Understanding Stable Boundary Layers: Progress and Challenges

1. 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)

2. What do we know now that we didn’t 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 didn’t 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 1980’s, LES of SBLs couldn’t be done. Many LES now converge in an encouraging way.

4. What do we know now that we didn’t 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.

5. 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 it’s 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?

7. 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.

8. Main unsolved problems (4): • Don’t 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?

9. Main unsolved problems (5): • What are we using as boundary-conditions for surface fluxes? We know that MO doesn’t 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.

10. 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).

11. 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.

12. 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.)

13. 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. (Can’t resolve sharp inversions with poor vertical resolution.) 2007/8 the International Polar Year

15. 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