Timothy M. Merlis's research while affiliated with Princeton University and other places

The climate simulation frontier of a global storm-resolving model (GSRM; or k -scale model because of its kilometer-scale horizontal resolution) is deployed for climate change simulations. The climate sensitivity, effective radiative forcing, and relative humidity changes are assessed in multiyear atmospheric GSRM simulations with perturbed sea-surface temperatures and/or carbon dioxide concentrations. Our comparisons to conventional climate model results can build confidence in the existing climate models or highlight important areas for additional research. This GSRM’s climate sensitivity is within the range of conventional climate models, although on the lower end as the result of neutral, rather than amplifying, shortwave feedbacks. Its radiative forcing from carbon dioxide is higher than conventional climate models, and this arises from a bias in climatological clouds and an explicitly simulated high-cloud adjustment. Last, the pattern and magnitude of relative humidity changes, simulated with greater fidelity via explicitly resolving convection, are notably similar to conventional climate models.

Plain Language Summary The main tool to investigate and make projections of the response of the climate system to warming is global climate models. These models usually have ∼100 km horizontal resolution and as a result need parameterizations in order to account for small‐scale processes. These parameterizations are a large source of uncertainty. Advancements in computational capabilities and technology allow us to run global models with high enough resolutions, called global storm‐resolving models (GSRMs), that some important small‐scale processes are explicitly resolved. In this study, we use two‐year‐long integrations of an atmosphere GSRM in present‐day and warmer climates and compare the precipitation response to more traditional global climate models. We find that the precipitation response of the GSRM lies within the range of the traditional models, except for the low percentiles of precipitation. A closer examination indicates that the main source of differences between the GSRM and traditional models occurs in midlatitude land regions, where the GSRM predicts more extensive drying of low precipitation percentiles compared to the traditional models. The GSRM also has an increase in precipitation extremes with warming that is somewhat weaker than that of traditional global climate models.

Cumulus entrainment substantially regulates the earth's climate but remains poorly constrained in global climate models. Recent studies have shown that cumulus bulk entrainment (or dilution) is particularly sensitive to continentality, with the entrainment rate in simulated maritime cumuli nearly double that of continental cumuli. This study examines the impacts of such land–ocean entrainment contrasts on the current climate using 21‐year simulations with the Geophysical Fluid Dynamics Laboratory's High‐Resolution Atmospheric Model (HIRAM). In response to a 25% reduction in the HIRAM entrainment parameter c0 over land, precipitation over tropical land regions increases by up to 40%. Along with directly facilitating enhanced convective precipitation, this c0 reduction induces an increase in soil moisture, which may contribute to a further enhancement of convective precipitation over land. A 25% c0 reduction over the oceans leads to more widespread modifications of convection patterns, with the strongest signal in the tropical Pacific. Deep convection shifts upstream (eastward) there, inducing enhanced large‐scale ascent over the central Pacific with compensating subsidence and reduced humidity and precipitation over the western Pacific (WP). Land–ocean variations in c0 project onto the Pacific Walker circulation, with the 25% land reduction strengthening it by 4% and the 25% ocean reduction weakening it by 14%. These changes are driven by variations in convective and large‐scale stratiform heating over the Pacific. While reduced c0 over land enhances diabatic heating in the Maritime Continent to strengthen the Walker circulation, reduced c0 over the oceans decreases diabatic heating in the WP to weaken the Walker circulation.

We develop a novel single-column model of clear-sky radiative-advective equilibrium where advective heating is internally determined by relaxing the column temperature and humidity toward fixed midlatitude profiles, consistent with an air-mass transformation perspective. The model reproduces observed polar temperature and advective heating rate profiles, and also captures many of the climate-change responses found in climate models. Exploring the model's physics, we show that the surface-based temperature inversion develops by ceding energy downwards to the surface, which then radiates this energy to space; we name this the ``surface radiator fin'' effect. We use the model to address three outstanding questions regarding polar climate change: (i) What mechanisms control polar lapse-rate change? (ii) What determines the known compensation between changes in dry and moist energy transport? and (iii) What is the most physically consistent way to decompose forcing and feedbacks at the poles? In answer to these questions, we show that: (i) Three mechanisms control the lapse-rate response to warming: weakening of the surface radiator fin, increased radiative cooling by free-tropospheric water vapor emission, and relaxation toward the external profile anomaly; all three increase the lapse rate as climate warms. (ii) Compensation between dry and moist advective heating results from a delicate balance between changes in the boundary layer and the free troposphere, with no constraints imposing precise compensation. (iii) Remote advective influence on the poles should be considered a forcing, while lapse-rate and advective heating changes should not be treated as separate feedbacks but rather as part of the temperature feedback.

Plain Language Summary Carbon dioxide (CO2), as an important greenhouse gas, is known to reduce the Earth's longwave emission, provoking a positive forcing that increases the net flow of energy into the Earth system. In this study, we discuss the cause of negative forcing, where CO2 increases longwave emission that happens most commonly in Antarctica and in some rare conditions in the Arctic and tropics. In contrast to conventional arguments that a near‐surface temperature increase with altitude is key to a negative CO2 forcing, we show that the stratospheric temperature and, in the tropics, clouds play a more important role. The results are based on temperature modification experiments and an analysis of the vertical structure of atmospheric emission changes. While a negative forcing does not mean the surface would cool since there are other important adjustments involved in the re‐establishment of energy balance, the results show the values of resolving the spectral dimension of radiation to quantify the radiative sensitivity to the near‐surface and stratosphere temperature structure.

Changes in tropical deep convection with global warming are a leading source of uncertainty for future climate projections. A comparison of the responses of active sensor measurements of cloud ice to interannual variability and next-generation global storm-resolving model (also known as k-scale models) simulations to global warming shows similar changes for events with the highest column-integrated ice. The changes reveal that the ice loading decreases outside the most active convection but increases at a rate of several percent per Kelvin surface warming in the most active convection. Disentangling thermodynamic and vertical velocity changes shows that the ice signal is strongly modulated by structural changes of the vertical wind field towards an intensification of strong convective updrafts with warming, suggesting that changes in ice loading are strongly influenced by changes in convective velocities, as well as a path toward extracting information about convective velocities from observations.

Changes in tropical deep convection with global warming are a leading source of uncertainty for future climate projections. A comparison of the responses of active sensor measurements of cloud ice to interannual variability and next-generation global storm-resolving model (also known as k-scale models) simulations to global warming shows similar changes for events with the highest column-integrated ice. The changes reveal that the ice loading decreases outside the most active convection but increases at a rate of several percent per Kelvin surface warming in the most active convection. Disentangling thermodynamic and vertical velocity changes shows that the ice signal is strongly modulated by structural changes of the vertical wind field towards an intensification of strong convective updrafts with warming, suggesting that changes in ice loading are strongly influenced by changes in convective velocities as well as a path toward extracting information about convective velocities from observations.

Plain Language Summary Given an increase in carbon dioxide concentration, individual radiative feedbacks stabilize or amplify the climate response. When diagnosed from comprehensive climate models, these feedbacks exhibit considerable variability, yet, single atmospheric columns have been successfully used as minimal models to quantify the effect of temperature and water vapor changes on global climate. Here, we bring together these perspectives by developing an analytical model for radiative feedbacks that, for projected changes, knows only of local surface temperature. That is, thermodynamic expressions for atmospheric temperature, water vapor, and sea‐ice albedo are combined with radiative kernels, which characterize the top‐of‐atmosphere radiative response to a small perturbation, to yield estimates of radiative feedbacks independent of the simulated atmospheric and cryospheric changes from a climate model. The analytical model captures the equator‐to‐pole and seasonal feedback structure, including in the ice‐covered polar regions, as well as the global feedback across an ensemble of global climate models. This research thus provides a framework for a quantitative understanding of radiative feedbacks from simple physics combined with geographic patterns of surface warming.

Atmospheric macroturbulence transports energy down the equator-to-pole gradient. This is represented by diffusion in energy balance models (EBMs), and EBMs have proven valuable to understanding and quantifying the pattern of surface temperature change. They typically assume climate-state independent diffusivity, chosen to well represent the current climate, and find that this is sufficient to emulate warming response in general circulation models (GCMs). Meanwhile, model diagnoses of GCM simulations have shown that the diffusivity changes with climate. There is also ongoing development for diffusivity theories based on atmospheric dynamics. Here, we examine the role that changes in diffusivity play in the large-scale equator-to-pole contrast in surface warming in EBMs, building on previous analytic EBM theories for polar amplified warming. New analytic theories for two formulations of climate-state dependent diffusivity capture the results of numerical EBM solutions. For reasonable choices of parameter values, the success of the new analytic theories reveals why the change of diffusivity is limited in response to radiative forcing and does not eliminate polar amplified warming.

An idealized aquaplanet moist global atmospheric model with realistic radiative transfer but no clouds and no convective parameterization is found to possess multiple climate equilibria. When forced symmetrically about the equator, in some cases the Inter Tropical Convergence Zone (ITCZ) migrates to an off‐equatorial equilibrium position. Mechanism denial experiments prescribing relative humidity imply that radiation‐circulation coupling is essential to this instability. The cross‐equatorial asymmetry occurs only when the underlying slab ocean is sufficiently deep and the atmosphere's spectral dynamical core is sufficiently coarse (∼T170 or less with our control parameters). At higher resolutions, initializing with an asymmetric state indicates metastability with very slow (thousands of days) return to hemispheric symmetry. There is some sensitivity to the model timestep, which affects the time required to transition to the asymmetric state, with little effect on the equilibrium climate. The instability is enhanced when the planetary boundary layer scheme favors deeper layers or by a prescribed meridional heat transport away from the equator within the slab. The instability is not present when the model is run with a convective parameterization scheme commonly utilized in idealized moist models. We argue that the instability occurs when the asymmetric heating associated with a spontaneous ITCZ shift drives a circulation that rises poleward of the perturbed ITCZ. These results serve as a warning of the potential for instability and non‐uniqueness of climate that may complicate studies with idealized models of the tropical response to perturbations in forcing.