Ability Of General Circulation Models Research Articles

The Sensitivity of Moisture Flux Partitioning in the Cold‐Point Tropopause to External Forcing

AbstractThe dryness of the stratosphere is the result of air entering through the cold tropical tropopause layer (TTL). However, our understanding of the moisture flux partitioning into water vapor and frozen hydrometeors is incomplete. This raises concerns regarding the ability of General Circulation Models to accurately predict changes in stratospheric water vapor following perturbations in the radiative budget due to volcanic aerosol or stratospheric geoengineering. We present the first results using a global storm‐resolving model investigating the sensitivity of moisture fluxes within the TTL to an additional heating source. We address the question how the partitioning of moisture fluxes into water vapor and frozen hydrometeors changes under perturbations. The analysis reveals the resilience of the TTL, keeping the flux partitioning constant even at an average cold‐point warming exceeding 8 K. In the control and perturbed simulations, water vapor contributes around 80% of the moisture entering the stratosphere.

An ensemble-based assessment of bias adjustment performance, changes in hydrometeorological predictors and compound extreme events in EAS-CORDEX

The effectiveness of adaptive measures tackling the effects of climate change is dependent on robust climate projections. This becomes even more important in the face of intensifying extreme events. One example of these events is flooding, which embodies a major threat to highly vulnerable coastal urban areas. This includes eastern Asia, where multiple coastal megacities are located, e.g. Shanghai and Shenzhen. While the ability of general circulation models (GCMs) and regional climate models (RCMs) to project atmospheric changes associated with these events has improved, systematic errors (biases) remain. This study therefore assess capabilities of improving the quality of regional climate projections for eastern Asia. This is performed by evaluating an ensemble consisting of bias adjustment methods, GCM-RCM model runs and future emission scenarios based on representative concentration pathways (RCP) obtained from EAS-CORDEX. We show that bias adjustment significantly improves the quality of model output and best results are obtained by applying quantile delta mapping. Based on these results we evaluate potential future changes in crucial hydrometeorological predictors, univariate extreme events and compound extreme events, focusing on high wind speeds and extreme precipitation. Key findings include an increase in daily maximum temperature of 1.5 to nearly 4 °C, depending on the scenario, as well as increased levels of precipitation under RCP 8.5. Furthermore, a distinct intensification of extreme events including high temperatures and heavy precipitation is detected and this increase exceeds the increase of the overall mean of these predictors. The annual number of compound events including heavy precipitation and extreme wind speeds shows a significant increase of up to 50% for RCP 8.5 in the South China Sea as well as the adjacent coastal areas.

Observed and CMIP5‐Simulated Radiative Flux Variability Over West Africa

AbstractWe explore the ability of general circulation models in the Coupled Model Intercomparison Project (CMIP5) to recreate observed seasonal variability in top‐of‐the‐atmosphere and surface radiation fluxes over West Africa. This tests CMIP5 models' ability to describe the radiative energy partitioning, which is fundamental to our understanding of the current climate and its future changes. We use 15 years of the monthly Clouds and the Earth's Radiant Energy System Energy Balanced and Filled (EBAF) product, alongside other satellite, reanalysis, and surface station products. We find that the CMIP5 multimodel mean is generally within the reference product range, with annual mean CMIP5 multimodel mean—EBAF of −0.5 W m−2 for top‐of‐the‐atmosphere reflected shortwave radiation, and 4.6 W m−2 in outgoing longwave radiation over West Africa. However, the range in annual mean of the model seasonal cycles is large (37.2 and 34.0 W m−2 for reflected shortwave radiation and outgoing longwave radiation, respectively). We use seasonal and regional contrasts in all‐sky fluxes to infer that the representation of the West African monsoon in numerical models affects radiative energy partitioning. Using clear‐sky surface fluxes, we find that the models tend to have more downwelling shortwave and less downwelling longwave radiation than EBAF, consistent with past research. We find models that are drier and have lower aerosol loading tend to show the largest differences. We find evidence that aerosol variability has a larger effect in modulating downwelling shortwave radiation than water vapor in EBAF, while the opposite effect is seen in the majority of CMIP5 models.

Changes in the occurrence of extreme precipitation events at the Paleocene–Eocene thermal maximum

Future global warming is widely anticipated to increase the occurrence of extreme precipitation events, but such hydrological changes have received limited attention within paleoclimate studies. Several proxy studies of the hydrological response to the Paleocene–Eocene Thermal Maximum hyperthermal, ∼56 Ma, have recently invoked changes in the occurrence of extreme precipitation events to explain observations, but these changes have not been studied for the geologic past using climate models. Here, we use a coupled atmosphere–ocean general circulation model, HadCM3L, to study regional changes in metrics for extreme precipitation across the onset of the PETM by comparing simulations performed with possible PETM and pre-PETM greenhouse gas forcings. Our simulations show a shift in the frequency–intensity relationship of precipitation, with extreme events increasing in importance over tropical regions including equatorial Africa and southern America. The incidence of some extreme events increases by up to 70% across the PETM in some regions. While the most extreme precipitation rates tend to relate to increases in convective precipitation, in some regions dynamic changes in atmospheric circulation are also of importance. Although shortcomings in the ability of general circulation models to represent the daily cycle of precipitation and the full range of extreme events precludes a direct comparison of absolute precipitation rates, our simulations provide a useful spatial framework for interpreting hydrological proxies from this time period. Our results indicate that changes in extreme precipitation behaviour may be decoupled from those in mean annual precipitation, including, for example in east Africa, where the change in mean annual precipitation is small but a large increase in the size and frequency of extreme events occurs. This has important implications for the interpretation of the hydrological proxy record and our understanding of climatic, as well as biogeochemical, responses to global warming events.

Interannual and low-frequency variability of Upper Indus Basin winter/spring precipitation in observations and CMIP5 models

An assessment is made of the ability of general circulation models in the CMIP5 ensemble to reproduce observed modes of low-frequency winter/spring precipitation variability in the region of the Upper Indus basin (UIB) in south-central Asia. This season accounts for about two thirds of annual precipitation totals in the UIB and is characterized by “western disturbances” propagating along the eastward extension of the Mediterranean storm track. Observational data are utilized for for spatiotemporal characterization of the precipitation seasonal cycle, to compute seasonalized spectra and finally, to examine teleconnections, in terms of large-scale patterns in sea-surface temperature (SST) and atmospheric circulation. Annual and lowpassed variations are found to be associated primarily with SST modes in the tropical and extratropical Pacific. A more obscure link to North Atlantic SST, possibly related to the North Atlantic Oscillation, is also noted. An ensemble of 31 CMIP5 models is then similarly assessed, using unforced preindustrial multi-century control runs. Of these models, eight are found to reproduce well the two leading modes of the observed seasonal cycle. This model subset is then assessed in the spectral domain and with respect to teleconnection patterns, where a range of behaviors is noted. Two model families each account for three members of this subset. The degree of within-family similarity in behavior is shown to reflect underlying model differences. The results provide estimates of unforced regional hydroclimate variability over the UIB on interannual and decadal scales and the corresponding far-field influences, and are of potential relevance for the estimation of uncertainties in future water availability.

Spontaneous QBO-like oscillations in an atmospheric model dynamical core

[1] The ability of general circulation models (GCMs) to simulate the quasi-biennial oscillation (QBO) is an important model characteristic. Typically, the moist convective parameterization is believed to be the key GCM component that triggers tropical waves, thereby forcing wave-mean flow interactions. We show that QBO-like oscillations can also be simulated in a dry dynamical core driven by the Held-Suarez forcing. No gravity wave drag parameterization is applied. The simulations utilize the semi-Lagrangian spectral transform dynamical core of National Center for Atmospheric Research's Community Atmosphere Model. The QBO-like signal has a long period between 42–45 months and occurs in the upper stratosphere; different from observations. However, the amplitudes, asymmetries, and meridional extent closely resemble the observed QBO. Wave-number frequency analysis shows that resolved equatorially trapped waves are abundant despite the absence of cumulus convection. A Transformed Eulerian-Mean analysis suggests that the divergence of the Eliassen-Palm flux and vertical advection provide most of the forcing counteracted by diffusion.

Skill, Correction, and Downscaling of GCM-Simulated Precipitation

The ability of general circulation models (GCMs) to correctly simulate precipitation is usually assessed by comparing simulated mean precipitation with observed climatologies. However, to what extent the skill in simulating average precipitation indicates how well the models represent temporal changes is unclear. A direct assessment of the latter is hampered by the fact that freely evolving climate simulations for past periods are not set up to reproduce the specific evolution of internal atmospheric variability. Therefore, model-to-real-world comparisons of time series of daily, monthly, or annual precipitation are not meaningful. Here, for the first time, the authors quantify GCM skill in simulating precipitation variability using simulations in which the temporal evolution of the large-scale atmospheric state closely matches that of the real world. This is achieved by nudging the atmospheric states in the ECHAM5 GCM, but crucially not the precipitation field itself, toward the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40). Global correlation maps between observed and simulated seasonal precipitation allow areas in which simulated future precipitation changes are likely to be meaningful to be identified. In many areas, correlations higher than 0.8 are found.This means also that in these regions the simulated precipitation is a very good predictor for the true precipitation, and thus a statistical correction of the simulated precipitation, which can include a downscaling component, can provide useful estimates for local-scale precipitation. The authors show that a simple scaling of the simulated precipitation performs well in a cross validation and thus appears to be a promising alternative to standard statistical downscaling approaches.

The ability of general circulation models to simulate tropical cyclones and their precursors over the North Atlantic main development region

The ability of General Circulation Models (GCMs) to generate Tropical Cyclones (TCs) over the North Atlantic Main Development Region (MDR; 10–20°N, 20–80°W; Goldenberg and Shapiro in J Clim 9:1169–1187, 1996) is examined through a subset of ocean–atmosphere coupled simulations from the World Climate Research Programme (WCRP) Coupled Model Intercomparison Project phase 3 (CMIP3) multimodel data set and a high-resolution (0.5°) Sea Surface Temperature (SST)-forced simulation from the Australian Conformal-Cubic Atmospheric Model GCM. The results are compared with National Center for Environmental Prediction (NCEP-2) and European Center for Medium Range Weather Forecasts Re-Analysis (ERA-40) reanalyses over a common period from 1980 to 1998. Important biases in the representation of the TC activity are encountered over the MDR. This study emphasizes the strong link in the GCMs between African Easterly Waves (AEWs) and TC activity in this region. However, the generation of AEWs is not a sufficient condition alone for the models to produce TCs. Precipitation over the Sahel, especially rainfall over the Fouta Djallon highlands (cf. Fig. 1), is playing a role in the generation of TCs over the MDR. The influence of large-scale fields such as SST, vertical wind shear and tropospheric humidity on TC genesis is also examined. The ability of TC genesis indices, such as the Genesis Potential Index and the Convective Yearly Genesis Potential, to represent TC activity over the MDR in simulations at low to high spatial resolutions is analysed. These indices are found to be a reasonable method for comparing cyclogenesis in different models, even though other factors such as AEW activity should also be considered.

Investigating the ability of general circulation models to capture the effects of Eurasian snow cover on winter climate

The ability of general circulation models (GCMs) to reproduce the observed strong correlations of Eurasian snow extent in the fall to wave activity and Northern Annular Mode anomalies in the following winter is studied. The observed correlations have been hypothesized to involve two parts: a Rossby wave pulse generated in the troposphere in response to snow‐forced surface cooling and a coupled zonal‐mean stratosphere‐troposphere response to this Rossby wave pulse involving eddy mean flow interactions. It is found that all coupled ocean atmosphere GCMs used within the Coupled Model Intercomparison Project 3 (CMIP3) fail to capture the observed correlations. Using the CMIP3 GCMs and two versions of a particular GCM forced by prescribed sea surface temperatures, possible reasons for this are considered. The snow forcing, as represented in the spatial extent and interannual variability of snow cover area, is found to be reasonable although somewhat weak in the GCMs, as is the relationship between snow cover and the zonal‐mean circulation. However, the anomaly of eddy geopotential height associated with Eurasian snow cover anomalies is found to be too localized longitudinally in the GCMs. It is proposed that the reduced longitudinal scale of the snow‐forced Rossby wave pulse prevents it from propagating into the stratosphere, thus inhibiting the observed wave‐driven stratosphere‐troposphere response to the pulse.

A Canadian Contribution to the Paleoclimate Model Intercomparison Project (PMIP)

PMIP (Paleoclimate Model lntercomparison Project) is designed to compare and improve the ability of General Circulation Models (GCMs) to simulate a wide range of climatic conditions including known features of past climatic states that are significantly different from present conditions. One of the first simulations of past climate conducted under this project targets the 6000 yr BP period for the following reasons: 1) deglaciation was complete and the last remnants of the Laurentide Ice Sheet had essentially disappeared by this time, 2) sea surface temperatures approached modern values, 3) the orbital insolation regime was the only major boundary condition significantly different from present, and 4) a number of study sites can already be used to provide preliminary paleoenvironmental reconstructions for this period. Contributions from the Canadian scientific community towards this experiment are presented, following a brief overview of Canadian modern environmental conditions which places the regional contributions into a broader perspective.

Snow Cover and Snow Mass Intercomparisons of General Circulation Models and Remotely Sensed Datasets

Abstract Confirmation of the ability of general circulation models (GCMs) to accurately represent snow cover and snow mass distributions is vital for climate studies. There must be a high degree of confidence that what is being predicted by the models is reliable, since realistic results cannot be assured unless they are tested against results from observed data or other available datasets. In this study, snow output from seven GCMs and passive-microwave snow data derived from the Nimbus-7 Scanning Multichannel Microwave Radiometer (SMMR) are intercompared. National Oceanic and Atmospheric Administration satellite data are used as the standard of reference for snow extent observations and the U.S. Air Force snow depth climatology is used as the standard for snow mass. The reliability of the SMMR snow data needs to be verified, as well, because currently this is the only available dataset that allows for yearly and monthly variations in snow depth. [The GCMs employed in this investigation are the United Ki...

Improvements in General Circulation Model performance in simulating Antarctic climate

Increasingly, many aspects of the study of Antarctica and the high southern latitudes are being aided by various types of numerical models. Among these are the General Circulation Models (GCMs), which are powerful tools that can be used to understand the maintenance of present atmospheric climate and determine its sensitivity to imposed changes. The changes in the ability of GCMs used over the last two decades to simulate aspects of atmospheric climate at high southern latitudes are traced and it is concluded there has been a steady improvement in model products. The task of assessing model climates in high southern latitudes is made difficult by the uncertainties in the data used for the climatological statistics. It is suggested that the quality of the climates produced by most modern GCMs in many aspects cannot be said to be poor, especially considering the uncertainties in ‘observed’ climate. There is obviously need for improvements in both modelling and observations. Finally, some topics are highlighted in which the formulation of models could be improved, with special reference to better treatment of physical processes at high southern latitudes.

An Essay on Climate Dynamics

Following a general survey of the evidence for climatic variation, the dynamical basis of climate and the modeling of climatic change are briefly reviewed. The ability of general circulation models and of climate models employing a greater degree of averaging or statistical representation to simulate climatic changes is limited by our incomplete knowledge of the physical system and by the models' computational demands. Future research in climate dynamics is seen to involve a mixture of modeling approaches in an expanded program of experimentation and analysis.