1. Effects of different surface types and human activities

2. Review of last lecture Reynolds averaging: Separation of mean and turbulent components u = U + u’, = 0 Intensity of turbulence: turbulent kinetic energy (TKE) Eddy fluxes Fx = -  /z The turbulent closure problem: Number of unknowns > Number of equations Surface layer: related to gradient Mixed layer: Local theories (K-theory): = - 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

3. Effects 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

4. Effects of vegetation Makes water/heat reservoir deeper (transport deep water out of soil) Enhances evaporation (leafs increase evaporation area) Dependent on vegetation type

5. Global distribution of different vegetation types (From Bonon et al. 2002)

6. Vegetation feedback Vegetation in turn is affected by environmental conditions (e.g. seasons, droughts, global warming)

7. 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

8. Local effect I: The heat island effect Nighttime: City warmer than surrounding rural area Daytime: City has same air temperature as rural area

9. 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

10. Mitigation of heat island effect Greening the city (streets and roof top) Change construction materials Reduce anthropogenic heat sources

11. Local effect II: Dispersion of air pollution Dispersion depends on stability of BL UnstableStableNeutral

12. Dispersion also affected by inversions Low level inversionUpper level inversion

13. Global effect: Anything released by human beings will be transported globally by atmospheric circulations and ocean circulations

14. The Global Carbon Cycle

15. 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.