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EOS 340 Atmospheric Science EOS 340 Atmospheric Science Rossby Waves Synoptic Scale Weather The general poleward pressure gradient should produce a pattern of uniform eastward (zonal) winds. This is certainly the case for the sub-tropical and polar jet streams which exist near the boundaries between the Hadley and Ferrel, and Ferrel and Polar cells respectively. But the Earth’s surface has large and imposing contrasts associated with continental and ocean boundaries. These boundaries introduce meridional (northsouth) undulations into the zonal flow. In has proven useful to describe a regions weather in terms of the re-occurring movement of near permanent air masses, fronts, highs and lows. Any locations “climate”, can usually be well defined by the proximity and movement of these dominant features. Air Masses There are six types of air mass, defined by combinations of their humidity and temperature at their source, Rossby Waves. The conservation of angular momentum across regions of the By Humidity: maritime (moist) and planet with different Coriolis (f) values, results in an continental (dry) environment that can support waves. Undulations in the zonal By Temperature: Tropical (warm) winds, (i.e. jet stream) can be described in terms of Rossby Polar (cold) Waves, with wave speeds given by, Arctic (very cold) c = U − βl 2 Marine Arctic: mA where U is the zonal wind speed, β=df/dy is the meridional Marine Polar: mP gradient in f, and l is the zonal wavelength. Often, these Marine Tropical: mT undulations appear to be “stationary (arrested) waves”, when c=0. This occurs when Fronts ls = U β Continental Arctic: cA Continental Polar: cP Continental Tropical: cT The largest and more significant atmospheric fronts develop when these major air masses meet, and advance on one another. Since humans are most interested in a change of weather, fronts are named for the advancing air mass. Long waves (l>ls) travel westward, while shorter waves (l<ls), travel eastward. The stationary waves give rise to near permanent ridges and troughs in the 500 mb pressure field. 52 53 EOS 340 Atmospheric Science EOS 340 Atmospheric Science Cold Front When Arctic or Polar air masses advance on Polar or Tropical air masses, respectively, then the front is a Cold Front. These fronts are usually steep and have strong uplift and winds associated with them. Precipitation may often start out as rain, only to turn to sleet and snow as the front advances. Weather maps sow these as a line with triangles pointing in the direction of the advancing cold air mass. Warm Front Produces when warmer, Tropical or Polar air advances onto colder Polar or Arctic air, respectively. Usually the uplift/slope is more gradual, and there may be shallow temperature inversions limiting convection. Identified by a line with semicircles, extending from the warm into the colder air mass. Mid-Latitude Cyclones An instability is a dynamical phenomena, akin to the random perturbations associated with turbulence, and typically develop along shear lines. When a zonal pressure gradient separating Polar and Tropical air masses becomes unstable, then the instability can grow, and a mid-latitude cyclone can develop. The Basic Model: 1) A kink (instability) develops in the polar front. 2) The kink grows as the westward cold, and eastward warm fronts begin to advance. 3) A low pressure regions develops at the kink, becoming the center of rotation, and a storm in born (cyclogenesis). 4) clouds and winds continue to develop along both fronts. 5) If the cold front catches the warm, and occluded front may develop. 6) The low pressure region may get cut off from the warmer, tropical air, limiting the source for further convection/pressure reduction, and the storm will dissipate. Occluded Front Because cold fronts may travel faster than warm front, they may catch and overtake/join the two fronts: an occluded front. This results in three air masses in close proximity: an advancing cold air mass, a slower warm air mass, and the departing cold air mass. A Cold occlusion occurs when the advancing cold air mass is colder than the departing cold air mass, otherwise it’s a warm occlusion. Identified by both triangles and semi-circles Two sources of energy exist for the storm. First is the release of with the warmer air on the side of the triangles. potential energy, as dense, cold air sinks underneath lighter, warm air. Second is the release of latent heat, as the warmer air is pushed up and water vapour condenses in to liquid water Stationary Front A front separating air masses, but neither is advancing on the droplets. other. Winds may still blow along frontal region. Triangles and semi-circles appear on opposite sides of front. 54 55