1 CHAPTER 10 EXTRATROPICAL CYCLONES FORMING EAST OF THE ROCKY MTS Where will a low pressure system center form at the surface relative to an upper air trough and ridge On the EAST side of the trough under the LEFT EXIT region of the jetstreak Consider a situation in which upper level divergence exceeds low level convergence There is a surface low that is weakening Left entrance region Left exit region Convergence Divergence Sinking motion Rising motion Increasing surface Decreasing surface pressure pressure Right entrance region Divergence Rising motion Decreasing surface pressure Right exit region Convergence Sinking motion Increasing surface pressure 2 Movement from Movement from trough to ridge ridge to trough Air speeds up Air slows down Divergence Convergence Rising motion Sinking motion Decreasing Increasing surface surface pressure pressure Both curvature and jetstreak influence flow Effects of Heating and cooling on development of highs and lows Diabatic processes involve heating and cooling usually dude to solar heating 1 Conduction contact with surface 2 Convection rising plumes 3 Radiation 4 Latent heat release Warming of the atmosphere will lead to the formation of low pressure at the surface Cooling of the surface will lead to the formation of high pressure at the surface Low pressure centers over the ocean tend to be stronger then low pressure centers over continents More evaporation occurs over the oceans then over land leading to more latent heating in the could and a reduction in surface pressure 3 Friction is reduced over water so frictional dissipation of the low occurs at a slower rate If an air column is heated through latent heat release how will the surface pressure change The surface pressure will LOWER when an air column is heated When air pressure is rising due to a surface low pressure system any latent heat release will INTENSIFY the surface low SUMMARY 1 Divergence Convergence wind speed and direction in upper level flow 2 Cyclonic Anticyclonic flow a Cyclonic counter clockwise and slower then geostrophic 3 Curvature effect flow around troughs ridges a Direction of divergence convergence 4 Role of friction surface low is diverted towards low pressure Development of High and Low Pressure Centers Dynamic processes curvature jetstreaks and friction and thermodynamic processes heating and cooling cause a redistribution of mass in the atmosphere and create high and low pressure centers We need to consider all of these processes curvature jetstreaks friction heating and cooling to understand why high and low pressure centers form in a particular location Low pressure at the surface usually forms initially due to upper level features such as troughs and jetstreaks High pressure at the surface usually forms due to cooling of the air Air flows in towards a low pressure center at the surface and rises This rising air is associated with clouds and precipitation The air then diverges at upper levels of the troposphere Air flows outward from a high pressure center at the surface leading to sinking motion above the high pressure center This sinking motion is typically associated with clear skies At upper levels air often converges above a high pressure center Curvature effect will result in CONVERGENCE at A Curvature effect will result in CONVERGENCE at E Curvature effect will result in N A at F Curvature effect will result in DIVERGENCE at C Jetstreak effect will result in CONVERGENCE at A 4 Jetstreak effect will result in N A at B Jetstreak effect will result in DIVERGENCE at C Jetstreak effect will result in CONVERGENCE at G C will have maximum UPPER LEVEL DIVERGENCE A will have maximum UPPER LEVEL CONVERGENCE C will develop a surface LOW A will develop a surface HIGH Convergence is a net INFLOW of air molecules into a region of the atmosphere and is associated with an INCREASE of surface pressure Assuming that the spacing between height contours remain constant an air parcel will SLOW DOWN as it move from a ridge to a trough at 300mb leading to CONVERGENCE between the ridge and trough Convergence of air at the surface will result in RISING MOTION in an air column which is associated with CLOUDS PRECIPITATION For a straight jetstreak at 300mb you would expect a sinking motion in the LEFT ENTRANCE REGION AND RIGHT EXIT REGION How does surface friction modify the pressure at the center of low and high pressure systems Surface friction causes air to flow into lows increasing the central pressure and out of highs decreasing their central pressure The pressure gradient force is strongest in the entrance region and the Coriolis force is strongest in the exit region of jetstreaks More evaporation occurs over the oceans than over land leading to more latent heating in the clouds and a reduction in surface pressure Friction is reduced over water so frictional dissipation of the low over water occurs at a slower rate Geostrophic Balance horizontal PGF Coriolis Force Above the boundary layer the atmosphere is also close to being in geostrophic balance 5 For exact balance isobars must be straight and parallel and air flows parallel to isobars A surface low has formed in SE Colorado Where is the trough jetstreak that formed this low located West of the low near Rockies Air masses are least likely to develop along coastlines A surface low has formed in a given location For this low to intensify of for its pressure to decrease further where should the trough axis be relative to this low West Prior to a cyclone 4 distinct air masses exist reflecting a temperature gradient between the equator and the pole Initial development of a cyclone Small wave introduces curvature jet streak can form enhancing divergence east of trough Cyclones for preferentially in 5 locations in North America Extratropical Cyclones Form along the jet stream Often look like a comma on a satellite image They attempt to balance temperature differenced between the pole and equator The attempt to balance temperature differences between upper and lower troposphere Transport war air northward and upward Transport cold air southward and downward Deposit latent heat into the atmosphe For an extratropical cyclone to intensity the upper level trough axis should be located to the West of the surface low 6 East of the cyclone center Warm front Wide region of clouds and warm moist air Clouds deepest closest to the front From south to north o Deep stratus with precipitation o Stratus without precipitation o