Forcing and feedback in the MPI-ESM-LR coupled model under abruptly quadrupled CO2

Radiative feedback mechanisms associated with temperature, water vapor, cloud, and surface albedo change determine climate sensitivity to radiative forcing. Here we use the linearized radiative kernel-technique in combination with a Gregory analysis to determine the strength and structure of feedbacks, as well as direct and adjusted CO2 forcings in the coupled Max Planck Institute Earth System Model at base resolution (MPI-ESM-LR). We show that the combined Kernel-Gregory approach yields an elegant separation of surface temperature-dependent feedbacks from contributions to radiative forcing by fast adjustments. MPI-ESM-LR exhibits a relatively large cloud adjustment of nearly 2 W m−2 in direct response to quadrupled CO2, with positive cloud adjustment evident throughout the tropics, subtropics and over most landmasses whereas midlatitude storm tracks contribute negatively. The model features a nonlinear regression of radiation imbalance to global mean surface temperature change, resulting in a significantly increasing effective climate sensitivity after about 20 years which is approximately at temperatures 4–5 K above preindustrial. This feature is not uncommon among climate models and is relevant for future climate projections. We analyze the contribution of the individual feedback processes to this behavior and discuss possible origins such as differential ocean warming patterns associated with deep-ocean heat uptake or state dependencies of the feedback processes

Improving a global model from the boundary layer: total turbulent energy and the neutral limit Prandtl number

Model intercomparisons have identified important deficits in the representation of the stable boundary layer by turbulence parametrizations used in current weather and climate models. However, detrimental impacts of more realistic schemes on the large-scale flow have hindered progress in this area. Here we implement a total turbulent energy scheme into the climate model ECHAM6. The total turbulent energy scheme considers the effects of Earth’s rotation and static stability on the turbulence length scale. In contrast to the previously used turbulence scheme, the TTE scheme also implicitly represents entrainment flux in a dry convective boundary layer. Reducing the previously exaggerated surface drag in stable boundary layers indeed causes an increase in southern hemispheric zonal winds and large-scale pressure gradients beyond observed values. These biases can be largely removed by increasing the parametrized orographic drag. Reducing the neutral limit turbulent Prandtl number warms and moistens low-latitude boundary layers and acts to reduce longstanding radiation biases in the stratocumulus regions, the Southern Ocean and the equatorial cold tongue that are common to many climate models

Understanding the intermodel spread in global-mean hydrological sensitivity

This paper assesses intermodel spread in the slope of global-mean precipitation change ΔP with respect to surface temperature change. The ambiguous estimates in the literature for this slope are reconciled by analyzing four experiments from phase 5 of CMIP (CMIP5) and considering different definitions of the slope. The smallest intermodel spread (a factor of 1.5 between the highest and lowest estimate) is found when using a definition that disentangles temperature-independent precipitation changes (the adjustments) from the slope of the temperature-dependent precipitation response; here this slope is referred to as the hydrological sensitivity parameter η. The estimates herein show that η is more robust than stated in most previous work. The authors demonstrate that adjustments and η estimated from a steplike quadrupling CO2 experiment serve well to predict ΔP in a transient CO2 experiment. The magnitude of η is smaller in the coupled ocean–atmosphere quadrupling CO2 experiment than in the noncoupled atmosphere-only experiment. The offset in magnitude due to coupling suggests that intermodel spread may undersample uncertainty. Also assessed are the relative contribution of η, the surface warming, and the adjustment on the spread in ΔP on different time scales. Intermodel variation of both η and the adjustment govern the spread in ΔP in the years immediately after the abrupt forcing change. In equilibrium, the uncertainty in ΔP is dominated by uncertainty in the equilibrium surface temperature response. A kernel analysis reveals that intermodel spread in η is dominated by intermodel spread in tropical lower tropospheric temperature and humidity changes and cloud changes

Cloud and boundary layer interactions over the Arctic sea ice in late summer

Observations from the Arctic Summer Cloud Ocean Study (ASCOS), in the central Arctic sea-ice pack in late summer 2008, provide a detailed view of cloud- atmosphere-surface interactions and vertical mixing processes over the sea-ice environment. Measurements from a suite of ground-based remote sensors, near-surface meteorological and aerosol instruments, and profiles from radiosondes and a helicopter are combined to characterize a weeklong period dominated by low-level, mixed-phase, stratocumulus clouds. Detailed case studies and statistical analyses are used to develop a conceptual model for the cloud and atmosphere structure and their interactions in this environment. Clouds were persistent during the period of study, having qualities that suggest they were sustained through a combination of advective influences and in-cloud processes, with little contribution from the surface. Radiative cooling near cloud top produced buoyancy-driven, turbulent eddies that contributed to cloud formation and created a cloud-driven mixed layer. The depth of this mixed layer was related to the amount of turbulence and condensed cloud water. Coupling of this cloud-driven mixed layer to the surface boundary layer was primarily determined by proximity. For 75%of the period of study, the primary stratocumulus cloud-driven mixed layer was decoupled from the surface and typically at a warmer potential temperature. Since the near-surface temperature was constrained by the ocean-ice mixture, warm temperatures aloft suggest that these air masses had not significantly interacted with the sea-ice surface. Instead, backtrajectory analyses suggest that these warm air masses advected into the central Arctic Basin from lower latitudes. Moisture and aerosol particles likely accompanied these air masses, providing necessary support for cloud formation. On the occasions when cloud-surface coupling did occur, back trajectories indicated that these air masses advected at low levels, while mixing processes kept the mixed layer in equilibrium with the near-surface environment. Rather than contributing buoyancy forcing for the mixed-layer dynamics, the surface instead simply appeared to respond to the mixedlayer processes aloft. Clouds in these cases often contained slightly higher condensed water amounts, potentially due to additional moisture sources from below

Compensation of hemispheric albedo asymmetries by shifts of the ITCZ and tropical clouds

Despite a substantial hemispheric asymmetry in clear-sky albedo, observations of Earth's radiation budget reveal that the two hemispheres have the same all-sky albedo. Here, aquaplanet simulations with the atmosphere general circulation model ECHAM6 coupled to a slab ocean are performed to study to what extent and by which mechanisms clouds compensate hemispheric asymmetries in clear-sky albedo. Clouds adapt to compensate the imposed asymmetries because the intertropical convergence zone (ITCZ) shifts into the dark surface hemisphere. The strength of this tropical compensation mechanism is linked to the magnitude of the ITCZ shift. In some cases the ITCZ shift is so strong as to overcompensate the hemispheric asymmetry in clear-sky albedo, yielding a range of climates for which the hemisphere with lower clear-sky albedo has a higher all-sky albedo. The ITCZ shift is sensitive to the convection scheme and the depth of the slab ocean. Cloud-radiative feedbacks explain part of the sensitivity to the convection scheme as they amplify the ITCZ shift in the Tiedtke (TTT) scheme but have a neutral effect in the Nordeng (TNT) scheme. A shallower slab ocean depth, and thereby reduced thermal inertia of the underlying surface and increased seasonal cycle, stabilizes the ITCZ against annual-mean shifts. The results lend support to the idea that the climate system adjusts so as to minimize hemispheric albedo asymmetries, although there is no indication that the hemispheres must have exactly the same albedo. © 2014 American Meteorological Society

The subtle origins of surface-warming hiatuses

During the first decade of the twenty-first century, the Earth’s surface warmed more slowly than climate models simulated1. This surface-warming hiatus is attributed by some studies to model errors in external forcing2, 3, 4, while others point to heat rearrangements in the ocean5, 6, 7, 8, 9, 10 caused by internal variability, the timing of which cannot be predicted by the models1. However, observational analyses disagree about which ocean region is responsible11, 12, 13, 14, 15, 16. Here we show that the hiatus could also have been caused by internal variability in the top-of-atmosphere energy imbalance. Energy budgeting for the ocean surface layer over a 100-member historical ensemble reveals that hiatuses are caused by energy-flux deviations as small as 0.08 W m−2, which can originate at the top of the atmosphere, in the ocean, or both. Budgeting with existing observations cannot constrain the origin of the recent hiatus, because the uncertainty in observations dwarfs the small flux deviations that could cause a hiatus. The sensitivity of these flux deviations to the observational dataset and to energy budget choices helps explain why previous studies conflict, and suggests that the origin of the recent hiatus may never be identified

Radiative convective equilibrium as a framework for studying the interaction between convection and its large-scale environment

An uncertain representation of convective clouds has emerged as one ofthe key barriers to our understanding of climate sensitivity. The largegap in resolved spatial scales between General Circulation Models (GCMs)and high resolution models has made a systematic study of convectiveclouds across model configurations difficult. It is shown here that thesimulated atmosphere of a GCM in Radiative Convective Equilibrium (RCE)is sufficiently similar across a range of domain sizes to justify theuse of RCE to study both a GCM and a high resolution model on the samedomain with the goal of improved constraints on the parameterizedclouds. Simulations of RCE with parameterized convection have beenanalyzed on domains with areas spanning more than two orders ofmagnitude (0.80-204x10(6)km(2)), all having the same grid spacing of13km. The simulated climates on different domains are qualitativelysimilar in their degree of convective organization, the precipitationrates, and the vertical structure of the clouds and water vapor, withthe similarity increasing as the domain size increases. Sea surfacetemperature perturbation experiments are used to estimate the climatefeedback parameter for the differently configured experiments, and thecloud radiative effect is computed to examine the role which clouds playin the response. Despite the similar climate states between the domainsthe feedback parameter varies by more than a factor of two; thehydrological sensitivity parameter is better behaved, varying by afactor of 1.4. The sensitivity of the climate feedback parameter todomain size is related foremost to a nonsystematic response of low-levelclouds as well as an increasingly negative longwave feedback on largerdomains

Reconciling conflicting accounts of local radiative feedbacks in climate models

The literature offers conflicting findings about which regions contribute most to increases in the global radiative feedback after a forcing increase. This paper explains the disagreement by discriminating between two common definitions of the local feedback, which use either local temperature or global temperature as their basis. Although the two definitions of feedback have been previously compared in aquaplanet models with slab oceans, here the definitions are compared for the first time in an atmosphere–ocean general circulation model (MPI-ESM1.2) integrated over four doublings of atmospheric CO2 concentrations. Large differences between the definitions can be seen in all feedbacks, but especially in the temperature and water vapor feedbacks. Differences of up to 10 W m22 K21 over the Southern Ocean can be explained by the pattern of surface warming, which weights the local feedbacks and reduces their contribution to the global mean. This finding is, however, dependent on the resolution of analysis, because the local-temperature definition is mathematically inconsistent across spatial scales. Furthermore, attempts to estimate the effect of “pattern weighting” by separating local feedbacks and warming patterns at the gridcell level fail, because the radiative change in key tropical regions is also determined by tropospheric stability via the global circulation. These findings indicate that studies of regional feedback change are more sensitive to methodological choices than previously thought, and that the tropics most likely dominate regional contributions to global radiative feedback change on decadal to centennial time scales. SIGNIFICANCE STATEMENT: Radiative feedbacks are processes that either intensify or damp global surface warming. We compare two ways to define local radiative feedbacks in a climate model and find that the choice of definition drastically impacts the results. Differences in feedback between the definitions are up to 10 W m22 K21 over the Southern Ocean; by comparison, the estimate of the true global feedback is around 21 W m22 K21. Also, one of the definitions is mathematically inconsistent across different scales of spatial aggregation. Our findings matter because they help to reconcile disagreement in previous studies about which regions dominate global radiative feedback change in model simulations of global warming. Ó 2022 American Meteorological Society

Employment, Utilization, and Development of Airborne Laser Scanning in Fenno-Scandinavian Archaeology-A Review

This paper gives a presentation of how airborne laser scanning (ALS) has been adopted in archaeology in the North over the period 2005-2019. Almost two decades have passed since ALS first emerged as a potential tool to add to the archaeologist's toolbox. Soon after, it attracted the attention of researchers within archaeological communities engaged with remote sensing in the Fenno-Scandinavian region. The first archaeological ALS projects gave immediate good results and led to further use, research, and development through new projects that followed various tracks. The bulk of the research and development focused on studying how well-suited ALS is for identifying, mapping, and documenting archaeological features in outfield land, mainly in forested areas. The poor situation in terms of lack of information on archaeological records in outfield areas has been challenging for research and especially for cultural heritage management for a long period of time. Consequently, an obvious direction was to study how ALS-based mapping of cultural features in forests could help to improve the survey situation. This led to various statistical analyses and studies covering research questions related to for instance effects on detection success of laser pulse density, and the size and shape of the targeted features. Substantial research has also been devoted to the development and assessment of semi-automatic detection of archaeological features based on the use of algorithms. This has been studied as an alternative approach to human desk-based visual analyses and interpretations of ALS data. This approach has considerable potential for detecting sites over large regions such as the vast roadless and unbuilt wilderness regions of northern Fennoscandia, and has proven highly successful. In addition, the current review presents how ALS has been employed for monitoring purposes and for landscape studies, including how it can influence landscape understanding. Finally, the most recent advance within ALS research and development has been discussed: testing of the use of drones for data acquisition. In conclusion, aspects related to the utilization of ALS in archaeological research and cultural heritage management are summarized and discussed, together with thoughts about future perspectives.Peer reviewe