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Storer, RL, Zhang GJ, Song XL.  2015.  Effects of convective microphysics parameterization on large-scale cloud hydrological cycle and radiative budget in tropical and midlatitude convective regions. Journal of Climate. 28:9277-9297.   10.1175/jcli-d-15-0064.1   AbstractWebsite

A two-moment microphysics scheme for deep convection was previously implemented in the NCAR Community Atmosphere Model version 5 (CAM5) by Song et al. The new scheme improved hydrometeor profiles in deep convective clouds and increased deep convective detrainment, reducing the negative biases in low and midlevel cloud fraction and liquid water path compared to observations. Here, the authors examine in more detail the impacts of this improved microphysical representation on regional-scale water and radiation budgets. As a primary source of cloud water for stratiform clouds is detrainment from deep and shallow convection, the enhanced detrainment leads to larger stratiform cloud fractions, higher cloud water content, and more stratiform precipitation over the ocean, particularly in the subtropics where convective frequency is also increased. This leads to increased net cloud radiative forcing. Over land regions, cloud amounts are reduced as a result of lower relative humidity, leading to weaker cloud forcing and increased OLR. Comparing the water budgets to cloud-resolving model simulations shows improvement in the partitioning between convective and stratiform precipitation, though the deep convection is still too active in the GCM. The addition of convective microphysics leads to an overall improvement in the regional cloud water budgets.

Zhang, GJ.  1994.  Effects of cumulus convection on the simulated monsoon circulation in a general circulation model. Monthly Weather Review. 122:2022-2038.   10.1175/1520-0493(1994)122<2022:eoccot>2.0.co;2   AbstractWebsite

The effect of cumulus convection on the Asian summer monsoon circulation is investigated, using a general circulation model. Two simulations for the summer months (June. July, and August) are performed, one para meterizing convection using a mass Bur scheme and the other without convective parameterization. The results show that convection has significant effects on the monsoon circulation and its associated precipitation. In the simulation with the mass flux convictive parameterization, precipitation in the western Pacific is decreased, together with a decrease in surface evaporation and wind speed. In the Indian monsoon region it is almost the opposite. Comparison with a simulation using moist convective adjustment to parameterize convection shows that the monsoon circulation and precipitation distribution in the no-convection simulation are very similar to those in the simulation with moist convective adjustment. The difference in the large-scale circulation with and without convective parameterization is interpreted in terms of convective stabilization of the atmosphere by convection, using dry and moist static energy budgets. It is shown that weakening of the low-level convergence in the western Pacific in the simulation with convection is closely associated with the stabilization of the atmosphere by convection, mostly through drying of the lower troposphere; changes in low-level convergence lead to changes in precipitation. The precipitation increase in the Indian monsoon region can be explained similarly.

Zhang, GJ.  2009.  Effects of entrainment on convective available potential energy and closure assumptions in convection parameterization. Journal of Geophysical Research-Atmospheres. 114   10.1029/2008jd010976   AbstractWebsite

This study investigates the effect of entrainment dilution on convective available potential energy (CAPE) and closure assumptions in convection parameterization using the sounding data from three Intensive Observation Periods (IOPs). It is shown that entrainment of the environmental air has a strong dilution effect on CAPE, and this effect depends on the degree of subsaturation of the entrained air: the drier the entrained air, the larger the effect. For CAPE-based closure assumptions, the dilute CAPE has a moderate correlation with convective removal of CAPE. While better than for undiluted CAPE, which is virtually uncorrelated with convective CAPE removal, this correlation is not satisfactory enough for convection closure. For quasi-equilibrium-based closures, while the free tropospheric quasi-equilibrium assumption is a superior closure for convection when undiluted CAPE is used, both the Arakawa-Schubert quasi-equilibrium closure and the free tropospheric quasi-equilibrium closure work well for dilute CAPE in all three IOPs studied. It is further shown from the CAPE definition and the large-scale temperature budget equation that for undiluted CAPE, the free tropospheric large-scale CAPE change and precipitation are approximately linearly related. The most important effect of entrainment dilution on CAPE and convection parameterization closure assumptions is to enhance the role of free tropospheric humidity, thereby diminishing the overwhelming role of boundary layer control on undiluted CAPE and its variation.

Collier, JC, Zhang GJ.  2007.  Effects of increased horizontal resolution on simulation of the North American monsoon in the NCAR CAM3: An evaluation based on surface, satellite, and reanalysis data. Journal of Climate. 20:1843-1861.   10.1175/jcl14099.1   AbstractWebsite

Simulation of the North American monsoon system by the National Center for Atmospheric Research (NCAR) Community Atmosphere Model (CAM3) is evaluated in its sensitivity to increasing horizontal resolution. For two resolutions, T42 and T85, rainfall is compared to Tropical Rainfall Measuring Mission (TRMM) satellite-derived and surface gauge-based rainfall rates over the United States and northern Mexico as well as rainfall accumulations in gauges of the North American Monsoon Experiment ( NAME) Enhanced Rain Gauge Network (NERN) in the Sierra Madre Occidental. Simulated upper-tropospheric mass and wind fields are compared to those from NCEP-NCAR reanalyses. The comparison presented herein demonstrates that tropospheric motions associated with the North American monsoon system are sensitive to increasing the horizontal resolution of the model. An increase in resolution from T42 to T85 results in changes to a region of large-scale midtropospheric descent found north and east of the monsoon anticyclone. Relative to its simulation at T42, this region extends farther south and west at T85. Additionally, at T85, the subsidence is stronger. Consistent with the differences in large-scale descent, the T85 simulation of CAM3 is anomalously dry over Texas and northeastern Mexico during the peak monsoon months. Meanwhile, the geographic distribution of rainfall over the Sierra Madre Occidental region of Mexico is more satisfactorily simulated at T85 than at T42 for July and August. Moisture import into this region is greater at T85 than at T42 during these months. A focused study of the Sierra Madre Occidental region in particular shows that, in the regional-average sense, the timing of the peak of the monsoon is relatively insensitive to the horizontal resolution of the model, while a phase bias in the diurnal cycle of monsoon season precipitation is somewhat reduced in the higher-resolution run. At both resolutions, CAM3 poorly simulates the month-to-month evolution of monsoon rainfall over extreme northwestern Mexico and Arizona, though biases are considerably improved at T85.

Zhang, GJ, Mu MQ.  2005.  Effects of modifications to the Zhang-McFarlane convection parameterization on the simulation of the tropical precipitation in the National Center for Atmospheric Research Community Climate Model, version 3. Journal of Geophysical Research-Atmospheres. 110   10.1029/2004jd005617   AbstractWebsite

[ 1] This study compares the simulation of tropical convection in the National Center for Atmospheric Research Community Climate Model, version 3 (CCM3), using the original and a revised convective parameterization closure in the Zhang-McFarlane scheme. The revised closure couples convection to the large-scale forcing in the free troposphere instead of to the convective available potential energy in the atmosphere as employed in the original closure. In addition, a relative humidity threshold is used for convection trigger. It is shown that the mean precipitation distribution in the tropical regions for both summer and winter is, in general, improved when the new closure is used. During June, July, and August the precipitation in the western Pacific monsoon region is significantly enhanced, alleviating the negative precipitation bias there in the model. The spurious precipitation in the Arabian Peninsula desert is completely eliminated. During December, January, and February the South Pacific Convergence Zone is enhanced considerably. All these changes are desirable in addressing important model deficiencies. The probability distributions of the precipitation intensity from the model simulations are compared with that from the Tropical Rainfall Measurement Mission (TRMM) data. It is shown that over 90% of the CCM3 precipitation is from light rain with rainfall rate less than 1 mm h(-1), whereas the simulation with the new closure and the TRMM observations show significant contribution ( 30 - 40%) from rainfall rates greater than 2 mm h(-1). Precipitation simulation over the western North Pacific summer monsoon region and the Arabian Peninsula was examined in detail to understand the causes of the precipitation biases in CCM3 over these regions. It is demonstrated that the convective available potential energy ( CAPE)- based closure limits the CAPE buildup at the beginning of the monsoon season, resulting in the under simulation of the western North Pacific monsoon precipitation. In the Arabian Peninsula the positive feedback between convection and surface evaporation leads to the spurious heavy precipitation center there. In addition to the new closure the use of relative humidity threshold is also found to be important to the improvement of the simulation.

Song, FF, Zhang GJ.  2016.  Effects of Southeastern Pacific sea surface temperature on the double-ITCZ bias in NCAR CESM1. Journal of Climate. 29:7417-7433.   10.1175/jcli-d-15-0852.1   AbstractWebsite

The double intertropical convergence zone (ITCZ) is a long-standing bias in the climatology of coupled general circulation models (CGCMs). The warm biases in southeastern Pacific (SEP) sea surface temperature (SST) are also evident in many CGCMs. In this study, the role of SEP SST in the double ITCZ is investigated by prescribing the observed SEP SST in the Community Earth System Model, version 1 (CESM1). Both the double ITCZ and dry equator problems are significantly improved with SEP SST prescribed. Both atmospheric and oceanic processes are involved in the improvements. The colder SST over the SEP decreases the precipitation, which enhances the southeasterly winds outside the prescribed SST region, cooling the ocean via increased evaporation. The enhanced descending motion over the SEP strengthens the Walker circulation. The easterly winds over the equatorial Pacific enhance upwelling and shoal the thermocline over the eastern Pacific. The changes of surface wind and wind curl lead to a weaker South Equatorial Countercurrent and stronger South Equatorial Current, preventing the warm water from expanding eastward, thereby improving both the double ITCZ and dry equator. The enhanced Walker circulation also increases the low-level wind convergence and reduces the wind speed in the tropical western Pacific, leading to warmer SST and stronger convection there. The stronger convection in turn leads to more cloud and reduces the incoming solar radiation, cooling the SST. These competing effects between radiative heat flux and latent heat flux make the atmospheric heat flux secondary to the ocean dynamics in the western Pacific warming.

Mu, MQ, Zhang GJ.  2008.  Energetics of Madden Julian oscillations in the NCAR CAM3: A composite view. Journal of Geophysical Research-Atmospheres. 113   10.1029/2007jd008700   AbstractWebsite

This study further examines the simulation of the tropical Madden-Julian oscillation (MJO) using the modified Zhang-McFarlane convection scheme in the National Center for Atmospheric Research Community Atmospheric Model version 3 (CAM3). The results demonstrate that modifications to the Zhang-McFarlane scheme lead to significantly enhanced MJO and much improvement of the MJO structures. However, the propagation speed is too fast compared to observations. The westward tilting structures of moisture flux convergence over the western Pacific warm pool are simulated very well by the modified CAM3 (CAM3m). The modified Zhang-McFarlane convection scheme also improves the energetic structures of the MJOs. With the climatological mean of the energetics from the composite MJO removed, the energy budget terms show clear eastward propagation following MJO movement in the CAM3m and ECMWF reanalysis (ERA40). The results further support that the interaction between convection and largescale circulation is important in the maintenance and growth of the MJO through perturbation kinetic energy (PKE) conversion from perturbation available potential energy (PAPE) generated by the correlation of specific volume (alpha) and large-scale heating (Q(1)). Too weak Q(1) and PAPE generated through (alpha, Q(1)) is responsible for the weak MJO and intraseasonal variability simulated by the standard CAM3. Too strong climatology mean of the energetics in the CAM3m may be caused by too strong stationary intraseasonal variability. Moisture flux convergence is responsible for the changes of specific humidity in the CAM3m and CAM3 during the cycle of the MJO over the Indian and western Pacific Oceans. These results indicate that interaction between convection, moisture and convergence in the lower troposphere may be responsible for the MJO development over the Indian and western Pacific Ocean in both observations and simulations. The weak intraseasonal variability and MJO simulated by the original Zhang-McFarlane deep convection scheme is attributed to the lack of coherent shallow convection ahead of deep convection in the MJO cycle.

Mu, MQ, Zhang GJ.  2006.  Energetics of Madden-Julian oscillations in the National Center for Atmospheric Research Community Atmosphere Model version 3 (NCAR CAM3). Journal of Geophysical Research-Atmospheres. 111   10.1029/2005jd007003   AbstractWebsite

This study analyzes the tropical intraseasonal variability and associated Madden-Julian oscillations (MJO) simulated in the National Center for Atmospheric Research Climate Model CAM3 using two versions of the Zhang-McFarlane convection scheme. The results are compared with those from the Xie-Arkin observations and European Center for Medium-Range Weather Forecasts (ECMWF) reanalysis. It is shown that modifications to the convection scheme in the model lead to substantial enhancement of the intraseasonal variability and MJO. The spatial scale of the precipitation anomalies associated with intraseasonal variability and MJO is also in better agreement with the Xie-Arkin observations and ECMWF reanalysis. By contrast the intraseasonal variability and MJO in the CAM3 are weak. The analysis of the energetics of intraseasonal variability shows that perturbation kinetic energy (PKE) and its sources and sinks through conversion from potential energy and mean flow and generation from wave energy flux are too strong in the modified CAM3 and too weak in the CAM3 when compared with the reanalysis. It also shows that different mechanisms are responsible for the PKE production in different locations. In convectively active regions, conversion from potential energy and vertical transport are important to the maintenance of the upper troposphere PKE; in convectively suppressed regions, horizontal wave energy flux convergence and barotropic conversion are important. The interaction between convection and large-scale circulation plays an important role in the maintenance of intraseasonal variability and MJO through PKE conversion from potential energy generated by convective heating.

Zhang, GJ, Cai M, Hu A.  2013.  Energy consumption and the unexplained winter warming over northern Asia and North America. Nature Clim. Change. 3:466-470.: Nature Publishing Group   10.1038/nclimate1803   AbstractWebsite

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Zhang, GJ, Wu XQ, Zeng XP, Mitovski T.  2016.  Estimation of convective entrainment properties from a cloud-resolving model simulation during TWP-ICE. Climate Dynamics. 47:2177-2192.   10.1007/s00382-015-2957-7   AbstractWebsite

The fractional entrainment rate in convective clouds is an important parameter in current convective parameterization schemes of climate models. In this paper, it is estimated using a 1-km-resolution cloud-resolving model (CRM) simulation of convective clouds from TWP-ICE (the Tropical Warm Pool-International Cloud Experiment). The clouds are divided into different types, characterized by cloud-top heights. The entrainment rates and moist static energy that is entrained or detrained are determined by analyzing the budget of moist static energy for each cloud type. Results show that the entrained air is a mixture of approximately equal amount of cloud air and environmental air, and the detrained air is a mixture of similar to 80 % of cloud air and 20 % of the air with saturation moist static energy at the environmental temperature. After taking into account the difference in moist static energy between the entrained air and the mean environment, the estimated fractional entrainment rate is much larger than those used in current convective parameterization schemes. High-resolution (100 m) large-eddy simulation of TWP-ICE convection was also analyzed to support the CRM results. It is shown that the characteristics of entrainment rates estimated using both the high-resolution data and CRM-resolution coarse-grained data are similar. For each cloud category, the entrainment rate is high near cloud base and top, but low in the middle of clouds. The entrainment rates are best fitted to the inverse of in-cloud vertical velocity by a second order polynomial.

Zhou, ZQ, Xie SP, Zhang GJ, Zhou WY.  2018.  Evaluating AMIP Skill in Simulating Interannual Variability over the Indo-Western Pacific. Journal of Climate. 31:2253-2265.   10.1175/jcli-d-17-0123.1   AbstractWebsite

Local correlation between sea surface temperature (SST) and rainfall is weak or even negative in summer over the Indo-western Pacific warm pool, a fact often taken as indicative of weak ocean feedback on the atmosphere. An Atmospheric Model Intercomparison Project (AMIP) simulation forced by monthly varying SSTs derived from a parallel coupled general circulation model (CGCM) run is used to evaluate AMIP skills in simulating interannual variability of rainfall. Local correlation of rainfall variability between AMIP and CGCMsimulations is used as a direct metric of AMIP skill. This "perfect model'' approach sidesteps the issue of model biases that complicates the traditional skill metric based on the correlation between AMIP and observations. Despite weak local SST-rainfall correlation, the AMIP-CGCM rainfall correlation exceeds a 95% significance level over most of the Indo-western Pacific warm pool, indicating the importance of remote (e.g., El Nino in the equatorial Pacific) rather than local SST forcing. Indeed, the AMIP successfully reproduces large-scale modes of rainfall variability over the Indo-western Pacific warm pool. Compared to the northwest Pacific east of the Philippines, the AMIP-CGCMrainfall correlation is low from the Bay of Bengal through the South China Sea, limited by internal variability of the atmosphere that is damped in CGCM by negative feedback from the ocean. Implications for evaluating AMIP skill in simulating observations are discussed.

Suhas, E, Zhang GJ.  2015.  Evaluating convective parameterization closures using cloud-resolving model simulation of tropical deep convection. Journal of Geophysical Research-Atmospheres. 120:1260-1277.   10.1002/2014jd022246   AbstractWebsite

Closure is an important component of a mass flux-based convective parameterization scheme, and it determines the amount of convection with the aid of a large-scale variable (closure variable) that is sensitive to convection. In this study, we have evaluated and quantified the relationship between commonly used closure variables and convection for a range of global climate model (GCM) horizontal resolutions, taking convective precipitation and mass flux at 600 hPa as measures for deep convection. We have used cloud-resolving model simulation data to create domain averages representing GCM horizontal resolutions of 128km, 64 km, 32 km, 16 km, 8 km, and 4km. Lead-lag correlation analysis shows that except moisture convergence and turbulent kinetic energy, none of the other closure variables evaluated in this study show any relationship with convection for the six subdomain sizes. It is found that the correlation between moisture convergence and convective precipitation is largest when moisture convergence leads convection. This correlation weakens as the subdomain size decreases to 8km or smaller. Although convective precipitation and mass flux increase with moisture convergence at a given subdomain size, as the subdomain size increases, the rate at which they increase becomes smaller. This suggests that moisture convergence-based closure should scale down the predicted mass flux for a given moisture convergence as GCM resolution increases.

Song, XL, Zhang GJ, Li JLF.  2012.  Evaluation of microphysics parameterization for convective clouds in the NCAR Community Atmosphere Model CAM5. Journal of Climate. 25:8568-8590.   10.1175/jcli-d-11-00563.1   AbstractWebsite

A physically based two-moment microphysics parameterization scheme for convective clouds is implemented in the NCAR Community Atmosphere Model version 5 (CAMS) to improve the representation of convective clouds and their interaction with large-scale clouds and aerosols. The explicit treatment of mass mixing ratio and number concentration of cloud and precipitation particles enables the scheme to account for the impact of aerosols on convection. The scheme is linked to aerosols through cloud droplet activation and ice nucleation processes and to stratiform cloud parameterization through convective detrainment of cloud liquid/ice water content (LWC/IWC) and droplet/crystal number concentration (DNC/CNC). A 5-yr simulation with the new convective microphysics scheme shows that both cloud LWC/IWC and DNC/CNC are in good agreement with observations, indicating the scheme describes microphysical processes in convection well. Moreover, the microphysics scheme is able to represent the aerosol effects on convective clouds such as the suppression of warm rain formation and enhancement of freezing when aerosol loading is increased. With more realistic simulations of convective cloud microphysical properties and their detrainment, the mid- and low-level cloud fraction is increased significantly over the ITCZ-southern Pacific convergence zone (SPCZ) and subtropical oceans, making it much closer to the observations. Correspondingly, the serious negative bias in cloud liquid water path over subtropical oceans observed in the standard CAMS is reduced markedly. The large-scale precipitation is increased and precipitation distribution is improved as well. The long-standing precipitation bias in the western Pacific is significantly alleviated because of microphysics-thermodynamics feedbacks.

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.

Suhas, E, Zhang GJ.  2014.  Evaluation of trigger functions for convective parameterization schemes using observations. Journal of Climate. 27:7647-7666.   10.1175/jcli-d-13-00718.1   AbstractWebsite

Realistic simulation of different modes of atmospheric variability ranging from diurnal cycle to interannual variation in global climate models (GCMs) depends crucially on the convection trigger criteria. In this study, using the data from constrained variational analysis by the Atmospheric System Research program for single-column models (SCM), the performance of the commonly used convective trigger functions in GCMs is evaluated based on the equitable threat score (ETS) value, a widely used forecast verification metric. From the ETS score, three consistently better-performing trigger functions were identified. They are based on the dilute and undilute convective available potential energy (CAPE) generation rate from large-scale forcing in the free troposphere (hereafter dCAPE) and parcel buoyancy at the lifting condensation level (Bechtold scheme). The key variables used to define these trigger functions are examined in detail. It is found that the dilute dCAPE trigger function performs the best consistently in both the tropical and midlatitude convective environment. Analysis of the composite fields of key variables of the trigger functions, based on the correct prediction, overprediction and underprediction of convection, and correct prediction of no-convection cases for convective onset, brings to light some critical factors responsible for the performance of the trigger functions. The lower-tropospheric advective forcing in dilute dCAPE trigger and vertical velocity in Bechtold trigger are identified to be the most importance ones. Suggestions are offered for further improvements.