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Markus Stowasser and Kevin Hamilton

Abstract

The relations between local monthly mean shortwave cloud radiative forcing and aspects of the resolved-scale meteorological fields are investigated in hindcast simulations performed with 12 of the global coupled models included in the model intercomparison conducted as part of the preparation for Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). In particular, the connection of the cloud forcing over tropical and subtropical ocean areas with resolved midtropospheric vertical velocity and with lower-level relative humidity are investigated and compared among the models. The model results are also compared with observational determinations of the same relationships using satellite data for the cloud forcing and global reanalysis products for the vertical velocity and humidity fields. In the analysis the geographical variability in the long-term mean among all grid points and the interannual variability of the monthly mean at each grid point are considered separately. The shortwave cloud radiative feedback (SWCRF) plays a crucial role in determining the predicted response to large-scale climate forcing (such as from increased greenhouse gas concentrations), and it is thus important to test how the cloud representations in current climate models respond to unforced variability.

Overall there is considerable variation among the results for the various models, and all models show some substantial differences from the comparable observed results. The most notable deficiency is a weak representation of the cloud radiative response to variations in vertical velocity in cases of strong ascending or strong descending motions. While the models generally perform better in regimes with only modest upward or downward motions, even in these regimes there is considerable variation among the models in the dependence of SWCRF on vertical velocity. The largest differences between models and observations when SWCRF values are stratified by relative humidity are found in either very moist or very dry regimes. Thus, the largest errors in the model simulations of cloud forcing are prone to be in the western Pacific warm pool area, which is characterized by very moist strong upward currents, and in the rather dry regions where the flow is dominated by descending mean motions.

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Elisa Manzini and Kevin Hamilton

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The excitation and propagation of equatorial planetary waves and inertia-gravity waves were studied by comparing simulations from the comprehensive GFDL troposphere-stratosphere-mesosphere SKYHI general circulation model (GCM) and from a linear primitive equation model with the same domain and numerical resolution. The basic state of the linear model is time dependent and is derived from the mean zonal wind and temperature obtained from a simulation with the full SKYHI model. The latent and convective heating fields of this SKYHI integration are used as the forcing for the linear model in a parallel simulation.

The wavelength and frequency characteristics of the prominent vertically propagating equatorial Kelvin and Rossby-gravity waves are remarkably similar in the linear model and in SKYHI. Amplitudes are also similar in the lower stratosphere, indicating that the latent and convective heating is the dominant mechanism producing equatorial wave activity in the GCM. The amplitude of these waves in the upper stratosphere and mesosphere is larger in the linear model than in SKYHI. Given that the linear and SKYHI models have comparable radiative damping and horizontal subgrid scale diffusion, it appears that the wave amplitudes in SKYHI are limited by some nonlinear saturation, possibly involving the subgrid-scale vertical mixing.

At low latitudes the linear model reproduces the flux of upward-propagating inertia-gravity waves seen in the full model. The results also show that a significant fraction of the inertia-gravity wave activity found in the midlatitude mesosphere of the SKYHI model can be accounted for by tropical convective heating.

The global-scale Rossby normal modes seen in observations were also identified in the analyses of westward-propagating planetary waves in both models. They are of realistic amplitude in the SKYHI simulation but are much weaker in the linear model. Thus, it appears that latent and convective heating is not the main source of excitation for the Rossby normal modes.

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Kevin Hamilton and J. D. Mahlman

Abstract

A study has been made of the evolution of the zonal-mean zonal wind and temperature in a multiyear integration of the 40-level, 3° × 3.6° resolution “SKYHI” general circulation model (GCM) that has been developed at GFDL. In the tropical upper stratosphere the mean wind variation is dominated by a strong semiannual oscillation (SAO). The peak SAO amplitude in the model is almost 25 m s−1 and occurs near the 1 mb level. The phase of the SAO near the stratopause is such that maximum westerlies occur shortly after the equinoxes. These features are in good agreement with the available observations. In addition the meridional width of the stratopause SAO in the GCM compares well with observations.

A diagnostic analysis of the zonal-mean momentum balance near the tropical stratopause was performed using the detailed fields archived during the GCM integration. It appears that the easterly accelerations in the model SAO are provided by a combination of (i) divergence of the meridional component of the Eliassen-Palm flux associated with quasi-stationary planetary waves and (ii) mean angular momentum advection by the residual meridional circulation. The effects of the residual circulation dominate in the summer hemisphere, while the eddy contributions are more important in the winter hemisphere. The westerly accelerations in the model SAO result from the convergence of the vertical momentum transport associated with gravity waves that have a broad distribution of space and time scales. Thus, in contrast to some simple theoretical models, large-scale equatorial Kelvin waves appear to play only a very minor role in the dynamics of the SAO in the SKYHI GCM.

A second equatorial SAO amplitude maximum was found in the tropical upper mesosphere of the GCM. This apparently corresponds to the mesopause SAO that has been identified in earlier observational studies. While the observed phase of this oscillation is reproduced in the model, the simulated amplitude is unrealistically small.

The model integration included the computation of the concentration of N2O. The results show a fairly realistic simulation of the semiannual variation of tropical stratospheric N2O mixing ratio seen in satellite observations.

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R. John Wilson and Kevin Hamilton

Abstract

This paper discusses the thermotidal oscillations in simulations performed with a newly developed comprehensive general circulation model of the Martian atmosphere. With reasonable assumptions about the effective thermal inertia of the planetary surface and about the distribution of radiatively active atmospheric aerosol, the model produces both realistic zonal-mean temperature distributions and a diurnal surface pressure oscillation of at least roughly realistic amplitude. With any reasonable aerosol distribution, the simulated diurnal pressure oscillation has a very strong zonal variation, in particular a very pronounced zonal wavenumber-2 modulation. This results from a combination of the prominent wave-2 component in the important boundary forcings (topography and surface thermal inertia) and from the fact that the eastward-propagating zonal wave-1 Kelvin normal mode has a period near 1 sol (a Martian mean solar day of 88 775 s). The importance of global resonance is explicitly demonstrated with a series of calculations in which the global mean temperature is arbitrarily altered. The resonant enhancement of the diurnal wave-1 Kelvin mode is predicted to be strongest in the northern summer season. In the model simulations there is also a strong contribution to the semidiurnal tide from a near-resonant eastward-propagating wave-2 Kelvin mode. It is shown that this is significantly forced by a nonlinear steepening of the diurnal Kelvin wave. The daily variations of near-surface winds in the model are also examined. The results show that the daily march of wind at any location depends strongly on the topography, even on the smallest horizontal scales resolved in the model (∼ few hundred km). The global tides also play an important role in determining the near-surface winds, especially so in very dusty atmospheric conditions.

The results for the diurnal and semidiurnal surface pressure oscillations in seasonal integrations of the model are compared in detail with the observations at the two Viking Lander sites (22°N and 48°N). The observations over much of the year can be reasonably reproduced in simulations with a globally uniform aerosol mixing ratio (and assuming more total aerosol in the northern winter season, when the largest dust storms are generally observed). There are features of the Viking observations that do not seem to be explainable in this way, however. In particular, in early northern summer, the model predicts amplitudes for the diurnal pressure oscillation at both lander sites that are at least a factor of 2 larger than observed. Results are presented showing that the low amplitudes observed could be explained if the dust distribution tended to be concentrated over the highlands, rather than being uniformly mixed. Annual cycle simulators with a version of the model with an interactive dust transport do in fact reveal the tendency of the circulation to organize so that larger dust mixing ratios occur over highlands, particularly near subsolar latitudes. When the model includes globally uniform surface dust injection and parameterized dust sedimentation, the annual cycle of the diurnal and semidiurnal tides at both lander sites can be rather well reproduced, except for the periods of global dust storms. The attempts to simulate the observed rapid evolution of the tidal pressure oscillations during the onset of a global dust atom also demonstrate the importance of a nonuniform dust concentration. Simulations with the version of the model incorporating interactive dust are able to roughly reproduce the Viking observations when a strong zonally uniform dust injection is prescribed in the Southern Hemisphere Tropics and subtropics.

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John N. Koshyk and Kevin Hamilton

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Horizontal kinetic energy spectra simulated by high-resolution versions of the Geophysical Fluid Dynamics Laboratory SKYHI middle-atmosphere general circulation model are examined. The model versions considered resolve heights between the ground and ∼80 km, and the horizontal grid spacing of the highest-resolution version is about 35 km. Tropospheric kinetic energy spectra show the familiar ∼−3 power-law dependence on horizontal wavenumber for wavelengths between about 5000 and 500 km and have a slope of ∼−5/3 at smaller wavelengths. Qualitatively similar behavior is seen in the stratosphere and mesosphere, but the wavelength marking the transition to the shallow regime increases with height, taking a value of ∼2000 km in the stratosphere and ∼4000 km in the mesosphere.

The global spectral kinetic energy budget for various height ranges is computed as a function of total horizontal wavenumber. Contributions to the kinetic energy tendency from nonlinear advective processes, from conversion of available potential energy, from mechanical fluxes through the horizontal boundaries of the region, and from parameterized subgrid-scale dissipation are all examined. In the troposphere, advective contributions are negative at large scales and positive over the rest of the spectrum. This is consistent with a predominantly downscale nonlinear cascade of kinetic energy into the mesoscale. The global kinetic energy budget in the middle atmosphere differs significantly from that in the troposphere, with the positive contributions at most scales coming predominantly from vertical energy fluxes.

The kinetic energy spectra calculated from two model versions with different horizontal resolution are compared. Differences between the spectra over the resolved range of the lower-resolution version are smallest in the troposphere and increase with height, owing mainly to large differences in the divergent components. The result suggests that the parameterization of dynamical subgrid-scale processes in middle-atmosphere general circulation models, as well as in high-resolution tropospheric general circulation models, may need to be critically reevaluated.

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Tiehan Zhou, Marvin A. Geller, and Kevin Hamilton

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Several idealized models of tropical upwelling are presented in order to clarify the roles of the nonlinear Hadley circulation and extratropical wave driving. In particular, it is shown that the Hadley circulation and wave-driven circulation interact to determine the nature of tropical upwelling. The authors explain several observed features such as maximum upwelling in the summer subtropics and the annual variation of the upwelling.

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Marvin A. Geller, Tiehan Zhou, and Kevin Hamilton

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Sensitivity tests of a mechanistic model of the mean meridional circulation driven by specified eddy forcing are conducted to investigate how the morphology of tropical upwelling in the lower stratosphere is related to the structure of the forcing expected to be associated with the stratospheric surf zone. The basic morphology of tropical upwelling is found to be similar among the mechanistic model forced with reasonable eddy fluxes, the Geophysical Fluid Dynamics Laboratory (GFDL) SKYHI GCM, U.K. Met Office (UKMO) analyses, and other climate models, indicating the robustness of the upwelling features. Atmospheric data are analyzed to characterize the interannual variability of wave drag. The influence of such variations on the interannual variability of tropical upwelling in the lower stratosphere is explored, which may help explain the observed interannual variability of stratospheric water vapor.

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Markus Stowasser, Kevin Hamilton, and George J. Boer

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The climatic response to a 5% increase in solar constant is analyzed in three coupled global ocean–atmosphere general circulation models, the NCAR Climate System Model version 1 (CSM1), the Community Climate System Model version 2 (CCSM2), and the Canadian Centre for Climate Modelling and Analysis (CCCma) Coupled General Circulation Model version 3 (CGCM3). For this simple perturbation the quantitative values of the radiative climate forcing at the top of the atmosphere can be determined very accurately simply from a knowledge of the shortwave fluxes in the control run. The climate sensitivity and the geographical pattern of climate feedbacks, and of the shortwave, longwave, clear-sky, and cloud components in each model, are diagnosed as the climate evolves. After a period of adjustment of a few years, both the magnitude and pattern of the feedbacks become reasonably stable with time, implying that they may be accurately determined from relatively short integrations.

The global-mean forcing at the top of the atmosphere due to the solar constant change is almost identical in the three models. The exact value of the forcing in each case is compared with that inferred by regressing annual-mean top-of-the-atmosphere radiative imbalance against mean surface temperature change. This regression approach yields a value close to the directly diagnosed forcing for the CCCma model, but a value only within about 25% of the directly diagnosed forcing for the two NCAR models. These results indicate that this regression approach may have some practical limitation in its application, at least for some models.

The global climate sensitivities differ among the models by almost a factor of 2, and, despite an overall apparent similarity, the spatial patterns of the climate feedbacks are only modestly correlated among the three models. An exception is the clear-sky shortwave feedback, which agrees well in both magnitude and spatial pattern among the models. The biggest discrepancies are in the shortwave cloud feedback, particularly in the tropical and subtropical regions where it is strongly negative in the NCAR models but weakly positive in the CCCma model. Almost all of the difference in the global-mean total feedback (and climate sensitivity) among the models is attributable to the shortwave cloud feedback component.

All three models exhibit a region of positive feedback in the equatorial Pacific, which is surrounded by broad areas of negative feedback. These positive feedback regions appear to be associated with a local maximum of the surface warming. However, the models differ in the zonal structure of this surface warming, which ranges from a mean El Niño–like warming in the eastern Pacific in the CCCma model to a far-western Pacific maximum of warming in the NCAR CCSM2 model. A separate simulation with the CCSM2 model, in which these tropical Pacific zonal gradients of surface warming are artificially suppressed, shows no region of positive radiative feedback in the tropical Pacific. However, the global-mean feedback is only modestly changed in this constrained run, suggesting that the processes that produce the positive feedback in the tropical Pacific region may not contribute importantly to global-mean feedback and climate sensitivity.

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Bei-Wei Lu, Lionel Pandolfo, and Kevin Hamilton

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A nonlinear principal component analysis (NLPCA) is applied to monthly mean zonal wind observations from January 1956 through December 2007 taken at seven pressure levels between 10 and 70 hPa in the stratosphere near the equator to represent the well-known quasi-biennial oscillation (QBO) and investigate its variability and structure. The NLPCA is conducted using a simplified two–hidden layer feed-forward neural network that alleviates the problems of nonuniqueness of solutions and data overfitting that plague nonlinear techniques of principal component analysis. The QBO is used as a test bed for the new compact model of NLPCA.

The two nonlinear principal components of the dataset of the equatorial stratospheric zonal winds, determined by the compact NLPCA, offer a clear picture of the QBO. In particular, their structure shows that the QBO phase consists of a predominant 28.3-month cycle that is modulated by an 11-yr cycle as well as by longer cycles. The differences in wind variability between westerly and easterly regimes and between Northern Hemisphere winter and summer seasons and the tendency for a seasonal synchronization of the QBO phases are well captured.

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Yuqing Wang, Li Zhou, and Kevin Hamilton

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A regional atmospheric model (RegCM) developed at the International Pacific Research Center (IPRC) is used to investigate the effect of assumed fractional convective entrainment/detrainment rates in the Tiedtke mass flux convective parameterization scheme on the simulated diurnal cycle of precipitation over the Maritime Continent region. Results are compared with observations based on 7 yr of the Tropical Rainfall Measuring Mission (TRMM) satellite measurements. In a control experiment with the default fractional convective entrainment/detrainment rates, the model produces results typical of most other current regional and global atmospheric models, namely a diurnal cycle with precipitation rates over land that peak too early in the day and with an unrealistically large diurnal range. Two sensitivity experiments were conducted in which the fractional entrainment/detrainment rates were increased in the deep and shallow convection parameterizations, respectively. Both of these modifications slightly delay the time of the rainfall-rate peak during the day and reduce the diurnal amplitude of precipitation, thus improving the simulation of precipitation diurnal cycle to some degree, but better results are obtained when the assumed entrainment/detrainment rates for shallow convection are increased to the value consistent with the published results from a large eddy simulation (LES) study. It is shown that increasing the entrainment/detrainment rates would prolong the development and reduce the strength of deep convection, thus delaying the mature phase and reducing the amplitude of the convective precipitation diurnal cycle over the land. In addition to the improvement in the simulation of the precipitation diurnal cycle, convective entrainment/detrainment rates also affect the simulation of temporal variability of daily mean precipitation and the partitioning of stratiform and convective rainfall in the model. The simulation of the observed offshore migration of the diurnal signal is realistic in some regions but is poor in some other regions. This discrepancy seems not to be related to the convective lateral entrainment/detrainment rate but could be due to the insufficient model resolution used in this study that is too coarse to resolve the complex land–sea contrast.

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