Slide 1objectivesPlotting conventions synoptic chartsExtratropical cyclones: case study example: November 1998Slide 5Amplification of the waveSlide 7Vorticity analysisSlide 9In summaryVertical velocity and 500hPa chartsSlide 12Frontal ZonesSlide 14Slide 15Secondary cold frontTemperature structureClose-up in the temperature and windsStationary frontsOccluded Front:Frontal systems have a tridimensional featureSlide 22Slide 23Slide 24Slide 25Slide 26MoistureExample of Dry line observed in a Td map:Slide 29Hourly observations: cold frontHourly observations: warm frontHourly observations: occluded frontSurface weatherSlide 34Slide 35Precipitation as seen by radarSlide 37Slide 38Slide 39Geog 166: weather systemsProf. Leila M. V. Carvalhoobjectives•The weather analysis will be focused first on the formation and evolution of extratropical cyclones •We will discuss a case-study that caused strong winds and heavy precipitation to parts of the central United States•It was unusually intense but typifies many features of winter storms in middle and high latitudesPlotting conventions synoptic chartsExtratropical cyclones: case study example: November 1998500hPa chart for 00UTCPolar Vortex is split into two regional cyclonic features (Russia and Northern Canada)Two ridges separate the two troughsSolid red lines denote axes of ridges and dashed red lines are axes of troughsTypical distance between two troughs : 50o of longitude or ~ 4000km500hPa charts (contours 60m intervals (dKm) and relative vorticity (blue shading 10-4s-1). Right sea level pressure 4hPa interval and 1000-500hPa thickness (colors) Baroclinic waves travel ~ 10m/s: wintertime climatological wind at 700Pa (steering level)Successive ridges (or troughs typically ass at a fixed point on Earth at roughly 4 days (but it can be sometimes 1 or 2 days apart depending on the steering flow)Baroclinic waves tend to be stronger over the oceans (as we have seen many times) but they can develop over landFrontal surfaces Note the intensification of the Low pressure system and how it “splits off the westerly to form a CUTOFF LOW (ISOLATED MINIMUM OF GEOPOTENTIAL HEIGHT FIELD)That implies the existence of a closed cyclonic circulationNote the tightening of the contours that imply the intensification of the winds around the lowNote the intensification of the low pressure at the surface Amplification of the wave The surface low is the EXTRATROPICAL CYCLONENote that pressure gradients increase with time showing the intensification of the low as the cold air moves southward and the warm air moves northward In the beginning the surface Low is eastward of the trough but as the 500hPa and low level cyclone amplify they come into vertical alignmentAdvection of temperature, amplification of the wave and the extratropical cycloneVorticity analysisyuxv indicates cyclonic vorticity related to the horizontal wind shear (u varies rapidly with y and v varies rapidly with x) these maxima move eastward 9 hours laterSurface Low pressure system drops from 998 hPa at 00UTC to 978 hPa at 18UTC (bottom) to 968 at 00UTC (not shown) Nov 11=> 30hPa in 24h!!! 3x more rapid than the typical cyclone500 hPa (thick black), 1000hPa height (thin black), 1000-500hPa thickness (dashed) Arrows: sense of the geostrophic wind Bottom: from text book: Atmospheric Circulation Systems: Palmen and Newton (p. 326)Amplification of the wave in the thickness field: advection of temperature by the windNote that the strengthening of the east-west gradient of temperature leads to the weakening of the North-south temperature gradient as the wave is growingSurface low aligned with the 500hPa trough: weakening of the horizontal temperature advectionIn summary•The geostrophic wind field evolves from a highly baroclinic pattern (high gradients and strong vertical wind shear) into a more equivalent varotropic pattern (less directional shear of the lower tropospheric geostrophic wind field in fully developed baroclinic waves•Strong winds but weaker temperature gradients marks the end of the intensificaiton phase in the life cycle of the cycloneVertical velocity and 500hPa chartsRapid Intensification Rising of the northward air in the region of warm advection  sinking air in the region of southward cold advection to the rear of the cycloneRising of warm air and sinking of cold air is indicative of a conversion of potential energy into kinetic energy In baroclinic waves the potential energy is associated with the east-west temp gradients and the kinetic energy with the meridional wind component•Note that the juxtaposition of this inward-spiraling rising and subsiding air (“yin-yang” pattern) influences the pattern of rain and cloudsFrontal Zones•Narrow bands marked by sharp horizontal gradients and sometimes outright discontinuities in wind and temperature•Frontogenesis: development of frontal zones (initiated by large-scale horizontal deformation fields•Mesoscale circulations in the plane perpendicular to the fronts are instrumental in increasing temperature contrasts and organizing the distribution of precipitation into bands parallel to the frontsWinds and pressure: cold frontBelow: sea level pressure, and surface winds •Pronounced wind shift, winds exhibit strong westerly component. To the east southerly wind component are dominant. Isobars bend sharply along the front•As a cold front pass in the NH there will be a shift in an anticyclonic sense (veering) from southerly to westerlyWinds and pressure: warm frontBelow: sea level pressure, and surface winds •Is a more subtle feature •Shows signal of advection around the surface low•As a warm front pass in the NH there will be a shift in an anticyclonic sense (veering) from southeasterly to southerly•Late stage there will be a junction of the two fronts: occluded front extends from the center of the surface low to a triple point when it meets the warm and cold front•When an occluded front pass a station the surface wind veers from southeasterly to southwesterlySecondary cold front•A front which forms behind a frontal cyclone and within a cold air mass, characterized by an appreciable horizontal temperature gradient•Cause also winds to veer as it pass over a stationTemperature structure•The zone of relatively uniform temperature to the southeast of the surface low is referred to as the warm sector of a cyclone•The cold front