wind: small scale and localwind: small scale and local...

Wind: Small Scale and LocalWind: Small Scale and Local SystemsyChapter 9Chapter 9

Part 1

Atmospheric scales of motionAtmospheric scales of motion• Scales of atmospheric circulations range fromScales of atmospheric circulations range from

meters or less to thousands of kilometers-millions of metersmillions of meters

• Time scales of motion are roughly proportional to the spatial scalesto the spatial scales

• Small turbulent eddies in turbulent boundary l l t d t i t i llayer last seconds to minutes- microscale

• Long waves such as upper level low now over the west coast last a week or more

• Hurricanes of moderate spatial scale-pmesoscale, but often last long- up to two weeks

Scales of motionScales of motion

• Mesoscale systems include thunderstorms, land/sea breezes, mountain/valley winds-last up to a day-often diurnally forced circulations

• Synoptic scale – surface high and low pressure y gsystems on weather maps

• Eddies-spinning, turbulent masses of airEddies spinning, turbulent masses of air

Boundary Layer Friction and TurbulenceBoundary Layer Friction and Turbulence• In smooth (laminar) flow, molecular viscosity –( ) y

molecular friction• In turbulent flow, eddy viscosity- much larger y y g

spatial scale• Near earth’s surface, eddy viscosity related to

surface roughness- trees, buildings, hills enhance surface roughness- frozen pond-low

f hsurface roughness• Surface roughness causes eddies with

fl t ti i d d d di tifluctuating wind speed and directions-mechanical turbulence

Re < 1Re < 1

Types of Flowaround a spherearound a sphere

ρFrpvd=Re

μ=Re

skg/m , viscosityfluid

mdiameter,particle

⋅=

=

μpd

Boundary Layer TurbulenceBoundary Layer Turbulence• Turbulence from surface heating is – thermalTurbulence from surface heating is thermal

turbulence• Where thermal turbulence dominates – mixing isWhere thermal turbulence dominates mixing is

called free convection• Where mechanical turbulence dominates –e e ec a ca tu bu e ce do ates

mixing by forced convection• Friction of earth’s surface decreases with height, c o o ea s su ace dec eases e g ,

usually negligible above 1000 m• Friction layer or planetary boundary layer (PBL)y p y y y ( )• In neutral to unstable atmosphere during

daytime heating- much mixing with surface air y g gand air at higher altitudes mixed

Effects of TurbulenceEffects of TurbulenceD i i d d t t f• Deeper mixing causes downward transport of momentum- stronger winds aloft mix down to lower levelslower levels

• At night, surface winds lighter- during inversions cut off from higher winds aloft- often get g gnocturnal jets above inversion layer

• Downwind of mountain ranges may have roll eddies or rotors extreme turbulenceeddies- or rotors- extreme turbulence

• Large, rapid change in wind speed with height, such as along jet streams may cause instabilitysuch as along jet streams may cause instability and turbulent eddies to develop- clear air turbulence often experienced while flying

Unstable

StableStable

Large Eddies: Rotors and WavesLarge Eddies: Rotors and Waves

Clear Air TurbulenceClear Air Turbulence

Local or mesoscale wind systemsLocal or mesoscale wind systems• On a diurnal basis, thermally induced mesosale

i l i i l d b /l db dcirculations include seabreeze/landbreeze and mountain/valley wind systems

• Sea and land breezes- heating over land creates high• Sea and land breezes- heating over land creates high pressure aloft- lower pressure aloft over cooler ocean water – pressure gradient force aloft

• PGF causes flow aloft toward water- convergence aloft causes surface high over waterT f l ft l d f l l d• Transfer aloft over land causes surface low over land-PGF at surface from ocean to land- sea breeze- return flow aloft completes circulationp

• Sea breeze strongest mid-afternoon- gradually turns to right (n. hemisphere) as coriolis force affects direction

Sea breeze circulationSea breeze circulation

Land breeze circulation-reverse of sea breeze

Land and Sea BreezesLand and Sea Breezes• Land breeze usually weaker than sea breeze

due to weaker temperature gradients-essentially non-existent along California coast in summer as water stays cooler than land

• If air is sufficiently moist, showers or y ,thunderstorms develop along sea breeze front (often in eastern US, Gulf Coast, rare on West ( , ,Coast)

• Florida- thunderstorms very common as AtlanticFlorida thunderstorms very common as Atlantic and Gulf of Mexico sea breezes converge and air is very moistair is very moist

Atlantic, Gulf sea breeze convergence

MODIS Aqua 1km True Color image for 2008/237 (08/24/08)