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Credit: udiprod, https://youtu.be/49JwbrXcPjc CORIOLIS FORCE

GEOSTROPHIC FLOWS AROUND HIGH & LOW PRESSURE HIGH LOW Rule of thumb : keep high pressure on the right low pressure on the left in NH

CONTINENTAL U.S. SURFACE ANALYSIS Cold front Warm front Stat ionary front

GLOBAL CLOUD AND PRECIPITATION MAP Bright colors indicate high cloud tops (low temperatures) shows Intertropical Convergence Zone (ITCZ) as longitudinal band near Equator intellicast.com

Hurricane Sandy, Oct. 18–28, 2012 (NASA) Q: The circular air motion of cyclones is a consequence of the fictitious Coriolis force in our observation reference frame on a rotating sphere. Yet we are familiar with satellite images of hurricanes (tropical cyclones). Why does the circular motion persist in the satellite reference frame? Satellites in geostationary orbit Credit: Lookang, wikipedia

ATMOSPHERIC CIRCULATION Ferrel cell Credit: NASA

SANTA ANA WINDS an example of compression heating Air from the high desert Dry and hot air Compression heating according to dry adiabatic lapse rate 10°C 2 km 20°C 30°C

… I recall being told, when I first moved to Los Angeles and was living on an isolated beach, that the Indians would throw themselves into the sea when the bad wind blew. I could see why. The Pacific turned ominously glossy during a Santa Ana period , and one woke in the night troubled not only by the peacocks screaming in the olive trees but by the eerie absence of surf. The heat was surreal . The sky had a yellow cast , the kind of light sometimes called "earthquake weather." My only neighbor would not come out of her house for days, and there were no lights at night, and her husband roamed the place with a machete . One day he would tell me that he had heard a trespasser, the next a rattlesnake. "On nights like that," Raymond Chandler once wrote about the Santa Ana, " every booze party ends in a fight. Meek little wives feel the edge of the carving knife and study their husbands' necks. Anything can happen ." That was the kind of wind it was. I did not know then that there was any basis for the effect it had on all of us, but it turns out to be another of those cases in which science bears out folk wisdom. The Santa Ana, which is named for one of the canyons it rushes through, is foehn wind, like the foehn of Austria and Switzerland and the khamsin of Israel. There are a number of persistent malevolent winds , perhaps the best know of which are the mistral of France and the Mediterranean sirocco , but a foehn wind has distinct characteristics : it occurs on the leeward slope of a mountain range and, although the air begins as a cold mass, it is warmed as it comes down the mountain and appears finally as a hot dry wind . Whenever and wherever foehn blows, doctors hear about headaches and nausea and allergies, about "nervousness," about "depression. " In Los Angeles some teachers do not attempt to conduct formal classes during a Santa Ana, because the children become unmanageable . In Switzerland the suicide rate goes up during the foehn, and in the courts of some Swiss cantons the wind is considered a mitigating circumstance for crime. Surgeons are said to watch the wind, because blood does not clot normally during a foehn. … Joan Didion on the Santa Anas

VERTICAL TRANSPORT: BUOYANCY Object ( - ) z z + Δz Fluid ( -′ ) Barometric law assumes ’ = ’′ e * ! = 0 (zero buoyancy) ’ ≠ ’′ produces buoyant acceleration upward or downward Consider an object (density - , volume " ) immersed in a fluid (density -′ ): Archimedes principle : Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object. P (z) > P (z + Δz) e pressure-gradient force on object directed upward Buoyancy results from pressure diff. Buoyancy acceleration (upward) : ! ' = ! # − / = - ( 0/ − -0/ -0 = - ( − - - / For air, so - ↑ as T ↓ - = . # / 0’ (Archimedes principle) The PGF is - ( 0/ 2 ) 3 Net force is PGF – gravity: - ( 0/ − -0/ and $ ! = & " "’ &" = & " & ’

ATMOSPHERIC LAPSE RATE AND STABILITY Γ = 9.8 K km -1 z “ Lapse rate ” = − !" !# 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 : T z T ATM (observed) What happens following release depends on the local (environment) atmospheric lapse rate : The stability of the atmosphere against vertical mixing is solely determined by its lapse rate. unstable unstable • upward buoyancy amplifies initial perturbation: atmosphere is unstable • zero buoyancy does not alter perturbation: atmosphere is neutral stable • downward buoyancy relaxes initial perturbation: atmosphere is stable inversion • ( “ inversion ” ): very stable 1 = − 3’ 34 = 5 6 ( = 9.8 K km )* − 45 *+, 46 > 8 − 45 *+, 46 = 8 − 45 *+, 46 < 8 45 *+, 46 > 0 − *+ '() *,

Temperature, T Unstable Height, z Γ T parcel Stable ATMOSPHERIC STABILITY T ATM Γ T parcel − 45 *+, 46 > 8 − 45 *+, 46 < 8 − ,- #$% ,. Temperature, T Height, z T ATM − ,- #$% ,.

WHAT DETERMINES THE LAPSE RATE OF THE ATMOSPHERE? • An atmosphere left to evolve adiabatically from an initial state would eventually tend to neutral conditions ( − &' !"# &( = / ) at equilibrium Initial equilibrium state: − $* !"# $+ = - z T ATM Γ • Fast vertical mixing in an unstable atmosphere maintains the lapse rate to Γ . z T Solar heating of surface & radiative cooling of air: unstable atmosphere Γ ATM z T initial final Γ buoyant motions relax unstable atmosphere back towards − $% !"# $& = / • Solar heating of surface and radiative cooling from the atmosphere disrupt that equilibrium and produce an unstable atmosphere: Observation of − 01 !"# 02 = 8 is sure indicator of an unstable atmosphere.

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

-20 -10 0 10 20 30 Temperature, o C 0 1 4 2 3 Altitude, km cloud boundary layer

T 0 Temperature Height ABSOLUTE STABILITY, ABSOLUTE INSTABILITY CONDITIONAL STABILITY CONDITIONALLY STABLE ABSOLUTELY STABLE ABSOLUTELY UNSTABLE Γ W Γ − dT ATM d z − dT ATM d z

T 0 Temperature Height Γ W Γ − dT ATM d z cloudy parcel When the cloudy parcel moves downward, at any given altitude its temperature will be lower than the environmental temperature. Therefore, it will keep going down – the cloudy parcel is thus unstable with respect to downward motion. Likewise, the cloudy parcel is unstable w.r.t. upward motion.

FRONTS WARM FRONT: WARM AIR COLD AIR WIND Front boundary; inversion COLD FRONT: COLD AIR WARM AIR WIND inversion

DIURNAL CYCLE OF SURFACE HEATING/COOLING ventilation of urban pollution z T 0 NIGHT MORNING Mixing depth 1 km Subsidence inversion NIGHT MORNING AFTERNOON MIDDAY Γ

SUBSIDENCE INVERSION typically ~2 km altitude

WHY ARE INVERSIONS IMPORTANT? • Inversions inhibit the vertical exchange of air and pollutants. • They trap pollutants like a lid on a pot. • Smog events in LA are typically associated with inversions.

THE LOW INVERSION LAYER OF L.A. SMOG

Example of atmospheric stability: Dilution of power plant plumes Match each power plant plume ( 1–4 ) to the corresponding atmospheric lapse rate ( A–D , solid lines). Power plant plumes Atmospheric lapse rates (the dashed line is the adiabatic lapse rate Γ ).

Example of atmospheric stability: Wet convection • Identify stable and unstable regions in the profile. • Consider an air parcel rising from A to B and forming a cloud at point B . Consider the following observed temperature profile: Assuming Γ w = 6 K km -1 , calculate the altitude to which the air parcel will rise before it becomes stable relative to the surrounding atmosphere.

FIRST-ORDER PARAMETERIZATION OF TURBULENT FLUX • Observed mean turbulent dispersion of pollutants is near-Gaussian parameterize it by analogy with molecular diffusion: Source Instantaneous plume Time-averaged envelope 0 1 Near-Gaussian profile • Typical values of K z : 10 2 cm 2 s -1 (very stable) to 10 7 cm 2 s -1 (very unstable); mean value for troposphere is ~ 10 5 cm 2 s -1 • Same parameterization (with different K x , K y ) is also applicable in horizontal direction but is less important (mean winds are stronger) turbulent diffusion coefficient (a.k.a. eddy diffusivity) Turbulent flux = − = 1 > 2 3 0 31

TYPICAL TIME SCALES FOR VERTICAL MIXING Estimate time ∆< to travel ∆6 by turbulent diffusion: 1 day 5 km 0 km 2 km “ Planetary boundary layer ” (PBL) Tropopause (~10 km) 1 week 1 month 10 years ∆$ = ∆% ! &' " with ? 4 ~10 5 cm 6 s )*

STRATOSPHERE - TROPOSPHERE EXCHANGE

BREWER - DOBSON CIRCULATION (BDC) Stratospheric wave driving of the the BDC is dominated by planetary-scale Rossby waves . Stationary planetary waves are generated by large-scale orography and land–sea contrasts , but transient planetary-scale and synoptic-scale waves also contribute to the wave driving, particularly in the lower stratosphere. To a lesser extent, small-scale gravity waves , forced by smaller mountains, convection, and frontal instabilities with length scales of roughly 10–1000 km, also contribute to the wave driving. In the upper stratosphere and mesosphere, however, gravity waves begin to play a more dominant role. Stratopause Tropopause Altitude Latitude Hadley Circulation

Strong convection redistributes heat and moisture in the tropical atmosphere. Distinctive anvil clouds form when hot, humid air (red) rises in a region called the convective core . As the air rises it cools, resulting in heavy precipitation. During the strong convection typical of the tropics the air can rise high into the troposphere (roughly 15 km altitude) where the column is truncated by high winds. This forms high-altitude cirrus clouds that stream out ahead of the storms. Cool air (still with a high relative humidity) drops out of the cap of cirrus clouds, which warms and dries as it falls. This process humidifies the air in the upper troposphere. Near the surface over the tropical oceans (beneath the boundary layer) the air is always humid and often filled with low-level clouds. (Image by Robert Simmon)

anvil cloud cumulus towers Photo on Feb. 5, 2008, by the International Space Station Expedition 16 crew (NASA) Over western Africa near the Senegal-Mali border, a fully formed anvil cloud with numerous smaller cumulonimbus towers rising near it CUMULONIMBUS CLOUD OVER AFRICA Perhaps the most impressive of cloud formations, cumulonimbus (from the Latin for “pile” and “rain cloud”) clouds form due to vigorous convection (rising and overturning) of warm, moist, and unstable air. Surface air is warmed by the sun-heated ground surface and rises; if sufficient atmospheric moisture is present, water droplets will condense as the air mass encounters cooler air at higher altitudes. The air mass itself also expands and cools as it rises due to decreasing atmospheric pressure, a process known as adiabatic cooling. This type of convection is common in tropical latitudes year-round and during the summer season at higher latitudes .