A Weak Temperature Gradient Framework to Quantify the Causes of Potential Intensity Variability in the Tropics

Potential intensity (PI) has been shown to have a linear sensitivity to sea surface temperature (SST) of about 8 m s−1 K−1, which is close to the sensitivity of PI in simulations subject to a weak temperature gradient (WTG) approximation. This suggests that most of the PI variance is associated with local rather than global SST variations. We verify that PI perturbations are approximately linear in SST, with slopes of 1.8 ± 0.2 m s−1 K−1 in radiative–convective equilibrium (RCE) and 9.1 ± 0.9 m s−1 K−1 in WTG. To do so, we simulate the sensitivity of both RCE and WTG states in a single-column model (SCM) perturbed by changing in turn CO2 concentration, aerosol concentrations, prescribed SST, and surface winds speeds. While PI is much more sensitive to SST in WTG than in RCE simulations, the SST itself is much less sensitive to radiative forcing in WTG than in RCE because of the absence of strong atmospheric response. Using these results, we develop a linear model, based on SST and midlevel saturation MSE perturbations, to partition SST and PI perturbations between local components occurring under a WTG constraint and global components that are representative of an RCE state. This model explains up to 95% of the variability of PI in reanalysis. The SCM-derived linear model coefficients are statistically indistinguishable from coefficients from a linear fit of reanalysis PI to SST and midlevel saturation MSE in most ocean basins. Our model shows that North Atlantic PI variations are explained almost entirely by local forcings in recent decades.

Decomposing the Drivers of Polar Amplification with a Single Column Model

The precise mechanisms driving Arctic amplification are still under debate. Previous attribution methods compute the vertically-uniform temperature change required to balance the top-of-atmosphere energy imbalance caused by each forcing and feedback, with any departures from vertically-uniform warming collected into the lapse-rate feedback. We propose an alternative attribution method using a single column model that accounts for the forcing-dependence of high latitude lapse-rate changes. We examine this method in an idealized General Circulation Model (GCM), finding that, even though the column-integrated carbon dioxide (CO2) forcing and water vapor feedback are stronger in the tropics, they contribute to polar-amplified surface warming as they produce bottom-heavy warming in high latitudes. A separation of atmospheric temperature changes into local and remote contributors shows that, in the absence of polar surface forcing (e.g., sea-ice retreat), changes in energy transport are primarily responsible for the polar amplified pattern of warming. The addition of surface forcing substantially increases polar surface warming and reduces the contribution of atmospheric dry static energy transport to the warming. This physically-based attribution method can be applied to comprehensive GCMs to provide a clearer view of the mechanisms behind Arctic amplification.

Improvements in Supercooled Liquid Water Simulations of Low-Level Mixed-Phase Clouds over the Southern Ocean Using a Single-Column Model

A high-resolution global atmospheric model, the nonhydrostatic icosahedral atmospheric model (NICAM), exhibited underestimation biases in low-level mixed-phase clouds in the midlatitudes and polar regions. The ice-cloud microphysics used in a single-moment bulk cloud microphysics scheme (NSW6) was evaluated and improved using a single-column model by reference to a double-moment bulk cloud microphysics scheme (NDW6). Budget analysis indicated that excessive action of the Bergeron–Findeisen and riming processes crucially reduced supercooled liquid water. In addition, the rapid production of rain directly reduced cloud water and indirectly reduced cloud water through the production of snow and graupel by riming. These biases in growth rates were found to originate from the number concentration diagnosis used in NSW6. The diagnosis based on the midlatitude cloud systems assumption was completely different from the one for low-level mixed-phase clouds. To alleviate underestimation biases, rain production, heterogeneous ice nucleation, vapor deposition by snow and graupel, and riming processes were revised. The sequential revisions of cloud microphysics alleviated the underestimation biases step by step without parameter tuning. The lifetime of cloud layers simulated by NSW6 was reasonably prolonged.

Using a single column model (SGRIST1.0) for connecting model physics and dynamics in the Global-to-Regional Integrated forecast SysTem (GRIST-A20.8)

A single column model (SGRIST1.0) is developed as a tool for coupling a full-physics package (from Community Atmosphere Model, version 5 (CAM5)) to the Global-to-Regional Integrated forecast System (GRIST). In a two-step approach, the full-physics package is first isolated and coupled to SGRIST1.0 for reducing the uncertainties associated with model physics and assessing its behavior, then assimilated by the model dynamical framework. In the first step, SGRIST1.0 serves as a tool for evaluating the physical parameterization suite in the absence of 3D dynamics. Three single column model test cases, including the tropical deep convection, shallow convection, and stratocumulus, demonstrate that the parameterization suite mimics the behaviors in the observations and the reference model (SCAM) outputs. Cloud fraction, cloud liquid, and some other micro- and macro-physical variables are sensitive to the model time step, suggesting time-step dependency of the corresponding parameterization schemes. The second step couples the physics package to the 3D dynamical modeling system, and the verified parameterization suite works well in GRIST. Two physics-dynamics coupling strategies are examined and found to have a clear impact on the intensity of the simulated storm. The incremental operator splitting strategy (ptend_f1_f1), produces a weaker storm than the pure operator splitting strategy (ptend_f2_sudden). Comparing these two splitting approaches, the ptend_f2_sudden coupling strategy has higher large-step stability than the ptend_f1_f1 option, but the intensity of the simulated storm is substantially reduced by ptend_f2_sudden provided that the time step becomes quite large. Some detailed model configuration strategies are suggested when using the CAM5 parameterization suite in GRIST.

The E3SM version 1 single-column model

The single-column model (SCM) functionality of the Energy Exascale Earth System Model version 1 (E3SMv1) is described in this paper. The E3SM SCM was adopted from the SCM used in the Community Atmosphere Model (CAM) but has evolved significantly since then. We describe changes made to the aerosol specification in the SCM, idealizations, and developments made so that the SCM uses the same dynamical core as the full general circulation model (GCM) component. Based on these changes, we describe and demonstrate the seamless capability to “replay” a GCM column using the SCM. We give an overview of the E3SM case library and briefly describe which cases may serve as useful proxies for replicating and investigate some long-standing biases in the full GCM runs while demonstrating that the E3SM SCM is an efficient tool for both model development and evaluation.