We describe Global Atmosphere 7.0 and Global Land 7.0 (GA7.0/GL7.0), the latest science configurations of the Met Office Unified Model (UM) and the Joint UK Land Environment Simulator (JULES) land ...surface model developed for use across weather and climate timescales. GA7.0 and GL7.0 include incremental developments and targeted improvements that, between them, address four critical errors identified in previous configurations: excessive precipitation biases over India, warm and moist biases in the tropical tropopause layer (TTL), a source of energy non-conservation in the advection scheme and excessive surface radiation biases over the Southern Ocean. They also include two new parametrisations, namely the UK Chemistry and Aerosol (UKCA) GLOMAP-mode (Global Model of Aerosol Processes) aerosol scheme and the JULES multi-layer snow scheme, which improve the fidelity of the simulation and were required for inclusion in the Global Atmosphere/Global Land configurations ahead of the 6th Coupled Model Intercomparison Project (CMIP6).
The southwesterly low-level jet (LLJ) located to the east of the Tibetan Plateau in southern China plays an important role in summertime convective initiation over north China. This study adopts a ...novel perspective and uses hindcast experiments in order to investigate the role of moisture in LLJ and associated heavy rainfall formation, employing a global atmospheric general circulation model (AGCM). In the sensitivity experiments, an increase of humidity in the inflow region leads to a weaker LLJ but stronger diurnal wind oscillations. The weaker LLJ is due to a decreased lower-tropospheric east-west pressure gradient resulting from a low pressure anomaly over southeastern China induced by deep convection and related condensational heating. On the other hand, the stronger diurnal variation of the LLJ originates from stronger day-and-night thermal differences over the sloping terrain, which is related to drier conditions over the mountain range. Moreover, the increased humidity and decreased LLJ counteract one another to impact precipitation in the outflow region. The change of precipitation is mainly determined by the altered moisture flux divergence. If the increase in humidity dominates, then the moisture flux convergence is enhanced and favors more precipitation over north China. Otherwise, if the decreased LLJ dominates, then the moisture flux convergence is reduced, which constrains precipitation. It is highlighted that the moist diabatic and dynamic processes are intimately coupled, and that a correct simulation of moisture flux convergence is vital for AGCMs to reproduce the LLJ-related precipitation, particularly the nocturnal precipitation peak, which is a deficiency in many current models.
THE ART AND SCIENCE OF CLIMATE MODEL TUNING Hourdin, Frédéric; Mauritsen, Thorsten; Gettelman, Andrew ...
Bulletin of the American Meteorological Society,
03/2017, Volume:
98, Issue:
3
Journal Article
Peer reviewed
Open access
The process of parameter estimation targeting a chosen set of observations is an essential aspect of numerical modeling. This process is usually named tuning in the climate modeling community. In ...climate models, the variety and complexity of physical processes involved, and their interplay through a wide range of spatial and temporal scales, must be summarized in a series of approximate submodels. Most submodels depend on uncertain parameters. Tuning consists of adjusting the values of these parameters to bring the solution as a whole into line with aspects of the observed climate. Tuning is an essential aspect of climate modeling with its own scientific issues, which is probably not advertised enough outside the community of model developers. Optimization of climate models raises important questions about whether tuning methods a priori constrain the model results in unintended ways that would affect our confidence in climate projections. Here, we present the definition and rationale behind model tuning, review specific methodological aspects, and survey the diversity of tuning approaches used in current climate models. We also discuss the challenges and opportunities in applying so-called objective methods in climate model tuning. We discuss how tuning methodologies may affect fundamental results of climate models, such as climate sensitivity. The article concludes with a series of recommendations to make the process of climate model tuning more transparent.
Tuning the climate of a global model Mauritsen, Thorsten; Stevens, Bjorn; Roeckner, Erich ...
Journal of advances in modeling earth systems,
03/2012, Volume:
4, Issue:
3
Journal Article
Peer reviewed
Open access
During a development stage global climate models have their properties adjusted or tuned in various ways to best match the known state of the Earth's climate system. These desired properties are ...observables, such as the radiation balance at the top of the atmosphere, the global mean temperature, sea ice, clouds and wind fields. The tuning is typically performed by adjusting uncertain, or even non‐observable, parameters related to processes not explicitly represented at the model grid resolution. The practice of climate model tuning has seen an increasing level of attention because key model properties, such as climate sensitivity, have been shown to depend on frequently used tuning parameters. Here we provide insights into how climate model tuning is practically done in the case of closing the radiation balance and adjusting the global mean temperature for the Max Planck Institute Earth System Model (MPI‐ESM). We demonstrate that considerable ambiguity exists in the choice of parameters, and present and compare three alternatively tuned, yet plausible configurations of the climate model. The impacts of parameter tuning on climate sensitivity was less than anticipated.
Key Points
Global climate models are tuned out of necessity
Model tuning is not often described and discussed
We attempt to de‐mystify tuning
The aerosol-cloud interactions due to black carbon (BC) aerosols, as well as the implied climate responses, are examined using an aerosol module in the coupled atmosphere-ocean general circulation ...model MPI-ESM. BC is simulated to enhance cloud droplet number concentration (CDNC) by 10-15% in the BC emission source regions, especially in the Tropics and mid-latitudes. Higher CDNC and reduced auto-conversion from cloud water to rain water explains the increased cloud water path over the tropical regions (30
S-30
N) in the model. In the global mean, the cloud water- as well as precipitation changes are negligibly small. The global-mean effective radiative forcing due to aerosol-cloud interactions for BC is estimated at
, which is attributable to the increase in CDNC burden and (regionally) cloud water in the model. Global mean temperature and rainfall response were found to be
and
, respectively, with significantly larger regional changes mainly in the downwind regions from BC sources.
Recent increases in computing power mean that atmospheric models for numerical weather prediction are now able to operate at grid spacings of the order of a few hundred meters, comparable to the ...dominant turbulence length scales in the atmospheric boundary layer. As a result, models are starting to partially resolve the coherent overturning structures in the boundary layer. In this resolution regime, the so‐called boundary layer “gray zone,” neither the techniques of high‐resolution atmospheric modeling (a few tens of meters resolution) nor those of traditional meteorological models (a few kilometers resolution) are appropriate because fundamental assumptions behind the parameterizations are violated. Nonetheless, model simulations in this regime may remain highly useful. In this paper, a newly formed gray zone boundary layer community lays the basis for parameterizing gray zone turbulence, identifies the challenges in high‐resolution atmospheric modeling and presents different gray zone boundary layer models. We discuss both the successful applications and the limitations of current parameterization approaches, and consider various issues in extending promising research approaches into use for numerical weather prediction. The ultimate goal of the research is the development of unified boundary layer parameterizations valid across all scales.
Key Points
The grid resolution of atmospheric models has become fine enough that are able to partially resolve turbulent motions in the boundary layer
This resolution regime comprises the turbulence “gray zone” where the assumptions behind traditional parameterizations are no longer valid
We review the current efforts by modelers to overcome the gray zone problem in order to provide useful simulations at high resolutions
Abstract
Global convection-permitting model simulations and remote sensing observations are used to investigate the interaction between organized convection, both moist and dry, and the atmospheric ...circulation in the case of an African easterly wave (AEW). The wave disturbance is associated with a quadrupole structure of divergence, with two convergence centers slightly ahead of the trough. Moisture transport from southeast of the trough to the area in front and lower midtropospheric moisture convergence precondition and organize convection. The main inflow into the squall-line cluster is from behind. The moisture-abundant inflow collides at the low level with monsoon air with high moist static energy and establishes a frontal line of updrafts at the leading edge of the propagating mesoscale convective system. A mantle of moisture surrounds the convective core. A potential vorticity budget analysis reveals that convective latent heating is driving the evolution of the wave but not in a quasi-steady way. The wave propagation includes a succession of convective bursts and subsequent dynamic adjustment processes. Dry convection associated with the Saharan air layer (SAL) and SAL intrusions into the wave trough together with vorticity advection can play a role in intensifying AEWs dynamically as they move from the West African coast across the Atlantic Ocean. Our analysis demonstrates that the synoptic-scale wave and convection are interlinked through mesoscale circulations on a continuum of scales. This implies that the relation between organized convection and the atmospheric circulation is intrinsically dynamic, which poses a particular challenge to subgrid convection parameterizations in numerical models.
The Radiative‐Convective Equilibrium Model Intercomparison Project (RCEMIP) is an intercomparison of multiple types of numerical models configured in radiative‐convective equilibrium (RCE). RCE is an ...idealization of the tropical atmosphere that has long been used to study basic questions in climate science. Here, we employ RCE to investigate the role that clouds and convective activity play in determining cloud feedbacks, climate sensitivity, the state of convective aggregation, and the equilibrium climate. RCEMIP is unique among intercomparisons in its inclusion of a wide range of model types, including atmospheric general circulation models (GCMs), single column models (SCMs), cloud‐resolving models (CRMs), large eddy simulations (LES), and global cloud‐resolving models (GCRMs). The first results are presented from the RCEMIP ensemble of more than 30 models. While there are large differences across the RCEMIP ensemble in the representation of mean profiles of temperature, humidity, and cloudiness, in a majority of models anvil clouds rise, warm, and decrease in area coverage in response to an increase in sea surface temperature (SST). Nearly all models exhibit self‐aggregation in large domains and agree that self‐aggregation acts to dry and warm the troposphere, reduce high cloudiness, and increase cooling to space. The degree of self‐aggregation exhibits no clear tendency with warming. There is a wide range of climate sensitivities, but models with parameterized convection tend to have lower climate sensitivities than models with explicit convection. In models with parameterized convection, aggregated simulations have lower climate sensitivities than unaggregated simulations.
Plain Language Summary
This study investigates tropical clouds and climate using results from more than 30 different numerical models set up in a simplified framework. The data set of model simulations is unique in that it includes a wide range of model types configured in a consistent manner. We address some of the biggest open questions in climate science, including how cloud properties change with warming and the role that the tendency of clouds to form clusters plays in determining the average climate and how climate changes. While there are large differences in how the different models simulate average temperature, humidity, and cloudiness, in a majority of models, the amount of high clouds decreases as climate warms. Nearly all models simulate a tendency for clouds to cluster together. There is agreement that when the clouds are clustered, the atmosphere is drier with fewer clouds overall. We do not find a conclusive result for how cloud clustering changes as the climate warms.
Key Points
Temperature, humidity, and clouds in radiative‐convective equilibrium vary substantially across models
Models agree that self‐aggregation dries the atmosphere and reduces high cloudiness
There is no consistency in how self‐aggregation depends on warming
Projections of future transient global temperature increase in climate models for a known forcing depend on the strength of the atmospheric feedbacks and the rate of transient ocean heat uptake. A ...Bayesian framework and an intermediate complexity climate model are used to calculate a probability density function (PDF) of the transient climate response (TCR), constrained by observations of global surface warming and ocean heat uptake. The PDF constrained by observations is wider than the TCR range of current climate models, and has a slightly lower mean. Uncertainties in the observed ocean warming are shown to potentially affect the TCR. It is proposed, however, that even if models were found to overestimate ocean heat uptake, correcting that bias would lead to revisions in surface temperature projections over the twenty‐first century that are smaller than the uncertainties introduced by poorly quantified atmospheric feedbacks.
An objective tracking algorithm is used to characterize the three‐dimensional structure of African Easterly Waves (AEWs) in ERA‐Interim reanalysis and a Met Office Unified Model (UM) simulation. A ...special focus is dedicated to the coupling of dynamical aspects of the wave and moist convection. The relation between baroclinic features of the wave and latent heating is explored. Latent heating at and slightly ahead of the wave trough is found to reinforce and sustain the anomalous wave circulation through potential vorticity (PV) generation and vortex stretching. The coupling of moist processes and the circulation takes place mainly through moisture convergence at lower mid‐tropospheric levels, between 850 and 500 hPa. These findings are confirmed and examined in more detail in a case‐study of a strong AEW based on high‐resolution UM simulations. PV tracers are used to investigate how different moist diabatic processes invigorate the wave. Again moisture anomalies are found to be the main contributors to generating small‐scale convergence centres and updraughts ahead of the trough at mid‐tropospheric levels. Although buoyancy effects are ultimately responsible for the convective uplift, the results suggest that mesoscale circulations associated with the AEW dynamics are crucial in creating the small‐scale moist static instabilities and vortices which are essential for the AEW maintenance. Boundarylayer mixing and advection from the northern Sahel may create pockets of high‐PV air around the trough in some instances, but this mechanism of wave sustainment needs further investigation.
The interaction between moist diabatic processes and the atmospheric circulation in African Easterly Waves (AEWs) is investigated in detail. Latent heating at and slightly ahead of the wave trough is found to reinforce and sustain the anomalous wave circulation through potential vorticity (PV) generation and vortex stretching. The coupling of moist processes and the circulation takes place mainly through moisture convergence at lower mid‐tropospheric levels, which generates small‐scale convergence centres and moist instabilities ahead of the trough.
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