The Heat Island EffectThe Heat Island Effect

Temperature distribution for Atlanta (from EPA)

Review of last lectureReview of last lecture

• General circulation models: Grid size. 3 usages. Name of the basic set of equations.

• 4 components of the climate system model.

• Mesoscale models: grid size. 3 characteristics.

How do human activities change the How do human activities change the global climate?global climate?

Human beings are changing the global climate system in three different ways:

• Change land cover (deforestation and afforestation)

• Release or cleanse pollutants (aerosols)• Release or cleanse greenhouse gases

Change of land coverChange of land cover

DesertificationDesertification

• Caused mainly be human activities and climate change

• Is one of the most significant global environmental problems

• About a billion people are under threat

Global desertification vulnerabilityGlobal desertification vulnerability

Change of PSDI in the last 100 yearsChange of PSDI in the last 100 years

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 different surface typesEffects of different surface types

Deeper heat reservoir (smaller T change) Deeper water reservoir (Wetter surface) Enhanced latent heat flux

Convective instability increases

Video: Urban heat island effect (by Video: Urban heat island effect (by Weather Channel)Weather Channel)

• http://www.youtube.com/watch?v=t-sXHl3l-rM

The heat island effectThe heat island effect

• Nighttime: City warmer than surrounding rural area• Daytime: City has same air temperature as rural area

Temperature distribution for AtlantaTemperature distribution for Atlanta

(from EPA)

Dependence of heat island effect on Dependence of heat island effect on populationpopulation

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• Decreased 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

Detection of Climate Change?Detection of Climate Change?

• We must contend with the problem of enhanced urban heat islands influencing the data, which means that records from large urban areas are not representative of the surrounding region.

• Atmospheric scientists are well aware of this source of bias in temperature records—which happens to be relatively small— and routinely account for its effect, either by discarding contaminated data or by adjusting values downward for affected stations.

SummarySummary

• Effects of vegetation: (1) makes heat/water reservoir deeper, (2) enhance evaporation, (3) grows and dies in response to environmental conditions

• Effects of different surface types: desert, city, grassland, forest, sea. Deeper heat/water reservoir and enhanced convective instability.

• Heat island effect. 7 causes