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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.

Song, XL, Zhang GJ.  2018.  The roles of convection parameterization in the formation of double ITCZ Syndrome in the NCAR CESM: I. Atmospheric Processes. Journal of Advances in Modeling Earth Systems. 10:842-866.   10.1002/2017ms001191   AbstractWebsite

Several improvements are implemented in the Zhang-McFarlane (ZM) convection scheme to investigate the roles of convection parameterization in the formation of double intertropical convergence zone (ITCZ) bias in the NCAR CESM1.2.1. It is shown that the prominent double ITCZ biases of precipitation, sea surface temperature (SST), and wind stress in the standard CESM1.2.1 are largely eliminated in all seasons with the use of these improvements in convection scheme. This study for the first time demonstrates that the modifications of convection scheme can eliminate the double ITCZ biases in all seasons, including boreal winter and spring. Further analysis shows that the elimination of the double ITCZ bias is achieved not by improving other possible contributors, such as stratus cloud bias off the west coast of South America and cloud/radiation biases over the Southern Ocean, but by modifying the convection scheme itself. This study demonstrates that convection scheme is the primary contributor to the double ITCZ bias in the CESM1.2.1, and provides a possible solution to the long-standing double ITCZ problem. The atmospheric model simulations forced by observed SST show that the original ZM convection scheme tends to produce double ITCZ bias in high SST scenario, while the modified convection scheme does not. The impact of changes in each core component of convection scheme on the double ITCZ bias in atmospheric model is identified and further investigated.

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

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

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.

Chung, CE, Zhang GJ.  2004.  Impact of absorbing aerosol on precipitation: Dynamic aspects in association with convective available potential energy and convective parameterization closure and dependence on aerosol heating profile. Journal of Geophysical Research-Atmospheres. 109   10.1029/2004jd004726   AbstractWebsite

[1] The Indian Ocean Experiment (INDOEX), conducted from 1995 to 2000 to document aerosols in south Asia during winter monsoon season, revealed the existence of a layer of highly absorbing aerosols in the lower troposphere. The observed aerosol has one of the two distinctly different vertical distributions: ( 1) aerosols concentrated in the planetary boundary layer (PBL) below 1.5 km and ( 2) elevated aerosol profile peaking around 3 km. Here we provide the dynamical basis for understanding the direct effects of absorbing aerosols on the large-scale precipitation and the role of the aerosol vertical distribution. This was done through a series of the south Asian aerosol experiments with the National Center for Atmospheric Research Community Climate Model (CCM3), together with different convection parameterization closures, and the Community Atmospheric Model (CAM2). It is found that the land surface temperature underneath the aerosol layer is sensitive to the aerosol vertical distribution: The lifted layer of aerosols results in a significant cooling of the underlying land surface, while the PBL profile makes very little cooling. The mechanism of the aerosol effect on precipitation distribution is investigated by examining the correspondence between the aerosol heating profile, changes of precipitation, and the atmospheric convective instability. The direct aerosol heating of the near-surface air increases the convective available potential energy ( CAPE), whereas the heating above the boundary layer decreases CAPE. Meanwhile, the regionally concentrated low-level aerosol heating tends to cause large-scale rising motion over time, which increases CAPE by decreasing the midlevel temperature. The net CAPE change is small for the lifted profile (i.e., profile elevated above PBL) because the CAPE increase by the midlevel cooling is counteracted by the CAPE decrease through the direct haze heating above the PBL. The precipitation increase averaged over the aerosol area is much larger when the PBL profile is used than when the lifted profile is used in the CCM3 with a CAPE-based convective parameterization closure. The sensitivity of the aerosol effect to convective parameterization closure is tested using a new closure, which is based on the environmental contribution to CAPE (CAPEe). It is shown that when this closure is used in CCM3, the precipitation increase averaged over the aerosol area is small regardless of the vertical profile. This is because the direct heating of either profile decreases CAPEe, opposing the CAPEe increase by the midlevel cooling.

Zurovac-Jevtic, D, Zhang GJ.  2003.  Development and test of a cirrus parameterization scheme using NCAR CCM3. Journal of the Atmospheric Sciences. 60:1325-1344.   10.1175/1520-0469(2003)060<1325:datoac>;2   AbstractWebsite

Recent research has shown that depending on the cloud properties, cirrus clouds can either increase or decrease the overall heating of the earth-atmosphere system. Hence, the representation of cirrus clouds in GCMs is recognized as an important contemporary problem. In this study a new diagnostic cirrus parameterization scheme is developed with the intention of improving the simulation of cirrus macro- and microphysical properties in large-scale models. The scheme allows both large-scale motions and convective detrainment to be a source of moisture for cirrus. Water vapor depletion is calculated as diffusional growth of ice crystals with known size distributions, and the effective fallout from a model layer is estimated using mass-weighted fall velocities of the bulk precipitation. The scheme was implemented and tested with the NCAR Community Climate Model (CCM3). The seasonal means of cirrus cloud cover and ice water contents over the warm pool region, as simulated with the new cirrus parameterization, appeared to be much more realistic than in the standard model version when compared to satellite and in situ data. In contrast to the high amount of optically thin cirrus at all cirrus levels simulated by the standard CCM3, cirrus formed with the new scheme are significantly thicker with a reduced amount in the lower part of the upper troposphere (approximately 10-14 km), whereas cirrus formed below the tropopause (approximately 14-17 km) stay thin but have higher cover. It has also been found that a more realistic precipitation treatment not only results in the formation of thicker anvil cirrus, but also increases the rain and evaporation rates in the middle troposphere. These results suggest that cirrus clouds can be an important potential water vapor source in the tropical troposphere.