CSTAR Presentation: Coastal Front Climatology

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CSTAR PresentationCoastal Front Climatology

• August 24, 2001
• Wyat K. Appel and Allen J. Riordan
• North Carolina State University

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Outline

• CSTAR Coastal Front Objectives
• Offshore Coastal Front ID Algorithm
• Onshore Passage Criteria
• Results (review)
• Case Composites
• New Results
• Future Work

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CSTAR CF Objectives/Methods

• Can large scale conditions predict
• offshore frontogenesis?
• onshore movement?
• Can we differentiate
• offshore fronts?
• onshore fronts?
• Are there clues for frontal movement?
• Methods
• objective CF climatology
• composites

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Data Sources

• National Data Buoy Center (NDBC)
• buoy 41001
• buoy 41002
• SAMSON (from NCDC)
• 1961 -1990, 1990-1995
• Cape Hatteras, Wilmington
• Raleigh

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Example of a Coastal Front
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Objective Coastal Front Climatology

• Used wind and temperature criteria at different
  land and ocean stations to identify coastal
  fronts offshore
• Used similar method to identify coastal front
  that make landfall
• Performed an objective climatology from 1984 -
  1994

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Offshore Coastal Front Detection

• Wind direction at 41001 must be between 20? and
  160? inclusive.
• Wind direction at Hatteras must be greater than
  330? and must also be 20? counterclockwise of
  wind at 41001.
• Wind speed at 41001 must be at least 4.1m/s
  (8kts).
• Air temperature at 41001 must be at least 3?C
  greater than Hatteras.
• Above criteria must be met for at least 6
  consecutive hours to be considered a coastal front

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Onshore Coastal Front Detection

• A Clockwise wind shift of at least 20? in one
  hour
• An increase in air temperature of at least 2?C
  during the same hour
• Wind direction between 30? and 180? after wind
  shift
• After wind shift, air temperature at coast at
  least 2ºC greater than Raleigh

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Results

• Offshore CF duration frequency

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Offshore CF Duration Frequency
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Results

• Offshore CF duration frequency

• Offshore hourly CF frequency

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Offshore Hourly CF Frequency
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Results

• Offshore CF duration frequency
• Offshore Hourly CF frequency

• RDU/ILM ? T time series

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? T Time Series

• offshore

• frontal passage

• Time series of ? T between Raleigh and Wilmington
  for a coastal front
• event on 21-22 January 1993 as detected by the
  algorithm.

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Results

• Offshore CF duration frequency
• Offshore Hourly CF frequency
• RDU/ILM ? T time series

• Classifications of coastal fronts
• Offshore vs. Onshore
• Diurnal
• CAD
• Strong vs. Weak
• frontogenesis
• ? T

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Coastal Front Type vs. CAD Type
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Case Composites Onshore

• Front is classified onshore if made landfall at
  Hatteras, Wilmington, or both
• A total of 73 onshore coastal fronts were found
  in the 11 year climatology
• Only cases that made landfall at both Hatteras
  and Wilmington were included in the composite
• Total of 29 cases make up the onshore composite

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Case Composites Offshore

• Front was classified offshore if it never made
  landfall at either of the coastal stations
• A total of 102 offshore fronts were found in the
  11 year climatology
• Only fronts that were detected offshore at both
  pairs of stations were included in the composite
• Total of 24 cases make up the offshore composite

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Case Composites Diurnal

• A coastal front was classified diurnal if it
  never made landfall and was not identified at 18Z
• They are short cases, lasting on average 9-15
  hours
• Typically form around 23Z and end around 13Z
• Represent the majority of all coastal fronts
  identified in the climatology, with 254 diurnal
  fronts found in the 11 year climatology
• A total of 69 cases make up the diurnal front
  composite

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New Results

• Majority of coastal fronts are associated with
  the classical dry onset, weak dry onset, and
  southern dry onset types of CAD
• Excluding diurnal fronts, 42 of coastal fronts
  made landfall
• 88 made landfall at Cape Hatteras
• 59 made landfall at Wilmington
• Coastal fronts appear more likely to make
  landfall during the spring than the fall
• 36 during the fall vs. 50 during the spring
• On average, landfall at Cape Hatteras precedes
  landfall at Wilmington by about 5 hours

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Future Work

• Characteristics of landfall, such as average
  increases in temperature and dewpoint at Cape
  Hatteras and Wilmington
• Case studies involving the different coastal
  front sub-types and the relation of coastal front
  behavior to different CAD sub-types
• Role of the Gulf Stream in coastal front
  development and behavior