Showing papers in "Journal of Advances in Modeling Earth Systems in 2018"

A higher-resolution version of the Max Planck Institute Earth System Model (MPI-ESM1.2-HR)

Abstract: The MPI‐ESM1.2 is the latest version of the Max Planck Institute Earth System Model and is the baseline for the Coupled Model Intercomparison Project Phase 6 and current seasonal and decadal climate predictions. This paper evaluates a coupled higher‐resolution version (MPI‐ESM1.2‐HR) in comparison with its lower‐resolved version (MPI‐ESM1.2‐LR). We focus on basic oceanic and atmospheric mean states and selected modes of variability, the El Nino/Southern Oscillation and the North Atlantic Oscillation. The increase in atmospheric resolution in MPI‐ESM1.2‐HR reduces the biases of upper‐level zonal wind and atmospheric jet stream position in the northern extratropics. This results in a decrease of the storm track bias over the northern North Atlantic, for both winter and summer season. The blocking frequency over the European region is improved in summer, and North Atlantic Oscillation and related storm track variations improve in winter. Stable Atlantic meridional overturning circulations are found with magnitudes of ~16 Sv for MPI‐ESM1.2‐HR and ~20 Sv for MPI‐ESM1.2‐LR at 26°N. A strong sea surface temperature bias of ~5°C along with a too zonal North Atlantic current is present in both versions. The sea surface temperature bias in the eastern tropical Atlantic is reduced by ~1°C due to higher‐resolved orography in MPI‐ESM‐HR, and the region of the cold‐tongue bias is reduced in the tropical Pacific. MPI‐ESM1.2‐HR has a well‐balanced radiation budget and its climate sensitivity is explicitly tuned to 3 K. Although the obtained reductions in long‐standing biases are modest, the improvements in atmospheric dynamics make this model well suited for prediction and impact studies.

Development and Validation of the Whole Atmosphere Community Climate Model With Thermosphere and Ionosphere Extension (WACCM‐X 2.0)

Abstract: Key developments have been made to the NCAR Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM‐X). Among them, the most important are the self‐consistent solution of global electrodynamics, and transport of O+ in the F‐region. Other ionosphere developments include time‐dependent solution of electron/ion temperatures, metastable O+ chemistry, and high‐cadence solar EUV capability. Additional developments of the thermospheric components are improvements to the momentum and energy equation solvers to account for variable mean molecular mass and specific heat, a new divergence damping scheme, and cooling by O(3P) fine structure. Simulations using this new version of WACCM‐X (2.0) have been carried out for solar maximum and minimum conditions. Thermospheric composition, density, and temperatures are in general agreement with measurements and empirical models, including the equatorial mass density anomaly and the midnight density maximum. The amplitudes and seasonal variations of atmospheric tides in the mesosphere and lower thermosphere are in good agreement with observations. Although global mean thermospheric densities are comparable with observations of the annual variation, they lack a clear semiannual variation. In the ionosphere, the low‐latitude E × B drifts agree well with observations in their magnitudes, local time dependence, seasonal, and solar activity variations. The prereversal enhancement in the equatorial region, which is associated with ionospheric irregularities, displays patterns of longitudinal and seasonal variation that are similar to observations. Ionospheric density from the model simulations reproduces the equatorial ionosphere anomaly structures and is in general agreement with observations. The model simulations also capture important ionospheric features during storms.

Using Machine Learning to Parameterize Moist Convection: Potential for Modeling of Climate, Climate Change, and Extreme Events

Abstract: The parameterization of moist convection contributes to uncertainty in climate modeling and numerical weather prediction. Machine learning (ML) can be used to learn new parameterizations directly from high-resolution model output, but it remains poorly understood how such parameterizations behave when fully coupled in a general circulation model (GCM) and whether they are useful for simulations of climate change or extreme events. Here, we focus on these issues using idealized tests in which an ML-based parameterization is trained on output from a conventional parameterization and its performance is assessed in simulations with a GCM. We use an ensemble of decision trees (random forest) as the ML algorithm, and this has the advantage that it automatically ensures conservation of energy and non-negativity of surface precipitation. The GCM with the ML convective parameterization runs stably and accurately captures important climate statistics including precipitation extremes without the need for special training on extremes. Climate change between a control climate and a warm climate is not captured if the ML parameterization is only trained on the control climate, but it is captured if the training includes samples from both climates. Remarkably, climate change is also captured when training only on the warm climate, and this is because the extratropics of the warm climate provides training samples for the tropics of the control climate. In addition to being potentially useful for the simulation of climate, we show that ML parameterizations can be interrogated to provide diagnostics of the interaction between convection and the large-scale environment.

The GFDL Global Atmosphere and Land Model AM4.0/LM4.0: 2. Model Description, Sensitivity Studies, and Tuning Strategies

Abstract: In Part II of this two-part paper, documentation is provided of key aspects of a version of the AM4.0/LM4.0 atmosphere/land model that will serve as a base for a new set of climate and Earth system models (CM4 and ESM4) under development at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). The quality of the simulation in AMIP (Atmospheric Model Intercomparison Project) mode has been provided in Part I. Part II provides documentation of key components and some sensitivities to choices of model formulation and values of parameters, highlighting the convection parameterization and orographic gravity wave drag. The approach taken to tune the model's clouds to observations is a particular focal point. Care is taken to describe the extent to which aerosol effective forcing and Cess sensitivity have been tuned through the model development process, both of which are relevant to the ability of the model to simulate the evolution of temperatures over the last century when coupled to an ocean model.

The International Land Model Benchmarking (ILAMB) System: Design, Theory, and Implementation

Abstract: Author(s): Collier, N; Hoffman, FM; Lawrence, DM; Keppel-Aleks, G; Koven, CD; Riley, WJ; Mu, M; Randerson, JT | Abstract: The increasing complexity of Earth system models has inspired efforts to quantitatively assess model fidelity through rigorous comparison with best available measurements and observational data products. Earth system models exhibit a high degree of spread in predictions of land biogeochemistry, biogeophysics, and hydrology, which are sensitive to forcing from other model components. Based on insights from prior land model evaluation studies and community workshops, the authors developed an open source model benchmarking software package that generates graphical diagnostics and scores model performance in support of the International Land Model Benchmarking (ILAMB) project. Employing a suite of in situ, remote sensing, and reanalysis data sets, the ILAMB package performs comprehensive model assessment across a wide range of land variables and generates a hierarchical set of web pages containing statistical analyses and figures designed to provide the user insights into strengths and weaknesses of multiple models or model versions. Described here is the benchmarking philosophy and mathematical methodology embodied in the most recent implementation of the ILAMB package. Comparison methods unique to a few specific data sets are presented, and guidelines for configuring an ILAMB analysis and interpreting resulting model performance scores are discussed. ILAMB is being adopted by modeling teams and centers during model development and for model intercomparison projects, and community engagement is sought for extending evaluation metrics and adding new observational data sets to the benchmarking framework.

The GFDL Global Atmosphere and Land Model AM4.0/LM4.0: 1. Simulation Characteristics With Prescribed SSTs

Abstract: In this two-part paper, a description is provided of a version of the AM4.0/LM4.0 atmosphere/ land model that will serve as a base for a new set of climate and Earth system models (CM4 and ESM4) under development at NOAA’s Geophysical Fluid Dynamics Laboratory (GFDL). This version, with roughly 100 km horizontal resolution and 33 levels in the vertical, contains an aerosol model that generates aerosol fields from emissions and a ‘‘light’’ chemistry mechanism designed to support the aerosol model but with prescribed ozone. In Part 1, the quality of the simulation in AMIP (Atmospheric Model Intercomparison Project) mode—with prescribed sea surface temperatures (SSTs) and sea-ice distribution—is described and compared with previous GFDL models and with the CMIP5 archive of AMIP simulations. The model’s Cess sensitivity (response in the top-of-atmosphere radiative flux to uniform warming of SSTs) and effective radiative forcing are also presented. In Part 2, the model formulation is described more fully and key sensitivities to aspects of the model formulation are discussed, along with the approach to model tuning.

Structure and Evolution of Mesoscale Convective Systems: Sensitivity to Cloud Microphysics in Convection-Permitting Simulations Over the United States

Abstract: Regional climate simulations over the continental United States were conducted for the 2011 warm season using the Weather Research and Forecasting model at convection-permitting resolution (4 km) with two commonly used microphysics parameterizations (Thompson and Morrison). Sensitivities of the simulated mesoscale convective system (MCS) properties and feedbacks to large-scale environments are systematically examined against high-resolution geostationary satellite and 3-D mosaic radar observations. MCS precipitation including precipitation amount, diurnal cycle, and distribution of hourly precipitation intensity are reasonably captured by the two simulations despite significant differences in their simulated MCS properties. In general, the Thompson simulation produces better agreement with observations for MCS upper level cloud shield and precipitation area, convective feature horizontal and vertical extents, and partitioning between convective and stratiform precipitation. More importantly, Thompson simulates more stratiform rainfall, which agrees better with observations and results in top-heavier heating profiles from robust MCSs compared to Morrison. A stronger dynamical feedback to the large-scale environment is therefore seen in Thompson, wherein an enhanced mesoscale vortex behind the MCS strengthens the synoptic-scale trough and promotes advection of cool and dry air into the rear of the MCS region. The latter prolongs the MCS lifetimes in the Thompson relative to the Morrison simulations. Hence, different treatment of cloud microphysics not only alters MCS convective-scale dynamics but also has significant impacts on their macrophysical properties such as lifetime and precipitation. As long-lived MCSs produced 2–3 times the amount of rainfall compared to short-lived ones, cloud microphysics parameterizations have profound impact in simulating extreme precipitation and the hydrologic cycle. Plain Language Summary Massive thunderstorms over the Great Plains of the United States have become more frequent and more intense in the past decades. As Earth continues to warm, changes in the characteristics of these massive thunderstorms, which often cause flooding and severe wind damage, have major societal implications. Climate models with spatial resolution comparable to weather forecasting models can now be used to simulate the complex physics in storms and reproduce their climatological properties. However, details of how to represent the cloud microphysical processes remain uncertain, with potential implications for long-term simulation of climate in regions of convective storms. This study examines the uncertainties associated with cloud microphysics of convective storms in the central United States by using two different microphysical representations and comparing results with a warm-season satellite and radar observations. Microphysical processes leading to a broader and more realistic storm rainfall areas favor prolonged lifetime of the storms and thus have greater effects on the evolution of the large-scale circulation and greater potential for storms producing floods, factors important for evaluating the effects of convective storms in a changing climate.

ICON-A, the Atmosphere Component of the ICON Earth System Model: I. Model Description

Abstract: ICON-A is the new icosahedral nonhydrostatic (ICON) atmospheric general circulation model in a configuration using the Max Planck Institute physics package, which originates from the ECHAM6 general circulation model, and has been adapted to account for the changed dynamical core framework. The coupling scheme between dynamics and physics employs a sequential updating by dynamics and physics, and a fixed sequence of the physical processes similar to ECHAM6. To allow a meaningful initial comparison between ICON-A and the established ECHAM6-LR model, a setup with similar, low resolution in terms of number of grid points and levels is chosen. The ICON-A model is tuned on the base of the Atmospheric Model Intercomparison Project (AMIP) experiment aiming primarily at a well balanced top-of atmosphere energy budget to make the model suitable for coupled climate and Earth system modeling. The tuning addresses first the moisture and cloud distribution to achieve the top-of-atmosphere energy balance, followed by the tuning of the parameterized dynamic drag aiming at reduced wind errors in the troposphere. The resulting version of ICON-A has overall biases, which are comparable to those of ECHAM6. Problematic specific biases remain in the vertical distribution of clouds and in the stratospheric circulation, where the winter vortices are too weak. Biases in precipitable water and tropospheric temperature are, however, reduced compared to the ECHAM6. ICON-A will serve as the basis of further development and as the atmosphere component to the coupled model, ICON-Earth system model (ESM). Plain Language Summary ICON-A is a new atmospheric model as needed for research on the general circulation of the atmosphere, or as atmospheric component in an Earth system model, as used in climate research. This article describes the construction of the atmospheric model, in particular how two major parts are coupled to each other: “dynamics” and “physics.” Dynamics is the part that solves the equations for the atmospheric motion, temperature, density, and concentrations of water vapor, cloud water, and cloud ice. Physics is the part that computes the changes in these fields related to processes like radiation, cloud condensation, or turbulence. These physical changes depend on the state of the atmosphere as computed by the dynamics, and the changes computed by physics force change in the dynamics. The article documents the details of this construction. Further, the article describes how the physics is tuned to obtain a good representation of the general circulation of the period 1979 to 1988 in comparison to observations. A more detailed evaluation of such simulations is presented in a companion article by Crueger et al. (2018, https://doi.org/10.1029/2017MS001233).

The Low-Resolution Version of HadGEM3 GC3.1: Development and Evaluation for Global Climate.

Abstract: A new climate model, HadGEM3 N96ORCA1, is presented that is part of the GC3.1 configuration of HadGEM3. N96ORCA1 has a horizontal resolution of ~135 km in the atmosphere and 1° in the ocean and requires an order of magnitude less computing power than its medium-resolution counterpart, N216ORCA025, while retaining a high degree of performance traceability. Scientific performance is compared both to observations and the N216ORCA025 model. N96ORCA1 reproduces observed climate mean and variability almost as well as N216ORCA025. Patterns of biases are similar across the two models. In the north-west Atlantic, N96ORCA1 shows a cold surface bias of up to 6K, typical of ocean models of this resolution. The strength of the Atlantic meridional overturning circulation (16 to 17 Sv) matches observations. In the Southern Ocean, a warm surface bias (up to 2K) is smaller than in N216ORCA025 and linked to improved ocean circulation. Model El Nino/Southern Oscillation and Atlantic Multidecadal Variability are close to observations. Both the cold bias in the Northern hemisphere (N96ORCA1) and the warm bias in the Southern hemisphere (N216ORCA025) develop in the first few decades of the simulations. As in many comparable climate models, simulated interhemispheric gradients of top-of-atmosphere radiation are larger than observations suggest, with contributions from both hemispheres. HadGEM3 GC3.1 N96ORCA1 constitutes the physical core of the UK Earth System Model (UKESM1) and will be used extensively in the Coupled Model Intercomparison Project 6 (CMIP6), both as part of UKESM1 and as a stand-alone coupled climate model.

Understanding Cloud and Convective Characteristics in Version 1 of the E3SM Atmosphere Model

Abstract: This study provides comprehensive insight into the notable differences in clouds and precipitation simulated by the Energy Exascale Earth System Model Atmosphere Model version 0 and version 1 (EAMv1). Several sensitivity experiments are conducted to isolate the impact of changes in model physics, resolution, and parameter choices on these differences. The overall improvement in EAMv1 clouds and precipitation is primarily attributed to the introduction of a simplified third-order turbulence parameterization Cloud Layers Unified By Binormals (along with the companion changes) for a unified treatment of boundary layer turbulence, shallow convection, and cloudmacrophysics, though it also leads to a reduction in subtropical coastal stratocumulus clouds. This lack of stratocumulus clouds is considerably improved by increasing vertical resolution from 30 to 72 layers, but the gain is unfortunately subsequently offset by other retuning to reach the top-of-atmosphere energy balance. Increasing vertical resolution also results in a considerable underestimation of high clouds over the tropical warm pool, primarily due to the selection for numerical stability of a higher air parcel launch level in the deep convection scheme. Increasing horizontal resolution from 1° to 0.25° without retuning leads to considerable degradation in cloud and precipitation fields, with much weaker tropical and subtropical shortand longwave cloud radiative forcing and much stronger precipitation in the intertropical convergence zone, indicating poor scale awareness of the cloud parameterizations. To avoid this degradation, significantly different parameter settings for the low-resolution (1°) and high-resolution (0.25°) were required to achieve optimal performance in EAMv1. Plain Language Summary The Energy Exascale Earth System Model (E3SM) is a new and ongoing U.S. Department of Energy (DOE) climate modeling effort to develop a high-resolution Earth system model specifically targeting next-generation DOE supercomputers to meet the science needs of the nation and the mission needs of DOE. The increase of model resolution along with improvements in representing cloud and convective processes in the E3SM atmosphere model version 1 has led to quite significant model behavior changes from its earlier version, particularly in simulated clouds and precipitation. To understand what causes the model behavior changes, this study conducts sensitivity experiments to isolate the impact of changes in model physics, resolution, and parameter choices on these changes. Results from these sensitivity tests and discussions on the underlying physical processes provide substantial insight into the model errors and guidance for future E3SM development.

Improved aerosol processes and effective radiative forcing in HadGEM3 and UKESM1

Abstract: Aerosol processes and, in particular, aerosol-cloud interactions cut across the traditional physical-Earth system boundary of coupled Earth system models and remain one of the key uncertainties in estimating anthropogenic radiative forcing of climate. Here we calculate the historical aerosol effective radiative forcing (ERF) in the HadGEM3-GA7 climate model in order to assess the suitability of this model for inclusion in the UK Earth system model, UKESM1. The aerosol ERF, calculated for the year 2000 relative to 1850, is large and negative in the standard GA7 model leading to an unrealistic negative total anthropogenic forcing over the twentieth century. We show how underlying assumptions and missing processes in both the physical model and aerosol parameterizations lead to this large aerosol ERF. A number of model improvements are investigated to assess their impact on the aerosol ERF. These include an improved representation of cloud droplet spectral dispersion, updates to the aerosol activation scheme, and black carbon optical properties. One of the largest contributors to the aerosol forcing uncertainty is insufficient knowledge of the preindustrial aerosol climate. We evaluate the contribution of uncertainties in the natural marine emissions of dimethyl sulfide and organic aerosol to the ERF. The combination of model improvements derived from these studies weakens the aerosol ERF by up to 50% of the original value and leads to a total anthropogenic historical forcing more in line with assessed values.

A Flexible and Efficient Radiation Scheme for the ECMWF Model

Abstract: This paper describes a new radiation scheme ecRad for use both in the model of the European Centre for Medium‐Range Weather Forecasts (ECMWF), and off‐line for noncommercial research. Its modular structure allows the spectral resolution, the description of cloud and aerosol optical properties, and the solver, to be changed independently. The available solvers include the Monte Carlo Independent Column Approximation (McICA), Tripleclouds, and the Speedy Algorithm for Radiative Transfer through Cloud Sides (SPARTACUS), the latter which makes ECMWF the first global model capable of representing the 3‐D radiative effects of clouds. The new implementation of the operational McICA solver produces less noise in atmospheric heating rates, and is 41% faster, which can yield indirect forecast skill improvements via calling the radiation scheme more frequently. We demonstrate how longwave scattering may be implemented for clouds but not aerosols, which is only 4% more computationally costly overall than neglecting longwave scattering and yields further modest forecast improvements. It is also shown how a sequence of radiation changes in the last few years has led to a substantial reduction in stratospheric temperature biases.

An Environmentally Forced Tropical Cyclone Hazard Model

Abstract: A new statistical-dynamical model is developed for estimating the long-term hazard of rare, high impact tropical cyclones events globally. There are three components representing the complete storm lifetime: an environmental index-based genesis model, a beta-advection track model, and an autoregressive intensity model. All three components depend upon the local environmental conditions, including potential intensity, relative sea surface temperature, 850 and 250 hPa steering flow, deep-layer mean vertical shear, 850 hPa vorticity, and midlevel relative humidity. The hazard model, using 400 realizations of a 32 year period (approximately 3,000 storms per realization), captures many aspects of tropical cyclone statistics, such as genesis and track density distribution. Of particular note, it simulates the observed number of rapidly intensifying storms, a challenging issue in tropical cyclone modeling and prediction. Using the return period curve of landfall intensity as a measure of local tropical cyclone hazard, the model reasonably simulates the hazard in the western north Pacific (coastal regions of the Philippines, China, Taiwan, and Japan) and the Caribbean islands. In other regions, the observed return period curve can be captured after a local landfall frequency adjustment that forces the total number of landfalls to be the same as that observed while allowing the model to freely simulate the distribution of intensities at landfall.

The KPP Boundary Layer Scheme for the Ocean: Revisiting Its Formulation and Benchmarking One-Dimensional Simulations Relative to LES

Abstract: We evaluate the Community ocean Vertical Mixing (CVMix) project version of the K-profile parameterization (KPP). For this purpose, one-dimensional KPP simulations are compared across a suite of oceanographically relevant regimes against large eddy simulations (LES). The LES is forced with horizontally uniform boundary fluxes and has horizontally uniform initial conditions, allowing its horizontal average to be compared to one-dimensional KPP tests. We find the standard configuration of KPP [Danabasoglu et al., 2006] consistent with LES across many forcing regimes, supporting the physical basis of KPP. Our evaluation motivates recommendations for ”best practices” for using KPP within ocean circulation models, and identifies areas where further research is warranted. Further, our test suite can be used as a baseline for evaluation of a broad suite of boundary layer models. The original treatment of KPP recommends the matching of interior diffusivities and their gradients to the KPP predicted values computed in the ocean surface boundary layer (OSBL). However, we find that difficulties in representing derivatives of rapidly changing diffusivities near the base of the OSBL can lead to loss of simulation fidelity. We propose two alternative approaches: (1) match to the internal predicted diffusivity along, (2) set the KPP diffusivity to zero at the OSBL base. Although computationally simpler, the second alternative is sensitive to implementation details and we offer methods to prevent the emergence of numerical high frequency noise. We find the KPP entrainment buoyancy flux to be sensitive to vertical grid resolution and details of how to diagnose the KPP boundary layer depth. We modify the KPP turbulent shear velocity parameterization to reduce resolution dependence. Additionally, our results show that the KPP parameterized non-local tracer flux is incomplete due to the assumption that it solely redistributes the surface tracer flux. However, examination of the LES vertical turbulent scalar flux budgets show that non-local fluxes can exist in the absence of surface tracer fluxes. This result motivates further studies of the non-local flux parameterization. Draft from March 9, 2018

Preindustrial Control Simulations With HadGEM3-GC3.1 for CMIP6

Abstract: Preindustrial control simulations with the third Hadley Centre Global Environmental Model,run in the Global Coupled configuration 3.1 of the Met Office Unified Model (HadGEM3-GC3.1) are presented at two resolutions. These are N216ORCA025, which has a horizontal resolution of 60 km in the atmosphere and 0.25° in the ocean, and N96ORCA1, which has a horizontal resolution of 130 km in the atmosphere and 1∘in the ocean. The aim of this study is to document the climate variability in these simulations, make comparisons against present-day observations (albeit under different forcing), and discuss differences arising due to resolution. In terms of interannual variability in the leading modes of climate variability the two resolutions behave generally very similarly. Notable differences are in the westward extent of El Nin oand the pattern of Atlantic multidecadal variability, in which N216ORCA025 compares more favorably to observations, and in the Antarctic Circumpolar Current, which is far too weak in N216ORCA025. In the North Atlantic region, N216ORCA025 has a stronger and deeper Atlantic Meridional Overturning Circulation, which compares well against observations, and reduced biases in temperature and salinity in the North Atlantic subpolar gyre. These simulations are being provided to the sixth Coupled Model Intercomparison Project(CMIP6) and provide a baseline against which further forced experiments may be assessed.

An Extended Eddy-Diffusivity Mass-Flux Scheme for Unified Representation of Subgrid-Scale Turbulence and Convection

Abstract: Large-scale weather forecasting and climate models are beginning to reach horizontal resolutions of kilometers, at which common assumptions made in existing parameterization schemes of subgrid-scale turbulence and convection-such as that they adjust instantaneously to changes in resolved-scale dynamics-cease to be justifiable Additionally, the common practice of representing boundary-layer turbulence, shallow convection, and deep convection by discontinuously different parameterizations schemes, each with its own set of parameters, has contributed to the proliferation of adjustable parameters in large-scale models Here we lay the theoretical foundations for an extended eddy-diffusivity mass-flux (EDMF) scheme that has explicit time-dependence and memory of subgrid-scale variables and is designed to represent all subgrid-scale turbulence and convection, from boundary layer dynamics to deep convection, in a unified manner Coherent up and downdrafts in the scheme are represented as prognostic plumes that interact with their environment and potentially with each other through entrainment and detrainment The more isotropic turbulence in their environment is represented through diffusive fluxes, with diffusivities obtained from a turbulence kinetic energy budget that consistently partitions turbulence kinetic energy between plumes and environment The cross-sectional area of up and downdrafts satisfies a prognostic continuity equation, which allows the plumes to cover variable and arbitrarily large fractions of a large-scale grid box and to have life cycles governed by their own internal dynamics Relatively simple preliminary proposals for closure parameters are presented and are shown to lead to a successful simulation of shallow convection, including a time-dependent life cycle

DNS and LES for Simulating Stratocumulus: Better Together

Abstract: We argue that combining direct numerical simulation (DNS) with large-eddy simulation (LES) and field studies could accelerate current lines of stratocumulus research. LES allows for a faster and more holistic study of the parameter space, but LES is sensitive to details of its formulation because the energetics are tied to unresolved processes in the cloud top region. One way to assess this sensitivity is through field studies. Another way is through DNS. In particular, DNS can be used to test the hypothesis that LES, even with an inadequate representation of the physics of cloud top entrainment, properly quantifies the sensitivity of cloud-topped boundary layers to changing environmental conditions. We support this argument by contrasting theoretical aspects of both techniques, by presenting first DNS results of a stratocumulus-topped boundary layer and discussing their convergence toward Reynolds number similarity, and by showing the consistency of DNS results with LES results and field measurements.

Thinning Can Reduce Losses in Carbon Use Efficiency and Carbon Stocks in Managed Forests Under Warmer Climate

Abstract: Forest carbon use efficiency (CUE, the ratio of net to gross primary productivity) represents the fraction of photosynthesis that is not used for plant respiration. Although important, it is often neglected in climate change impact analyses. Here we assess the potential impact of thinning on projected carbon cycle dynamics and implications for forest CUE and its components (i.e., gross and net primary productivity and plant respiration), as well as on forest biomass production. Using a detailed process-based forest ecosystem model forced by climate outputs of five Earth System Models under four representative climate scenarios, we investigate the sensitivity of the projected future changes in the autotrophic carbon budget of three representative European forests. We focus on changes in CUE and carbon stocks as a result of warming, rising atmospheric CO2 concentration, and forest thinning. Results show that autotrophic carbon sequestration decreases with forest development, and the decrease is faster with warming and in unthinned forests. This suggests that the combined impacts of climate change and changing CO2 concentrations lead the forests to grow faster, mature earlier, and also die younger. In addition, we show that under future climate conditions, forest thinning could mitigate the decrease in CUE, increase carbon allocation into more recalcitrant woody pools, and reduce physiological-climate-induced mortality risks. Altogether, our results show that thinning can improve the efficacy of forest-based mitigation strategies and should be carefully considered within a portfolio of mitigation options.

Energetic Constraints on the ITCZ Position in Idealized Simulations With a Seasonal Cycle

Abstract: The atmospheric energy budget has recently been shown to provide powerful constraints on the position and shifts of the zonal and annual mean intertropical convergence zone (ITCZ), which lies close to the latitude of zero vertically integrated energy transport (energy flux equator, EFE). Relatively little work has however explored the applicability of the energetic framework to ITCZ shifts on shorter timescales. This study investigates to what extent the EFE tracks the ITCZ on subseasonal timescales in idealized aquaplanet simulations with different mixed layer depths. It is shown that the ITCZ always lags the EFE, even in the simulation with the shallowest mixed layer depth, making it possible for the EFE and the ITCZ to reside on opposite sides of the equator. At these times, which occur as the winter cross‐equatorial Hadley circulation retreats from the summer hemisphere, the required energy balance is achieved not through shifts of the Hadley cell's ascending branch and ITCZ to track the EFE but through changes in the cell's vertical structure into one of negative gross moist stability (GMS). For any given position of the ascending branch, the winter cell is much weaker as it retreats from than as it expands into the summer hemisphere and develops a shallow return flow at mid‐to‐lower tropospheric levels where the moist static energy reaches its minimum, hence favoring a negative GMS. It is argued that the asymmetry between the expanding and retreating phases of the winter Hadley cell is linked to the nonlinear seasonal evolution of near‐surface temperatures.

Increase in Precipitation Efficiency With Surface Warming in Radiative-Convective Equilibrium

Abstract: The precipitation efficiency of convection (ε) plays an important role in simple models of the tropical atmosphere as well as in global climate models’ projections of future climate changes, but remains poorly understood and poorly constrained. A particularly urgent question is how ε will change in warmer climates. To address these issues, this study investigates the precipitation efficiency in simulations of radiative-convective equilibrium with a cloud-resolving model forced by a wide range of sea surface temperatures (SSTs). Two different domains are considered: a small, doubly periodic domain, and a 2-D (x-z) “mock-Walker” domain with a sinusoidal SST profile that resembles the equatorial Pacific, and the sensitivities of the results to the microphysical scheme and to the horizontal resolution are also explored. It is found that ε generally increases with warming in the small domain simulations because of increases in the efficiency with which cloud condensate is converted into precipitation, with changes in the re-evaporation of falling precipitation playing a secondary role. This picture is complicated in the 2-D simulations by substantial changes in the degree of convective organization as the underlying SSTs are varied. ε is found to decrease as convection becomes more organized, because convective organization results in relatively more low clouds, which have small (≤0.1) precipitation efficiencies, and relatively less high clouds, which have larger (∼0.4) precipitation efficiencies. Plain Language Summary The precipitation efficiency of convection (ε) quantifies the fraction of water that condenses in a cloud that reaches the surface as precipitation. Recent work has shown that changes in ε can play an important role in determining the warming of climate models in response to increases in atmospheric carbon dioxide concentrations, and ε is also a key factor in theories for the dynamics of the tropical atmosphere. Despite this importance, however, ε is poorly understood and poorly constrained. In this study, we take a first step to addressing this issue by investigating how precipitation efficiency behaves in idealized simulations of the tropical atmosphere, in which the underlying sea surface temperature is varied across a wide range of values. We find that ε generally increases with warming because clouds become denser and so form precipitation more easily, though in some cases ε decreases because of changes in the large-scale flow of the tropical atmosphere.

Constraining Aging Processes of Black Carbon in the Community Atmosphere Model Using Environmental Chamber Measurements

Abstract: The direct radiative forcing of black carbon aerosol (BC) on the Earth system remains unsettled, largely due to the uncertainty with physical properties of BC throughout their lifecycle. Here we show that ambient chamber measurements of BC properties provide a novel constraint on the crude BC aging representation in climate models. Observational evidence for significant absorption enhancement of BC can be reproduced when the aging processes in the four-mode version of the Modal Aerosol Module (MAM4) aerosol scheme in the Community Atmosphere Model version 5 are calibrated by the recent in situ chamber measurements. An observation-based scaling method is developed in the aging timescale calculation to alleviate the influence of biases in the simulated model chemical composition. Model sensitivity simulations suggest that the different monolayer settings in the BC aging parameterization of MAM4 can cause as large as 26% and 24% differences in BC burden and radiative forcing, respectively. We also find that an increase in coating materials (e.g., sulfate and secondary organic aerosols) reduces BC lifetime by increasing the hygroscopicity of the mixture but enhances its absorption, resulting in a net increase in BC direct radiative forcing. Our results suggest that accurate simulations of BC aging processes as well as other aerosol species are equally important in reducing the uncertainty of BC forcing estimation.

The Relative Influence of Atmospheric and Oceanic Model Resolution on the Circulation of the North Atlantic Ocean in a Coupled Climate Model

Abstract: It is often unclear how to optimally choose horizontal resolution for the oceanic and atmospheric components of coupled climate models, which has implications for their ability to make best use of available computational resources. Here we investigate the effect of using different combinations of horizontal resolutions in atmosphere and ocean on the simulated climate in a global coupled climate model (Alfred Wegener Institute Climate Model [AWI‐CM]). Particular attention is given to the Atlantic Meridional Overturning Circulation (AMOC). Four experiments with different combinations of relatively high and low resolutions in the ocean and atmosphere are conducted. We show that increases in atmospheric and oceanic resolution have clear impacts on the simulated AMOC, which are largely independent. Increased atmospheric resolution leads to a weaker AMOC. It also improves the simulated Gulf Stream separation; however, this is only the case if the ocean is locally eddy resolving and reacts to the improved atmosphere. We argue that our results can be explained by reduced mean winds caused by higher cyclone activity. Increased resolution of the ocean affects the AMOC in several ways, thereby locally increasing or reducing the AMOC. The finer topography (and reduced dissipation) in the vicinity of the Caribbean basin tends to locally increase the AMOC. However, there is a reduction in the AMOC around 45°N, which relates to the reduced mixed layer depth in the Labrador Sea in simulations with refined ocean and changes in the North Atlantic current pathway. Furthermore, the eddy‐induced changes in the Southern Ocean increase the strength of the deep cell.

Evaluation and Intercomparison of Five North American Dry Deposition Algorithms at a Mixed Forest Site.

Abstract: To quantify differences between dry deposition algorithms commonly used in North America, five models were selected to calculate dry deposition velocity (V d) for O3 and SO2 over a temperate mixed forest in southern Ontario, Canada, where a 5-year flux database had previously been developed. The models performed better in summer than in winter with correlation coefficients for hourly V d between models and measurements being approximately 0.6 and 0.3, respectively. Differences in mean V d values between models were on the order of a factor of 2 in both summer and winter. All models produced lower V d values than the measurements of O3 in summer and SO2 in summer and winter, although the measured V d may be biased. There was not a consistent tendency in the models to overpredict or underpredict for O3 in winter. Several models produced magnitudes of the diel variation of V d (O3) comparable to the measurements, while all models produced slightly smaller diel variations than the measurements of V d (SO2) in summer. A few models produced larger diel variations than the measurements of V d for O3 and SO2 in winter. Model differences were mainly due to different surface resistance parameterizations for stomatal and nonstomatal uptake pathways, while differences in aerodynamic and quasi-laminar resistances played only a minor role. It is recommended to use ensemble modeling results for ecosystem impact assessment studies, which provides mean values of all the used models and thus can avoid too much overestimations or underestimations.

Boundary Layer Diabatic Processes, the Virtual Effect, and Convective Self-Aggregation

Abstract: Author(s): Yang, D | Abstract: The atmosphere can self-organize into long-lasting large-scale overturning circulations over an ocean surface with uniform temperature. This phenomenon is referred to as convective self-aggregation and has been argued to be important for tropical weather and climate systems. Here we present a boundary layer centric framework based on the available potential energy budget of convective self-aggregation. We show that boundary layer diabatic processes dominate the available potential energy production and are, therefore, essential to convective self-aggregation. We further show that the enhanced virtual effect of water vapor can lead to convective self-aggregation.

Regional Climate Simulations With the Community Earth System Model

Abstract: The spectral element (SE) variable-resolution (VR) mesh dynamical core is tested in developmental versions of the Community Earth System Model version 2 (CESM2). The SE dynamical core is tested in baroclinic wave, aquaplanet and full physics configurations to evaluate variable-resolution simulations against uniform high and uniform low-resolution simulations. Different physical parameterization suites are also evaluated to gauge their sensitivity to resolution. Dry dynamical core variable-resolution cases compare well to high-resolution tests. More recent versions of the atmospheric physics, including cloud schemes for CESM2, are less sensitive to changes in horizontal resolution. Most of the sensitivity is due to sensitivity to time step and interactions between deep convection and large-scale condensation, which is expected from the closure methods. The resulting full physics SE-VR model produces a similar climate to the global lowresolution mesh and similar high-frequency statistics in the high-resolution region. The SE-VR simulations are able to reproduce uniform high-resolution results, making them an effective tool for regional climate simulations at lower computational cost. Some biases are reduced (orographic precipitation in Western United States), but biases do not necessarily go away at high resolution (e.g., summertime surface temperatures). Variable-resolution grids are a viable alternative to traditional nesting for regional climate studies and are available in CESM2. Plain Language Summary This manuscript describes comprehensive tests of a numerical climate model that has high horizontal resolution in one region. This enables high-resolution simulations of climate, and extreme weather events that occur on small scales to be simulated at lower computational costs. Results indicate that the model represents low-resolution climate well, and also reproduces extreme climate statistics in the region with high resolution. We conclude that the variable resolution model is a good way to simulate and predict regional climate.