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Zhang, GJ.  1997.  A further study on estimating surface evaporation using monthly mean data: Comparison of bulk formulations. Journal of Climate. 10:1592-1600.   10.1175/1520-0442(1997)010<1592:afsoes>2.0.co;2   AbstractWebsite

This study further examines the errors in estimating surface evaporation using monthly mean surface atmospheric variables from the moored buoys in the equatorial Pacific. Results from two significantly different bulk aerodynamic formulas are compared. It is shown that for both formulas the errors are within 2-3 W m(-2) on average, representing a relative error of less than 3% in the equatorial Pacific. Although small, this error varies systematically across the equatorial Pacific and depends on the bulk formula used. It is also demonstrated that the individual nonlinear contributions associated with the strong dependence of exchange coefficient on surface wind to monthly mean surface evaporation are significant. However, they largely cancel each other, leaving small net effect.

Zhang, GJ, Cho HR.  1991.  Parameterization of the vertical transport of momentum by cumulus clouds. Part II: Application. Journal of the Atmospheric Sciences. 48:2448-2457.   10.1175/1520-0469(1991)048<2448:potvto>2.0.co;2   AbstractWebsite

The parameterization theory developed in Part I is applied to compute the vertical transport of momentum by cumulus clouds for the average of six convective periods in Phase III of GATE. Special attention is paid to the role of perturbation pressure field in vertical momentum transport. Good agreement between the parameterized and the observed apparent momentum sources is obtained. In general, cumulus convection tends to decrease the vertical wind shear of the environment. It is found that the cloud-mean momentum varies significantly with height in an environment with vertical wind shear, and the pressure gradient force is mostly responsible for this variation. Sensitivity tests of the parameterization scheme show that the cloud-mean momentum obtained is fairly insensitive to some poorly represented parameters in the cloud model. Results using this scheme are compared with those using the Schneider and Lindzen scheme. Appreciable improvement is found with the use of the new scheme.

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

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

Zhang, GJ.  2003.  Lagrangian study of cloud properties and their relationships to meteorological parameters over the US southern Great Plains. Journal of Climate. 16:2700-2716.   10.1175/1520-0442(2003)016<2700:lsocpa>2.0.co;2   AbstractWebsite

Hourly satellite cloud data from 18 June to 18 July 1997 over the U. S. southern Great Plains are analyzed to study the scale-dependent cloud properties and their relationships to atmospheric conditions. The observed clouds are classified into high, midlevel, and low clouds according to their top heights. For each cloud type, contribution to the total cloud amount from clouds of different sizes is determined using a Lagrangian cloud classification scheme. It is found that in this continental, convectively active environment, more than half of the total cloud amount is from high clouds, of which 80% comes from clouds with area >4 x 10(4) km(2). For midlevel clouds, more than 50% of the contribution to cloud amount is from small clouds (e. g., cloud area,4 x 10(4) km(2)). Almost all of the low clouds with significant contribution to cloud amount have spatial scales,4 x 10(4) km(2). This suggests that most of the midlevel and low clouds are of subgrid scale to a typical GCM resolution (T42 or T63). It is further found that cloud radiative properties, such as cloud albedo, outgoing longwave radiation, and cloud radiative forcing, have strong scale dependence. Bigger clouds are brighter and have lower outgoing longwave radiation. These results indicate that contributions to the observed cloud radiative forcing are dominated by large cloud systems. The diurnal variation of the cloud properties is also examined. Using concurrent meteorological analysis from NCEP, possible relationships between cloud properties and prevailing meteorological conditions were sought. It is found that clear relationships exist between cloud properties, such as cloud amount and albedo, and the layer-averaged relative humidity, and the relationships vary with cloud scale. In addition, cloud properties for high clouds are well correlated to vertical velocity in the upper troposphere. More large and highly reflective clouds tend to occur in regions of upward motion. Low clouds have a clear correspondence with the lower-tropospheric static stability and temperature. Large and thick clouds prefer to exist where the lower-tropospheric air is cold, statically more stable, and has high relative humidity.

Zhang, GJ, McPhaden MJ.  1995.  The relationship between sea surface temperature and latent heat flux in the equatorial Pacific. Journal of Climate. 8:589-605.   10.1175/1520-0442(1995)008<0589:trbsst>2.0.co;2   AbstractWebsite

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

Zhang, GJ.  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.

Zhang, GJ.  2002.  Convective quasi-equilibrium in midlatitude continental environment and its effect on convective parameterization. Journal of Geophysical Research-Atmospheres. 107   10.1029/2001jd001005   AbstractWebsite

[1] The quasi-equilibrium assumption proposed by Arakawa and Schubert assumes that convection is controlled by the large-scale forcing in a statistical sense, in such a way that the stabilization of the atmosphere by convection is in quasi-equilibrium with the destabilization by the large-scale forcing. The assumption was developed largely based on observations in the tropical maritime environment and has not been evaluated in midlatitudes. This study examines the quasi-equilibrium assumption in midlatitude continental convection environment using summertime observations from the Southern Great Plains of the United States. Two complementary approaches are taken for this purpose. The first one compares the net time rate of change of convective available potential energy to that due to the large-scale forcing. The second one examines the contributions to the net change of CAPE from the boundary layer air and the free tropospheric air above. Results from both the approaches indicate that the quasi-equilibrium assumption is not well suited for midlatitude continental convection. It is shown that the net change of CAPE is comparable to and largely comes from that due to thermodynamic changes of the boundary layer air, while the contribution from the free troposphere above the boundary layer is negligible. The analysis also shows that the role of convective inhibition to suppress convection is the most pronounced when the large-scale forcing in the free troposphere is weak. On the basis of these and other observations, a modification to the quasi-equilibrium assumption is proposed. It assumes that convective and large-scale processes in the free troposphere above the boundary layer are in balance, so that contribution from the free troposphere to changes in CAPE is negligible. This assumption is then tested using the single column model of the NCAR CCM3 by modifying the closure in the CCM3 convection scheme. Such a modification significantly improves the single column model simulation. The applicability of this new quasi-equilibrium assumption to tropical convection environment is also discussed.

Zhang, GJ.  1995.  The sensitivity of surface-energy balance to convective parameterization in a general circulation model. Journal of the Atmospheric Sciences. 52:1370-1382.   10.1175/1520-0469(1995)052<1370:TSOSEB>2.0.CO;2   AbstractWebsite

This study investigates the interaction between convection and the surface energy fluxes, and its sensitivity to convective parameterization schemes using a general circulation model. Two simulations of the global circulation averaged annually from 1 June 1985 to 31 May 1986 are performed, with particular emphasis on the tropical Pacific. In the control simulation, a convective scheme that parameterizes convection based on low-level moisture convergence is used. A second experiment employs a parameterization scheme that uses the time rate of change of convective available potential energy (CAFE) to determine convection. When the low-level moisture convergence is used as the closure for convective parameterization, convection and low-level convergence occur near the landmass of Southeast Asia. The large-scale circulation is such that a fairly strong surface wind that provides moisture to fuel convection is located in the western tropical Pacific warm pool regions, giving rise to relatively high latent heat flux there. When the time rate of change of CAFE is used to close convective parameterization, convection and its associated low-level large-scale convergence and weak surface wind speed occur in the warm pool region, resulting in low latent heat flux there. Response of surface solar radiative flux to convection is found to be the largest in the surface energy budget. Convection affects surface radiation through the generation of clouds. In the experiment, more clouds are produced over the tropical oceans, leading to less solar radiation received on the surface. More clouds in the experiment also lead to less net emission of longwave radiation from the ocean surface due to the cloud greenhouse effect, albeit the magnitude is much smaller than that for solar radiation. The large changes in surface latent heat and solar radiative fluxes from the control run to the experiment suggest that the surface energy balance in the atmospheric general circulation model is highly sensitive to convective parameterization.

Zhang, GJ, Mu MQ.  2005.  Simulation of the Madden-Julian oscillation in the NCAR CCM3 using a revised Zhang-McFarlane convection parameterization scheme. Journal of Climate. 18:4046-4064.   10.1175/jcli3508.1   AbstractWebsite

This study presents the simulation of the Madden-Julian oscillation (MJO) in the NCAR CCM3 using a modified Zhang-McFarlane convection parameterization scheme. It is shown that, with the modified scheme, the intraseasonal (20-80 day) variability in precipitation, zonal wind, and outgoing longwave radiation (OLR) is enhanced substantially compared to the standard CCM3 simulation. Using a composite technique based on the empirical orthogonal function (EOF) analysis, the paper demonstrates that the simulated MJOs are in better agreement with the observations than the standard model in many important aspects. The amplitudes of the MJOs in 850-mb zonal wind, precipitation, and OLR are comparable to those of the observations, and the MJOs show clearly eastward propagation from the Indian Ocean to the Pacific. In contrast, the simulated MJOs in the standard CCM3 simulation are weak and have a tendency to propagate westward in the Indian Ocean. Nevertheless, there remain several deficiencies that are yet to be addressed. The time period of the MJOs is shorter, about 30 days, compared to the observed time period of 40 days. The spatial scale of the precipitation signal is smaller than observed. Examination of convective heating from both deep and shallow convection and its relationship with moisture anomalies indicates that near the mature phase of the MJO, regions of shallow convection developing ahead of the deep convection coincide with regions of positive moisture anomalies in the lower troposphere. This is consistent with the recent observations and theoretical development that shallow convection helps to precondition the atmosphere for MJO by moistening the lower troposphere. Sensitivity tests are performed on the individual changes in the modified convection scheme. They show that both change of closure and use of a relative humidity threshold for the convection trigger play important roles in improving the MJO simulation. Use of the new closure leads to the eastward propagation of the MJO and increases the intensity of the MJO signal in the wind field, while imposing a relative humidity threshold enhances the MJO variability in precipitation.

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

This study examines the response of the climate simulation by the National Center for Atmospheric Research Community Climate Model(CCM3) to the introduction of the Zhang and McFarlane convective parameterization in the model. It is shown that in the CCM3 the simulated surface climate in the tropical convective regimes, especially in the western Pacific warm pool, is markedly improved, yielding a much better agreement with the recent observations. The systematic bias in the surface evaporation, surface wind stress over the tropical Pacific Ocean in previous model simulations is significantly reduced, owing to the better simulation of the surface flow. Experiments using identical initial and boundary conditions, but different convection schemes, are performed to isolate the role of the convection schemes and to understand the interaction between convection and the large scale circulation in a convecting atmosphere. The comparison of the results from these experiments in the western Pacific warm pool suggests that use of the Zhang and McFarlane scheme makes a significant contribution to the improved climate simulation in CCM3. The simulated atmosphere using the Zhang and McFarlane scheme exhibits a quasi-equilibrium between convection and the large-scale processes. When this scheme is removed from the CCM3, such a quasi-equilibrium is no longer observed. In addition, the simulated thermodynamic structures, the surface evaporation, and surface winds in the Pacific warm pool become very similar to those in the CCM2 climate. Examination of the temporal evolution of the various fields demonstrates that the stabilization of the atmosphere using the new convection Scheme takes place during the transition from nonequilibrium to quasi equilibrium at the beginning of the time integration. After quasi equilibrium is reached, the atmosphere is warmer and more stable compared to the run without the new scheme. Associated with the more stable stratification, the atmospheric circulation becomes weaker, thus the surface winds and evaporation are weaker because of the coupling between thermodynamics and dynamics in the tropical troposphere.

Zhang, GJ, McFarlane NA.  1991.  Convective stabilizations in midlatitudes. Monthly Weather Review. 119:1915-1928.   10.1175/1520-0493(1991)119<1915:CSIM>2.0.CO;2   AbstractWebsite

The upper-air sounding data from PRE-STORM are used to investigate the convective stabilization effect on the large-scale atmosphere. To facilitate comparison between different stages of cumulus convection, the data are divided into four categories: environment, presystem, insystem, and postsystem. It is found that the convective available potential energy of the atmosphere is reduced substantially after cumulus convection, most of which is consumed during the transition from presystem to insystem. Examination of the temperature and moisture changes during cumulus convection suggests that cooling and drying in the subcloud layer are the most important factors in stabilizing the atmosphere. In general, virtual potential temperature profiles in all categories are close to reversible moist adiabats below the 600-mb level and nearly parallel to moist pseudoadiabats above it. The effect of entrainment on parcel buoyancy is also studied. It is found that a small amount of entrainment of ambient air can lead to a pronounced decrease of parcel buoyancy. Furthermore, for diluted parcel ascent, the convective available potential energy is greater for the insystem category than for the postsystem one, whereas the opposite is true for undiluted parcel ascent.

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.  2003.  Convective momentum transport and perturbation pressure field from a cloud-resolving model simulation. Journal of the Atmospheric Sciences. 60:1120-1139.   10.1175/1520-0469(2003)060<1120:cmtapp>2.0.co;2   AbstractWebsite

This study uses a 2D cloud-resolving model to investigate the vertical transport of horizontal momentum and to understand the role of a convection-generated perturbation pressure field in the momentum transport by convective systems during part of the Tropical Ocean and Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE) Intensive Observation Period. It shows that convective updrafts transport a significant amount of momentum vertically. This transport is downgradient in the easterly wind regime, but upgradient during a westerly wind burst. The differences in convective momentum transport between easterly and westerly wind regimes are examined. The perturbation pressure gradient accounts for an important part of the apparent momentum source. In general it is opposite in sign to the product of cloud mass flux and the vertical wind shear, with smaller magnitude. Examination of the dynamic forcing to the pressure field demonstrates that the linear forcing representing the interaction between the convective updrafts and the large-scale wind shear is the dominant term, while the nonlinear forcing is of secondary importance. Thus, parameterization schemes taking into account the linear interaction between the convective updrafts and the large-scale wind shear can capture the essential features of the perturbation pressure field. The parameterization scheme for momentum transport by Zhang and Cho is evaluated using the model simulation data. The parameterized pressure gradient force using the scheme is in excellent agreement with the simulated one. The parameterized apparent momentum source is also in good agreement with the model simulation. Other parameterization methods for the pressure gradient are also discussed.

Zhang, GJ, Ramanathan V, McPhaden MJ.  1995.  Convection-evaporation feedback in the equatorial Pacific. Journal of Climate. 8:3040-3051.   10.1175/1520-0442(1995)008<3040:cefite>2.0.co;2   AbstractWebsite

The coupling between convection and surface evaporation is investigated to assess the importance of evaporative cooling in regulating the tropical sea surface temperature. It is found that such a coupling is scale dependent. On timescales of several days, convective activity enhances surface evaporation, which together with the decrease of surface solar radiation, acts to cool the sea surface, However, on scales of climatic interest, convection acts to reduce surface evaporation. High sea surface temperature gives rise to more convective activity, which through interaction with the large-scale circulation, increases the low-level large-scale convergence and decreases the surface wind, leading to low evaporation in spite of the increased surface-air humidity difference. Therefore, although individual convective events can significantly enhance surface evaporation on short timescales, the long-term average effect of convection is to suppress surface evaporation at high SST due to its interaction with the large-scale circulation. One potential implication of this result is that evaporative cooling on climate timescales may not provide a negative feedback on the sea surface temperature of warm oceans with convectively disturbed tropospheres.

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.

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

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

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

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

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.

Zhang, GJ, Zurovac-Jevtic D, Boer ER.  1999.  Spatial characteristics of the tropical cloud systems: comparison between model simulation and satellite observations. Tellus Series a-Dynamic Meteorology and Oceanography. 51:922-936.   10.1034/j.1600-0870.1999.00026.x   AbstractWebsite

A Lagrangian cloud classification algorithm is applied to the cloud fields in the tropical Pacific simulated by a high-resolution regional atmospheric model. The purpose of this work is to assess the model's ability to reproduce the observed spatial characteristics of the tropical cloud systems. The cloud systems are broadly grouped into three categories: deep clouds, mid-level clouds and low clouds. The deep clouds are further divided into mesoscale convective systems and non-mesoscale convective systems. It is shown that the model is able to simulate the total cloud cover for each category reasonably well. However, when the cloud cover is broken down into contributions from cloud systems of different sizes, it is shown that the simulated cloud size distribution is biased toward large cloud systems, with contribution from relatively small cloud systems significantly under-represented in the model for both deep and mid-level clouds. The number distribution and area contribution to the cloud cover from mesoscale convective systems are very well simulated compared to the satellite observations, so are low clouds as well. The dependence of the cloud physical properties on cloud scale is examined. It is found that cloud liquid water path, rainfall, and ocean surface sensible and latent heat fluxes have a clear dependence on cloud types and scale. This is of particular interest to studies of the cloud effects on surface energy budget and hydrological cycle. The diurnal variation of the cloud population and area is also examined. The model exhibits a varying degree of success in simulating the diurnal variation of the cloud number and area. The observed early morning maximum cloud cover in deep convective cloud systems is qualitatively simulated. However, the afternoon secondary maximum is missing in the model simulation. The diurnal variation of the tropospheric temperature is well reproduced by the model while simulation of the diurnal variation of the moisture held is poor. The implication of this comparison between model simulation and observations on cloud parameterization is discussed.

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, 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.

Zhang, GJ, Grossman RL.  1996.  Surface evaporation during the central equatorial Pacific experiment: A climate-scale perspective. Journal of Climate. 9:2522-2537.   10.1175/1520-0442(1996)009<2522:sedtce>2.0.co;2   AbstractWebsite

This study is directed to evaluating the feedback between evaporation (F-L) and sea surface temperature (T-s) in the equatorial Pacific Ocean by looking at the components that control dF(L)/dT(s), the variation of evaporation with T-s. First eddy correlation evaporation estimates obtained during long (similar to 1000-1500 km), low-level (30 m) traverses of the central equatorial Pacific by research aircraft during the Central Equatorial Pacific Experiment (CEPEX) are analyzed. From this limited dataset, extension to climate space- and timescales is made by comparing the aircraft measurements to bulk aerodynamic estimates to F-L using mean values from both the aircraft and Tropical Atmosphere-Ocean buoys. Variation of surface evaporation with T-s is shown to be affected not only to surface saturation humidity deficit and its dependence on T-s, but also by variations of wind speed with T-s. Depending on the relative importance of the two contributions, surface evaporation can either increase or decrease with T-s. Intercomparison between the aircraft data and the buoy data indicates that the humidity deficit effect is dominant during CEPEX, and in low T-s, where surface winds are only weakly related to T-s: the effect of wind speed variation with T-s is much more important in the 2-yr buoy data for T-s greater than or equal to 301 K. The discrepancy between the evaporation feedback in CEPEX and that from the 2-yr buoy data is shown to be largely due to oversampling of high winds and high evaporation during CEPEX for 302 less than or equal to T, < 303 K. The long-term buoy data show that for T-s < 301 K, dF(L)/dT(s) = +9 W m(-2) K-1, while for 304 K > T-c greater than or equal to 301 K, dF(L)/dT(s) = -13 W m(-2) K-1. Furthermore, observations of F-L are well below the values necessary for evaporation to be the primary limiting factor in the regulation of T-s in the equatorial Pacific.

Zhang, GJ, Cho HR.  1991.  Parameterization of the vertical transport of momentum by cumulus clouds. Part I: Theory. Journal of the Atmospheric Sciences. 48:1483-1492.   10.1175/1520-0469(1991)048<1483:potvto>2.0.co;2   AbstractWebsite

A scheme is developed to parameterize the vertical transport of momentum by cumulus clouds, in which the effect of a cloud-induced pressure field is included. In addition, a new form for momentum exchange between clouds and their environment is used. The scheme incorporates a simple cloud model which includes both updraft and downdraft and specifies the cloud dynamic fields to enable us to determine the mean pressure gradient force across the clouds. It is shown that in a typical environment with vertical wind shear, the pressure gradient force is along the direction of the wind shear. The equation governing cloud mean momentum together with the diagnostic equation for cloud-induced pressure field is solved by iteration to evaluate quantitatively the horizontal pressure gradient force across the cloud and determine the cloud mean momentum. Application of the parameterization scheme to GATE convective events will be presented in Part 11 of this paper.

Zhang, GJ, Song XL.  2010.  Convection parameterization, tropical Pacific double ITCZ, and upper-ocean biases in the NCAR CCSM3. Part II: Coupled feedback and the role of ocean heat transport. Journal of Climate. 23:800-812.   10.1175/2009jcli3109.1   AbstractWebsite

This study investigates the coupled atmosphere-ocean feedback and the role of ocean dynamic heat transport in the formation of double ITCZ over the tropical Pacific in the NCAR Community Climate System Model, version 3 (CCSM3) and its alleviation when a revised Zhang-McFarlane (ZM) convection scheme is used. A hierarchy of coupling strategy is employed for this purpose. A slab ocean model is coupled with the atmospheric component of the Community Atmosphere Model, version 3 (CAM3) to investigate the local feedback between the atmosphere and the ocean. It is shown that the net surface energy flux differences in the southern ITCZ region between the revised and original ZM scheme seen in the stand-alone CAM3 simulations can cool the SST by up to 1.5 degrees C. However, the simulated SST distribution is very sensitive to the prescribed ocean heat transport required in the slab ocean model. To understand the role of ocean heat transport, the fully coupled CCSM3 model is used. The analysis of CCSM3 simulations shows that the altered ocean dynamic heat transport when the revised ZM scheme is used is largely responsible for the reduction of SST bias in the southern ITCZ region, although surface energy flux also helps to cool the SST in the first few months of the year in seasonal variation. The results, together with those from Part I, suggest that the unrealistic simulation of convection over the southern ITCZ region in the standard CCSM3 leads to the double-ITCZ bias through complex coupled interactions between atmospheric convection, surface winds, latent heat flux, cloud radiative forcing, SST, and upper-ocean circulations. The mitigation of the double-ITCZ bias using the revised ZM scheme is achieved by altering this chain of interactions.

Zhang, GJ.  2003.  Convective quasi-equilibrium in the tropical western Pacific: Comparison with midlatitude continental environment. Journal of Geophysical Research-Atmospheres. 108   10.1029/2003jd003520   AbstractWebsite

[1] This study examines the nature of convective quasi-equilibrium in the tropical western Pacific using the Tropical Ocean and Global Atmosphere/Coupled Ocean Atmosphere Response Experiment (TOGA COARE) sounding data. Results are compared with those in midlatitudes using data from the southern Great Plains in the United States. It is shown that precipitation is highly correlated to the free tropospheric large-scale forcing in both the tropics and midlatitudes. In the tropics, but not in the midlatitudes, precipitation is also highly correlated to the total large-scale forcing that includes surface fluxes. This is because the surface sensible and latent heat fluxes are small over the tropical ocean; thus their contribution to the total large-scale forcing is relatively small compared to the free tropospheric forcing. The opposite is true for midlatitude continental convection. It is also shown that the free tropospheric quasi-equilibrium between convection and large-scale forcing recently proposed by the author for midlatitude continental convection is equally accurate for tropical oceanic convection. The quasi-equilibrium proposed by Arakawa and Schubert is less accurate in depicting convective stabilization for data averaged from 3 to 24 hours. On timescales longer than 24 hours the two are comparable. Comparisons are made with other studies that use the Arakawa-Schubert quasi-equilibrium concept, and seemingly contradictory results are reconciled.