Lecture Notes

Friday, September 05, 2002

Surface analysis: The pressure field

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Objectives

• plot contours of sea-level pressure and 3-hour pressure tendency using plotted station data
• analyse surface maps to determine effect of observed synoptic systems upon weather

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Notes

Contour maps

• The atmosphere is a continuous medium, and weather stations sample only part of it. Contour analyses provide a continuous field to work with.
  • isopleths: contours of any quantity
  • isobars: contours of constant pressue
  • isallobars: contours of constant pressure tendency
  • isotherms: contours of constant temperature
  • isentropes: lines of constant potential temperature
• Contour maps of the sealevel pressure (SLP) field show important relationships to cloud fields, precipitation, and surface winds.
  • Sea-level pressure contours define the synoptic-scale eddies and associated sub-synoptic scale troughs and ridges that generate most weather.
  • Winds tend to circulate cyclonicly around low pressure centers (identified with L's) and anticyclonicly around high pressure centers (identified with H's). This is counter-clockwise and clockwise in Northern Hemisphere, respectively. [Click for illustration]
  • Winds tend to blow parallel to isobars. Due to surface friction they blow slightly down pressure gradients (i.e. from high to low pressure). [Click for illustration]
  • Troughs and ridges extend oblongly from pressure centers.
  • In general, troughs and ridges track eastward across North America. The Western part of the continent just east of the Rockies (Colorado, Alberta, Mackenzie Basin) is an active region of cyclone and anticyclone formation. Regions of pressure fall (rise) associated with advancing troughs (ridges) and receding ridges (troughs) correpond to precitation and cloud formation (clearing). [Click for illustration]
  • Sea-level pressure tendency can be used to track the instantaneous path of synoptic-scale cyclones and anticyclones with their attendant troughs and ridges, helping to identify zones of rising (negative pressure tendencies) and subsiding air (positive pressure tendencies) associated with cloud formation and clearing, respectively.

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Contouring Exercises

• There is an excellent interactive contouring tutorial at the University of Wisconsin.
• Using the CMC surface analysis for 12Z 5 September 2003 ([gif] [ps] [pdf]), plot isallobars (contours of constant pressure tendency) at ..., -3, -2, -1, 1, 2, 3, ... mb/3-hr associated with the eastward advancing trough over Hudson's Bay. A pencil and eraser is highly recommended.
  Hints:
  • Your field should look similar to the 12-hour GEM regional forecast initiated 00Z 5 Sept 2003 and verifying for 12Z, (the GEM model is the Canadian operational model. [gif] [pdf]
  • Start by circling maximum values and work your way out. The following is a partially completed analysis of the highest contour levels: [Click for illustration]

Handouts

Links

•  Canadian Meteorological Centre (Operational Analysis Charts and Bulletins)
• National Weather Service (USA)  [U.S.]  [Misc.]
•  Universilty Center for Atmospheric Research (UCAR)
•  Unisys