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Guang J. Zhang
,
Jeffrey T. Kiehl
, and
Philip J. Rasch

Abstract

This study examines the response of the climate simulation by the National Center for Atmospheric Research Community Climate Model (CCM3) to the introduction of the Zhang and McFarlane convective parameterization in the model. It is shown that in the CCM3 the simulated surface climate in the tropical convective regimes, especially in the western Pacific warm pool, is markedly improved, yielding a much better agreement with the recent observations. The systematic bias in the surface evaporation, surface wind stress over the tropical Pacific Ocean in previous model simulations is significantly reduced, owing to the better simulation of the surface flow.

Experiments using identical initial and boundary conditions, but different convection schemes, are performed to isolate the role of the convection schemes and to understand the interaction between convection and the large-scale circulation in a convecting atmosphere. The comparison of the results from these experiments in the western Pacific warm pool suggests that use of the Zhang and McFarlane scheme makes a significant contribution to the improved climate simulation in CCM3. The simulated atmosphere using the Zhang and McFarlane scheme exhibits a quasi-equilibrium between convection and the large-scale processes. When this scheme is removed from the CCM3, such a quasi-equilibrium is no longer observed. In addition, the simulated thermodynamic structures, the surface evaporation, and surface winds in the Pacific warm pool become very similar to those in the CCM2 climate.

Examination of the temporal evolution of the various fields demonstrates that the stabilization of the atmosphere using the new convection scheme takes place during the transition from nonequilibrium to quasi equilibrium at the beginning of the time integration. After quasi equilibrium is reached, the atmosphere is warmer and more stable compared to the run without the new scheme. Associated with the more stable stratification, the atmospheric circulation becomes weaker, thus the surface winds and evaporation are weaker because of the coupling between thermodynamics and dynamics in the tropical troposphere.

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Sungsu Park
,
Christopher S. Bretherton
, and
Philip J. Rasch

Abstract

This paper provides a description of the integrated representation for the cloud processes in the Community Atmosphere Model, version 5 (CAM5). CAM5 cloud parameterizations add the following unique characteristics to previous versions: 1) a cloud macrophysical structure with horizontally nonoverlapped deep cumulus, shallow cumulus, and stratus in each grid layer, where each of which has its own cloud fraction, and mass and number concentrations for cloud liquid droplets and ice crystals; 2) stratus–radiation–turbulence interactions that allow CAM5 to simulate marine stratocumulus solely from grid-mean relative humidity without relying on a stability-based empirical formula; 3) prognostic treatment of the number concentrations of stratus liquid droplets and ice crystals, with activated aerosols and detrained in-cumulus condensates as the main sources and with evaporation, sedimentation, and precipitation of stratus condensate as the main sinks; and 4) radiatively active cumulus and snow. By imposing consistency between diagnosed stratus fraction and prognosed stratus condensate, unrealistically empty or highly dense stratus is avoided in CAM5. Because of the activation of the prognostic aerosols and the parameterizations of the radiation and stratiform precipitation production as a function of the cloud droplet size, CAM5 simulates various aerosol indirect effects as well as the direct effects: that is, aerosols affect both the radiation budget and the hydrological cycle.

Detailed analysis of various simulations indicates that CAM5 improves upon CAM3/CAM4 in global performance as well as in physical formulation. However, several problems are also identified in CAM5, which can be attributed to deficient regional tuning, inconsistency between various physics parameterizations, and incomplete treatment of physics. Efforts are continuing to further improve CAM5.

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Peter G. Hess
,
David S. Battisti
, and
Philip J. Rasch

Abstract

How the surface boundary heating (sea surface temperature) and cumulus adjustment process affect the location, structure, energetics, and dynamics of the intertropical convergence zones (ITCZs) is investigated. A series of experiments is performed with a general circulation model where the lower boundary is specified to be water at a fixed sea surface temperature (SST), an aqua planet. All experiments are run using equinoctal insolation with no longitudinal variation in SST. Two different convective parameterization schemes (Kuo and moist convective adjustment) and several different zonally symmetric SST distributions are used in these experiments.

The location of the ITCZ is found to be sensitive to the convective parameterization scheme and the SST distribution. The model with the moist convective adjustment scheme produces an ITCZ over the tropical SST maximum, even under conditions where the SST gradient is weak. By contrast, the model with the Kuo convective parameterization is not as sensitive to SST distribution: the model with the Kuo scheme yields two ITCZs straddling the equator at approximately 7° latitude for a wide variety of SST distributions, including when the warmest water is located on the equator. The location of the ITCZ affects the structure and strength of both the time-mean Hadley cells and the subtropical jets. Furthermore, the equatorial wave spectrum is strongly influenced by the type of cumulus parameterization scheme used. The “convective characteristics” for each parameterization scheme are presented in detail to elucidate the influence of the large-scale environment on convection.

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Prajvala Kurtakoti
,
Wilbert Weijer
,
Milena Veneziani
,
Philip J. Rasch
, and
Tarun Verma

Abstract

Bjerknes compensation (BJC) refers to the anticorrelation observed between atmospheric and oceanic heat transport (AHT/OHT) variability, particularly on decadal to longer time scales that may be important to the predictability of the climate system. This study investigates the spread in BJC across fully coupled simulations of phase 6 of the Coupled Model Intercomparison Project (CMIP6) and critical processes (particularly related to sea ice and clouds) that may contribute to that spread. BJC on decadal to longer time scales is confirmed across all the simulations evaluated, and it is strongest in the Northern Hemisphere (NH) between 60° and 70°N. At these latitudes, BJC appears to be primarily driven by the exchange of turbulent fluxes (sensible and latent) in the Greenland, Iceland, and Barents Seas. Metrics to break down how sea ice and clouds uniquely modify the radiative balance of the polar atmosphere during anomalous OHT events are presented. These metrics quantify the impacts of sea ice and clouds on surface and top of atmosphere (latent, sensible, longwave, and shortwave radiative) energy fluxes. Cloud responses tend to counter the clear sky impacts over the Marginal Ice Zone (MIZ). It is further shown that the degree of BJC present in a simulation at high latitudes is heavily influenced by the sensitivity of the sea ice to OHT, which is most influential over the MIZ. These results are qualitatively robust across models and explain the intermodel spread in NH BJC in the preindustrial control experiment.

Open access
Aaron Donohoe
,
Ed Blanchard-Wrigglesworth
,
Axel Schweiger
, and
Philip J. Rasch

Abstract

The sea ice-albedo feedback (SIAF) is the product of the ice sensitivity (IS), that is, how much the surface albedo in sea ice regions changes as the planet warms, and the radiative sensitivity (RS), that is, how much the top-of-atmosphere radiation changes as the surface albedo changes. We demonstrate that the RS calculated from radiative kernels in climate models is reproduced from calculations using the “approximate partial radiative perturbation” method that uses the climatological radiative fluxes at the top of the atmosphere and the assumption that the atmosphere is isotropic to shortwave radiation. This method facilitates the comparison of RS from satellite-based estimates of climatological radiative fluxes with RS estimates across a full suite of coupled climate models and, thus, allows model evaluation of a quantity important in characterizing the climate impact of sea ice concentration changes. The satellite-based RS is within the model range of RS that differs by a factor of 2 across climate models in both the Arctic and Southern Ocean. Observed trends in Arctic sea ice are used to estimate IS, which, in conjunction with the satellite-based RS, yields an SIAF of 0.16 ± 0.04 W m−2 K−1. This Arctic SIAF estimate suggests a modest amplification of future global surface temperature change by approximately 14% relative to a climate system with no SIAF. We calculate the global albedo feedback in climate models using model-specific RS and IS and find a model mean feedback parameter of 0.37 W m−2 K−1, which is 40% larger than the IPCC AR5 estimate based on using RS calculated from radiative kernel calculations in a single climate model.

Free access
Tao Zhang
,
De-Zheng Sun
,
Richard Neale
, and
Philip J. Rasch

Abstract

The asymmetry between El Niño and La Niña is a key aspect of ENSO that needs to be simulated well by models in order to fully capture the role of ENSO in the climate system. Here the asymmetry between the two phases of ENSO in five successive versions of the Community Climate System Model (CCSM1, CCSM2, CCSM3 at T42 resolution, CCSM3 at T85 resolution, and the latest CCSM3 + NR, with the Neale and Richter convection scheme) is evaluated. Different from the previous studies, not only is the surface signature of ENSO asymmetry examined, but so too is its subsurface signature. By comparing the differences among these models as well as the differences between the models and the observations, an understanding of the causes of the ENSO asymmetry is sought.

An underestimate of the ENSO asymmetry is noted in all of the models, but the latest version with the Neale and Richter scheme (CCSM3 + NR) is getting closer to the observations than the earlier versions. The net surface heat flux is found to damp the asymmetry in the SST field in both the models and observations, but the damping effect in the models is weaker than that in the observations, thus excluding a role of the surface heat flux in contributing to the weaker asymmetry in the SST anomalies associated with ENSO. Examining the subsurface signatures of ENSO—the thermocline depth and the associated subsurface temperature for the western and eastern Pacific—reveals the same bias; that is, the asymmetry in the models is weaker than that in the observations.

The analysis of the corresponding Atmospheric Model Intercomparison Project (AMIP) runs in conjunction with the coupled runs suggests that the weaker asymmetry in the subsurface signatures in the models is related to the lack of asymmetry in the zonal wind stress over the central Pacific, which in turn is due to a lack of sufficient asymmetry in deep convection (i.e., the nonlinear dependence of the deep convection on SST). In particular, the lack of a westward shift in the deep convection in the models in response to a cold phase SST anomaly is found as a common factor that is responsible for the weak asymmetry in the models. It is also suggested that a more eastward extension of the deep convection in response to a warm phase SST anomaly may also help to increase the asymmetry of ENSO. The better performance of CCSM3 + NR is apparently linked to an enhanced convection over the eastern Pacific during the warm phase of ENSO. Apparently, either a westward shift of deep convection in response to a cold phase SST anomaly or an increase of convection over the eastern Pacific in response to a warm phase SST anomaly leads to an increase in the asymmetry of zonal wind stress and therefore an increase in the asymmetry of subsurface signal, favoring an increase in ENSO asymmetry.

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Byron A. Boville
,
Philip J. Rasch
,
James J. Hack
, and
James R. McCaa

Abstract

The parameterizations of clouds and precipitation processes have been revised considerably in the Community Atmosphere Model version 3 (CAM3) compared to its predecessors, CAM2 and the Community Climate Model version 3 (CCM3). The parameterizations in CAM3 are more realistic in their representation of processes affecting cloud liquid and ice particles and represent the linkages between processes more completely. This paper describes the changes to the representation of clouds in CAM3, including the partitioning of cloud water between liquid and ice phases, the determination of particle sizes and sedimentation rates, the phase and evaporation rate of precipitation, and the calculation of the cloud fraction.

Parameterization changes between CCM3 and CAM2 introduced a significant cold bias at the tropical tropopause, resulting in a dry bias for stratospheric water vapor. Tests of the sensitivity of the tropical temperature profile and the tropical tropopause temperature to individual process changes suggested that the radiative balance at the tropopause was altered by improvements in both clouds and relative humidity below. Radiative equilibrium calculations suggested that the cold bias could be removed by improving the representation of subvisible cirrus clouds. These results motivated the complete separation of the representation of liquid and ice cloud particles and an examination of the processes that determine their sources and sinks. As a result of these changes, the tropopause cold bias has been almost eliminated in CAM3.

The total cloud condensate variable, used in CAM2, has been separated into cloud liquid and cloud ice variables in CAM3. Both sedimentation and large-scale transport of the condensate variables are now included. Snowfall is computed explicitly and the latent heat of fusion has been included for all freezing and melting processes. Both deep and shallow convection parameterizations now detrain cloud condensate directly into the stratiform clouds instead of evaporating the detrained condensate into the environment. The convective parameterizations are not easily modified to include the latent heat of fusion. Therefore, the determination of the phase of convective precipitation, and of detrained condensate, is added as a separate step. Evaporation is included for sedimenting cloud particles and for all sources of precipitation.

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Ben Kravitz
,
Douglas G. MacMartin
,
Philip J. Rasch
, and
Andrew J. Jarvis

Abstract

The authors describe a new method of comparing different climate forcing agents (e.g., CO2 concentration, CH4 concentration, and total solar irradiance) in climate models that circumvents many of the difficulties associated with explicit calculations of efficacy. This is achieved by introducing an explicit feedback loop external to a climate model that adjusts one forcing agent to balance another while keeping global-mean surface temperature constant. The convergence time of this feedback loop can be adjusted, allowing for comparisons of forcing agents to be achieved with relatively short simulations. Comparisons between forcing agents are highly linear in concordance with predicted scaling relationships; for example, the global-mean climate response to a doubling of the CO2 concentration is equivalent to that of a 2.1% change in total solar irradiance. This result is independent of the magnitude of the forcing agent (within the range of radiative forcings considered here) and is consistent across two different climate models.

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Keith M. Hines
,
David H. Bromwich
,
Philip J. Rasch
, and
Michael J. Iacono

Abstract

To evaluate and improve the treatment of clouds and radiation by the climate models of the National Center for Atmospheric Research (NCAR), simulations by the NCAR Community Climate Model version 3 (CCM3), as well as the recently released Community Atmosphere Model version 2 (CAM2), are examined. The Rasch and Kristjánsson prognostic cloud condensate scheme, which is now the standard scheme for CAM2, is included in a version of CCM3 and evaluated. Furthermore, the Rapid Radiative Transfer Model (RRTM), which alleviates the deficit in downward clear-sky longwave radiation, is also included in a version of CCM3. The new radiation scheme in CAM2 also alleviates the clear-sky longwave bias, although RRTM is not included. The impact of the changes is especially large over the interior of Antarctica. The changes induced by the introduction of the prognostic cloud scheme are found to have a much larger impact on the CCM3 simulations than do those from the introduction of RRTM. The introduction of the prognostic cloud scheme increases cloud emissivity in the upper troposphere, reduces cloud emissivity in the lower troposphere, and results in a better vertical distribution of cloud radiative properties over interior Antarctica. The climate simulations have a very large cold bias in the stratosphere, especially during summer. There are significant deficiencies in the simulation of Antarctic cloud radiative effects. The optical thickness of Antarctic clouds appears to be excessive. This contributes to a warm bias in surface temperature during winter and a deficit in downward shortwave radiation during summer. Some biases for Antarctica are larger for CCM3 with the prognostic cloud condensate scheme than with the standard diagnostic clouds. When the mixing ratio threshold for autoconversion from suspended ice cloud to falling precipitation is reduced toward a more realistic value, the Antarctic clouds are thinned and some of the biases are reduced. To improve the surface energy balance, not only must the radiative effects of clouds be improved, it is also necessary to improve the representation of sensible heat flux. Insufficient vertical resolution of the frequently very shallow, very stable surface boundary layer apparently contributes to an excessive heat flux from the atmosphere to the surface during winter. The representations of Antarctic clouds and radiation by the new NCAR CAM2 are not clearly improved compared to those of the earlier CCM3. For example, the surface albedo over Antarctica is descreased in CAM2 and Community Climate System Model version 2 (CCSM2) simulations in comparison to CCM3 simulations, contributing to a summer warm bias in tropospheric temperature for the former.

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Hailong Wang
,
Casey D. Burleyson
,
Po-Lun Ma
,
Jerome D. Fast
, and
Philip J. Rasch

Abstract

Long-term Atmospheric Radiation Measurement (ARM) datasets collected at the three tropical western Pacific (TWP) sites are used to evaluate the ability of the Community Atmosphere Model (CAM5) to simulate the various types of clouds, their seasonal and diurnal variations, and their impact on surface radiation. A number of CAM5 simulations are conducted at various horizontal grid spacing (around 2°, 1°, 0.5°, and 0.25°) with meteorological constraints from analysis or reanalysis. Model biases in the seasonal cycle of cloudiness are found to be weakly dependent on model resolution. Positive biases (up to 20%) in the annual mean total cloud fraction appear mostly in stratiform ice clouds. Higher-resolution simulations do reduce the positive bias in ice clouds, but they inadvertently increase the negative biases in convective clouds and low-level liquid clouds, leading to a positive bias in annual mean shortwave fluxes at the sites, as high as 65 W m−2 in the 0.25° simulation. Such resolution-dependent biases in clouds can adversely lead to biases in ambient thermodynamic properties and, in turn, produce feedback onto clouds. Both the model and observations show distinct diurnal cycles in total, stratiform, and convective cloud fractions; however, they are out of phase by 12 h and the biases vary by site. The results suggest that biases in deep convection affect the vertical distribution and diurnal cycle of stratiform clouds through the transport of vapor and/or the detrainment of liquid and ice. The approach used here can be easily adapted for the evaluation of new parameterizations being developed for CAM5 or other global or regional models.

Open access