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Yang, B, Qian Y, Lin G, Leung LR, Rasch PJ, Zhang GJ, McFarlane SA, Zhao C, Zhang YC, Wang HL, Wang MH, Liu XH.  2013.  Uncertainty quantification and parameter tuning in the CAM5 Zhang-McFarlane convection scheme and impact of improved convection on the global circulation and climate. Journal of Geophysical Research-Atmospheres. 118:395-415.   10.1029/2012jd018213   AbstractWebsite

In this study, we applied an uncertainty quantification (UQ) technique to improve convective precipitation in the global climate model, the Community Atmosphere Model version 5 (CAM5), in which the convective and stratiform precipitation partitioning is very different from observational estimates. We examined the sensitivity of precipitation and circulation to several key parameters in the Zhang-McFarlane deep convection scheme in CAM5, using a stochastic importance-sampling algorithm that can progressively converge to optimal parameter values. The impact of improved deep convection on the global circulation and climate was subsequently evaluated. Our results show that the simulated convective precipitation is most sensitive to the parameters of the convective available potential energy consumption time scale, parcel fractional mass entrainment rate, and maximum downdraft mass flux fraction. Using the optimal parameters constrained by the observed Tropical Rainfall Measuring Mission, convective precipitation improves the simulation of convective to stratiform precipitation ratio and rain-rate spectrum remarkably. When convection is suppressed, precipitation tends to be more confined to the regions with strong atmospheric convergence. As the optimal parameters are used, positive impacts on some aspects of the atmospheric circulation and climate, including reduction of the double Intertropical Convergence Zone, improved East Asian monsoon precipitation, and improved annual cycles of the cross-equatorial jets, are found as a result of the vertical and horizontal redistribution of latent heat release from the revised parameterization. Positive impacts of the optimal parameters derived from the 2 degrees simulations are found to transfer to the 1 degrees simulations to some extent. Citation: Yang, B. et al. (2013), Uncertainty quantification and parameter tuning in the CAM5 Zhang-McFarlane convection scheme and impact of improved convection on the global circulation and climate, J. Geophys. Res. Atmos., 118, 395-415, doi:10.1029/2012JD018213.

Song, FF, Zhang GJ.  2018.  Understanding and improving the scale dependence of trigger functions for convective parameterization using cloud-resolving model data. Journal of Climate. 31:7385-7399.   10.1175/jcli-d-17-0660.1   AbstractWebsite

As the resolution of global climate model increases, whether trigger functions in current convective parameterization schemes still work remains unknown. In this study, the scale dependence of undilute and dilute dCAPE, Bechtold, and heated condensation framework (HCF) triggers is evaluated using the cloud-resolving model (CRM) data. It is found that all these trigger functions are scale dependent, especially for dCAPE-type triggers, with skill scores dropping from similar to 0.6 at the lower resolutions (128, 64, and 32 km) to only similar to 0.1 at 4 km. The average convection frequency decreases from 14.1% at 128 km to 2.3% at 4 km in the CRM data, but it increases rapidly in the dCAPE-type triggers and is almost unchanged in the Bechtold and HCF triggers across resolutions, all leading to large overpredictions at higher resolutions. In the dCAPE-type triggers, the increased frequency is due to the increased rate of dCAPE greater than the threshold (65 J kg(-1) h(-1)) at higher resolutions. The box-and-whisker plots show that the main body of dCAPE in the correct prediction and overprediction can be separated from each other in most resolutions. Moreover, the underprediction is found to be corresponding to the decaying phase of convection. Hence, two modifications are proposed to improve the scale dependence of the undilute dCAPE trigger: 1) increasing the dCAPE threshold and 2) considering convection history, which checks whether there is convection prior to the current time. With these modifications, the skill at 16 km, 8 km, and 4 km can be increased from 0.50, 0.27, and 0.15 to 0.70, 0.61, and 0.53, respectively.

Li, G, Zhang GJ.  2008.  Understanding biases in shortwave cloud radiative forcing in the national center for atmospheric research community atmosphere model (CAM3) during El Nino. Journal of Geophysical Research-Atmospheres. 113   10.1029/2007jd008963   AbstractWebsite

This study aims to understand the weak response of shortwave cloud radiative forcing (SWCF) to El Nino in the NCAR CAM3. Observations from ERBE and CERES show strong negative SWCF in the central and eastern equatorial Pacific during El Nino. The standard CAM3 simulation at T42 resolution severely underestimates this response, with even wrong sign in the eastern Pacific. However, an experimental simulation at the same resolution, but with a modified convection parameterization scheme, simulates the cloud shortwave response to El Nino well, although the improvement in the eastern Pacific is not as significant as in the western and central Pacific. To unravel the mechanistic differences in SWCF response to El Nino between the two simulations, the authors analyze the cloud amount, cloud liquid water path (LWP), cloud ice water path (IWP), and convective and large-scale precipitation. It is shown that positive LWP anomalies are mainly responsible for the improved SWCF response to El Nino in the experimental simulation. Interaction among deep convection, shallow convection and low-level clouds is explored to explain this result. Negative LWP anomalies, largely due to reduced cloud water content and amount of low clouds during El Nino in the standard CAM3, weaken the SWCF response. Comparison with a higher-resolution simulation of CAM3 at T85 shows that the T85 simulation produces realistic SWCF response through greatly increased cloud water and ice content in the middle and upper troposphere, while reduced low-level cloud water content remains a problem.

Song, XL, Wu XQ, Zhang GJ, Arritt RW.  2008.  Understanding the effects of convective momentum transport on climate simulations: The role of convective heating. Journal of Climate. 21:5034-5047.   10.1175/2008jci12187.1   AbstractWebsite

A simplified general circulation model (GCM), consisting of a complete dynamical core, simple specified physics. and convective momentum transport (CMT) forcing. is used to understand the effects of CMT on climate simulations with a focus on the role of convective heating in the response of circulation to the CMT forcing. It is found that the convective heating dominates the meridional circulation response and dynamical processes dominate the zonal wind response to the CMT forcing in the tropics: the simplified model reproduces sonic of the key features of CMT-induced circulation changes observed in the full GCM in the tropics. These results suggest that the CMT-induced zonal and meridional circulation changes in the tropics in the full GCM are dominated by dynamical processes and the convective heating, respectively. Inclusion of the CMT in the model induce,,, a marked change in convective heating, which negatively correlates with the change in vertical velocity. indicating the existence of CMT-induced convective heating-circulation feedback. The sensitivity experiment with the removal of mean convective heating feedback demonstrates that the convective heating affects the response of the meridional circulation to the CMT forcing through the CMT-induced convective heating-circulation feedback.

Collier, JC, Zhang GJ.  2005.  US warm-season rainfall in NCAR CAM3: An event-oriented perspective. Geophysical Research Letters. 32   10.1029/2005gl024217   AbstractWebsite

A modified form of the Community Atmosphere Model, ver. 3 (CAM3) developed at the National Center for Atmospheric Research (NCAR) is validated in its warm-season mean precipitation diurnal cycle for two regions of the United States. For all grid boxes of each region, simulated and observed precipitation records over four four-month periods are separated into discrete precipitation events. These events are binned into mutually-exclusive categories, and the diurnal harmonic for each category is estimated. In this way, the model is validated over the spectrum of precipitation episodes, and biases in the overall seasonal-mean diurnal cycle can be attributed to particular kinds of events. The results of the study indicate that the model's total seasonal precipitation is overwhelmingly weighted in the extremely long events and that these events contain the source of any biases in the seasonal mean.

Zhang, GJ.  1995.  Use of monthly mean data to compute surface turbulent fluxes in the tropical Pacific. Journal of Climate. 8:3084-3090.   10.1175/1520-0442(1995)008<3084:uommdt>;2   AbstractWebsite

This study investigates to what extent monthly mean surface meteorological variables can be used to estimate surface turbulent fluxes in the equatorial Pacific. The two-year data from the TOGA TAO moored buoy array are used to compute the monthly mean surface fluxes using both daily mean and monthly mean data as input to the bulk aerodynamic formulas. A unique feature of the dataset is that it covers the western Pacific warm pool, a region of climatologically low surface winds and active convection. The results show that the ocean surface sensible and latent heat fluxes can be estimated using the monthly mean data to a very high accuracy and that the momentum flux is generally underestimated by about 10% when the monthly mean data instead of daily data is used. This study, together with previous studies by other researchers, suggests that the monthly mean data such as those derived from satellite measurements can be used to estimate the surface heat, moisture, and momentum fluxes over the global oceans.