The Arctic boundary layer

The Arctic boundary layer:
Characteristics and properties
Steven Cavallo
June 1, 2006
Boundary layer meteorology
Overview of the Arctic boundary layer
An annual overview of observational characteristics from SHEBA
The wintertime ABL:
•Characteristics of the near-surface inversion
•Arctic haze
The summertime ABL:
•Measurements from AOE 2001
•Arctic stratus clouds
Observations from SHEBA
SHEBA (Surface Heat Budget of the
Arctic Ocean Experiment) was a year-long
field campaign from October 1997October 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.
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)
SHEBA Surface Energy Budget
1-month average values
•Surface gains heat from AprilSeptember
•Average longwave flux always
•Shortwave maximum occurred
when albedo was a minimum
Melting  albedo decreases 
more shortwave reaches surface
•Bowen ratio (Hs/Hl) large during
Persson et al. 2002
SHEBA Surface Energy Budget
Daily mean values
• Positive net radiation in
winter during cloudy
• 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
The wintertime ABL
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
•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
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
•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
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:
1) Cloud absorption by solar radiation (Herman and Goody
2) Weak ascent and entrainment form upper layer, lower layer
an advective fog (Tsay and Jayaweera 1984)
3) Weak rising vertical motion is most conducive for layering
(McInnes and Curry 1995)
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)
Arctic Stratus Clouds (ASC)
3) McInnes and Curry 1995 (cont’d)
W = 0 cm/s
Control, w = 0.2 cm/s
W = 1 cm/s
No radiation
No drizzle
•Weak, rising motion produces
most favorable conditions for
layered clouds
W = -1 cm/s
•Radiation enhances
condensation from cloud-top
cooling in upper layer
•Sensible heat loss to
underlying sea-ice produces a
stable fog/low cloud layer
No radiation or drizzle
• 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.
Arctic Stratus Clouds (ASC)
1) Herman and Goody 1976
Cloud optical thickness
Cloud depth
2) Tsay and Jayaweera 1984
Temperature profile close to
saturated lapse rate inside cloud
Warm, moist air aloft
Cold surface temperatures
Thick clouds will
absorb enough
radiation to cause
evaporation in the