AOSS 414: Weather Systems 22 February 2016 cspgeography.blogspot.com Exam Information • One hour in length – Will start at 12:40pm or as soon as everyone is here • Closed notes – No derivations • Format – A few definitions – A few calculations – A few short answer Exam Topics • Energetics • Atmospheric structure and development of general circulation • Conversion of APE to KE • Life cycle of mid-latitude cyclone • Bjerknes Polar Front Theory • Airmasses • Bergeron Classifications • Thermodynamic diagrams/plotting/calculations • Inversion types • Frontal Structures • Cold, warm, occluded (potential temperature cross sections) • Clouds and precipitation • Frontogenesis/Frontolysis Definitions Occluded Fronts Note: These are just an examples. The actual shape of the occluded front will depend on the shape of the frontal surfaces which will vary case by case. Warm Occlusion 3 km Before Occlusion θ4 θ4 2 km θ3 Warm θ3 1 km θ2 Cool θ1 SFC Cold After Occlusion θ = const 3 km θ4 θ θ 2 km 3 Trough of warm air 4 θ3 1 km θ2 θ1 SFC TROWAL (TRough Of Warm air ALoft) Example of vertical motion associated with warm occluded front Shaded area is vertical velocity in cm/sec (Martin 2006) Warm Occlusion Upper cold front precedes surface occluded front by 100 to 200 miles. Again, precipitation is found in advance of surface front and is more closely aligned with TROWAL than surface front. Cold Occlusion 3 km Before Occlusion θ4 2 km θ 3 Warm θ θ4 1 km 2 θ3 θ 1 Cold SFC Cool After Occlusion θ = const 3 km θ4 Trough of warm air 2 km θ3 θ2 1 km θ1 SFC Cold Occlusion In central US, more common than warm occluded fronts. Upper warm front follows surface occluded front by 50 to 100 miles, and precipitation is observed well behind surface front. (Quasi-) Stationary Fronts • A quasi-stationary front is a front along which one air mass is not appreciably replacing another air mass. – Front has slowed or stopped such that its velocity is less than five knots. – Isobars (lines of constant pressure) are nearly parallel to the stationary front and thus the wind shift across front is nearly 180º. • Frictional effects result in some flow toward frontal surface – Result: Lifting of warm air over colder air • Weak waves (aloft and at surface) may cause some undulation back and forth, thus the additive term “quasi”. – Previously described temperature structures somewhat evident. Stable Stationary Front • The observed weather associated with stationary fronts is a function of frontal slope and the stability of the ascending warm air mass. Unstable Stationary Front • If the warm air mass is conditionally unstable, the vertical extent of clouds is enhanced – More persistent rains and chance of thunderstorms. • Within the cold air mass of a shallow stationary front, extensive fog and low ceilings may develop if air becomes saturated by falling rain and drizzle. Lake/Sea Breeze Front • Diurnal phenomenon – Land-water surface temperature gradients form due to solar heating • This results in reversal (with height) of horizontal pressure gradients and thus formation of a Offshore flow thermally induced circulation cell. Onshore flow P4 P3 P2 P1 Warm Cool Land Water ~ Three miles inland along WI/IL Border Movement of cold air Differences in frontal slopes are most pronounced in lowest 300 meters. Above ~300 meters, slopes for warm and cold fronts are quite similar. Movement of warm air: Anafronts and Katafronts • Anafront – A front at which the warm air is ascending the frontal surface up to high altitudes. • Katafront – A front (usually a cold front) at which the warm air descends the frontal surface (except, presumably, in the lowest layers). Source: AMS Glossary of Meteorology Common frontal types in Central US • From AMS Glossary of Meteorology: Frontogenesis: 1. The initial formation of a front or frontal zone. 2. More generally, an increase in the horizontal gradient of an air mass property, principally density, and the development of the accompanying features of the wind field that typify a front. • In class, we have identified fronts by large changes in the potential temperature across a “frontal zone”. – Thus temperature gradient can be used as one measure of frontal zone strength Frontolysis: 1. The dissipation of a front or frontal zone. 2. More generally, a decrease in the horizontal gradient of an air mass property, principally density, and the dissipation of accompanying features of the wind field that typify a front. AOSS 414: Weather Systems A Quick Primer on Surface Map Analysis Isoplething • Connects equal values of a particular meteorological variable. • Highlights areas of relative maxima and minima 30 30 29 30 30ºF 30ºF 33 32 40 40 39 40 40ºF 40ºF 43 44 46 50 50 49 50 50ºF 50ºF 52 Isoplething – A slightly more realistic example • Connects equal values of a particular meteorological variable. • Highlights areas of relative maxima and minima 29 30ºF 30ºF 33 32 39 40ºF 40ºF 43 44 46 49 50ºF 50ºF 52 Different Types of Isopleths • A line of continuous…. – Temperature = Isotherm – Potential Temperature = Isentrope – Dew Point = Isodrosotherm – Wind Speed = Isotach – Pressure = Isobar – Snowfall or Snow Depth = Isochion Isoplething • Basic rules for isoplething/contouring – Contours pass through stations that have the same value as the contour, and values are higher on one side and lower on the other side of the contour – Contours are drawn at specific intervals – Examples: temperatures (5˚C), sea-level pressure (4 mb) – Label contours – Contours either form closed curves or go off the edge of the map…they NEVER end in the middle of the map! – Do not draw contours far beyond where data exists – Contours do not fork – Contours for a given variable never cross each other – Only analyze for features for which you have the data/evidence that supports their existence. Isoplething 1032 mb 1029 1032 mb 1033 1028 mb 1032 1026 1025 L 1033 1030 1034 1029 1030 1033 1032 mb 1032 mb What you will be doing today: - Contour pressure field (that is, draw isobars) - Contour interval will be every four mb - Start at 1000mb and contour every four above and below - You will eventually use these lines to “find the fronts” Isoplething • Where have we broken some rules? H 1032 mb 1029 1032 mb 1033 1032 1026 1025 L 1033 1030 1034 1029 1030 1033 Isoplething You don’t know what is happening out here, so don’t assume trend continues! Only designate known CENTERS of pressure. 1032 mb 1029 H 1032 mb 1033 1028 mb 1032 Label, 1026 Label, 1025 Label… L 1033 1030 1034 1029 1030 1033 You don’t know what is happening out here, either, so don’t connect! Isoplething It is okay to smooth out your isopleths (example: yellow dotted line), as the green dashed line suggests that you know more than you actually do. The yellow dashed line is likely more realistic, though use common sense (ie., don’t eliminate data trends that look real…. just don’t get too wavy). 1032 mb 1029 1032 mb 1033 1028 mb 1032 1026 1025 L 1033 1030 1034 1029 1032 mb 1030 1033 Also, when present, use the wind field to help your placement of centers of pressure. Cloud Cover Reports Plotting Symbol METAR Code CLR FEW OR SCT BKN OVC VV Station Models – Wind Barbs (Indicate the speed of the wind and the direction that it is coming from) Station Model – Present Weather and Obscurations Drizzle Rain/ Tstorms Snow Assorted Analysis of Fronts Position front on warm side of region of increased temperature gradient. Use moisture gradient and wind direction shift to assist when possible. Shade precipitation areas in green. Highlight fog (yellow), T-storms (red) and freezing rain/drizzle (red) symbols. Contouring Cloud Cover • Ceiling = lowest cloud layer that is broken or overcast • Contour/shade areas with overcast conditions. • Scallop areas with broken conditions. The AOSS 440 way 16 993 **10 19 990 **11 18 000 18 23 995 20 999 18 24 998 18 18 23 006 27 004 24 004 18 18 18 Highlighting Present Weather • Green: snow, rain • Red: freezing drizzle, freezing rain or thunderstorms • Yellow: Fog or Mist The AOSS 440 way 16 993 **10 19 990 **11 18 000 18 23 995 20 999 18 24 998 18 18 23 006 27 004 24 004 18 18 18 An example of Isoplething (Example: Isobars) Label all contours, but don’t label centers of pressure unless you have a closed contour. High Pressure System H Relative maximum in pressure pattern, usually accompanied by at least one closed isobar, and normally with an outward, clockwise circulation from its center (NH). Low Pressure System L Relative minimum in pressure pattern, usually accompanied by at least one closed isobar, and normally with an inward, counter-clockwise circulation from its center (NH). Surface Fronts What I would like you to do for Wednesday…. • Contour sea-level pressure field using an interval of 4-mb. – Start with 1000-mb and work both ways – In other words: 996-mb, 1000-mb, 1004-mb …. • Properly locate/label all centers of pressure and fronts – We can spend time working on the fronts in class if need be • NOTE: These will be handed in for credit.