Lake Arrowhead 16 June 2005 Stable Boundary Layers working group Bjorn first suggests relaxing to a

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Lake Arrowhead 16 June 2005Stable Boundary
Layers working groupBjorn first suggests
relaxing to a mixed-layer model. Bert then argues
for the K-profile model. And then we found a
look-up table and celebrated our success with a
case of beer 

• SBL Team Bob Beare, Wayne Angevine, Bert
  Holtslag, Branko Kosovic, Julie Lundquist,
  Thorsten Mauritsen, Bjorn Stevens, Gunilla
  Svensson, Brian (UCLA)

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What do we know now that we didnt know 10 years
ago

• There are different kinds of stable boundary
  layers long-lived and nighttime boundary layers
  (and then flow over cold pools).
• There is an emerging awareness among the climate
  community that SBLs are a modeling problem. (IPCC
  ACIA (Arctic Climate Impact Assessment) reports
  specifically address SBLs.)
• We have developed a stable boundary layer
  community via collaborations like CASES, SHEBA,
  GABLS.

3
What do we know now that we didnt know 10 years
ago (2)

• Observations in the SBL have increased
  dramatically, and recognition of the non-local
  effects on SBLs is addressed.
• Some theoretical advances have been made (e.g.
  Derbyshire 99, Van de Wiel 2002) since the early
  pioneers (Zilitinkevich, Nieuwstadt, Wyngaard
  Brost).
• We are making progress in using LES as a standard
  tool in diagnosing stable boundary layers. In
  the 1980s, LES of SBLs couldnt be done. Many
  LES now converge in an encouraging way.

4
What do we know now that we didnt know 10 years
ago (3)

• Some progress has been made on understanding the
  transitions to and from SBLs. The evening
  transition is generally understood to be gradual
  and starts early. The morning transition is
  understood to be driven by entrainment.
• Some success in SBL simulation has been seen
  using sharp-tails at high-resolution, especially
  in fog conditions.

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Main unsolved problems (1)

• Why do synoptic-scale models degrade when they
  implement our more specific understandings of
  SBLs (e.g. sharp-tails)? Is it Ekman pumping or a
  complicated nonlinear interaction of multiple
  messy things. (And what precision is required for
  declaring success?)

6
Main unsolved problems (2)

• We are having problems generalizing the different
  types of SBLs. Lots of different kinds of SBLS,
  so its difficult to generalize (and simulate in
  the laboratory). How much do we need to
  categorize the unique phenomena before we can
  untangle them and generalize?

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Main unsolved problems (3)

• Some observations for testing existing
  formulations are lacking or unobservable. For
  example, how to define dynamical forcing (e.g.
  geostrophic wind, subsidence) or boundary
  conditions (e.g. roughness lengths).
• Although the diurnal cycle is important across
  many communities, transitions are not completely
  understood.

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Main unsolved problems (4)

• Dont have enough observations for model testing
  and validation representativity is an issue
  need more profiles need volume averages need
  fluxes over larger scales need reliable flux
  measurements in the surface layer
• Not much laboratory experimentation (too hard?)
• What is the impact of the SBLs on trace gas
  budgets and atmospheric chemistry?

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Main unsolved problems (5)

• What are we using as boundary-conditions for
  surface fluxes? We know that MO doesnt work very
  close to the surface. LES has the same problem.
• Long-term funding to address SBL problems or to
  establish and perpetuate collaborative efforts
  (especially between observationalists/ modelers/
  theoreticians) has been systematically lacking,
  particularly in the US.

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Low-hanging fruit (1)

• Modeling studies Use PCMDI/AMIP climate
  simulations (2m Arctic temperatures, low-level
  cloud albedo) to look at the influence of the
  stability functions and determine if SBL matters
  and how much. (Need to tease out the compensating
  errors.) Also, coupled ocean-atmosphere modeling
  efforts (ARCMIP regional climate models over the
  SHEBA area) modeling.
• Modeling Coastal sbl problem (warm air over cold
  water) could be solved with brute force
  (sufficient horizontal and vertical resolution in
  modeling).      

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Low-hanging fruit (2)

• Modeling Numerical experiment playing with
  length scales between the SBL and free
  troposphere could address issue of cyclone
  intensity is the SBL effect overemphasized?
• Observations and Modeling The long-lived
  boundary layer might be where the most progress
  can be made in high-latitude regions, we can
  integrate over a long enough time and over large
  enough spaces. We suggest establishing a
  long-term comprehensive observing site as a
  laboratory.  

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Low-hanging fruit (3)

• Observations and Modeling Focus on the nocturnal
  boundary layer mine the extensive datasets
  already collected. More collaboration between
  observationalists and modelers is needed to
  distill datasets into model-relevant
  boundary-layer parameters (sfc fluxes,
  boundary-layer height, dynamic forcing, etc.).
  Should also ensure collection of observations in
  different locations for model-tuning exercises.
• Modeling Forecast skill scores could be defined
  to include SBL parameters (2m T, PBL height, wind
  angle, etc.)

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Low-hanging fruit (4)

• Modeling More exploration of the question of
  vertical resolution over land could be fruitful.
• Modeling Can we declare success regarding the
  (highly-caveated) weakly-stable nocturnal
  boundary layer in mid-latitudes over land? Height
  predictions may be OK how about fluxes? And how
  precise do we need to be?

14
Hypothetical future fruit on trees that have just
been planted

• Observations Collect datasets with different
  stabilities, surface fluxes, boundary-layer
  heights, geostrophic winds, for testing models
  with. Profiles of winds, temperature, TKE, would
  be preferable.
• For climate modeling, the identification of the
  low-level inversions in the Arctic, which is due
  to interaction with the surface, is important.
  (Cant resolve sharp inversions with poor
  vertical resolution.) 2007/8 the International
  Polar Year     

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Complex problems

• Need a simplified expression of the
  strongly-stratified intermittent stable boundary
  layer.
• Gravity waves
• Katabatic flows and density currents
• Advection of turbulence (non-local)
• Enclosed basins and their stagnant decoupled cold
  pools