Impact of the NCEP TKE‐Based Eddy‐Diffusivity Mass‐Flux Boundary Layer Scheme on the Climatology and Warming Response of GFDL AM4.0 Model

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Abstract
National Centers for Environmental Prediction turbulent kinetic energy (TKE)‐based eddy‐diffusivity mass‐flux (EDMF) scheme is implemented in Geophysical Fluid Dynamics Laboratory atmospheric model (AM4.0) for improving the physical consistency of subgrid‐scale planetary boundary layer (PBL) turbulence parameterization. The mass flux (MF) component represents vertically coherent convective structures responsible for countergradient transport in the upper PBL, which the original AM4.0's ED‐only scheme cannot represent. Consequently, AM4.0 with EDMF produces a deeper and more well‐mixed PBL, leading to better zonal‐mean vertical temperature and humidity profiles and reduced near‐surface wet bias over subtropical and midlatitude oceans. Other model performance changes are generally minor, such as similar biases in global top of atmosphere (TOA) net radiation and shortwave cloud radiative effects, small and compensating changes in low cloud amount and cloud liquid water path, improved low‐level equatorial easterlies but deteriorated extratropical westerlies, slightly increased global‐mean precipitation, and weaker TOA radiative response to uniform sea surface warming. Three adaptations of EDMF are important for its performance at AM4.0's relatively coarse vertical resolution: limiting the overshoot of MF updraft above PBL‐top, reducing the ED‐induced mixing across PBL‐top, and disabling the MF transport of TKE. Low clouds and their radiative effects are also sensitive to four EDMF parameters that control the ED in the lower and upper PBL respectively, the TKE dissipation rate, and the lateral entrainment of MF updraft and downdraft. An automatic linear tuning of these parameters slightly improves the radiative bias, especially for the coastal stratocumulus. More substantial improvements likely require formulation updates of the EDMF scheme and its coupling with other AM4.0 model components.
Funding Information
  • NOAA Research (NA18OAR4320123)
  • NOAA Research (NA23OAR4320198)

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