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Sandeep Sahany, J. David Neelin, Katrina Hales, and Richard B. Neale

Abstract

Properties of the transition to strong deep convection, as previously observed in satellite precipitation statistics, are analyzed using parcel stability computations and a convective plume velocity equation. A set of alternative entrainment assumptions yields very different characteristics of the deep convection onset boundary (here measured by conditional instability and plume vertical velocity) in a bulk temperature–water vapor thermodynamic plane. In observations the threshold value of column water vapor above which there is a rapid increase in precipitation, referred to as the critical value, increases with temperature, but not as quickly as column saturation, and this can be matched only for cases with sufficiently strong entrainment. This corroborates the earlier hypothesis that entraining plumes can explain this feature seen in observations, and it places bounds on the lower-tropospheric entrainment. Examination of a simple interactive entrainment scheme in which a minimum turbulent entrainment is enhanced by a dynamic entrainment (associated with buoyancy-induced vertical acceleration) shows that the deep convection onset curve is governed by the prescribed minimum entrainment. Results from a 0.5° resolution version of the Community Climate System Model, whose convective parameterization includes substantial entrainment, yield a reasonable match to satellite observations in several respects. Temperature–water vapor dependence is seen to agree well with the plume calculations and with offline simulations performed using the convection scheme of the model. These findings suggest that the convective transition characteristics, including the onset curve in the temperature–water vapor plane, can provide a substantial constraint for entrainment assumptions used in climate model deep convective parameterizations.

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Sandeep Sahany, J. David Neelin, Katrina Hales, and Richard B. Neale

Abstract

Tropical deep convective transition characteristics, including precipitation pickup, occurrence probability, and distribution tails related to extreme events, are analyzed using uncoupled and coupled versions of the Community Climate System Model (CCSM) under present-day and global warming conditions. Atmospheric Model Intercomparison Project–type simulations using a 0.5° version of the uncoupled model yield good matches to satellite retrievals for convective transition properties analyzed as a function of bulk measures of water vapor and tropospheric temperature. Present-day simulations with the 1.0° coupled model show transition behavior not very different from that seen in the higher-resolution uncoupled version. Frequency of occurrence of column water vapor (CWV) for precipitating points shows reasonable agreement with the retrievals, including the longer-than-Gaussian tails of the distributions. The probability density functions of precipitating grid points collapse toward similar form when normalized by the critical CWV for convective onset in both historical and global warming cases. Under global warming conditions, the following statements can be made regarding the precipitation statistics in the simulation: (i) as the rainfall pickup shifts to higher CWV with warmer temperatures, the critical CWV for the current climate is a good predictor for the same quantity under global warming with the shift given by straightforward conditional instability considerations; (ii) to a first approximation, the probability distributions shift accordingly, except that (iii) frequency of occurrence in the longer-than-Gaussian tail increases considerably, with implications for occurrences of extreme events; and, thus, (iv) precipitation conditional averages on CWV and tropospheric temperature tend to extend to higher values.

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Richard B. Neale, Jadwiga H. Richter, and Markus Jochum

Abstract

The NCAR Community Climate System Model, version 3 (CCSM3) exhibits persistent errors in its simulation of the El Niño–Southern Oscillation (ENSO) mode of coupled variability. The amplitude of the oscillation is too strong, the dominant 2-yr period too regular, and the width of the sea surface temperature response in the Pacific too narrow, with positive anomalies extending too far into the western Pacific. Two changes in the parameterization of deep convection result in a significant improvement to many aspects of the ENSO simulation. The inclusion of convective momentum transport (CMT) and a dilution approximation for the calculation of convective available potential energy (CAPE) are used in development integrations, and a striking improvement in ENSO characteristics is seen. An increase in the periodicity of ENSO is achieved by a reduction in the strength of the existing “short-circuited” delayed-oscillator mode. The off-equatorial response is weaker and less tropically confined, largely as a result of the CMT and an associated redistribution of zonal momentum. The Pacific east–west structure is improved in response to the presence of convective dilution and cooling provided by increased surface fluxes. The initiation of El Niño events is fundamentally different. Enhanced intraseasonal surface stress variability leads to absolute surface westerlies and a cooling–warming dipole between the Philippine Sea and western Pacific. Lag-regression analysis shows that intraseasonal variability may play a significant role in event initiation and maintenance as opposed to being a benign response to increased SSTs. Recent observational evidence appears to support such a leading relationship.

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Lei Zhou, Richard B. Neale, Markus Jochum, and Raghu Murtugudde

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Two modifications are made to the deep convection parameterization in the NCAR Community Climate System Model, version 3 (CCSM3): a dilute plume approximation and an implementation of the convective momentum transport (CMT). These changes lead to significant improvement in the simulated Madden–Julian oscillations (MJOs). With the dilute plume approximation, temperature and convective heating perturbations become more positively correlated. Consequently, more available potential energy is generated and the intraseasonal variability (ISV) becomes stronger. The organization of ISV is also improved, which is manifest in coherent structures between different MJO phases and an improved simulation of the eastward propagation of MJOs with a reasonable eastward speed. The improved propagation can be attributed to a better simulation of the low-level zonal winds due to the inclusion of CMT. The authors posit that the large-scale zonal winds are akin to a selective conveyor belt that facilitates the organization of ISVs into highly coherent structures, which are important features of observed MJOs. The conclusions are supported by two supplementary experiments, which include the dilute plume approximation and CMT separately.

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Aneesh C. Subramanian, Markus Jochum, Arthur J. Miller, Raghu Murtugudde, Richard B. Neale, and Duane E. Waliser

Abstract

This study assesses the ability of the Community Climate System Model, version 4 (CCSM4) to represent the Madden–Julian oscillation (MJO), the dominant mode of intraseasonal variability in the tropical atmosphere. The U.S. Climate Variability and Predictability (CLIVAR) MJO Working Group’s prescribed diagnostic tests are used to evaluate the model’s mean state, variance, and wavenumber–frequency characteristics in a 20-yr simulation of the intraseasonal variability in zonal winds at 850 hPa (U850) and 200 hPa (U200), and outgoing longwave radiation (OLR). Unlike its predecessor, CCSM4 reproduces a number of aspects of MJO behavior more realistically.

The CCSM4 produces coherent, broadbanded, and energetic patterns in eastward-propagating intraseasonal zonal winds and OLR in the tropical Indian and Pacific Oceans that are generally consistent with MJO characteristics. Strong peaks occur in power spectra and coherence spectra with periods between 20 and 100 days and zonal wavenumbers between 1 and 3. Model MJOs, however, tend to be more broadbanded in frequency than in observations. Broad-scale patterns, as revealed in combined EOFs of U850, U200, and OLR, are remarkably consistent with observations and indicate that large-scale convergence–convection coupling occurs in the simulated MJO.

Relations between MJO in the model and its concurrence with other climate states are also explored. MJO activity (defined as the percentage of time the MJO index exceeds 1.5) is enhanced during El Niño events compared to La Niña events, both in the model and observations. MJO activity is increased during periods of anomalously strong negative meridional wind shear in the Asian monsoon region and also during strong negative Indian Ocean zonal mode states, in both the model and observations.

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Haoming Chen, Tianjun Zhou, Richard B. Neale, Xiaoqing Wu, and Guang Jun Zhang

Abstract

The performance of an interim version of the NCAR Community Atmospheric Model (CAM3.5) in simulating the East Asian summer monsoon (EASM) is assessed by comparing model results against observations and reanalyses. Both the climate mean states and seasonal cycle of major EASM components are evaluated. Special attention is paid to the sensitivity of model performance to changes in the convection scheme. This is done by analyzing four CAM3.5 runs with identical dynamical core and physical packages but different modifications to their convection scheme, that is, the original Zhang–McFarlane () scheme, Neale et al.’s modification (), Wu et al.’s modification (), and Zhang’s modification (). The results show that CAM3.5 can capture the major climate mean states and seasonal features of the EASM circulation system, including reasonable simulations of the Tibetan high in the upper troposphere and the western Pacific subtropical high (WPSH) in the middle and lower troposphere. The main deficiencies are found in monsoon rainfall and the meridional monsoon cell. The weak meridional land–sea thermal contrasts in the model contribute to the weaker monsoon circulation and to insufficient rainfall in both tropical and subtropical regions of EASM. The seasonal migration of rainfall, as well as the northward jump of the WPSH from late spring to summer, is reasonably simulated, except that the northward jump of the monsoon rain belt still needs improvement. Three runs using modified schemes generally improve the model performance in EASM simulation compared to the control run. The monsoon rainfall distribution and its seasonal variation are sensitive to modifications of the convection scheme, which is most likely due to differences in closure assumptions. , which uses a convective available potential energy (CAPE)-based closure assumption, performs better in tropical regions where the rainfall is closely related to CAPE. However, and , which use quasi-equilibrium (QE) closure, have more realistic subtropical rainfall in the mei-yu/baiu/changma front region, mainly because the rainfall in the subtropics is more sensitive to the rate of destabilization by the large-scale flow.

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Melissa Gervais, John R. Gyakum, Eyad Atallah, L. Bruno Tremblay, and Richard B. Neale

Abstract

An intercomparison of the distribution and extreme values of daily precipitation between the National Center for Atmospheric Research Community Climate System Model, version 4 (CCSM4) and several observational/reanalysis data sources are conducted over the contiguous United States and southern Canada. The use of several data sources, from gridded station, satellite, and reanalysis products, provides a measure of errors in the reference datasets. An examination of specific locations shows that the global climate model (GCM) distributions closely match the observations along the East and West Coasts, with larger discrepancies in the Great Plains and Rockies. In general, the distribution of model precipitation is more positively skewed (more light and less heavy precipitation) in the Great Plains and the eastern United States compared to gridded station observations, a recurring error in GCMs. In the Rocky Mountains the GCMs generally overproduce precipitation relative to the observations and furthermore have a more negatively skewed distribution, with fewer lower daily precipitation values relative to higher values. Extreme precipitation tends to be underestimated in regions and time periods typically characterized by large amounts of convective precipitation. This is shown to be the result of errors in the parameterization of convective precipitation that have been seen in previous model versions. However, comparison against several data sources reveals that model errors in extreme precipitation are approaching the magnitude of the disparity between the reference products. This highlights the existence of large errors in some of the products employed as observations for validation purposes.

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Richard B. Neale, Jadwiga Richter, Sungsu Park, Peter H. Lauritzen, Stephen J. Vavrus, Philip J. Rasch, and Minghua Zhang

Abstract

The Community Atmosphere Model, version 4 (CAM4), was released as part of the Community Climate System Model, version 4 (CCSM4). The finite volume (FV) dynamical core is now the default because of its superior transport and conservation properties. Deep convection parameterization changes include a dilute plume calculation of convective available potential energy (CAPE) and the introduction of convective momentum transport (CMT). An additional cloud fraction calculation is now performed following macrophysical state updates to provide improved thermodynamic consistency. A freeze-drying modification is further made to the cloud fraction calculation in very dry environments (e.g., the Arctic), where cloud fraction and cloud water values were often inconsistent in CAM3. In CAM4 the FV dynamical core further degrades the excessive trade-wind simulation, but reduces zonal stress errors at higher latitudes. Plume dilution alleviates much of the midtropospheric tropical dry biases and reduces the persistent monsoon precipitation biases over the Arabian Peninsula and the southern Indian Ocean. CMT reduces much of the excessive trade-wind biases in eastern ocean basins. CAM4 shows a global reduction in cloud fraction compared to CAM3, primarily as a result of the freeze-drying and improved cloud fraction equilibrium modifications. Regional climate feature improvements include the propagation of stationary waves from the Pacific into midlatitudes and the seasonal frequency of Northern Hemisphere blocking events. A 1° versus 2° horizontal resolution of the FV dynamical core exhibits superior improvements in regional climate features of precipitation and surface stress. Improvements in the fully coupled mean climate between CAM3 and CAM4 are also more substantial than in forced sea surface temperature (SST) simulations.

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Daehyun Kim, Prince Xavier, Eric Maloney, Matthew Wheeler, Duane Waliser, Kenneth Sperber, Harry Hendon, Chidong Zhang, Richard Neale, Yen-Ting Hwang, and Haibo Liu

Abstract

Process-oriented diagnostics for Madden–Julian oscillation (MJO) simulations are being developed to facilitate improvements in the representation of the MJO in weather and climate models. These process-oriented diagnostics are intended to provide insights into how parameterizations of physical processes in climate models should be improved for a better MJO simulation. This paper proposes one such process-oriented diagnostic, which is designed to represent sensitivity of simulated convection to environmental moisture: composites of a relative humidity (RH) profile based on precipitation percentiles. The ability of the RH composite diagnostic to represent the diversity of MJO simulation skill is demonstrated using a group of climate model simulations participating in phases 3 and 5 of the Coupled Model Intercomparison Project (CMIP3 and CMIP5). A set of scalar process metrics that captures the key physical attributes of the RH diagnostic is derived and their statistical relationship with indices that quantify the fidelity of the MJO simulation is tested. It is found that a process metric that represents the amount of lower-tropospheric humidity increase required for a transition from weak to strong rain regimes has a robust statistical relationship with MJO simulation skill. The results herein suggest that moisture sensitivity of convection is closely related to a GCM’s ability to simulate the MJO.

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Julio T. Bacmeister, Michael F. Wehner, Richard B. Neale, Andrew Gettelman, Cecile Hannay, Peter H. Lauritzen, Julie M. Caron, and John E. Truesdale

Abstract

Extended, high-resolution (0.23° latitude × 0.31° longitude) simulations with Community Atmosphere Model versions 4 and 5 (CAM4 and CAM5) are examined and compared with results from climate simulations conducted at a more typical resolution of 0.9° latitude × 1.25° longitude. Overall, the simulated climate of the high-resolution experiments is not dramatically better than that of their low-resolution counterparts. Improvements appear primarily where topographic effects may be playing a role, including a substantially improved summertime Indian monsoon simulation in CAM4 at high resolution. Significant sensitivity to resolution is found in simulated precipitation over the southeast United States during winter. Some aspects of the simulated seasonal mean precipitation deteriorate notably at high resolution. Prominent among these is an exacerbated Pacific “double ITCZ” bias in both models. Nevertheless, while large-scale seasonal means are not dramatically better at high resolution, realistic tropical cyclone (TC) distributions are obtained. Some skill in reproducing interannual variability in TC statistics also appears.

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