospheric circulations transported globally by atmospheric circulations and ocean circulationsand ocean circulations

The Global Carbon CycleThe Global Carbon Cycle

SummarySummary• 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.