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Zhang, GJ, Vogelmann AM, Jensen MP, Collins WD, Luke EP.  2010.  Relating satellite-observed cloud properties from MODIS to meteorological conditions for marine boundary layer clouds. Journal of Climate. 23:1374-1391.   10.1175/2009jcli2897.1   AbstractWebsite

This study examines 6 yr of cloud properties observed by the Moderate Resolution Imaging Spectroradiometer (MODIS) on board the NASA Terra satellite in five prominent marine boundary layer (MBL) cloud regions (California, Peru, Canary, Angola, and Australia) and investigates their relationships with near-surface meteorological parameters obtained from NCEP reanalyses. About 62 000 independent scenes are used to examine the instantaneous relationships between cloud properties and meteorological parameters that may be used for global climate model (GCM) diagnostics and parameterization. Cloud liquid water path (LWP) generally increases with lower-tropospheric stability (LTS) and lifting condensation level (LCL), whereas cloud drizzle frequency is favored by weak LTS and negligible cold air advection. Cloud fraction (CF) depends strongly on variations in LTS, and to a lesser extent on surface air temperature advection and LCL, although the relationships vary from region to region. The authors propose capturing the effects of these three parameters on CF via their linear combination in terms of a single parameter, the effective lower-tropospheric stability (eLTS). Results indicate that eLTS offers a marked improvement over LTS alone in explaining the median CF variations within the different study regions. A parameterization of CF in terms of eLTS is provided, which produces results that are improved over those of Klein and Hartmann's LTS-only parameterization. However, the new parameterization may not predict the observed variability correctly, and the authors propose a method that might address this shortcoming via a statistical approach.

Zhang, GJ, McPhaden MJ.  1995.  The relationship between sea surface temperature and latent heat flux in the equatorial Pacific. Journal of Climate. 8:589-605.   10.1175/1520-0442(1995)008<0589:trbsst>;2   AbstractWebsite

Moored buoy data from the equatorial Pacific are analyzed to investigate the relationship between sea surface temperature (SST) and latent heat flux from the ocean. It is found that at low SST the latent heat flux increases with SST; at high SST the latent heat flux decreases with increasing SST, a relationship that cannot be explained by thermodynamic considerations alone, Analysis of the wind speeds and humidity differences between the surface air and the saturation humidity at the sea surface temperature indicates that while at low SST the humidity difference primarily determines the latent heat flux, and at high SST a sharp decrease in wind speed is mostly responsible for the low latent heat flux. A mechanism that leads to low latent heat flux at high SST is suggested; it involves the interaction between convection and the large-scale circulation. The longitudinal distribution of SST, wind speed, humidity difference, and latent heat flux is found to be similar to that in previous studies. In the eastern Pacific, SST is the lowest, the wind speed is large, and the humidity difference is low; in the western Pacific, SST is the highest, whereas the wind speed is low and the humidity difference is large. Latent heat flux increases from the eastern Pacific westward, reaching a maximum in the central Pacific, and then decreases toward the western Pacific warm pool. Through analyses of the data on different timescales, we found that the atmospheric processes leading to low latent heat flux over warm SST were mainly operative on seasonal timescales (periods longer than 90 days). On shorter timescales (periods of 30-90 days), the influence of intraseasonal Madden and Julian waves was evident. On this timescale, the relationship between SST and latent heat flux was characterized by a 10-day lag between atmospheric forcing (primarily related to winds) and the local oceanic response in the western and central Pacific. In the eastern Pacific cold tongue, SST and latent heat flux variations were nearly in phase on this timescale, indicating an atmospheric response to oceanic forcing. For periods less than 30 days, SST variations associated with tropical instability waves were likewise shown to be important in forcing latent heat flux variations in the eastern Pacific cold tongue.

Zhang, GJ, Kiehl JT, Rasch PJ.  1998.  Response of climate simulation to a new convective parameterization in the National Center for Atmospheric Research community climate model (CCM3). Journal of Climate. 11:2097-2115.   10.1175/1520-0442-11.8.2097   AbstractWebsite

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.

Song, XL, Zhang GJ.  2014.  Role of climate feedback in El Nino-Like SST response to global warming. Journal of Climate. 27:7301-7318.   10.1175/jcli-d-14-00072.1   AbstractWebsite

Under global warming from the doubling of CO2, the equatorial Pacific experiences an El Nino-like warming, as simulated by most global climate models. A new climate feedback and response analysis method (CFRAM) is applied to 10 years of hourly output of the slab ocean model (SOM) version of the NCAR Community Climate System Model, version 3.0, (CCSM3-SOM) to determine the processes responsible for this warming. Unlike the traditional surface heat budget analysis, the CFRAM can explicitly quantify the contributions of each radiative climate feedback and of each physical and dynamical process of a GCM to temperature changes. The mean bias in the sum of partial SST changes due to each feedback derived with CFRAM in the tropical Pacific is negligible (0.5%) compared to the mean SST change from the CCSM3-SOM simulations, with a spatial pattern correlation of 0.97 between the two. The analysis shows that the factors contributing to the El Nino-like SST warming in the central Pacific are different from those in the eastern Pacific. In the central Pacific, the largest contributor to El Nino-like SST warming is dynamical advection, followed by PBL diffusion, water vapor feedback, and surface evaporation. In contrast, in the eastern Pacific the dominant contributor to El Nino-like SST warming is cloud feedback, with water vapor feedback further amplifying the warming.

Zhang, GJ, McFarlane NA.  1995.  Role of convective scale momentum transport in climate simulation. Journal of Geophysical Research-Atmospheres. 100:1417-1426.   10.1029/94jd02519   AbstractWebsite

This paper studies the effect of convective-scale momentum transport in climate simulation using a comprehensive parameterization scheme. A unique feature of the scheme is the inclusion of the perturbation pressure field induced by convection and its effect on the cloud momentum transport. Through two experiments of seasonal simulations, it is shown that the perturbation pressure forcing on the cloud momentum transport accounts for a significant part of the total convective momentum source/sink, indicating that the cloud momentum field is substantially modulated by the convection-induced pressure field. The overall effect of convective momentum transport is to reduce the vertical wind shear in both the zonal and the meridional directions. The response of the large-scale circulation to convective momentum transport is very significant. The zonally averaged zonal wind decreases by as much as 5 ms(-1) in a broad area in the upper tropical troposphere and the midlatitudes of the winter hemisphere. The Hadley circulation becomes stronger as a result of the zonal momentum transport. In general, inclusion of convective momentum transport leads to a much better simulation of the wind fields in both the upper and the lower troposphere. The temperature and moisture changes as a result of the inclusion of convective momentum transport are also examined in this study. The tropical troposphere is warmer and more moist due to the enhanced Hadley circulation. However, considering the uncertainties of the climatological analyses, most of these thermodynamic changes only make marginal improvement to the simulation.

Li, LJ, Wang B, Zhang GJ.  2015.  The role of moist processes in shortwave radiative feedback during ENSO in the CMIP5 models. Journal of Climate. 28:9892-9908.   10.1175/jcli-d-15-0276.1   AbstractWebsite

The weak negative shortwave (SW) radiative feedback (sw) during El Nino-Southern Oscillation (ENSO) over the equatorial Pacific is a common problem in the models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5). In this study, the causes for the (sw) biases are analyzed using three-dimensional cloud fraction and liquid water path (LWP) provided by the 17 CMIP5 models and the relative roles of convective and stratiform rainfall feedbacks in (sw) are explored. Results show that the underestimate of SW feedback is primarily associated with too negative cloud fraction and LWP feedbacks in the boundary layers, together with insufficient middle and/or high cloud and dynamics feedbacks, in both the CMIP and Atmospheric Model Intercomparsion Project (AMIP) runs, the latter being somewhat better. The underestimations of SW feedbacks are due to both weak negative SW responses to El Nino, especially in the CMIP runs, and strong positive SW responses to La Nina, consistent with their biases in cloud fraction, LWP, and dynamics responses to El Nino and La Nina. The convective rainfall feedback, which is largely reduced owing to the excessive cold tongue in the CMIP runs compared with their AMIP counterparts, contributes more to the difference of SW feedback (mainly under El Nino conditions) between the CMIP and AMIP runs, while the stratiform rainfall plays a more important role in SW feedback during La Nina.

Li, LJ, Wang B, Zhang GJ.  2014.  The role of nonconvective condensation processes in response of surface shortwave cloud radiative forcing to El Nino warming. Journal of Climate. 27:6721-6736.   10.1175/jcli-d-13-00632.1   AbstractWebsite

The weak response of surface shortwave cloud radiative forcing (SWCF) to El Nino over the equatorial Pacific remains a common problem in many contemporary climate models. This study shows that two versions of the Grid-Point Atmospheric Model of the Institute of Atmospheric Physics (IAP)/State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG) (GAMIL) produce distinctly different surface SWCF response to El Nino. The earlier version, GAMIL1, underestimates this response, whereas the latest version, GAMIL2, simulates it well. To understand the causes for the different SWCF responses between the two simulations, the authors analyze the underlying physical mechanisms. Results indicate the enhanced stratiform condensation and evaporation in GAMIL2 play a key role in improving the simulations of multiyear annual mean water vapor (or relative humidity), cloud fraction, and incloud liquid water path (ICLWP) and hence in reducing the biases of SWCF and rainfall responses to El Nino due to all of the improved dynamical (vertical velocity at 500 hPa), cloud amount, and liquid water path (LWP) responses. The largest contribution to the SWCF response improvement in GAMIL2 is from LWP in the Nino-4 region and from low-cloud cover and LWP in the Nino-3 region. Furthermore, as a crucial factor in the low-cloud response, the atmospheric stability change in the lower layers is significantly influenced by the nonconvective heating variation during La Nina.

Song, XL, Zhang GJ.  2018.  The roles of convection parameterization in the formation of double ITCZ Syndrome in the NCAR CESM: I. Atmospheric Processes. Journal of Advances in Modeling Earth Systems. 10:842-866.   10.1002/2017ms001191   AbstractWebsite

Several improvements are implemented in the Zhang-McFarlane (ZM) convection scheme to investigate the roles of convection parameterization in the formation of double intertropical convergence zone (ITCZ) bias in the NCAR CESM1.2.1. It is shown that the prominent double ITCZ biases of precipitation, sea surface temperature (SST), and wind stress in the standard CESM1.2.1 are largely eliminated in all seasons with the use of these improvements in convection scheme. This study for the first time demonstrates that the modifications of convection scheme can eliminate the double ITCZ biases in all seasons, including boreal winter and spring. Further analysis shows that the elimination of the double ITCZ bias is achieved not by improving other possible contributors, such as stratus cloud bias off the west coast of South America and cloud/radiation biases over the Southern Ocean, but by modifying the convection scheme itself. This study demonstrates that convection scheme is the primary contributor to the double ITCZ bias in the CESM1.2.1, and provides a possible solution to the long-standing double ITCZ problem. The atmospheric model simulations forced by observed SST show that the original ZM convection scheme tends to produce double ITCZ bias in high SST scenario, while the modified convection scheme does not. The impact of changes in each core component of convection scheme on the double ITCZ bias in atmospheric model is identified and further investigated.

Zhang, GJ.  2003.  Roles of tropospheric and boundary layer forcing in the diurnal cycle of convection in the U.S. Southern Great Plains. Geophysical Research Letters. 30   10.1029/2003gl018554   AbstractWebsite

This study examines the roles of the tropospheric large-scale forcing, surface sensible and latent heat fluxes and convective inhibition in the diurnal variation of convection in the U. S. Southern Great Plains using data from the Atmospheric Radiation Measurement program. It is shown that the diurnal variation of the tropospheric large-scale forcing has a strong in-phase relationship with convection, whereas the diurnal variations of surface sensible and latent heat fluxes as well as the thermodynamic properties of the near-surface air are nearly out of phase with that of convection. Both the single column version and the full global model of the NCAR CCM3 are used to test the roles of the tropospheric and boundary layer forcing in the observed diurnal variation of convection. When convection is parameterized based on the tropospheric large-scale forcing, the diurnal variation of convection is in good agreement with the observations.