Low-resolution General Circulation Model Research Articles

Application of HadCM3@Bristolv1.0 simulations of paleoclimate as forcing for an ice-sheet model, ANICE2.1: set-up and benchmark experiments

Abstract. Fully coupled ice-sheet–climate modelling over 10 000–100 000-year timescales at high spatial and temporal resolution remains beyond the capability of current computational systems. Forcing an ice-sheet model with precalculated output from a general circulation model (GCM) offers a middle ground, balancing the need to accurately capture both long-term processes, in particular circulation-driven changes in precipitation, and processes requiring a high spatial resolution like ablation. Here, we present and evaluate a model set-up that forces the ANICE 3-D thermodynamic ice-sheet–shelf model calculating the four large continental ice sheets (Antarctica, Greenland, North America, and Eurasia) with precalculated output from two steady-state simulations with the HadCM3 (GCM) using a so-called matrix method of coupling both components, whereby simulations with various levels of pCO2 and ice-sheet configuration are combined to form a time-continuous transient climate forcing consistent with the modelled ice sheets. We address the difficulties in downscaling low-resolution GCM output to the higher-resolution grid of an ice-sheet model and account for differences between GCM and ice-sheet model surface topography ranging from interglacial to glacial conditions. Although the approach presented here can be applied to a matrix with any number of GCM snapshots, we limited our experiments to a matrix of only two snapshots. As a benchmark experiment to assess the validity of this model set-up, we perform a simulation of the entire last glacial cycle from 120 kyr ago to present day. The simulated eustatic sea-level drop at the Last Glacial Maximum (LGM) for the combined Antarctic, Greenland, Eurasian, and North American ice sheets amounts to 100 m, in line with many other studies. The simulated ice sheets at the LGM agree well with the ICE-5G reconstruction and the more recent DATED-1 reconstruction in terms of total volume and geographical location of the ice sheets. Moreover, modelled benthic oxygen isotope abundance and the relative contributions from global ice volume and deep-water temperature agree well with available data, as do surface temperature histories for the Greenland and Antarctic ice sheets. This model strategy can be used to create time-continuous ice-sheet distribution and sea-level reconstructions for geological periods up to several million years in duration, capturing climate-model-driven variations in the mass balance of the ice sheet.

Using synoptic type analysis to understand New Zealand climate during the Mid-Holocene

Abstract. Diagnosing the climate of New Zealand from low-resolution General Circulation Models (GCMs) is notoriously difficult due to the interaction of the complex topography and the Southern Hemisphere (SH) mid-latitude westerly winds. Therefore, methods of downscaling synoptic scale model data for New Zealand are useful to help understand past climate. New Zealand also has a wealth of palaeoclimate-proxy data to which the downscaled model output can be compared, and to provide a qualitative method of assessing the capability of GCMs to represent, in this case, the climate 6000 yr ago in the Mid-Holocene. In this paper, a synoptic weather and climate regime classification system using Empirical Orthogonal Function (EOF) analysis of GCM and reanalysis data was used. The climate regimes are associated with surface air temperature and precipitation anomalies over New Zealand. From the analysis in this study, we find at 6000 BP that increased trough activity in summer and autumn led to increased precipitation, with an increased north-south pressure gradient ("zonal events") in winter and spring leading to drier conditions. Opposing effects of increased (decreased) temperature are also seen in spring (autumn) in the South Island, which are associated with the increased zonal (trough) events; however, the circulation induced changes in temperature are likely to have been of secondary importance to the insolation induced changes. Evidence from the palaeoclimate-proxy data suggests that the Mid-Holocene was characterized by increased westerly wind events in New Zealand, which agrees with the preference for trough and zonal regimes in the models.

Combining Argo profiles with a general circulation model in the North Atlantic. Part 2: Realistic transports and improved hydrography, between spring 2002 and spring 2003

A set of Argo profiles collected in the North Atlantic between May 2002 and April 2003 is combined with a low-resolution general circulation model (GCM) using the adjoint method. Fitting the real hydrographic observations leads to vast improvements in the model circulation, including the sea surface height and the meridional heat transport. We find striking differences in basin-scale transports compared with previous assimilation experiments that use the same GCM and a similar spatial resolution. Based on forward modeling studies, it is argued that these differences are due to different assimilation experiment durations. Over 1 year, the hydrography interpolated with the GCM from Argo profiles better represents the contemporary structures than does a long-term averaged climatology. The GCM dynamics are robust enough to distinguish between contemporary hydrography and climatological hydrography.

Multiple Oscillatory Modes of the Global Ocean Circulation

For the first time, the (linear) stability of the global ocean circulation has been determined explicitly. In a low-resolution general circulation model, a steady state is computed directly by solving the elliptic boundary value problem. The stability of this solution is determined by solving the generalized eigenvalue problem. Although the steady global circulation is (linearly) stable, there are two interesting oscillatory modes among the least stable ones, with periods of about 3800 and 2300 yr. These modes are characterized by buoyancy anomalies that propagate through the ocean basins as they are advected by the global overturning circulation. The millennial timescale is set by the time it takes for anomalies to travel, at depth, from the North Atlantic to the North Pacific. Further analyses confirm that the advective feedback between the steady flow and buoyancy anomalies is an essential process in the propagation mechanism. The growth rate of the millennial modes is controlled by vertical mixing. It is argued that these internal ocean modes may be a relevant mechanism for global climate variability on millennial timescales.

On advection and diffusion of plankton in coarse resolution ocean models

There is a widespread use of general circulation models (GCMs) to study the importance of various physical and biological processes for the primary production, and to understand the structure of the oceanic food web. However, it is shown that low-resolution GCMs may produce too high advection speeds for a plankton bloom that invade an area with potentially high plankton growth rates. Under the prescribed situation, the advection and diffusion act to initiate an exponential growth of plankton in a large grid cell implying that the plankton signal is quickly carried over the entire grid cell. When a certain concentration is reached, it may seed the surrounding grid cells with phytoplankton, and the procedure is repeated. Accordingly, the phytoplankton bloom can travel faster in the low-resolution model than the ocean currents that carry them. Some numerical experiments show that the resolution of the model needs to be at least on the order of Δ x∼ U/ γ P or D/γ P —whichever is largest—to describe the plankton dynamics in an adequate way. Here, U is the typical horizontal advection velocity, D is the horizontal diffusivity for plankton, and γ P represents the growth rate of phytoplankton. With U=0.1 m/s and γ P=1 day −1, it follows that the resolution needs to be at least 8 km to describe the phytoplankton dynamics in a reasonably correct way. Most GCM experiments use coarser resolution; accordingly, the results regarding the advection of plankton in these models should thus be viewed with some caution.

Effects of nested area size upon regional climate model simulations

This paper presents a numerical study on the 1998 summer rainfall over the Yangtze River valley in central and eastern China, addressing effect of a nested area size on simulations in terms of the technique of nesting a regional climate model (RCM) upon a general circulation model (GCM). Evidence suggests that the size exerts greater impacts upon regional climate of the country, revealing that a larger nested size is superior to a small one for simulation in mitigating errors of GCM-provided lateral boundary forcing. Also, simulations show that the RCM should incorporate regions of climate systems of great importance into study and a low-resolution GCM yields more pronounced errors as a rule when used in the research of the Tibetan Plateau, and, in contrast, our PσRCM can do a good job in describing the plateau’s role in a more realistic and accurate way. It is for this reason that the tableland should be included in the nested area when the RCM is employed to investigate the regional climate. Our PσRCM nesting upon a GCM reaches more realistic results compared to a single GCM used.

Sensitivity of tropical storms simulated by a general circulation model to changes in cumulus parametrization

AbstractA number of recent studies have examined the statistics of tropical storms simulated by general circulation models (GCMs) forced by observed sea surface temperatures. Many GCMs have demonstrated an ability to simulate some aspects of the observed interannual variability of tropical storms, in particular, variability in storm frequency. This has led to nascent attempts to use GCMs as part of programs to produce operational seasonal forecasts of tropical‐storm numbers.In this study, the sensitivity of the statistics of GCM‐simulated tropical storms to changes in the model's physical parametrizations is examined. After preliminary results indicated that these statistics were most sensitive to details of the convective parametrization, GCM simulations with identical dynamical cores but different convective parametrizations were created. The parametrizations examined included moist convective adjustment, two variants of the Arakawa‐Schubert scheme, and several variants of the relaxed Arakawa‐Schubert (RAS) scheme; the impact of including a shallow‐convection parametrization was also examined.The simulated tropical‐storm frequency, intensity, structure, and interannual variability were all found to exhibit significant sensitivities to changes in convective parametrization. A particularly large sensitivity was found when the RAS and Arakawa‐Schubert parametrizations were modified to place restrictions on the production of deep convection.Climatologies of the GCM tropical atmosphere and composites of tropical storms were examined to address the question of whether the tropical‐storm statistics were directly impacted on by changes in convection associated with tropical storms, or if they were indirectly affected by parametrization‐induced changes in the tropical mean atmosphere. A number of results point to the latter being the primary cause. A regional hurricane model, initialized with mean states from the GCM simulation climatologies, is used to further investigate this point. Particularly compelling is the fact that versions of the RAS scheme that produce significantly less realistic simulations of tropical storms nevertheless produce a much more realistic interannual variability of storms, apparently due to an improved tropical mean climate.A careful analysis of the background convective available potential energy (CAPE) is used to suggest that this quantity is particularly relevant to the occurrence of tropical storms in the low‐resolution GCMs, although this may not be the case with observations. If the tropical CAPE is too low, tropical storms in the low‐resolution GCMs cannot form with realistic frequency.

The Impact of an Envelope Orography on Low-Frequency Variability and Blocking in a Low-Resolution General Circulation Model

Abstract Sensitivity experiments with a perpetual January version of a low-revolution general circulation model (GCM) are conducted to investigate the influence of different, nonzero specifications of orography on low-frequency variability (LFV) and blocking in the Northern Hemisphere. Two 1200-day integrations are compared to examine the impact of an enhanced “envelope” orography. An ensemble of eight independent 90-day realizations is extracted from each simulation. Distributions of ensemble-mean statistics for the two simulations are presented, along with estimates of the statistical significance of the differences. The use of an envelope orography leads to significant changes in the distribution of the LFV (periods 10–90 days) over the Northern Hemisphere. When the LFV is partitioned into contributions from intramonthly ( 10–30-day periods) and intermonthly (30–90-day periods) fluctuations, it is found that the envelope orography significantly alters the distributions of intramonthly scale variability...

Mechanisms by Which Surface Drying Perturbs Tropical Precipitation Fields

Abstract The observed precipitation climatology in austral summer has a pronounced longitudinal gradient across Africa and South America. A low-resolution general circulation model (GCM) with a simple continent centered on the equator is used to understand how the presence of the land surface generates this gradient, and the role of surface wetness in determining its magnitude. In the model, precipitation is enhanced on the east coast of the continent in the summer hemisphere tropics with magnitude and location independent of surface wetness. Precipitation rates are lower in the continental interior and in the west as the surface becomes drier, resulting in a longitudinal precipitation gradient that is similar to observations. Modeled low-level moisture convergence and wind convergence anomalies mimic the precipitation anomalies over all but the driest land surfaces. A linearized primitive equation model is used to identify the physical mechanisms responsible for the GCM's dynamical response. Dry convecti...

Tropical Waves in a GCM with Zonal Symmetry

Abstract Tropical wave phenomena have been examined in the last 520 days of two 15-year runs of a low-resolution general circulation model (CCMO). The model boundary conditions were simplified to all-land, perpetual equinox, and no topography. The two runs were for fixed soil moisture at 75% and 0% , the so-called “wet” and “dry” models. Both models develop well-defined ITCZs with low-level convergence erratically concentrated along the equator. Highly organized eastward-propagating waves are detectable in both models with different wave speeds depending on the presence of moisture. The wave amplitudes (in, e.g., vertical velocity) are many orders of magnitude stronger in the wet model. The waves have a definite transverse nature as precipitation (low-level convergence) patches tend to move systematically north and south across the equator. In the wet model the waves are distinctly nondispersive and the transit time for passage around the earth is about 50 days, consistent with the Madden–Julian frequency...

Systematic and Random Error in an Extended-Range Forecasting Experiment

Abstract A dynamical extended-range forecast consisting of a series of six monthly forecasts for each of the eight Januarys from 1979 to 1986 is carried out with the Canadian Climate Centre low-resolution general circulation model. Results, in terms of the 500-rnb height field, are presented for the mean January systematic and random error and for the skill of the January mean forecast. The evolution of error as a function of time and spatial scale is investigated and the equations governing the growth of systematic and random error are derived and evaluated. The mean January systematic error in the 500-mb height field is modest and is largely in other than the zonal structures. Systematic-error variance is about an order of magnitude smaller than random-error variance and is concentrated in the larger scales of the flow. The January mean anomaly correlation indicates marginal forecast skill, as well as some connection between the spread of the forecasts and the skill of the average forecast. The budget e...

The Impact of Orography on Blocking Frequency in a General Circulation Model

Sensitivity experiments with a perpetual January version of a low-resolution general circulation model are conducted to investigate the influence of mountains on blocking in the Northern Hemisphere. Two 1200-day integrations, one with and the other without mountains, are compared. The absence of orography severely reduces the total number of days on which blocking is present anywhere over the Northern Hemisphere. It decreases the number of blocking episodes and shortens the duration of an average event. Strong blocking events do occur in the no-mountains simulation, however, and their structure and local maintenance characteristics are similar to strong blocks in the control run. It is concluded that orographic forcing greatly aids the blocking process in the model but is not crucial for the existence of model blocks.

Model Experiments on the Impact of Pacific Sea Surface Temperature Anomalies on Blocking Frequency

Abstract Sensitivity experiments with a perpetual January version of a low-resolution general circulation model are conducted to investigate the influence of Pacific sea surface temperature (SST) anomalies on blocking in the Northern Hemisphere. Six 1200-day integrations, each of which contains a different specified SST anomaly superimposed upon a fixed climatological SST distribution, are compared with a 1200-day control simulation to examine the impact of both equatorial Pacific and midlatitude North Pacific anomalous forcing. An ensemble of eight independent 90-day realizations is extracted from each simulation. Distributions of the ensemble-mean differences in the total number of blocking days from the control simulation are presented, along with estimates of the statistic significance of these differences. Pacific SST anomalies do not strongly affect the total number of days in which a block is present anywhere over the North Pacific basin, but they apparently can affect the preferred regions over th...

Effects of soil moisture on the sensitivity of a climate model to earth orbital forcing at 9000 yr BP

The sensitivity of climate to orbitally-related changes in solar radiation at 9000 yr BP (before present) is examined using fixed and interactive soil moisture versions of a low resolution general circulation model. In both versions of the model increased solar radiation for June–August at 9000 yr BP (compared to present) produced enhanced northern monsoons and warmer continental interiors in comparison to present whereas decreased solar radiation at 9000 yr BP in December–February produced weaker southern monsoons. The increased rainfall in the northern tropics in summer increased soil moisture and runoff at 9000 yr BP in the interactive model; in the southern hemisphere decreased rainfall in summer led to decreased soil moisture and runoff. Conditions in summer became drier (decreased soil moisture and runoff) at 9000 yr BP in the northern extratropics. The experiments showed that the magnitude (but not the sign) of model sensitivity to 9000 yr BP radiation is altered by the effects of interactive soil moisture. Decreased soil moisture (about 20%) over northern Eurasia in the interactive model led to smaller evaporative increases, greater temperature increases and greater reduction of precipitation than for the model with fixed soil moisture. Over northern tropical lands, slightly smaller temperature increases and greater evaporation and precipitation increases in the interactive model are linked to the simulation of increased soil moisture at 9000 yr BP. The differences in sensitivity between the two versions of the model over northern Eurasia are statistically significant at the 95% level whereas those for the tropics are not. Overall, the results of the simulations are generally supported by the geologic evidence for 9000 yr BP; however, the evidence lacks sufficient precision and the model resolution is too coarse for detailed model/data comparisons and for assessment of the relative accuracy of the two 9000 yr BP experiments. The computed sensitivities of temperature and soil moisture to 9000 yr BP radiation differ from those simulated under equilibrium conditions in the various general circulation model experiments for increased atmospheric concentration of CO2. In contrast to the effects of the enhanced seasonal cycle of solar radiation at 9000 yr BP, a CO2 increase causes a broad warming of both the ocean and land with little modification of land/ocean temperature difference. The experiments for 9000 yr BP indicate a clearer signal for summer drying than is obtained in the experiments for increased CO2. The results suggest that the 9000 yr BP climate may be of limited utility as an analog to future ‘warm’ climates.

The Effects of Improved Parameterizations for Orography, Snowcover, Surface Fluxes and Condensational Processes on the Climate of a Low Resolution GCM

The sensitivity of a low resolution, spectral general circulation model (GCM) to specification of physical processes is examined using a new version of the model with refined parameterizations. Specific refinements in parameterization include: 1) smoothing the original orography to greatly diminish undesirable topographic “ripples” occurring near high mountain ranges; 2) adding snowcover on the Tibetan plateau and representing winter snowcover in middle latitudes more realistically; 3) decreasing the land ground wetness and adjusting the drag coefficient and parameters governing condensation-moist convective adjustment. Results of comparative 5-year integrations show that better parameterization in the low resolution model produces significant improvement in simulation without resorting to the use of higher horizontal or vertical resolution. The combined changes in ground wetness, drag coefficient and condensation-moist convective parameters produce more realistic zonal banding of precipitation belts and a better representation of continental precipitation relative to the ocean. In addition, mass is more nearly conserved and mean sea level pressure and temperature patterns are in better agreement with observations than in the previous model. Major deficiencies in simulation that are not improved include zonal jet and stratospheric temperature structures. Overall, the improvements in simulation suggest a wider applicability of the low resolution model for use in climate sensitivity studies. Analysis of sensitivity experiments assessing specific effects of parameterization indicate that decreased ground

The Effects of Topography on the Atmospheric Energetics in. a Low-Resolution General Circulation Model

Abstract An analysis is made of the effect of orography on the atmospheric energetics in a low-resolution general circulation model to determine the temporal and scale dependency of these effects. The numerical model is a global, spectral, primitive equation model of the atmosphere with five equally spaced sigma levels in the vertical and triangular truncation at wavenumber 10 in the horizontal. A one-year seasonal simulation of the general circulation without mountains is compared to the results from a five-year seasonal simulation of the general circulation with mountains. The statistical significance of the topographic effects is evaluated by comparing them to magnitudes of model interannual variability determined from the five-year control simulation. A small, but important, portion of the changes due to topography are significant. At northern extratropical latitudes, the increases of eddy activities and baroclinic instability in summer resulting from incorporation of the effect of the mountains give ...

A Global Low-Order Spectral General Circulation Model. Part II: Diagnosis of the Seasonal Energetics

The purpose of this paper is to document the seasonal variation of the atmospheric energy cycle simulated by a low-resolution general circulation model in order to assess the model's strengths and weaknesses and better define its usefulness for future sensitivity studies. The numerical model is a global, spectral, primitive equation model with five equally-spaced sigma levels in the vertical and triangular truncation at wavenumber 10 in the horizontal. Included in the model are: orography; time-varying (but prescribed) sea surface temperature, snow cover, and solar declination angle; parameterizations of radiation, convection, condensation, diffusion, and surface transports; and a surface heat budget. The model simulates the observed vertically-integrated energetics of the troposphere reasonably well and the values are comparable to other general circulation models. At northern extratropical latitudes, the very long waves, wavenumbers 1 through 3, which are associated with thermal and orographic zonal asymmetries, dominate the model energy cycle. The annual cycle prevails, with maximum values approximately one month after the winter solstice. These model aspects are similar to those documented observationally. Analysis of the time variation of the model energetics details the presence of asymmetric and steplike variations superimposed on the annual cycles at northern extratropical latitudes. The. model statistics also reveal important latitudinal variations. In particular, more sinusoidal seasonal cycles and a predominance of energy at wavenumbers 4 through 7 are found at southern extratropical latitudes, and the thermally direct east-west circulations, associated with land-sea and precipitation zonal asymmetries, are important in the tropical energy cycle. The major discrepancies in the model simulation of the atmospheric energy cycle are the overestimation of zonal and eddy available potential energy and the conversion between them, an exaggerated semiannual component in the seasonal cycle of eddy available potential energy, a deficiency of eddy kinetic energy at the jet core level, and an underestimation of the transients. The model limitations leading to these discrepancies are discussed, as well as improvements made in subsequent versions of the model.

The Sensitivity of the African-Asian Monsoonal Climate to Orbital Parameter Changes for 9000 Years B.P. in a Low-Resolution General Circulation Model

Abstract The earth's orbital parameters, precession, obliquity and eccentricity, produce solar radiation differences (compared to present) of ∼7% at the solstices 9000 years before present (B.P.): more radiation in June-July-August, less in December-January-February. When this amplified seasonal cycle of solar radiation is used to drive a low-resolution general circulation model, an intensified monsoon circulation is simulated for Northern Hemisphere summer. The annual- and global-average land surface temperature and the annual- and global-average precipitation are the same for the simulated 9000 years B.P. climate and the present climate. Certain features of the simulated monsoon climate from this orbital-parameter sensitivity experiment agree with the paleoclimatic evidence.