Week 6 - boundary layer, land-sea breeze, urban meterology, air pollution

Planetary boundary layer (PBL)

• Where we live, where we experience weather

• The transition zone between the Earth and the free atmosphere, and often has unique characteristics

• Different aerosol concentration in and above Boundary layer

• PBL is lowest layer of the atmosphere, influenced by interactions with the Earth’s surface

• Eddies, turbulence and ‘mixing’

• “Laminar flow” above PBL

• Friction is why it’s usually less windy at the surface

Laminar flow

Laminar flow is smooth, non-turbulent air movement, usually in stable layers of the atmosphere

Energy balance

When the surface is wet, it takes energy to evaporate, cooling the air and reducing the boundary layer height

PBL: Roughness Length

Wind is stronger at higher altitudes

z(0) = 0.1 x height of sfc elements

Hot surface

Unstable PBL

Cold surface

Stable PBL

Rising air

More mixing

Wind-shear

Difference in wind speed and/or direction with height

Less vertical wind shear

Less vertical wind shear with an unstable PBL (warm surface), as rising air extends the boundary layer higher, and rising air ‘mixes’ with some of the air in the free atmosphere above.

The result is more uniform winds in the PBL and lower free atmosphere.

More vertical wind shear

More vertical wind shear with a stable PBL (cool surface), as near-surface winds in the PBL are “decoupled” from winds above in the free atmosphere.

Stability suppresses vertical ‘mixing’, so there’s a greater difference in winds between the boundary layer and free atmosphere.. so more wind shear

Planetary boundary layer: surface layer

Only refers to the lowest part of troposphere

Layer in direct contact with the surface

Interfacial layer → lowest few cm is - where molecular transport more effective.

Turbulent transport → dominant

Lapse rate is normally super-adiabatic

Planetary boundary layer: mixed layer

Layer above the surface layer

• Uniform mixing of heat, moisture and momentum

• Begins to develop around half an hour after sunrise and grows deeper into the afternoon

• Warmer, drier air from the free atmosphere above can be entrained into the mixed layer

Birds fly in this layer!

Entrained

“Dragged in and mixed from outside”

• Air from outside a turbulent region is drawn into it and mixed in

• The mixed layer is the turbulent part of the boundary near the boundary layer near the surface

• The free atmosphere above is more stable and less turbulent

• Sometimes, turbulent eddies at the top of the mixed layer pull down (entrain) warmer, drier air from above

• This air mixes with the cooler, moisture air below, changing temperature and humidity in the boundary layer

Planetary boundary layer: Entrainment later

• Stable layer above the mixed layer

• Acts as a lid, or ‘cap’ to the rising thermals

• Usually an inversion layer

• Warmer, drier air from the free atmosphere above can be entrained into the mixed layer below

Planetary boundary layer: nocturnal and residual layer

Nocturnal layer develops just above the surface, grows deeper into the night as more air is progressively cooled

• Layer is stable

• Inversion layer

Residual layer above is what remains of the daytime mixed layer

Seasonal cycle

• Seasonal variations in the depth of the boundary layer

• Depth of boundary layer is usually greater in summer, and less in winter

Morning - Diurnal cycle of boundary layer

Morning (sunrise to mid-morning)

• surface starts to heat → warm air begins to rise

• turbulence increases → boundary layer grows rapidly

• transition from stable to unstable conditions

Daytime - Diurnal cycle of boundary layer

Daytime (late morning to afternoon)

• surface heating is strongest → strong convection turbulence

• fully developed convective mixed layer

• boundary layer reaches maximum depth (~1-3km depending on conditions

Evening - Diurnal cycle of boundary layer

• Surface cools → turbulence weakens

• Convection mixing ceases

• A stable layer forms near the ground

• The rest of the mixed layer becomes a residual layer

Nighttime - Diurnal cycle of boundary layer

• Stable boundary layer dominates near the surface

• Little vertical mixing → pollutants and moisture can accumulate

• Winds slow near the surface due to friction and stable stratification

• Radiative cooling of the surface continues throughout the night

Land-sea breeze

Hot days when the sea is cool

• Air just above land heats faster than water

• Air above land surface warms, volume expands and density decreases

• Air pressure at surface will decrease

• Warm less dense air will rise

• PGF pushes air from high to low pressure

Land breeze

Land is cooler, sea is warmer

• Air just above land cools faster than water

• As the air just above the land surface cools, its volume contracts and the density increases

• Air pressure at the surface will increase

• The cool, denser air will sink

• PGF pushes air from high to low pressure

Sea breeze convergence

• Sea breeze converges with the prevailing wind over land, air will rise → leads to cloud and precipitation

• Sea breeze convergence is a major driver of showers and thunderstorms

• Common near the coast in all parts of the world

• Noticeable over peninsulas and islands in the tropics

Anthropogenic heat release

Heat derived from and radiated by industry, domestic vehicles and air conditioning exhaust, primarily in cities

‘Fabric’ of a city

Non-reflective building materials absorb much of the incident solar radiation, retaining as heat, then emitting back into the air

Lack of vegetation

Less evapotranspiration, and less release of water vapour

Urban street ‘canyons’

Trapping of heat energy, reduced sky view factor

Urban spaces

artificial → characteristics make them warmer than more naturally green spaces

Urban wind

More friction, wind speed slower than those in rural areas

Channelling between buildings - wind tunnel

Sky view factor (SVF)

Ground heats air above

More sky is visible, the more long-wave radiation emitted

Cooler vicinity (esp at night)

Direct effect of aerosols

Short-wave solar radiation absorbed lowering amounts received at surface

Reduces urban heat island intensity during the day

Indirect effects of aerosols

Act as cloud condensation nuclei (CCN)

More CCN results in longer-lasting clouds, but less precipitation

Air pollution sources

72% of carbon monoxide CO emissions

70% of nitrogen oxides NOx emissions

28% of volatile organic compounds VOC emissions

31% of emissions of particles smaller than 2.5 microns

27% of emissions of particles smaller than 10 microns

6% of all sulfur dioxide SO2

**NOx and VOC combine to create O3

Air pollution contributing factors

• Multiple sources of pollution

• Stationary high-pressure systems

• Light surface winds

• Subsidence temperature inversion

• Shallow mixing layer (in the boundary layer)

• Valleys

• Clear nights

• Smog

Air pollution: Topography

Pollution concentrations in valleys tends to be highest during colder months

At night, cold air ‘drains’ downhill and ‘pools’ in low-lying valleys