NCAR General Circulation Model Research Articles

The MJO in a Coarse-Resolution GCM with a Stochastic Multicloud Parameterization

Abstract The representation of the Madden–Julian oscillation (MJO) is still a challenge for numerical weather prediction and general circulation models (GCMs) because of the inadequate treatment of convection and the associated interactions across scales by the underlying cumulus parameterizations. One new promising direction is the use of the stochastic multicloud model (SMCM) that has been designed specifically to capture the missing variability due to unresolved processes of convection and their impact on the large-scale flow. The SMCM specifically models the area fractions of the three cloud types (congestus, deep, and stratiform) that characterize organized convective systems on all scales. The SMCM captures the stochastic behavior of these three cloud types via a judiciously constructed Markov birth–death process using a particle interacting lattice model. The SMCM has been successfully applied for convectively coupled waves in a simplified primitive equation model and validated against radar data of tropical precipitation. In this work, the authors use for the first time the SMCM in a GCM. The authors build on previous work of coupling the High-Order Methods Modeling Environment (HOMME) NCAR GCM to a simple multicloud model. The authors tested the new SMCM-HOMME model in the parameter regime considered previously and found that the stochastic model drastically improves the results of the deterministic model. Clear MJO-like structures with many realistic features from nature are reproduced by SMCM-HOMME in the physically relevant parameter regime including wave trains of MJOs that organize intermittently in time. Also one of the caveats of the deterministic simulation of requiring a doubling of the moisture background is not required anymore.

The MJO and Convectively Coupled Waves in a Coarse-Resolution GCM with a Simple Multicloud Parameterization

Abstract The adequate representation of the dominant intraseasonal and synoptic-scale variability in the tropics, characterized by the Madden–Julian oscillation (MJO) and convectively coupled waves, is still problematic in current operational general circulation models (GCMs). Here results are presented using the next-generation NCAR GCM—the High-Order Methods Modeling Environment (HOMME)—as a dry dynamical core at a coarse resolution of about 167 km, coupled to a simple multicloud parameterization. The coupling is performed through a judicious choice of heating vertical profiles for the three cloud types—congestus, deep, and stratiform—that characterize organized tropical convection. Important control parameters that affect the types of waves that emerge are the background vertical gradient of the moisture and the stratiform fraction in the multicloud parameterization, which set the strength of large-scale moisture convergence and unsaturated downdrafts in the wake of deep convection, respectively. Three numerical simulations using different moisture gradients and different stratiform fractions are considered. The first experiment uses a large moisture gradient and a small stratiform fraction and provides an MJO-like example. It results in an intraseasonal oscillation of zonal wavenumber 2, moving eastward at a constant speed of roughly 5 m s−1. The second uses a weaker background moisture gradient and a large stratiform fraction and yields convectively coupled Rossby, Kelvin, and two-day waves, embedded in and interacting with each other; and the third experiment combines the small stratiform fraction and the weak background moisture gradient to yield a planetary-scale (wavenumber 1) second baroclinic Kelvin wave. While the first two experiments provide two benchmark examples that reproduce several key features of the observational record, the third is more of a demonstration of a bad MJO model solution that exhibits very unrealistic features.

Accuracy issues of the existing thermospheric wind models: can we rely on them in seeking solutions to wind-driven problems?

Abstract. We address the question of the ability of empirical and general circulation model neutral wind profiles in the lower thermosphere to reproduce the observed characteristics of the winds in that part of the atmosphere. The winds in that altitude range are critical for electrodynamic processes, but evaluations of the model winds are generally difficult because of the sparse observational data, which makes an evaluation of the wind predictions over large areas difficult or impossible. In this paper, we use a recently identified characteristic of the winds in the lower thermosphere, namely the enhanced winds and strong shears between 95 and 115 km altitude, as a test of the models, at least in a statistical sense. Our results show that the Horizontal Wind Model (HWM) significantly underestimates the maximum winds and shears in the lower thermosphere, although it has reasonable agreement with the average winds. The NCAR general circulation model used in this study also underestimates the maximum winds and shears significantly when run with standard resolution, as well as producing an unrealistic increase of the wind speed with height. The agreement between the model and the observations improves significantly however, in a statistical sense, when the altitude resolution is increased. The improved height resolution in the model appears to produce a greater improvement in the model predictions than any of the other factors that we examined, such as improving the geomagnetic forcing or the forcing at the lower boundary.

New results on equatorial thermospheric winds and the midnight temperature maximum

Abstract. Optical observations of thermospheric winds and temperatures determined with high resolution measurements of Doppler shifts and Doppler widths of the OI 630-nm equatorial nightglow emission have been made with improved accuracy at Arequipa, Peru (16.4° S, 71.4° W) with an imaging Fabry-Perot interferometer. An observing procedure previously used at Arecibo Observatory was applied to achieve increased spatial and temporal sampling of the thermospheric wind and temperature with the selection of eight azimuthal directions, equally spaced from 0 to 360°, at a zenith angle of 60°. By assuming the equivalence of longitude and local time, the data obtained using this technique is analyzed to determine the mean neutral wind speeds and mean horizontal gradients of the wind field in the zonal and meridional directions. The new temperature measurements obtained with the improved instrumental accuracy clearly show the midnight temperature maximum (MTM) peak with amplitudes of 25 to 200 K in all directions observed for most nights. The horizontal wind field maps calculated from the mean winds and gradients show the MTM peak is always preceded by an equatorward wind surge lasting 1–2 h. The results also show for winter events a meridional wind abatement seen after the MTM peak. On one occasion, near the September equinox, a reversal was observed during the poleward transit of the MTM over Arequipa. Analysis inferring vertical winds from the observed convergence yielded inconsistent results, calling into question the validity of this calculation for the MTM structure at equatorial latitudes during solar minimum. Comparison of the observations with the predictions of the NCAR general circulation model indicates that the model fails to reproduce the observed amplitude by a factor of 5 or more. This is attributed in part to the lack of adequate spatial resolution in the model as the MTM phenomenon takes place within a scale of 300–500 km and ~45 min in local time. The model shortcoming is also attributed in part to the need for the model to include a hydrodynamical mechanism to describe the merging of the zonal wind with the meridional tidal winds that converge onto the geographical equator. Finally, a conclusion of this work is that the MTM compressional heating takes place along the perimeter of the pressure bulge rather than within the bulge, an issue previously not appreciated.

A seasonal study of mesospheric temperatures and emission intensities at Adelaide and Alice Springs

Aerospace imagers operating at Alice Springs (23°42′S, 133°53′E) and Adelaide (34°55′S, 138°36′E) have collected more than 4‐year of OH and O2 atmospheric emission data. Images taken over the course of each moonless night at 5‐min intervals were used to determine OH Meinel (6, 2) and O2 Atmospheric (0, 1) band emission intensities and temperatures, as well as atmospheric gravity wave parameters. The NCAR general circulation model TIME‐GCM was run for years 2002–2005 for comparison with these data. The data presented here show the interannual variability of OH and O2A emissions at two sites, Alice Springs and Adelaide, over a 4‐a period. It was found that the TIME‐GCM successfully reproduces many observed features of the data, including equinoctial maxima associated with the diurnal tide, a 6‐h phase shift between OH temperature and intensity maxima, and springtime OH intensity enhancements at Alice Springs. However, the model tends to underestimate the depth of the summertime temperature minimum at both sites, possibly due to inadequate specification of the seasonal variation of gravity waves in the model. The model does, however, successfully describe many of the mesospheric changes observed during the 2002 stratospheric warming event.

Modeling Water Vapor in the Upper Troposphere and Lower Stratosphere

Upper troposphere and lower stratosphere (UTLS) water vapor is investigated using a general circulation model, the Community Atmosphere Model 3.0 (CAM3.0). Seasonal variability in UTLS water vapor, temperature and zonal wind, based on model simulation results for the period 1991-2000, are analyzed. Results are validated against satellite data from the Halogen Occultation Experiment (HALOE) and ERA-40 reanalyzes from ECMWF. The model captures the seasonal cycle in temperature as well as water vapor. The zonal wind deviates from the reanalysis data in the tropics as the model is not able to reproduce the Quasi-Biennial Oscillation (QBO). Outside the tropics, the zonal wind corresponds very well with the ERA-40 zonal wind. The model is able to reproduce the seasonal signal in the tropical stratospheric water vapor; i.e., the Tape Recorder Signal. This indicates a realistic Brewer-Dobson circulation in the model. However, the Tape Recorder signal attenuates too fast compared to the HALOE data, suggesting a too strong horizontal mixing between the tropical stratosphere and mid latitudes. CAM3.0 shows considerable improvements in UTLS temperatures as well as water vapor compared to earlier generations of the NCAR general circulation models.

Preferred Regime Transition Routes and Evidence for an Unstable Periodic Orbit in a Baroclinic Model

The phase space structure of the attractor of a baroclinic quasigeostrophic model is analyzed in terms of mean tendencies in dominant EOF space. The dominant EOFs as well as the probability density functions in the planes spanned by the dominant EOFs are similar to corresponding features of the NCAR general circulation model, the Community Climate Model (CCM0). It is found that the mean motion is characterized by a cyclic motion involving the Northern Hemisphere annular mode (NAM), the Pacific–North American (PNA) pattern, and a third pattern. States with positive (negative) projection onto this third pattern are characterized by a mean tendency toward increasing (decreasing) NAM and PNA amplitudes. The time scale of the cycle is about 20 days, and it compares well with an observed westward-propagating wave. Three regimes are identified in the space of the dominant EOFs. Preferred transition routes between these regimes exist and correspond to particular phases of the cycle. The authors hypothesize that the cycle corresponds to an unstable periodic orbit that is a remnant of a heteroclinic connection between the stationary points underlying the regimes.

Atmospheric ozone as a climate gas

Observations during the past few decades indicate that the distribution of atmospheric ozone has been changing, notably decreases in the lower stratosphere around the globe and increases in the upper troposphere in the mid-to-high latitudes of the Northern Hemisphere. These changes can perturb the radiative forcing of the Earth-atmosphere climate system and thus raise the issue of atmospheric ozone as a climate gas. Here, we demonstrate the importance of ozone to climate by comparing general circulation model (GCM) simulated present climates between two atmospheric ozone climatologies: a version currently used in the NCAR GCM's (CCM1 and GENESIS) and an updated version. The new ozone climatology includes two improvements: the use of recent years' measurements from satellite (TOMS and SAGE) and ground-based ozonesondes and the consideration of longitudinal variations. Although the effect on the simulated global, annual mean surface air temperature is calculated to be small, the regional influences are quite substantial, especially at the middle and high latitudes of the Northern Hemisphere during winter. This suggests that atmospheric ozone is important in simulating the regional climate and observed ozone changes in the last few decades may have played an important role in affecting the climate. Note that to assess the climatic effect of ozone changes requires the GCM's to include interactive radiation-chemistry-dynamics interaction so that the ozone distribution can be consistently calculated.

GCM Simulations of the Three-Dimensional Propagation of Stationary Waves

Abstract Plumb's formulation of the stationary wave activity flux is used to determine how well versions of the GFDL and NCAR general circulation models simulate the sources, sinks, and horizontal propagation of atmospheric stationary waves, which play an important role in determining regional climate. The wave activity flux provides insight into the simulation of nondynamic as well as dynamic processes in these models. Model performances for current climate simulations are evaluated with respect to NMC analyses averaged over 1978–1990. The models fare best when the stationary wave forcing is strongest, that is, in the wintertime Northern Hemisphere, where they reproduce the observed three-branch structure of upward wave activity flux. For the Northern Hemisphere summer and the Southern Hemisphere in both summer and winter, the models show less agreement with observations, although they do simulate the generally downward flux observed during Northern Hemisphere summer, which the analysis suggests is cause...

Assessment of NCAR general circulation model precipitation in comparison with observations

In this study precipitation fields from six different present-day climate experiments are compared to observed data. The six NCAR general circulation model experiments are: (a) prescribed sea-surface temperature (SST) variable hydrology CCM1, (b) prescribed SST fixed hydrology CCM1, (c) mixed layer CCM1, (d) prescribed SST w/ BATS CCM0B (e) mixed layer CCM0A, and (f) mean annual CCM0A. In general, predicted precipitation compares more favorably with observations for the case of prescribed SSTs, than for the overly zonal energy balance ocean experiments. However, despite shortcomings, the energy balance ocean experiments have utility for climate change experiments due to their lack of bias associated with specifying ocean temperature distributions. The most favorable comparisons are in the subtropics and mid-latitudes, followed by the tropics and poles. The model is best at reproducing large scale features such as the sub-tropical deserts and mid-latitude frontal precipitation. It is poorest at stimulating features that are dependent upon sub-resolution processes such as orographic uplift and convection. The model reproduces the major observed trends between the solstice seasons, such as lower land/ocean precipitation fractions in DJF, although many regional inaccuracies exist. Continental precipitation is generally excessive in the model, although the global combined continental and oceanic (CAO) averages are quite close to the observed value. Observed precipitation typically accounts for between 0.3 and 0.4 of the global spatial variance in the model data.

Assessment of NCAR general circulation model precipitation in comparison with observations

In this study precipitation fields from six different present-day climate experiments are compared to observed data. The six NCAR general circulation model experiments are: (a) prescribed sea-surface temperature (SST) variable hydrology CCM1, (b) prescribed SST fixed hydrology CCM1, (c) mixed layer CCM1, (d) prescribed SST w/ BATS CCM0B (e) mixed layer CCM0A, and (f) mean annual CCM0A. In general, predicted precipitation compares more favorably with observations for the case of prescribed SSTs, than for the overly zonal energy balance ocean experiments. However, despite shortcomings, the energy balance ocean experiments have utility for climate change experiments due to their lack of bias associated with specifying ocean temperature distributions. The most favorable comparisons are in the subtropics and mid-latitudes, followed by the tropics and poles. The model is best at reproducing large scale features such as the sub-tropical deserts and mid-latitude frontal precipitation. It is poorest at stimulating features that are dependent upon sub-resolution processes such as orographic uplift and convection. The model reproduces the major observed trends between the solstice seasons, such as lower land/ocean precipitation fractions in DJF, although many regional inaccuracies exist. Continental precipitation is generally excessive in the model, although the global combined continental and oceanic (CAO) averages are quite close to the observed value. Observed precipitation typically accounts for between 0.3 and 0.4 of the global spatial variance in the model data.

The Response of a Linear Model of the Tropical Pacific to Surface Winds from the NCAR General Circulation Model

Abstract An experiment in which surface wind stress data from the National Center for Atmospheric Research global circulation model (GCM) was used to drive a simple model of the tropical Pacific is described. First, a 15-year integration of the GCM was conducted, forcing the model with observed sea surface temperatures (SSTs) for the period 1958–72 (the FORCED case). A parallel integration was conducted using long-term monthly mean SSTs (the CONTROL case). All other boundary forcing in both GCM simulations was identical. Surface wind stress date from each GCM integration were then used to drive the Florida State University 1½ layer, reduced gravity model of the tropical Pacific. A separate integration of the ocean model was conducted using observed wind stress for the period 1961–72 (the OBSTRESS case). The goal of this research was to evaluate the response of a sophisticated GCM to tropical Pacific SST variability in terms of the surface wind stress field, and to investigate the sensitivity of a simple w...

Response of the NCAR General Circulation Model to Prescribed Changes in Ocean Surface Temperature. Part II: Midlatitude and Subtropical Changes

Abstract The sensitivity of a six-layer NCAR atmospheric general circulation model (GCM) to a variety of idealized, very large amplitude, midlatitude and subtropical North Pacific Ocean surface temperature (OST) anomalies is analyzed. In the Pacific sector, the model exhibits a differential sensitivity depending on the latitudinal position of the imposed anomaly. Typically, the model response is a combination of a relative direct thermal circulation, an alteration in the pattern of cyclonic activity, and a selective wave response dependent on the planetary waves present in the unmodified control case. The extent to which the background planetary waves affect the model response is dependent on several factors including latitude-dependent features of the control case and the relative position of the anomaly. Analogous experiments with a simple quasi-geostrophic model are useful in isolating important physical and dynamical processes, and thereby assist in the interpretation of the GCM results. An analysis o...

Regional Climatic Effects of Large-Scale Thermal Pollution: Simulation Studies with the NCAR General Circulation Model

Abstract January and July experiments were performed with the NCAR general circulation model (GCM) to assess the potential climatic impact of the thermal energy released from a projected United States cast coast megalopolis circa 2000 A.D. The model has six layers in the vertical and a 5° latitude-longitude horizontal resolution. The ocean surface temperatures were held fixed with respect to time in both experiments at the appropriate observed climatological values for each month. To determine the statistical significance of the model response, sets of random perturbation experiments were performed for each month to obtain a measure of the model noise level (i.e., the estimated standard deviation of monthly means). Larger surface temperature changes are found in the January thermal pollution experiment. with a maximum of 12°C in the vicinity of the beat input. Smaller but still significant changes with a maximum of 3°C are found in the July experiment. Significant changes in precipitation and soil moistur...

A Space-Time Spectral Comparison of the NCAR and GFDL General Circulation Models to the Atmosphere

Abstract Spectra computed for the midlatitude troposphere indicate that both the NCAR and GFDL models have less variance than the atmosphere in most space and time scales. The deficiency is most serious for low-frequency planetary waves near 50°N. The low-frequency planetary-scale fluctuations also show less tendency for westward movement than is observed. In both models, especially NCAR's, the 4–5 day wavenumber 1 variance is relatively too large. The GFDL spectra are generally closer to observed values than the NCAR spectra, probably because of the GFDL model's higher horizontal and vertical resolution. The NCAR spectra show an abnormal level of variance at a period of 5 days in zonal wavenumbers 8–12.

Response of the NCAR General Circulation Model to North Pacific Sea Surface Temperature Anomalies

Abstract The general circulation model (GCM) of the National Center for Atmospheric Research was used to investigate the effects of anomalous sea surface temperature (SST) patterns in the mid-latitudes of the North Pacific Ocean on atmospheric circulations. One of the most frequently observed SST anomaly (SSTA) patterns, a “cold pool” of water in the west and a “warm pool” in the cast (from 35 to 55°N) with magnitudes ±4°C of the temperature of the surrounding water, was superimposed on the seasonally varying oceanic temperature as the thermal boundary condition in the North Pacific of the atmospheric GCM for the anomaly experiment Three similar experiments, one the control, two the noise and three an exaggerated SSTA, were run simultaneously. Time integrations for all cases were carried out for 120 days, beginning on 15 January. Preliminary results indicate that cyclonic activities are strengthened above the warm SSTA and suppressed above the cold SSTA. A direct thermal circulation is induced in the vert...

An Energetics Analysis of Forecast Experiments with the NCAR General Circulation Model

Abstract The energetics in numerical weather forecast experiments with the NCAR general circulation model have been analyzed. The 6-layer, 5-degree, second-generation global model was used to make two 10-day forecasts with the same initial conditions. The two experiments differed primarily in the methods of convective parameterization. Hemispheric integrals of the model energies and energy transformations are presented in the context of their approach to a quasi-equilibrium climatology. Spectral and spatial analyses of the eddy energies and transformations provide further insight into the model response to the initial conditions. After the initial adjustment, the eddy kinetic energy appears to lag the conversion from eddy available potential energy to eddy kinetic energy by at least 48 h in the long waves (wavenumbers 1–4) and by approximately 24 h in the baroclinic waves (wavenumbers 5–7), whereas little or no time lag is apparent in the short waves (wavenumbers 8–12). The sensitivity of the forecast ene...

Comments on “A Single Forecast Comparison between the NCAR and GFDL General Circulation Models”

Comments on “A Single Forecast Comparison between the NCAR and GFDL General Circulation Models”

Energetics Diagnosis of the NCAR General Circulation Model

A comprehensive energetics analysis has been performed on the NCAR general circulation model. The analysis involves January and July simulation experiments with the 6-layer, 5-degree, second-generation model with two different convective schemes. Spectral analysis of the energy transformations in the wave-number domain was performed separately on a global and hemispheric basis as well as for the tropics and mid-latitudes. Latitudinal distributions of energy variables were also examined. A qualitative agreement with observational estimates is generally recognized in the transformations of eddy energies. Quantitatively, however, the eddy energies, conversions and energy transfer between wavenumbers are weaker than observational estimates. It is noteworthy that substantial differences exist in the energetics of the two versions of the model with different convective schemes.

Response of the NCAR General Circulation Model to Prescribed Changes in Ocean Surface Temperature Part I: Mid-Latitude Changes

Four numerical experiments are analyzed to determine the three-dimensional response of the NCAR general circulation model to large prescribed changes in mid-latitude North Pacific Ocean surface temperature. The ocean surface temperature (OST) boundary conditions are subjected to changes of opposite sign in the eastern and west-central portions of the North Pacific Ocean. The maximum amplitude of the temperature changes is either 12°C or 4°C. The model atmosphere response in the North Pacific sector includes changes in amplitude and vertical tilt of the long waves, an increased direct thermal circulation (i.e., warm air rises over the positive OST change and cold air sinks over the negative OST change), and locally enhanced westerlies to the north of the positive OST change. Cyclones form and/or intensify over the positive OST change and tend to be absent or weak over the negative OST change. The mid-tropospheric response extends downstream from the prescribed change region, and the response both over and downstream from the region depends strongly on the longitude of the prescribed changes. Many features of the response are statistically significant, although generally not over the continental United States. The amplitude and phase of the mid-tropospheric long waves (zonal wavenumbers 1–4) are also affected. The prescribed change response is largest and of greatest statistical significance when the prescribed change is very large (12°C maximum amplitude) but is also frequently detectable when the prescribed change is one-third as large (4°C maximum amplitude). A comparable experiment involving a prescribed North Atlantic OST change produces a similar mid-tropospheric response.