4. Atmospheric transport. Daniel J. Jacob, Atmospheric Chemistry, Harvard University, Spring 2017

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1 4. Atmospheric transport Daniel J. Jacob, Atmospheric Chemistry, Harvard University, Spring 2017

2 Forces in the atmosphere: Gravity g Pressure-gradient ap = ( 1/ ρ ) dp / dx for x-direction (also y, z directions) Coriolis ac = 2ωvsin λ to R of direction of motion (NH) or L (SH) Friction a f = kv Angular velocity ω = 2π/24h Wind speed v Latitude λ Friction coefficient Equilibrium of forces: k In vertical: barometric law In horizontal: geostrophic flow parallel to isobars In horizontal, near surface: flow tilted to region of low pressure a p a c v P P + P a p a f ac v P P + P

3 GEOSTROPHIC FLOW: equilibrium between pressure-gradient and Coriolis forces Isobar a p a c steady parallel to isobars speed ~ pressure gradient a p a p a c N hemisphere example

4 air rises precipitation air sinks dry weather Surface Low Surface High Link to current weather map

5 Satellite in geostationary orbit

6 Cyclone tracks,

7 THE HADLEY CIRCULATION (1735): global sea breeze COLD Trade winds HOT COLD Explains: Intertropical Convergence Zone (ITCZ) Wet tropics, dry poles Easterly trade winds in the tropics But Meridional transport of air between Equator and poles results in strong winds in the longitudinal direction because of conservation of angular momentum; this results eventually in unstable conditions.

8 TROPICAL HADLEY CELL Easterly trade winds in the tropics at low altitudes Subtropical anticyclones at about 30 o latitude Westerlies at mid-latitudes

9 CLIMATOLOGICAL SURFACE WINDS AND PRESSURES (January) Current global satellite maps

10 CLIMATOLOGICAL SURFACE WINDS AND PRESSURES (July)

11 TIME SCALES FOR HORIZONTAL TRANSPORT (TROPOSPHERE) 1-2 months 2 weeks 1-2 months 1 year

12 VERTICAL TRANSPORT: BUOYANCY Consider an object (density ρ) immersed in a fluid (density ρ ): Fluid (ρ ) a p Object (ρ) g z+ z Buoyancy acceleration (upward) : a b=ap -g z p(z) > p(z+δz) pressure-gradient force on object directed upward ρ ρ = ρ g For air, M p a ρ= so ρ as T RT Barometric law assumes T = T a b = 0 (zero buoyancy) T T produces buoyant acceleration upward or downward

13 ATMOSPHERIC LAPSE RATE AND STABILITY Lapse rate = -dt/dz z stable unstable inversion unstable Γ = 9.8 K km -1 z Consider an air parcel at z lifted to z+dz and released. It cools upon lifting (expansion). Assuming lifting to be adiabatic, the cooling follows the adiabatic lapse rate Γ : ATM (observed) T g Γ= dt / dz = = 9.8 K km C p What happens following release depends on the local lapse rate dt ATM /dz: -dt ATM /dz > Γ upward buoyancy amplifies initial perturbation: atmosphere is unstable -dt ATM /dz = Γ zero buoyancy does not alter perturbation: atmosphere is neutral -dt ATM /dz < Γ downward buoyancy relaxes initial perturbation: atmosphere is stable dt ATM /dz > 0 ( inversion ): very stable -1 The stability of the atmosphere against vertical mixing is solely determined by its lapse rate.

14 WHAT DETERMINES THE LAPSE RATE OF THE ATMOSPHERE? An atmosphere left to evolve adiabatically from an initial state would eventually tend to neutral conditions (-dt/dz = Γ ) at equilibrium Solar heating of surface and radiative cooling from the atmosphere disrupts that equilibrium and produces an unstable atmosphere: z z z ATM Γ T Γ ATM T initial final Γ T Initial equilibrium state: - dt/dz = Γ Solar heating of surface/radiative cooling of air: unstable atmosphere buoyant motions relax unstable atmosphere back towards dt/dz = Γ Fast vertical mixing in an unstable atmosphere maintains the lapse rate to Γ. Observation of -dt/dz = Γ is sure indicator of an unstable atmosphere.

15 IN CLOUDY AIR PARCEL, HEAT RELEASE FROM H 2 O CONDENSATION MODIFIES Γ Wet adiabatic lapse rate Γ W = 2-7 K km -1 z T RH Latent heat release as H 2 O condenses Γ W = 2-7 K km -1 Γ W 100% RH > 100%: Cloud forms Γ = 9.8 K km -1 Γ

16 4 3 Altitude, km 2 cloud planetary boundary layer (PBL) Temperature, o C

17 SUBSIDENCE INVERSION typically 2 km altitude

18 DIURNAL CYCLE OF SURFACE HEATING/COOLING: ventilation of urban pollution z Planetary Boundary Layer (PBL) depth Subsidence inversion MIDDAY 1 km Γ Mixing depth NIGHT 0 MORNING T NIGHT MORNING AFTERNOON

19 VERTICAL PROFILE OF TEMPERATURE Mean values for 30 o N, March Radiative cooling (ch.7) - 3 K km -1 Altitude, km +2 K km -1 Radiative heating: O 3 + hν O 2 + O O + O 2 + M O 3 +M Radiative cooling (ch.7) K km -1 Latent heat release Surface heating heat

20 TYPICAL TIME SCALES FOR VERTICAL MIXING tropopause (10 km) 1 month 10 years planetary 2 km boundary layer 0 km 1 day