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2018
Lu, CS, Sun C, Liu YG, Zhang GJ, Lin YL, Gao WH, Niu SJ, Yin Y, Qiu YJ, Jin LJ.  2018.  Observational relationship between entrainment rate and environmental relative humidity and implications for convection parameterization. Geophysical Research Letters. 45:13495-13504.   10.1029/2018gl080264   AbstractWebsite

Entrainment rate is a critical but highly uncertain quantity in convective parameterizations; especially, the effects of environmental relative humidity on entrainment rate are controversial, or even opposite, in different studies. Analysis of aircraft observations of cumuli from the Routine AAF (Atmospheric Radiation Measurement [ARM] Aerial Facility) Clouds with Low Optical Water Depths (CLOWD) Optical Radiative Observations (RACORO) and Rain in Cumulus over the Ocean (RICO) field campaigns shows that entrainment rate is positively correlated with relative humidity. Physical analysis shows that higher relative humidity promotes entrainment by reducing buoyancy in the cloud cores and by weakening downdrafts near the cloud cores. The reduced buoyancy in the cloud cores and weakened downdrafts surrounding the cores further reduce updrafts in the cloud cores; the cloud cores with smaller updrafts are more significantly affected by their environment, resulting in larger entrainment rate. The relationship between entrainment rate and relative humidity is consistent with the buoyancy sorting concept widely used in convection parameterizations. The results provide reliable in situ observations to improve parameterizations of entrainment rate. Plain Language Summary Cumulus clouds affect vertical distributions of atmospheric energy and mass and further affect weather and climate. Near cloud edges, environmental air can be entrained into clouds. Entrainment rate describes how fast environmental air is entrained, which affects the growth and dissipation of clouds. However, our understanding of the factors affecting entrainment rate is far from established. Especially, different studies found that the effects of environmental relative humidity on entrainment rate could be opposite. Based on in situ observations of cumulus clouds, it is found that higher relative humidity causes larger entrainment rate. Physical analysis shows that relative humidity affects entrainment rate through its effects on thermodynamic and dynamical structures in and outside cumulus clouds. Mathematical artifacts in the calculation of entrainment rate are ruled out.

Liu, YC, Fan JW, Xu KM, Zhang GJ.  2018.  Analysis of cloud-resolving model simulations for scale dependence of convective momentum transport. Journal of the Atmospheric Sciences. 75:2445-2472.   10.1175/jas-d-18-0019.1   AbstractWebsite

We use 3D cloud-resolving model (CRM) simulations of two mesoscale convective systems at midlatitudes and a simple statistical ensemble method to diagnose the scale dependency of convective momentum transport (CMT) and CMT-related properties and evaluate a parameterization scheme for the convection-induced pressure gradient (CIPG) developed by Gregory et al. Gregory et al. relate CIPG to a constant coefficient multiplied by mass flux and vertical mean wind shear. CRM results show that mass fluxes and CMT exhibit strong scale dependency in temporal evolution and vertical structure. The upgradient-downgradient CMT characteristics for updrafts are generally similar between small and large grid spacings, which is consistent with previous understanding, but they can be different for downdrafts across wide-ranging grid spacings. For the small to medium grid spacings (4-64 km), Gregory et al. reproduce some aspects of CIPG scale dependency except for underestimating the variations of CIPG as grid spacing decreases. However, for large grid spacings (128-512 km), Gregory et al. might even less adequately parameterize CIPG because it omits the contribution from either the nonlinear-shear or the buoyancy forcings. Further diagnosis of CRM results suggests that inclusion of nonlinear-shear forcing in Gregory et al. is needed for the large grid spacings. For the small to median grid spacings, a modified Gregory et al. with the three-updraft approach help better capture the variations of CIPG as grid spacing decreases compared to the single updraft approach. Further, the optimal coefficients used in Gregory et al. seem insensitive to grid spacings, but they might be different for updrafts and downdrafts, for different MCS types, and for zonal and meridional components.

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.

Yang, MM, Zhang GJ, Sun DZ.  2018.  Precipitation and moisture in four leading CMIP5 models: Biases across large-scale circulation regimes and their attribution to dynamic and thermodynamic factors. Journal of Climate. 31:5089-5106.   10.1175/jcli-d-17-0718.1   AbstractWebsite

As key variables in general circulation models, precipitation and moisture in four leading models from CMIP5 (phase 5 of the Coupled Model Intercomparison Project) are analyzed, with a focus on four tropical oceanic regions. It is found that precipitation in these models is overestimated in most areas. However, moisture bias has large intermodel differences. The model biases in precipitation and moisture are further examined in conjunction with large-scale circulation by regime-sorting analysis. Results show that all models consistently overestimate the frequency of occurrence of strong upward motion regimes and peak descending regimes of 500-hPa vertical velocity JCLI-D-17-0718.1 regime, models produce too much precipitation compared to observation and reanalysis. But for moisture, their biases differ from model to model and also from level to level. Furthermore, error causes are revealed through decomposing contribution biases into dynamic and thermodynamic components. For precipitation, the contribution errors in strong upward motion regimes are attributed to the overly frequent . In the weak upward motion regime, the biases in the dependence of precipitation on probability density function (PDF) make comparable contributions, but often of opposite signs. On the other hand, the biases in column-integrated water vapor contribution are mainly due to errors in the frequency of occurrence of , while thermodynamic components contribute little. These findings suggest that errors in the frequency of occurrence are a significant cause of biases in the precipitation and moisture simulation.

Guo, XH, Lu CS, Zhao TL, Liu YG, Zhang GJ, Luo S.  2018.  Observational study of the relationship between entrainment rate and relative dispersion in deep convective clouds. Atmospheric Research. 199:186-192.   10.1016/j.atmosres.2017.09.013   AbstractWebsite

This study investigates the influence of entrainment rate (lambda) on relative dispersion (epsilon) of cloud droplet size distributions (CDSD) in the 99 growing precipitating deep convective clouds during TOGA-COARE. The results show that entrainment suppresses epsilon, which is opposite to the traditional understanding that entrainment-mixing broadens CDSD. To examine how the relationship between epsilon and lambda is affected by droplets with different sizes, CDSDs are divided into three portions with droplet radius < 3.75 mu m (N-1), radius in the range of 3.75-12.75 mu m (N-2) and 12.75-23.25 mu m (N-3), respectively. The results indicate that although the droplet concentration at different sizes generally decrease simultaneously as lambda increases, the variation of standard deviation (sigma) depends mainly on N-3, while the mean radius (r(m)) decreases with decreasing N-3, but increases with decreasing N-1. So the influence of entrainment on CDSD causes a more dramatical decrease in a than that in r(m), and further leads to the decrease of a as entrainment enhances. In addition, a conceptual model of CDSD evolution during entrainment mixing processes is developed to illustrate the possible scenarios entailing different relationships between a and lambda. The number concentration of small droplets and the degree of evaporation of small droplets are found to be key factors that shift the sign (i.e., positive or negative) of the epsilon-lambda relationship.

2017
Song, FF, Zhang GJ.  2017.  Impact of tropical SSTs in the North Atlantic and Southeastern Pacific on the Eastern Pacific ITCZ. Journal of Climate. 30:1291-1305.   10.1175/jcli-d-16-0310.1   AbstractWebsite

During boreal spring, observations show a double ITCZ over the eastern Pacific, with the northern ITCZ stronger than the southern ITCZ. However, it is opposite in most climate models. It is also evident that there exists a cold bias in tropical North Atlantic (TNA) sea surface temperature (SST) and a warm bias in southeastern Pacific (SEP) SST. In this study, the influences of TNA and SEP SSTs on the double-ITCZ bias are investigated by prescribing the observed SST in these regions in the NCAR CESM1. Results show that when TNA SST is prescribed, the northern ITCZ is substantially enhanced and the southern ITCZ is moderately reduced, although the SST response in these regions is small. When the SEP SST is prescribed, the southern ITCZ is reduced considerably. When bothTNAand SEP SSTs are prescribed, the double-ITCZ bias is reduced by similar to 68%. Moisture budget analysis suggests that dynamics, mainly the low-level convergence change, determines the above precipitation changes. Based on a mixed layer model, changes in low-level convergence are shown to be determined by surface pressure P-s changes. With prescribed TNA/SEP SSTs, SST gradients change the P-s in the region directly via the Lindzen-Nigam mechanism. The corresponding low-level circulation changes affect the 850-hPa thermodynamic state in a wider region, which in turn not only strengthens the SST-induced P-s change locally but also leads to P-s changes remotely, including the northern ITCZ region. Furthermore, the low-level convergence changes the vertical structure of moist static energy, altering the atmospheric stability and modulating precipitation distribution.

2016
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.

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.

2015
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.

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.

Guo, XH, Lu CS, Zhao TL, Zhang GJ, Liu YG.  2015.  An observational study of entrainment rate in deep convection. Atmosphere. 6:1362-1376.   10.3390/atmos6091362   AbstractWebsite

This study estimates entrainment rate and investigates its relationships with cloud properties in 156 deep convective clouds based on in-situ aircraft observations during the TOGA-COARE (Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment) field campaign over the western Pacific. To the authors' knowledge, this is the first study on the probability density function of entrainment rate, the relationships between entrainment rate and cloud microphysics, and the effects of dry air sources on the calculated entrainment rate in deep convection from an observational perspective. Results show that the probability density function of entrainment rate can be well fitted by lognormal, gamma or Weibull distribution, with coefficients of determination being 0.82, 0.85 and 0.80, respectively. Entrainment tends to reduce temperature, water vapor content and moist static energy in cloud due to evaporative cooling and dilution. Inspection of the relationships between entrainment rate and microphysical properties reveals a negative correlation between volume-mean radius and entrainment rate, suggesting the potential dominance of homogeneous mechanism in the clouds examined. In addition, entrainment rate and environmental water vapor content show similar tendencies of variation with the distance of the assumed environmental air to the cloud edges. Their variation tendencies are non-monotonic due to the relatively short distance between adjacent clouds.

2014
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.

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.

2013
Cai, QQ, Zhang GJ, Zhou TJ.  2013.  Impacts of shallow convection on MJO simulation: A moist static energy and moisture budget analysis. Journal of Climate. 26:2417-2431.   10.1175/jcli-d-12-00127.1   AbstractWebsite

The role of shallow convection in Madden-Julian oscillation (MJO) simulation is examined in terms of the moist static energy (MSE) and moisture budgets. Two experiments are carried out using the NCAR Community Atmosphere Model, version 3.0 (CAM3.0): a "CTL'' run and an "NSC'' run that is the same as the CTL except with shallow convection disabled below 700 hPa between 20 degrees S and 20 degrees N. Although the major features in the mean state of outgoing longwave radiation, 850-hPa winds, and vertical structure of specific humidity are reasonably reproduced in both simulations, moisture and clouds are more confined to the planetary boundary layer in the NSC run. While the CTL run gives a better simulation of the MJO life cycle when compared with the reanalysis data, the NSC shows a substantially weaker MJO signal. Both the reanalysis data and simulations show a recharge-discharge mechanism in the MSE evolution that is dominated by the moisture anomalies. However, in the NSC the development of MSE and moisture anomalies is weaker and confined to a shallow layer at the developing phases, which may prevent further development of deep convection. By conducting the budget analysis on both the MSE and moisture, it is found that the major biases in the NSC run are largely attributed to the vertical and horizontal advection. Without shallow convection, the lack of gradual deepening of upward motion during the developing stage of MJO prevents the lower troposphere above the boundary layer from being preconditioned for deep convection.

2011
Song, XL, Zhang GJ.  2011.  Microphysics parameterization for convective clouds in a global climate model: Description and single-column model tests. Journal of Geophysical Research-Atmospheres. 116   10.1029/2010jd014833   AbstractWebsite

An efficient two-moment microphysics parameterization scheme for convective clouds is developed to improve the representation of convective clouds and its interactions with stratiform clouds and aerosol in global climate models (GCMs). The scheme explicitly treats mass mixing ratio and number concentration of four hydrometeor species (cloud water, cloud ice, rain, and snow) and describes several microphysical processes, including autoconversion, self-collection, collection between hydrometeor species, freezing, cloud ice nucleation, droplet activation, and sedimentation. Thus this physically based scheme is suitable for investigating the interaction between convection and aerosol and the indirect aerosol effect on climate. An evaluation of the scheme in the single-column version of NCAR Community Atmospheric Model version 3.5 (CAM3.5) with the Tropical Warm Pool-International Cloud Experiment (TWP-ICE) data shows that the simulation of cloud microphysical properties in convective core is significantly improved, indicating that the new parameterization describes the microphysical processes in convection reasonably well. The contribution from convective detrainment to large-scale cloud ice and liquid water budgets is enhanced greatly. With more realistic convective cloud microphysical properties and their detrainment, the surface stratiform precipitation, which is seriously underestimated in the model, is increased by a factor of roughly 2.5, and therefore is much closer to the observations. In addition, the simulations of net surface shortwave radiation flux, OLR, specific humidity, and temperature are also improved to some extent. Sensitivity experiments show that the microphysics scheme is moderately sensitive to model vertical resolution, updraft vertical velocity, and numerics, but less so to the lower boundary conditions of hydrometeor budget equations. The experiments with climatological aerosol distribution show that convective precipitation is suppressed with increasing aerosol amount, consistent with some available observations.

2009
Zhang, GJ, Song XL.  2009.  Interaction of deep and shallow convection is key to Madden-Julian Oscillation simulation. Geophysical Research Letters. 36   10.1029/2009gl037340   AbstractWebsite

This study investigates the role of the interaction between deep and shallow convection in MJO simulation using the NCAR CAM3. Two simulations were performed, one using a revised Zhang-McFarlane convection scheme for deep convection and the Hack scheme for shallow convection, and the other disallowing shallow convection below 700 mb in the tropical belt. The two simulations produce dramatically different MJO characteristics. While the control simulation produces realistic MJOs, the simulation without shallow convection has very weak MJO signals in the Indian Ocean and western Pacific. Composite analysis finds that shallow convection serves to precondition the lower troposphere by moistening it ahead of deep convection. It also produces enhanced low-level mass convergence below 850 mb ahead of deep convection. This work, together with previous studies, suggests that a correct simulation of the interaction between deep and shallow convection is key to MJO simulation in global climate models. Citation: Zhang, G. J., and X. Song (2009), Interaction of deep and shallow convection is key to Madden-Julian Oscillation simulation, Geophys. Res. Lett., 36, L09708, doi:10.1029/2009GL037340.

Collier, JC, Zhang GJ.  2009.  Aerosol direct forcing of the summer Indian monsoon as simulated by the NCAR CAM3. Climate Dynamics. 32:313-332.   10.1007/s00382-008-0464-9   AbstractWebsite

In this study, the effects of aerosols on the simulation of the Indian monsoon by the NCAR Community Atmosphere Model CAM3 are measured and investigated. Monthly mean 3D mass concentrations of soil dust, black and organic carbons, sulfate, and sea salt, as output from the GOCART model, are interpolated to mid-month values and to the horizontal and vertical grids of CAM3. With these mid-month aerosol concentrations, CAM3 is run for a period of approximately 16 months, allowing for one complete episode of the Indian monsoon. Responses to the aerosols are measured by comparing the mean of an ensemble of aerosol-induced monsoon simulations to the mean of an ensemble of CAM3 simulations in which aerosols are omitted, following the method of Lau et al. (2006) in their experiment with the NASA finite volume general circulation model. Additionally, an ensemble of simulations of CAM3 using climatological mid-month aerosol concentrations from the MATCH model is composed for comparison. Results of this experiment indicate that the inclusion of aerosols results in drops in surface temperature and increases in precipitation over central India during the pre-monsoon months of March, April, and May. The presence of aerosols induces tropospheric shortwave heating over central India, which destabilizes the atmosphere for enhanced convection and precipitation. Reduced shortwave heating and enhanced evaporation at the surface during April and May results in reduced terrestrial emission to cool the lower troposphere, relative to simulations with no aerosols. This effect weakens the near-surface cyclonic circulation and, consequently, has a negative feedback on precipitation during the active monsoon months of June and July.

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.

2008
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.

Boyle, J, Klein S, Zhang G, Xie S, Wei X.  2008.  Climate model forecast experiments for TOGA COARE. Monthly Weather Review. 136:808-832.   10.1175/2007mwr2145.1   AbstractWebsite

Short-term (1-10 day) forecasts are made with climate models to assess the parameterizations of the physical processes. The time period for the integrations is that of the intensive observing period (IOP) of the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE). The models used are the National Center for Atmospheric Research (NCAR) Community Climate Model, version 3.1 (CAM3.1); CAM3.1 with a modified deep convection parameterization; and the Geophysical Fluid Dynamics Laboratory (GFDL) Atmospheric Model, version 2 (AM2). The models were initialized using the state variables from the 40-yr ECMWF Re-Analysis (ERA-40). The CAM deep convective parameterization fails to demonstrate the sensitivity to the imposed forcing to simulate precipitation patterns associated with the Madden-Julian oscillations (MJOs) present during the period. AM2 and modified CAM3.1 exhibit greater correspondence to the observations at the TOGA COARE site, suggesting that convective parameterizations that have some type of limiter (as do AM2 and the modified CAM3.1) simulate the MJO rainfall with more fidelity than those without. None of the models are able to fully capture the correct phasing of westerly wind bursts with respect to precipitation in the eastward-moving MJO disturbance. Better representation of the diabatic heating and effective static stability profiles is associated with a better MJO simulation. Because the models' errors in the forecast mode bear a resemblance to the errors in the climate mode in simulating the MJO, the forecasts may allow for a better way to dissect the reasons for model error.

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.

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.

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.

2007
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.

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.