THE HADLEY CIRCULATION (1735): global sea breeze HOT COLD Explains: Intertropical Convergence Zone (ITCZ) Wet tropics, dry poles Problem: does not account for Coriolis force. Meridional transport of air between Equator and poles would result in unstable longitudinal motion.
GLOBAL CLOUD AND PRECIPITATION MAP 20 Feb (intellicast.com)
TROPICAL HADLEY CELL Easterly “trade winds” in the tropics at low altitudes Subtropical anticyclones at about 30 o latitude
CLIMATOLOGICAL SURFACE WINDS AND PRESSURES (January)
CLIMATOLOGICAL SURFACE WINDS AND PRESSURES (July)
TIME SCALES FOR HORIZONTAL TRANSPORT (TROPOSPHERE) 2 weeks 1-2 months 1 year
QUESTIONS 1. Is the general atmospheric circulation stronger (i.e., are the winds faster) in the winter or in the summer hemisphere? 2. Is pollution from North America more likely to affect Hawaii in winter or in summer? 3. Concentrations of CO 2, krypton-85, and other gases emitted mainly in the northern hemisphere DECREASE with altitude in the northern hemisphere but INCREASE with altitude in the southern hemisphere. Explain.
VERTICAL TRANSPORT: BUOYANCY What is buoyancy? Object ( z z+ z Fluid ( ’) Balance of forces: Note: Barometric law assumed a neutrally buoyant atmosphere with T = T’ T T’ would produce bouyant acceleration
ATMOSPHERIC LAPSE RATE AND STABILITY T z = 9.8 K km -1 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 : z “Lapse rate” = -dT/dz ATM (observed) What happens following release depends on the local lapse rate –dT ATM /dz: -dT ATM /dz > upward buoyancy amplifies initial perturbation: atmosphere is unstable -dT ATM /dz = zero buoyancy does not alter perturbation: atmosphere is neutral -dT ATM /dz < downward buoyancy relaxes initial perturbation: atmosphere is stable dT ATM /dz > 0 (“inversion”): very stable unstable inversion unstable stable The stability of the atmosphere against vertical mixing is solely determined by its lapse rate
EFFECT OF STABILITY ON VERTICAL STRUCTURE
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 = at equilibrium Solar heating of surface disrupts that equilibrium and produces an unstable atmosphere: Initial equilibrium state: - dT/dz = z T z T Solar heating of surface: unstable atmosphere ATM ATM z T initial final buoyant motions relax unstable atmosphere to –dT/dz = Fast vertical mixing in an unstable atmosphere maintains the lapse rate to Observation of -dT/dz = is sure indicator of an unstable atmosphere.
IN CLOUDY AIR PARCEL, HEAT RELEASE FROM H 2 O CONDENSATION MODIFIES RH > 100%: Cloud forms “Latent” heat release as H 2 O condenses 9.8 K km -1 W 2-7 K km -1 RH 100% T z WW Wet adiabatic lapse rate W = 2-7 K km -1
VERTICAL PROFILE OF TEMPERATURE Mean values for 30 o N, March Altitude, km Surface heating Latent heat release Radiative cooling (ch.7) K km -1 2 K km K km -1 Radiative cooling (ch.7) Radiative heating: O 3 + h O 2 + O O + O 2 + M O 3 +M heat
SUBSIDENCE INVERSION typically 2 km altitude
QUESTIONS 1. An atmosphere with W < -dT/dz < - is called "conditionally unstable" ( W is the wet adiabatic lapse rate). Why? 2. Kuwaiti oil fires during the Persian Gulf war produced large clouds of soot a few km above the Earth's surface. Soot absorbs solar radiation. How would you effect such clouds to affect atmospheric stability? 3. Vertical profiles of concentrations of species emitted at the surface often show a "C-shape", particularly in the tropics, with high concentrations in the lower and upper troposphere and low concentrations in the middle troposphere. How would you explain such a profile?
DIURNAL CYCLE OF SURFACE HEATING/COOLING: z T 0 1 km MIDDAY NIGHT MORNING Mixing depth Subsidence inversion NIGHTMORNINGAFTERNOON
FRONTS WARM FRONT: WARM AIRCOLD AIR WIND Front boundary; inversion COLD FRONT: COLD AIR WARM AIR WIND inversion
TYPICAL TIME SCALES FOR VERTICAL MIXING Estimate time t to travel z by turbulent diffusion: 0 km 2 km 1 day “planetary boundary layer” tropopause 5 km (10 km) 1 week 1 month 10 years