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CHAPTER 4: ATMOSPHERIC TRANSPORT

CHAPTER 4: ATMOSPHERIC TRANSPORT. Forces in the atmosphere: Gravity Pressure-gradient Coriolis Friction. to R of direction of motion (NH) or L (SH). Equilibrium of forces:. In vertical: barometric law In horizontal: geostrophic flow parallel to isobars. g p. P. v. P + D P.

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CHAPTER 4: ATMOSPHERIC TRANSPORT

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  1. CHAPTER 4: ATMOSPHERIC TRANSPORT Forces in the atmosphere: • Gravity • Pressure-gradient • Coriolis • Friction to R of direction of motion (NH) or L (SH) Equilibrium of forces: In vertical: barometric law In horizontal: geostrophic flow parallel to isobars gp P v P + DP gc In horizontal, near surface: flow tilted to region of low pressure gp P v gf P + DP gc

  2. Air converges near the surface in low pressure centers, due to the modification of geostrophic flow under the influence of friction. Air diverges from high pressure centers. At altitude, the flows are reversed: divergence and convergence are associated with lows and highs respectively

  3. HOT COLD COLD THE HADLEY CIRCULATION (1735): global sea breeze 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.

  4. TROPICAL HADLEY CELL • Easterly “trade winds” in the tropics at low altitudes • Subtropical anticyclones at about 30o latitude

  5. CLIMATOLOGICAL SURFACE WINDS AND PRESSURES(January)

  6. TODAY’S GLOBAL CLOUD MAP Bright colors indicate the tallest clouds Intellicast.com

  7. CLIMATOLOGICAL SURFACE WINDS AND PRESSURES(July) where I’ll be this summer

  8. CLIMATOLOGICAL SURFACE WINDS AND PRESSURES(January)

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

  10. VERTICAL TRANSPORT: BUOYANCY Consider an object (density ρ) immersed in a fluid (density ρ’): z+Dz Object (r) Fluid (r’) P(z) > P(z+Δz) epressure-gradient force on object directed upward z g Buoyancy acceleration (upward) : so ρ↑ as T↓ For air, Barometric law assumes T = T’ e gb = 0(zero buoyancy) T T’ produces buoyant acceleration upward or downward

  11. “Lapse rate” = -dT/dz ATMOSPHERIC LAPSE RATE AND STABILITY 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 rateG : z G = 9.8 K km-1 stable z unstable What happens following release depends on the local lapse rate –dTATM/dz: • -dTATM/dz > Ge upward buoyancy amplifies initial perturbation: atmosphere is unstable • -dTATM/dz = Ge zero buoyancy does not alter perturbation: atmosphere is neutral • -dTATM/dz < Ge downward buoyancy relaxes initial perturbation: atmosphere is stable • dTATM/dz > 0 (“inversion”): very stable ATM (observed) inversion unstable T The stability of the atmosphere against vertical mixing is solely determined by its lapse rate.

  12. TEMPERATURE SOUNDING AT CHATHAM, MAToday (Feb 14, 2013) at 12Z (7 am) Dew point Temperature Aadiabatic lapse rate weather.unisys.com

  13. 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 = G ) at equilibrium • Solar heating of surface and radiative cooling from the atmosphere disrupts that equilibrium and produces an unstable atmosphere: z z z final G ATM G ATM initial G T T T Initial equilibrium state: - dT/dz = G Solar heating of surface/radiative cooling of air: unstable atmosphere buoyant motions relax unstable atmosphere back towards –dT/dz = G • Fast vertical mixing in an unstable atmosphere maintains the lapse rate to G. Observation of -dT/dz = Gis sure indicator of an unstable atmosphere.

  14. IN CLOUDY AIR PARCEL, HEAT RELEASE FROM H2O CONDENSATION MODIFIES G Wet adiabatic lapse rate GW = 2-7 K km-1 z T RH 100% GW “Latent” heat release as H2O condenses GW = 2-7 K km-1 RH > 100%: Cloud forms G G = 9.8 K km-1

  15. 4 3 Altitude, km 2 1 0 -20 -10 0 10 20 30 Temperature, oC cloud boundary layer

  16. SUBSIDENCE INVERSION typically 2 km altitude

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

  18. VERTICAL PROFILE OF TEMPERATUREMean values for 30oN, March Radiative cooling (ch.7) - 3 K km-1 Altitude, km +2 K km-1 Radiative heating: O3 + hn →O2 + O O + O2 + M →O3+M heat Radiative cooling (ch.7) Latent heat release - 6.5 K km-1 Surface heating

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

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