Grey Zone Discussion Group

1. Grey Zone Discussion Group Convective Parameterization: Progress and Challenges

2. What is the Grey Zone? • Multiple grey zones: “deep convective grey zone,” horizontal grid size around 3-30 km; “turbulence/shallow convective grey zone”, horizontal grid size around 0.3-3 km. • Deep convective grey zone: Important sub-grid convective structures not resolved, but likelihood that equilibrium assumptions important for closure do not hold. • “Soft grey zone” averages over distances greater than characteristic between-cloud separations. “Hard grey zone” averages over cloud length scales.

3. Parameterizations and Resolution • No clear consensus on what is meant by “scale-aware,” “scale-sensitive,” “scale-adaptive,” parameterization. • Possible definitions of these terms include: • parameterizations that systematically transition from parameterized to resolved flow as resolution increases; • parameterizations that simply respond to changes in resolved flow as resolution changes; • parameterizations that explicitly include information on grid partitioning (updraft fraction) (e.g., Arakawa-Wu formulation) or depend explicitly on grid size (e.g., Smagorinsky diffusion)

4. Current Practices and Research Strategies • Current modeling practices for deep convection in the grey zone: Most 10-20 km grid-size models use deep cumulus parameterizations, though there are a few exceptions. General agreement that these parameterizations require stochastic elements, though there is no clear sense of how to do this. • Current modeling practices for shallow convection in the grey zone: For models with horizontal grid sizes of 0.3-3 km, parameterization for shallow cumulus convection and large-eddy turbulence remains important and should have stochastic elements. • Current research consists of multiple modeling groups pursuing simulations of various test cases, using a variety of parameterization strategies.

5. Global Models at the Shallow Cumulus Grey Zone • Global climate models at the turbulence/shallow Cu grey zone (e.g., 3-km grid size): Evidence suggests they may do deep convection but not low clouds, so fail as climate models. If sub-grid turbulence (shallow Cu, Sc) can do low clouds (but note challenges to doing all-phase microphysics in HOC approaches), could be transformative.

6. Are O(3 km) Models Even Suitable for Deep Convection? • Limitations of 3-km models for modeling deep convection: unrealistic entrainment/detrainment, initiation problems, struggle to simulate weak squall lines, but can perform better than parametrized convection in some cases.

7. Challenges and Imperatives • Global models are likely to be in 5-50 km grid-size range for weather and climate. Significant concerns about some negative impacts on forecasts related to our limited understanding of the grey zone. • There has been limited application of advances already made. As a result, many parameterizations of the grey zone have been based on heuristic approaches guided by nothing more than practical considerations, e.g., running without parameterization may produce more realistic simulation, even though model-resolved motions are larger than observed phenomena, e.g., running at 1.5 km grid size to simulate BL rolls (too stable, too strong, Reynolds number too low….stochastic fixes?) • Organizing, testable hypotheses lacking at present for grey zone…limits research strategy.