Martin Köhler

A Dual Mass Flux Framework for Boundary Layer Convection. Part I: Transport

Abstract This study considers the question of what is the least complex bulk mass flux framework that can still conceptually reproduce the smoothly varying coupling between the shallow convective cloud layer and the subcloud mixed layer. To this end, the model complexity of the classic single bulk mass flux scheme is enhanced. Inspired by recent large-eddy simulation results, the authors argue that two relatively minor but key conceptual modifications are already sufficient to achieve this goal: (i) retaining a dry transporting updraft in the moist limit and (ii) applying continuous updraft area partitioning to this dual mass flux (DualM) framework. The dry updraft represents all internal mixed layer updrafts that terminate near the mixed layer top, whereas the moist updraft represents all updrafts that condense and rise out of the mixed layer as buoyant cumulus clouds. The continuous area partitioning between the dry and moist updraft is a function of moist convective inhibition above the mixed layer top...

CGILS: Results from the First Phase of an International Project to Understand the Physical Mechanisms of Low Cloud Feedbacks in Single Column Models

CGILS—the CFMIP-GASS Intercomparison of Large Eddy Models (LESs) and single column models (SCMs)—investigates the mechanisms of cloud feedback in SCMs and LESs under idealized climate change perturbation. This paper describes the CGILS results from 15 SCMs and 8 LES models. Three cloud regimes over the subtropical oceans are studied: shallow cumulus, cumulus under stratocumulus, and well-mixed coastal stratus/stratocumulus. In the stratocumulus and coastal stratus regimes, SCMs without activated shallow convection generally simulated negative cloud feedbacks, while models with active shallow convection generally simulated positive cloud feedbacks. In the shallow cumulus alone regime, this relationship is less clear, likely due to the changes in cloud depth, lateral mixing, and precipitation or a combination of them. The majority of LES models simulated negative cloud feedback in the well-mixed coastal stratus/stratocumulus regime, and positive feedback in the shallow cumulus and stratocumulus regime. A general framework is provided to interpret SCM results: in a warmer climate, the moistening rate of the cloudy layer associated with the surface-based turbulence parameterization is enhanced; together with weaker large-scale subsidence, it causes negative cloud feedback. In contrast, in the warmer climate, the drying rate associated with the shallow convection scheme is enhanced. This causes positive cloud feedback. These mechanisms are summarized as the “NESTS” negative cloud feedback and the “SCOPE” positive cloud feedback (Negative feedback from Surface Turbulence under weaker Subsidence—Shallow Convection PositivE feedback) with the net cloud feedback depending on how the two opposing effects counteract each other. The LES results are consistent with these interpretations.

Comparison of river basin hydrometeorology in ERA-Interim and ERA-40 reanalyses with observations

[1] The changes between the ERA-40 and ERA-Interim in the seasonal cycle of primarily temperature, precipitation and evaporation, the surface radiation budget, and the cloud fields are evaluated over three river basins, the Amazon, Mississippi, and Mackenzie, for the period 1990–2001, using a variety of surface observational data sets and the International Satellite Cloud Climatology Project data. In ERA-Interim over the Amazon, the unrealistic interannual drift of precipitation has been reduced, and annual precipitation is largely unbiased, although the seasonal amplitude of precipitation remains too small. However, ERA-Interim has a large cold 2-m temperature bias. The clear-sky surface shortwave flux in ERA-Interim is lower than that in ERA-40 and closer to observations. Low cloud cover has increased dramatically in ERA-Interim, and total reflective cloud cover has a larger positive bias in comparison with observations. The ratio of the precipitation heating of the atmosphere to the surface shortwave cloud forcing is much higher in the observations than that in both reanalyses. The diurnal cycle of precipitation has improved somewhat with the removal of a spurious early morning peak. For the Mississippi and Mackenzie river basins, the spin-up of precipitation in 24-h forecasts has been greatly reduced. Temperature biases are small in both reanalyses, but summer precipitation and evaporation exceed observational estimates. For the Mississippi river basin, reflective cloud cover in ERA-Interim has increased in winter and decreased in summer compared with that in ERA-40, giving a closer fit to the observations in both seasons. For the Mackenzie river basin, similar reflective cloud changes in ERA-Interim improve the fit to the observations in summer but not in winter.