The Arctic boundary layer: Characteristics and properties

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The Arctic boundary layer Characteristics and
properties
Steven Cavallo June 1, 2006 Boundary layer
meteorology
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Overview of the Arctic boundary layer (ABL)

• An annual overview of observational
  characteristics from SHEBA
• The wintertime ABL
• Characteristics of the near-surface inversion
• Leads
• Arctic haze
• The summertime ABL
• Measurements from AOE 2001
• Arctic stratus clouds

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Observations from SHEBA

• SHEBA (Surface Heat Budget of the Arctic Ocean
  Experiment) was a year-long field campaign from
  October 1997- October 1998.
• Measurements were taken from a site on the ice
  with a 20-m tower, drifting with the ice flow
  more than 1400 km over the year.

• Temperature and humidity profiles taken at the
  surface, and 5 other various heights ranging from
  2-18 m, with sampling rates of 5 s.
• Winds, friction velocity, and sensible and latent
  heat fluxes measured at the same levels above
  with a 10 Hz sampling rate.

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Observations from SHEBA

• 10-m temperatures (solid black line) were at
  times at much as 5?C warmer than surface
  temperatures (dotted black line) in winter
• Summer melt season began May 29
• Relative humidity always near saturation, but
  lowest in summer

• Compared to climatology, SHEBA results were
  similar, except for a pronounced period of above
  normal temperatures in the early spring

(Perrson et al. 2002)
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SHEBA Surface Energy Budget
1-month average values

• Surface gains heat from April-September
• Average longwave flux always negative
• Shortwave maximum occurred when albedo was a
  minimum
• ????Melting ? albedo decreases ?
  more shortwave reaches surface
• Bowen ratio (Hs/Hl) large during winter

Persson et al. 2002
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SHEBA Surface Energy Budget
Daily mean values

• Positive net radiation in winter during cloudy
  periods

• Longwave smallest under clear skies when
  radiation can escape into space

• Spikes in sensible heat flux during winter
  from leads (large open cracks in the ice)

Persson et al. 2002
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The wintertime ABL
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The wintertime ABL

• During the winter, there is little to no solar
  radiation.
• Snow and ice covered surface emits longwave
  radiation upwards faster than the atmosphere,
  allowing a near-surface temperature inversion to
  develop.
• ?Stable, shallow BL during the winter

Shaw 1995

• Inversion very shallow to the ground in winter,
  sometimes as strong as 5?C in lowest 18 m from
  surface.

Persson et al. 2002
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Leads

• An internal boundary layer is created due to
  convective eddies transporting heat and moisture
  upward over and downwind of the lead

? Heat fluxes can be predicted from the fetch
over a lead
Andreas 1980
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The summertime ABL
Arctic Ocean Experiment (AOE) August 2001

• Measurements from an ice breaker ship called
  Oden, moored to ice near the NP
• Temperatures quite variable in free troposphere,
  but rather homogeneous near surface
• Temperature inversion base most frequently 200
  m
• Inversion thickness most frequently 200-500 m
• Inversion strength 4-6?C most frequently, but
  sometimes 18?C

Figures from Tjernström et al. 2004
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Arctic Stratus Clouds (ASC)

• Three main types of summer Arctic boundary layer
  structure observed (Curry et al. 1988)
• 1) Cloud-topped mixed layer from surface to base
  of inversion
• 2) Stable BL with several layers of thin, patchy
  clouds
• 3) Stable, foggy BL with a cloud-topped mixed
  layer above
• Three main ideas as to why there is a layering
• Cloud absorption by solar radiation (Herman and
  Goody 1976)
• Weak ascent and entrainment form upper layer,
  lower layer an advective fog (Tsay and Jayaweera
  1984)
• Weak rising vertical motion is most conducive for
  layering (McInnes and Curry 1995)

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Arctic Stratus Clouds (ASC)
McInnes and Curry 1995
Initialized with observations from the Arctic
Stratus Experiment (ASE) in 1980 over the
Beaufort Sea, a high resolution 1-D model with
2nd order turbulence closure was used to simulate
the evolution of an Arctic BL.
Mean initial conditions (solid) and after 2 hours
of model integration (dashed)
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Arctic Stratus Clouds (ASC)
3) McInnes and Curry 1995 (contd)
Control, w 0.2 cm/s
W 0 cm/s
No radiation
W 1 cm/s
No drizzle

• Weak, rising motion produces most favorable
  conditions for layered clouds
• Radiation enhances condensation from cloud-top
  cooling in upper layer
• Sensible heat loss to underlying sea-ice produces
  a stable fog/low cloud layer

W -1 cm/s
No radiation or drizzle
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Summary

• Temperature inversion is characteristic all year
  due to ice and snow covered surface Wintertime
  it is shallow and based at the surface, while in
  the summertime it is most frequently 200 m above
  surface.
• Temperatures do not exceed much beyond 0?C in
  summer near the surface due to energy being used
  for latent heat release.
• Sensible heat fluxes generally much larger than
  latent heat fluxes, especially in the winter, and
  is generally upward except at times during the
  summer.
• Leads can can cause significant fluctuations in
  sensible and latent heat fluxes These
  fluctuations can be predicted using by knowing
  the near-surface wind speeds and fetch.
• The summertime ABL often consists of layered
  stratus clouds, for reasons not clearly
  understood, but related mostly to vertical
  velocity and radiative transfer.

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Arctic Stratus Clouds (ASC)
1) Herman and Goody 1976
Cloud optical thickness
Thick clouds will absorb enough radiation to
cause evaporation in the middle

Cloud depth
2) Tsay and Jayaweera 1984
Temperature profile close to saturated lapse rate
inside cloud
Warm, moist air aloft
Cold surface temperatures