~3-Hour Delay
Peak land precipitation shifts later, closer to observed afternoon timing.
Model Development and Evaluation
JAMES (2024)
Improved Precipitation Diurnal Cycle in GFDL Climate Models With Non-Equilibrium Convection
A non-equilibrium shallow-deep convective closure improves land precipitation diurnal phase and amplitude by better coupling CAPE buildup and convective release timing.
AM4 + CM4
Diurnal Cycle
Non-Equilibrium Convection
DOI: 10.1029/2024MS004315
~50% Bias Reduction
Diurnal phase bias is reduced by about half compared with satellite references.
Targeted Improvement
Mean state and variability are only modestly changed outside the diurnal cycle.
Paper Citation
Zhang, B., L. J. Donner, M. Zhao, and Z. Tan, 2024: Improved precipitation diurnal cycle in GFDL climate models with non-equilibrium convection. Journal of Advances in Modeling Earth Systems, 16, e2024MS004315. https://doi.org/10.1029/2024MS004315
Scientific Logic
- Question: Can non-equilibrium coupling between shallow and deep convection correct the land precipitation diurnal-cycle bias?
- Method: Implementation of a new closure in AM4/CM4 with coupled shallow-deep convection and evaluation against observed diurnal structure.
- Mechanism: Allowing deep convection to lag rapidly varying boundary-layer forcing improves the phase relation between CAPE buildup and convective release.
- Main Findings: The revised closure substantially improves timing and amplitude of summertime land precipitation diurnal cycles with limited degradation elsewhere.
Key Findings
- The revised closure reduces the common too-early afternoon convection bias over land.
- Improvements are retained in both atmosphere-only and coupled model configurations.
- Out-of-equilibrium convection physics yields better diurnal realism with limited large-scale degradation.
Scientific Question
Can convection closure be reformulated so GCMs better capture observed subdiurnal land convection timing without degrading broader climate behavior?
Method
- Introduce non-equilibrium interaction between shallow and deep convection in convective closure.
- Apply consistently in atmosphere-only AM4 and coupled CM4 configurations.
- Evaluate diurnal phase, amplitude, and broader climatological impacts versus observations.
Closure Framework
The non-equilibrium closure augments deep-convective CAPE relaxation using shallow-convective CAPE tendencies to represent subdiurnal boundary-layer control.
\[ \left(\frac{\partial \text{CAPE}}{\partial t}\right)_{\text{deep}} = -\frac{\text{CAPE} - \text{CAPE}_0}{\tau} - \left(\frac{\partial \text{CAPE}}{\partial t}\right)_{\text{shal}} \]
Key Animation
CAPE Tendency Cancellation Is the Core Mechanism
Shallow-convective CAPE tendency (left) and non-convective boundary-layer CAPE tendency (right) largely cancel, delaying deep-convective peak timing over land.
Figures from the Study
Diagnostic Figure 1
Diagnostic Figure 2
