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Wang, X, Zhang GJ.  2019.  Evaluation of the quasi-biweekly oscillation over the South China Sea in early and late summer in CAM5. Journal of Climate. 32:69-84.   10.1175/jcli-d-18-0072.1   AbstractWebsite

Low-frequency intraseasonal oscillations in the tropical atmosphere in general circulation models (GCMs) were studied extensively in many previous studies. However, the simulation of the quasi-biweekly oscillation (QBWO), which is an important component of the intraseasonal oscillations, in GCMs has not received much attention. This paper evaluates the QBWO features over the South China Sea in early [May-June (MJ)] and late [August-September (AS)] summer in the National Center for Atmospheric Research (NCAR) Community Atmosphere Model, version 5.3 (CAM5), using observations and reanalysis data. Results show that the major features of the spatial distribution of the QBWO in both MJ and AS are simulated reasonably well by the model, although the amplitude of the variation is overestimated. CAM5 captures the local oscillation in MJ and the westward propagation in AS of the QBWO. Although there are important biases in geographical location and intensity in MJ, the model represents the QBWO horizontal and vertical structure qualitatively well in AS. The diagnosis of the eddy vorticity budget is conducted to better understand the QBWO activities in the model. Both horizontal advection of relative vorticity and that of planetary vorticity (Coriolis parameter) are important for the local evolution of the QBWO in MJ in observations as well as model simulation, whereas advection of planetary vorticity contributes to the westward propagation of QBWO vorticity anomalies in AS. Since the Coriolis parameter f only changes with latitude, this suggests that the correct simulation of anomalous meridional wind is a key factor in the realistic simulation of the QBWO in the model.

Wang, Y, Zhang GJ, He YJ.  2017.  Simulation of precipitation extremes using a stochastic convective parameterization in the NCAR CAM5 under different resolutions. Journal of Geophysical Research-Atmospheres. 122:12875-12891.   10.1002/2017jd026901   AbstractWebsite

With the incorporation of the Plant-Craig stochastic deep convection scheme into the Zhang-McFarlane deterministic parameterization in the Community Atmospheric Model version 5 (CAM5), its impact on extreme precipitation at different resolutions (2 degrees, 1 degrees, and 0.5 degrees) is investigated. CAM5 with the stochastic deep convection scheme (experiment (EXP)) simulates the precipitation extreme indices better than the standard version (control). At 2 degrees and 1 degrees resolutions, EXP increases high percentile (>99th) daily precipitation over the United States, Europe, and China, resulting in a better agreement with observations. However, at 0.5 degrees resolution, due to enhanced grid-scale precipitation with increasing resolution, EXP overestimates extreme precipitation over southeastern U.S. and eastern Europe. The reduced biases in EXP at each resolution benefit from a broader probability distribution function of convective precipitation intensity simulated. Among EXP simulations at different resolutions, if the spatial averaging area over which input quantities used in convective closure are spatially averaged in the stochastic convection scheme is comparable, the modeled convective precipitation intensity decreases with increasing resolution, when gridded to the same resolution, while the total precipitation is not sensitive to model resolution, exhibiting some degree of scale-awareness. Sensitivity tests show that for the same resolution, increasing the size of spatial averaging area decreases convective precipitation but increases the grid-scale precipitation.

Wang, MC, Zhang GJ.  2018.  Improving the simulation of tropical convective cloud-top heights in CAM5 with CloudSat observations. Journal of Climate. 31:5189-5204.   10.1175/jcli-d-18-0027.1   AbstractWebsite

Using 4 years of CloudSat data, the simulation of tropical convective cloud-top heights (CCTH) above 6 km simulated by the convection scheme in the Community Atmosphere Model, version 5 (CAM5), is evaluated. Compared to CloudSat observations, CAM5 underestimates CCTH by more than 2 km on average. Further analysis of model results suggests that the dilute CAPE calculation, which has been incorporated into the convective parameterization since CAM4, is a main factor restricting CCTH to much lower levels. After removing this restriction, more convective clouds develop into higher altitudes, although convective clouds with tops above 12 km are still underestimated significantly. The environmental conditions under which convection develops in CAM5 are compared with CloudSat observations for convection with similar CCTHs. It is shown that the model atmosphere is much more unstable compared to CloudSat observations, and there is too much entrainment in CAM5. Since CCTHs are closely associated with cloud radiative forcing, the impacts of CCTH on model simulation are further investigated. Results show that the change of CCTH has important impacts on cloud radiative forcing and precipitation. With increased CCTHs, there is more cloud radiative forcing in tropical Africa and the eastern Pacific, but less cloud radiative forcing in the western Pacific. The contribution to total convective precipitation from convection with cloud tops above 9 km is also increased substantially.

Wang, Y, Zhang GJ, Craig GC.  2016.  Stochastic convective parameterization improving the simulation of tropical precipitation variability. Geophysical Research Letters. 43:6612-6619.   10.1002/2016gl069818   AbstractWebsite

The Plant-Craig (PC) stochastic convective parameterization scheme is implemented into the National Center for Atmospheric Research Community Atmosphere Model version 5 (CAM5) to couple with the Zhang-McFarlane deterministic convection scheme. To evaluate its impact on tropical precipitation simulation, two experiments are conducted: one with the standard CAM5 and the other with the stochastic scheme incorporated. Results show that the PC stochastic parameterization decreases the frequency of weak precipitation and increases the frequency of strong precipitation, resulting in better agreement with observations. The most striking improvement is in the probability distribution function (PDF) of precipitation intensity, with the well-known too-much-drizzle problem in CAM5 largely eliminated. In the global tropical belt, the precipitation intensity PDF from the simulation agrees remarkably well with that of Tropical Rainfall Measuring Mission observations. The stochastic scheme also yields a similar magnitude of intraseasonal variability of precipitation to observations and improves the simulation of the eastward propagating intraseasonal signals of precipitation and zonal wind.

Wang, Y, Zhang GJ, Jiang YQ.  2018.  Linking stochasticity of convection to large-scale vertical velocity to improve Indian Summer Monsoon Simulation in the NCAR CAM5. Journal of Climate. 31:6985-7002.   10.1175/jcli-d-17-0785.1   AbstractWebsite

The Plant-Craig (PC) stochastic convective parameterization scheme is modified by linking the stochastic generation of convective clouds to the change of large-scale vertical pressure velocity at 500 hPa with time so as to better account for the relationship between convection and the large-scale environment. Three experiments using the National Center for Atmospheric Research (NCAR) Community Atmosphere Model, version 5 (CAM5), are conducted: one with the default Zhang-McFarlane deterministic convective scheme, another with the original PC stochastic scheme, and a third with the modified PC stochastic scheme. Evaluation is focused on the simulation of the Indian summer monsoon (ISM), which is a long-standing challenge for all current global circulation models. Results show that the modified stochastic scheme better represents the annual cycle of the climatological mean rainfall over central India and the mean onset date of ISM compared to other simulations. Also, for the simulations of ISM intraseasonal variability for quasi-biweekly and 30-60-day modes, the modified stochastic parameterization produces more realistic propagation and magnitude, especially for the observed northeastward movement of the 30-60-day mode, for which the other two simulations show the propagation in the opposite direction. Causes are investigated through a moisture budget analysis. Compared to the other two simulations, the modified stochastic scheme with an appropriate representation of convection better represents the patterns and amplitudes of large-scale dynamical convergence and moisture advection and thus corrects the monsoon cycle associated with their covariation during the peaks and troughs of intraseasonal oscillation.

Wang, Y, Zhang GJ.  2016.  Global climate impacts of stochastic deep convection parameterization in the NCAR CAM5. Journal of Advances in Modeling Earth Systems. 8:1641-1656.   10.1002/2016ms000756   AbstractWebsite

In this study, the stochastic deep convection parameterization of Plant and Craig (PC) is implemented in the Community Atmospheric Model version 5 (CAM5) to incorporate the stochastic processes of convection into the Zhang-McFarlane (ZM) deterministic deep convective scheme. Its impacts on deep convection, shallow convection, large-scale precipitation and associated dynamic and thermodynamic fields are investigated. Results show that with the introduction of the PC stochastic parameterization, deep convection is decreased while shallow convection is enhanced. The decrease in deep convection is mainly caused by the stochastic process and the spatial averaging of input quantities for the PC scheme. More detrained liquid water associated with more shallow convection leads to significant increase in liquid water and ice water paths, which increases large-scale precipitation in tropical regions. Specific humidity, relative humidity, zonal wind in the tropics, and precipitable water are all improved. The simulation of shortwave cloud forcing (SWCF) is also improved. The PC stochastic parameterization decreases the global mean SWCF from 252.25 W/m(2) in the standard CAM5 to 248.86 W/m(2), close to 247.16 W/m(2) in observations. The improvement in SWCF over the tropics is due to decreased low cloud fraction simulated by the stochastic scheme. Sensitivity tests of tuning parameters are also performed to investigate the sensitivity of simulated climatology to uncertain parameters in the stochastic deep convection scheme.

Wu, XQ, Deng LP, Song XL, Vettoretti G, Peltier WR, Zhang GJ.  2007.  Impact of a modified convective scheme on the Madden-Julian Oscillation and El Nino-Southern Oscillation in a coupled climate model. Geophysical Research Letters. 34   10.1029/2007gl030637   AbstractWebsite

The connection between the intraseasonal Madden-Julian Oscillation (MJO) and interannual El Nino-Southern Oscillation (ENSO) has been proposed and investigated for the last two decades. However, many fully coupled atmosphere-ocean general circulation models (GCMs) are still unable to simulate many important characteristics of these two phenomena partly due to the great uncertainty in the representation of subgrid-scale cloud systems. We report herein the simulation of an El Nino in a fully coupled GCM with a modified convection scheme, which captures many of the observed features of the 1997/1998 El Nino event. The representation of convection in the coupled model plays a major role in modeling both interannual ENSO and intraseasonal MJO variability in closer accord with observations, and in reproducing the evolution of 1997/1998 El Nino-type events.

Wu, XQ, Deng LP, Song XL, Zhang GJ.  2007.  Coupling of convective momentum transport with convective heating in global climate simulations. Journal of the Atmospheric Sciences. 64:1334-1349.   10.1175/jas3894.1   AbstractWebsite

The effects of convective momentum transport (CMT) on global climate simulations are examined in this study. Comparison between two sets of 20-yr (1979-98) integration using the NCAR Community Climate Model version 3 (CCM3) illustrates that the inclusion of CMT in the convection scheme systematically modifies the climate mean state over the equatorial region. The convective momentum tendencies slow down the equatorward flow at higher latitudes near the surface and weaken the equatorial convergence and convection. This reduces the convective heating and drying around the equator and produces an improved meridional distribution within the upward branch of the Hadley circulation. The major heating peak during the boreal winter is moved to south of the equator at about 10 degrees S, which is closer to the heat budget residuals of the ECMWF reanalysis data. The responses of meridional wind to the reduced heating result in the secondary meridional circulation within the intertropical convergence zone.

Wu, XQ, Liang XZ, Zhang GJ.  2003.  Seasonal migration of ITCZ precipitation across the equator: Why can't GCMs simulate it? Geophysical Research Letters. 30   10.1029/2003gl017198   AbstractWebsite

The secondary meridional circulation induced by convective momentum transport (CMT) within the ascending branch of the Hadley circulation is a missing dynamical mechanism that can cause common failure of general circulation models (GCMs) in simulating seasonal migration of the intertropical convergence zone (ITCZ) precipitation maximum across the equator. This failure is manifested by the model bias that the precipitation peak remains north of the equator during November-March. The CMT-induced secondary circulation, characterized by strong downward motion along the equatorial belt and upward motion south and north of the belt, tends to modify the meridional distribution of precipitation with the strongest impacts during boreal winter and spring. A 20-year GCM simulation with the CMT parameterization successfully reproduces the observed seasonal migration of the ITCZ precipitation across the equator with the peaks near 8degreesN during boreal summer and near 8degreesS during boreal winter.