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Taylor, PC, Cai M, Hu AX, Meehl J, Washington W, Zhang GJ.  2013.  A Decomposition of Feedback Contributions to Polar Warming Amplification. Journal of Climate. 26:7023-7043.   10.1175/jcli-d-12-00696.1   AbstractWebsite

Polar surface temperatures are expected to warm 2-3 times faster than the global-mean surface temperature: a phenomenon referred to as polar warming amplification. Therefore, understanding the individual process contributions to the polar warming is critical to understanding global climate sensitivity. The Coupled Feedback Response Analysis Method (CFRAM) is applied to decompose the annual- and zonal-mean vertical temperature response within a transient 1% yr(-1) CO2 increase simulation of the NCAR Community Climate System Model, version 4 (CCSM4), into individual radiative and nonradiative climate feedback process contributions. The total transient annual-mean polar warming amplification (amplification factor) at the time of CO2 doubling is +2.12 (2.3) and +0.94 K (1.6) in the Northern and Southern Hemisphere, respectively. Surface albedo feedback is the largest contributor to the annual-mean polar warming amplification accounting for +1.82 and +1.04 K in the Northern and Southern Hemisphere, respectively. Net cloud feedback is found to be the second largest contributor to polar warming amplification (about +0.38 K in both hemispheres) and is driven by the enhanced downward longwave radiation to the surface resulting from increases in low polar water cloud. The external forcing and atmospheric dynamic transport also contribute positively to polar warming amplification: +0.29 and +0.32 K, respectively. Water vapor feedback contributes negatively to polar warming amplification because its induced surface warming is stronger in low latitudes. Ocean heat transport storage and surface turbulent flux feedbacks also contribute negatively to polar warming amplification. Ocean heat transport and storage terms play an important role in reducing the warming over the Southern Ocean and Northern Atlantic Ocean.

Park, HS, Chiang JCH, Lintner BR, Zhang GJ.  2010.  The delayed effect of major El Nino events on Indian monsoon rainfall. Journal of Climate. 23:932-946.   10.1175/2009jcli2916.1   AbstractWebsite

Previous studies have shown that boreal summer Indian monsoon rainfall is, on average, significantly above normal after major El Nino events. In this study, the underlying causes of this rainfall response are examined using both observational analysis and atmospheric general circulation model (AGCM) simulations. Moist static energy budgets for two strong El Nino events (1982/83 and 1997/98), estimated from monthly 40-yr European Centre for Medium-Range Weather Forecasts Re-Analysis (ERA-40), suggest that stronger low-level moisture transport and reduced moist stability associated with a warmer north Indian Ocean (NIO) can increase monsoon rainfall, despite a weakened monsoon circulation. The trade-off between a dynamically weaker monsoon and moist processes favoring enhanced monsoonal rainfall is broken during the late monsoon season (August-September) as the warm NIO enhances surface latent heat flux and the monsoon circulation relaxes back to the climatological mean. The monsoon circulation strength and the moist processes work together in the late season, which explains the observed tendency for monsoonal rainfall increases during the late monsoon season after strong winter El Nino conditions. Idealized AGCM experiments with a fixed-depth ocean mixed layer demonstrate that the remnant but weaker-than-peak warm SSTs in the eastern equatorial Pacific during spring and the early summer following winter El Ninos substantially contribute to the NIO warming. The results suggest that local air-sea interactions in the tropical Indian Ocean after winter El Nino are strongly dependent on the details of El Nino's decaying trend.

Zurovac-Jevtic, D, Zhang GJ.  2003.  Development and test of a cirrus parameterization scheme using NCAR CCM3. Journal of the Atmospheric Sciences. 60:1325-1344.   10.1175/1520-0469(2003)060<1325:datoac>;2   AbstractWebsite

Recent research has shown that depending on the cloud properties, cirrus clouds can either increase or decrease the overall heating of the earth-atmosphere system. Hence, the representation of cirrus clouds in GCMs is recognized as an important contemporary problem. In this study a new diagnostic cirrus parameterization scheme is developed with the intention of improving the simulation of cirrus macro- and microphysical properties in large-scale models. The scheme allows both large-scale motions and convective detrainment to be a source of moisture for cirrus. Water vapor depletion is calculated as diffusional growth of ice crystals with known size distributions, and the effective fallout from a model layer is estimated using mass-weighted fall velocities of the bulk precipitation. The scheme was implemented and tested with the NCAR Community Climate Model (CCM3). The seasonal means of cirrus cloud cover and ice water contents over the warm pool region, as simulated with the new cirrus parameterization, appeared to be much more realistic than in the standard model version when compared to satellite and in situ data. In contrast to the high amount of optically thin cirrus at all cirrus levels simulated by the standard CCM3, cirrus formed with the new scheme are significantly thicker with a reduced amount in the lower part of the upper troposphere (approximately 10-14 km), whereas cirrus formed below the tropopause (approximately 14-17 km) stay thin but have higher cover. It has also been found that a more realistic precipitation treatment not only results in the formation of thicker anvil cirrus, but also increases the rain and evaporation rates in the middle troposphere. These results suggest that cirrus clouds can be an important potential water vapor source in the tropical troposphere.

Subramanian, AC, Zhang GJ.  2014.  Diagnosing MJO hindcast biases in NCAR CAM3 using nudging during the DYNAMO field campaign. Journal of Geophysical Research-Atmospheres. 119:7231-7253.   10.1002/2013jd021370   AbstractWebsite

This study evaluates the Madden-Julian Oscillation (MJO) hindcast skill and investigates the hindcast biases in the dynamic and thermodynamic fields of the National Center for Atmospheric Research Community Atmosphere Model version 3. The analysis is based on the October 2011 MJO event observed during the Dynamics of the Madden-Julian Oscillation field campaign. The model captures the MJO initiation but, compared to the observations, the hindcast has a faster MJO phase speed, a dry relative humidity bias, a stronger zonal wind shear, and a weaker MJO peak amplitude. The MJO hindcast is then nudged toward the European Centre for Medium-Range Weather Forecast Reanalysis fields of temperature, specific humidity, horizontal winds, and surface pressure. The nudging tendencies highlight the model physics parameterization biases, such as not enough convective diabatic heating during the MJO initiation, not enough upper tropospheric stratiform condensation, and lower tropospheric reevaporation during the mature and decay phases and a strong zonal wind shear during the MJO evolution. To determine the role of temperature, specific humidity, and horizontal winds in the model physics parameterization errors, six additional nudging experiments are carried out, with either one or two of the fields allowed to evolve freely while the others are nudged. Results show that convection and precipitation increase when temperature or specific humidity are unconstrained and decrease when horizontal winds evolve freely or temperature alone is constrained to reanalysis. Budget analysis of moist static energy shows that the nudging tendency compensates for different process biases during different MJO phases. The diagnosis of such nudging tendencies provides a unique objective way to identify model physics biases, which usefully guides the model physics parameterization development.

Song, XL, Wu XQ, Zhang GJ, Arritt RW.  2008.  Dynamical effects of convective momentum transports on global climate simulations. Journal of Climate. 21:180-194.   10.1175/2007jcli1848.1   AbstractWebsite

Dynamical effects of convective momentum transports (CMT) on global climate simulations are investigated using the NCAR Community Climate Model version 3 (CCM3). To isolate the dynamical effects of the CMT, an experimental setup is proposed in which all physical parameterizations except for the deep convection scheme are replaced with idealized forcing. The CMT scheme is incorporated into the convection scheme to calculate the CMT forcing, which is used to force the momentum equations, while convective temperature and moisture tendencies are not passed into the model calculations in order to remove the physical feedback between convective heating and wind fields. Excluding the response of complex physical processes, the model with the experimental setup contains a complete dynamical core and the CMT forcing. Comparison between two sets of 5-yr simulations using this idealized general circulation model (GCM) shows that the Hadley circulation is enhanced when the CMT forcing is included, in agreement with previous studies that used full GCMs. It suggests that dynamical processes make significant contributions to the total response of circulation to CMT forcing in the full GCMs. The momentum budget shows that the Coriolis force, boundary layer friction, and nonlinear interactions of velocity fields affect the responses of zonal wind field, and the adjustment of circulation follows an approximate geostrophic balance. The adjustment mechanism of meridional circulation in response to ageostrophic CMT forcing is examined. It is found that the strengthening of the Hadley circulation is an indirect response of the meridional wind to the zonal CMT forcing through the Coriolis effect, which is required for maintaining near-geostrophic balance.