General Circulation/FrontsFall 2009Meteorology 311Precipitation Types• Rain• Snow– growth of ice crystals through deposition, accretion, and aggregation.• Freezing Rain – Rain freezes when it hits the surface.•Sleet•Sleet– Usually starts out as snow or ice.– Falls through a warm layer and melts.– Falls through a lower layer below freezing and freezes again. Common with warm fronts.• Graupel– Growth of ice crystal through accretion.– Accretion not significant enough to be called snow.Quick Quiz• Question #1: What is the primary cause of the seasons?• Question #2: When is the Earth the closest to the Sun?• Question #3: What is the significance of an equinox?• Question #4: What is the significance of a solstice?General Circulation• Seasons– Tilt of the Earth’s axis.– Equatorial regions receive much more solar energy than poles.• Systems form and bring equatorial heat toward the pole and polar cold air toward the equator.the pole and polar cold air toward the equator.• One the long term, the Earth has a general circulation that is rather persistent.– What we (mid-latitude) think of as weather is generally the small-scale perturbations on the larger circulatation.Single Cell Model• Hadley (1735)• Trying to describe why sailors experienced zonal winds in the lower latitudes.latitudes.• Assumed planet was covered by a single ocean and a fixed sun that remained over the equator.Single Cell Model• Strong heating• Air expanded vertically• Diverged toward both polespoles• Sank back to surface at the poles• Returned to the equator.Single Cell Model• Rotation of earth caused deflection to right in N.H. and left in S.H..• Differences in heating give rise to persistent large-scale motions.large-scale motions.– Thermally direct circulations.• Zonal winds can result from the deflection of meridional winds.Three-cell model• Ferrel (1865)• Hadley: tropics and sub-tropics• Ferrel: mid-latitudes (30-60º)• PolarHadley Cell• Strong solar heating at equator.• Equatorial low and the ITCZ.– Inter-Tropical Convergence Zone• Upper troposphere moves poleward to sub-tropics (20-30º).• Air acquires increasing zonal component.– Very strong. Circles the earth several times.Hadley Cell• Air sinks toward surface as it cools.• Forms sub-tropical high.• Air warms adiabatically, results in warm, cloud results in warm, cloud free conditions.– Weak pressure gradients and light winds.• Strongest in the winter when temperature gradient is the strongest.Polar Cell• Air mass moves from polar high to sub-polar low.• Slightly warmer air at sub-polar locations rises.sub-polar locations rises.• Very cold air at poles causes the polar high.• Coriolis results in the polar easterlies.Ferrel Cell• Indirect cell– Caused by turning of the two adjacent cells.• Coriolis results in Westerlies.Westerlies.Polar Front• Winds meet with cold polar winds heading toward the equator.• Differences in tropopause height (temperature gradient) cause stream of fast moving air (jet fast moving air (jet stream).• Polar front is primary mechanism that brings weather to the mid-latitudes.• Sub-tropical jet.Three-cell vs. Reality• Hadley cell does a good job of low latitude motions.• Ferrel and polar cells not quite as well represented.– Central U.S. is dominated by a southerly flow during the summer.–Polar easterlies only emerge in long term averages.–Polar easterlies only emerge in long term averages.• Upper level model is not realistic at all– Ferrel cell implies easterly motion in the upper troposphere: Overwhelming westerly wind.• Three-cell model provides a starting point for a more detailed account.– Doesn’t consider land-ocean contrasts or topography.Semi-permanent CellsJanuaryJulyAir Masses• Air masses form when air remains over a given region of a substantial length of time.• Air takes on characteristic of that region.• Labeled by mositure (c,m), temperature (T,P,A).mPmPcPcAtemperature (T,P,A).• Why don’t characteristic air masses originate over mid-latitudes?• What are boundaries between air masses called?cPmTmTmTcTFronts• Fronts separate different air mass types.• Thickness gradient defines the approximate location of the front.– Front is located on the warm side of the gradient.• Type of front depends on movement of the colder air.Fronts - Location• Wind shift line• Pressure trough• Temperature discontinuity• Dew Point temperature discontinutity• Pressure tendency pattern• Horizontal visibility variations• Horizontal variation in precipitation typeFronts – 5 types• Cold• Warm•Stationary•Stationary• Cold Occlusion• Warm OcclusionFront SymbolsNomenclature• Frontogenesis: Convergent low-level flow in the presence of a thickness gradient at low-levels.•Frontolysis: Divergent low-level flow •Frontolysis: Divergent low-level flow around an existing front.Active/Inactive Fronts• Active front: The warm air has a wind component with respect to the frontal motion toward the front.– Over a significant depth of the atmosphere.• Inactive front: no warm air component toward the front.Active/Inactive FrontCold Front • Temperature– Colder at the surface behind the front with lower thicknesses. (See exception in book).• Dew Point– Lower dew point temperatures found behind the front.• Winds–South-Westerly ahead of front, often gusty near the front–South-Westerly ahead of front, often gusty near the front– Winds veer to north or northwest as front passes, and increase in strength.• Pressure tendency– Pressure falls ahead of the front, rises behind the front.– Pressure trough exists along front.• Precipitation types– Deep convection ahead, possible shallow convection behind.Warm Front• Temperature– Cold air ahead of warm front. Temperature gradient is smaller than warm front (friction).• Dew Point– Lower dew points ahead of front, although larger RH values are typically found here.• Winds–South-easterly to northeasterly ahead of front in cool air. Little –South-easterly to northeasterly ahead of front in cool air. Little gustiness.– Winds veer to southwest or south as front passes.• Pressure tendency– Pressure falls ahead slowly ahead of warm fronts unless trough is intensifying rapidly.– Steady or slowly rising behind the front.• Precipitation types– Often steady ahead of front. No precipitation