Wind: Small Scale and Local Systems Chapter 9 Part 1

Wind: Small Scale and Local Systems Chapter 9 Part 1

Atmospheric scales of motion Scales of atmospheric circulations range from meters or less to thousands of kilometers- millions of meters Time scales of motion are roughly proportional to the spatial scales Small turbulent eddies in turbulent boundary layer 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- mesoscale, but often last long- up to two weeks

Scales 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 systems on weather maps Eddies-spinning, spinning, turbulent masses of air

Boundary Layer Friction and Turbulence In smooth (laminar) flow, molecular viscosity molecular friction In turbulent flow, eddy viscosity- much larger spatial scale Near earth s surface, eddy viscosity related to surface roughness- trees, buildings, hills enhance surface roughness- frozen pond-low surface roughness Surface roughness causes eddies with fluctuating ti wind speed and directionsmechanical turbulence

Re < 1 Types of Flow around a sphere Re = d p v r μ ρ F d p = particle diameter, m μ = fluid viscosity, kg/m s

Boundary Layer Turbulence Turbulence from surface heating is thermal turbulence Where thermal turbulence dominates mixing is called free convection Where e mechanical ca turbulence dominates mixing by forced convection Friction of earth s surface decreases eases with height, usually negligible above 1000 m Friction layer or planetary boundary layer (PBL) In neutral to unstable atmosphere during daytime heating- much mixing with surface air and air at higher altitudes mixed

Effects of Turbulence Deeper mixing i causes downward d transport t of momentum- stronger winds aloft mix down to lower levels At night, surface winds lighter- during inversions cut off from higher winds aloft- often get nocturnal jets above inversion layer Downwind of mountain ranges may have roll eddies- or rotors- extreme turbulence Large, rapid change in wind speed with height, such as along jet streams may cause instability and turbulent eddies to develop- clear air turbulence often experienced while flying

Stable Unstable

Large Eddies: Rotors and Waves

Clear Air Turbulence

Local or mesoscale wind systems On a diurnal basis, thermally induced mesosale circulations i include seabreeze/landbreeze and mountain/valley wind systems 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 water Transfer aloft over land causes surface low over land- PGF at surface from ocean to land- sea breeze- return flow aloft completes circulation Sea breeze strongest mid-afternoon- gradually turns to right (n. hemisphere) as coriolis force affects direction

Sea breeze circulation

Land breeze circulationreverse of sea breeze

Land and Sea Breezes Land breeze usually weaker than sea breeze due to weaker temperature gradientsessentially non-existent along California coast in summer as water stays cooler than land If air is sufficiently moist, showers or thunderstorms develop along sea breeze front (often in eastern US, Gulf Coast, rare on West Coast) Florida- thunderstorms very common as Atlantic and Gulf of Mexico sea breezes converge and air is very moist

Atlantic, Gulf sea breeze convergence

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