CCM3 Simulations Research Articles

Simulation of the Madden–Julian Oscillation in the NCAR CCM3 Using a Revised Zhang–McFarlane Convection Parameterization Scheme

Abstract 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.

A Comparison of Tropical Precipitation Simulated by the Community Climate Model with That Measured by the Tropical Rainfall Measuring Mission Satellite

This study evaluates the simulation of tropical precipitation by the Community Climate Model, version 3, (CCM3) developed at the National Center for Atmospheric Research. Monthly mean precipitation rates from an ensemble of CCM3 simulations are compared to those computed from observations of the Tropical Rainfall Measuring Mission (TRMM) satellite over a 44-month period. On regional and subregional scales, the comparison fares well over much of the Eastern Hemisphere south of 108S and over South America. However, model‐ satellite differences are large in portions of Central America and the Caribbean, the southern tropical Atlantic, the northern Indian Ocean, and the western equatorial and southern tropical Pacific. Since precipitation in the Tropics is the primary source of latent energy to the general circulation, such large model‐satellite differences imply large differences in the amount of latent energy released. Differences tend to be seasonally dependent north of 108N, where model wet biases occur in realistic wet seasons or model-generated artificial wet seasons. South of 108N, the model wet biases exist throughout the year or have no recognizable pattern.

Continental runoff dynamics in the Community Climate System Model 2 (CCSM2) control simulation

We present an analysis of the river transport model (RTM) implementation in a fully coupled Earth system model, the Community Climate System Model, version 2 (CCSM2). The results of a 350‐year CCSM2 control simulation were compared to observational river flow data and previous simulations using the Community Climate Model, version 3 (CCM3). River discharge into the Arctic Ocean improved significantly over earlier CCM3 simulations. Significant interannual variability was evident in the control simulation, although at times it exceeded the observational variability. The CCSM2 river discharges to the ocean during the peak discharge month of June for the Amazon, Amur, Ob, St. Lawrence, and Ganges Rivers were found to be within 25% of observations. CCSM2 produced negative river discharges for the St. Lawrence, Parana, and Ob Rivers for some months of the year because of the methods CCSM2 used to calculate runoff resulting from lakes, wetlands, and glaciers. Precipitation and snow cover analyses explained some of the improvements of CCSM2 over CCM3 as well as lingering sources of inaccuracy in the river discharges. The lack of water impoundment in both CCSM2 and CCM3 was shown to be another source of discrepancy between model river discharge and observed streamgage data. We discuss the differences and similarities between CCSM2 river discharges and observations within the context of extant freshwater resource management, observational measurement techniques, and spatial and temporal scales.

2×CO 2 Eastern Asia regional responses in the RSM/CCM3 modeling system

A global to regional modeling system has been developed to evaluate precipitation under doubled CO 2. The National Centers for Environmental Prediction (NCEP) regional spectral model (RSM) is initialized and forced by current and doubled CO 2 simulations from the NCAR community climate model (CCM3). Three RSM simulations, RSM0, RSM1, and RSM2, with resolution of 280, 50 and 15 km, are examined. The RSM0 setup resolution matches the T42 CCM3 simulations. The RSM2 simulation is centered over Taiwan. Due to incompatibility of the model physics, noticeable differences between RSM0 and CCM3 are found, especially in wintertime, which suggests that simulation from RSM0, rather than CCM3, should be used to contrast high-resolution regional variations produced by RSM1 or RSM2 simulations. While the spatial distributions of RSM1 and RSM2 simulations over Taiwan are greatly improved over the CCM3 simulation, the intensity of the unique wintertime drizzle is overestimated, especially in RSM2. There is also a spurious northward extension of the precipitation pattern from the subtropical warm-pool region. Thus the regional response to doubled CO 2, which consists of more summerlike wintertime precipitation characteristics over the northeastern and eastern sides of Taiwan, with increased intensity mostly in the extreme events, is still in doubt and must be examined with improved global and regional models.

Evaluation of upper tropospheric water vapor in the NCAR Community Climate Model (CCM3) using modeled and observed HIRS radiances

Upper tropospheric water vapor (UTWV) simulated by the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM3) is evaluated by comparing modeled, clear‐sky, brightness temperatures (BTs) to those observed from space by the High‐Resolution Infrared Radiation Sounder (HIRS). The climate model was modified to utilize a highly accurate longwave radiation model (RRTM) and a separate radiance module, which contains physics consistent with RRTM, to calculate BTs in the National Oceanic and Atmospheric Administration (NOAA‐7) HIRS 6.7 μm water vapor channel (CH12) and the 14.2 μm temperature channel (CH04). The CCM3 simulations follow the Atmospheric Model Intercomparison Project (AMIP) protocol for the period 1982–1984 to provide a basis for evaluating the water vapor distribution over a wide range of atmospheric conditions. BT differences of 2 K or less in CH04 indicate that CH12 differences are primarily due to water vapor rather than cloud effects or temperature variations. Regionally, CCM3 exhibits considerable positive and negative differences of 5–10 K in CH12 that are apparent in both convective and dry subtropical areas. This suggests that significant moist and dry discrepancies in UTWV amount of 50% or more are present in CCM3. These biases are also shown to persist through several annual cycles and at shorter timescales, and they are only slightly influenced by the improved longwave radiation model, which suggests a dynamical cause. Comparison to HIRS radiances provides a considerably more effective and comprehensive means of globally evaluating general circulation model (GCM)‐simulated UTWV on various timescales than surface‐based water vapor measurements.

Impacts of Diurnal Cycle of SST on the Intraseasonal Variation of Surface Heat Flux over the Western Pacific Warm Pool

Based on the method of estimating the diurnal amplitude of sea surface temperature (SST) as a function of daily averaged wind speed, precipitation and peak surface radiation, a parameterization scheme of diurnal cycle of SST is developed in the present study, Integrations to National Center for Atmospheric Research (NCAR) Community Climate Model (CCM3), separately forced by observed weekly SSTs with and without diurnal cycle of SST, are compared. Surface observation obtained from the Improved Meteorology (IMET) buoy during TOGA COARE Intensive Observation Period is applied to verify the SST parameterization, as well as a validation to the results from the CCM3 simulation. It is shown that the superposition of diurnal cycle of SST to the forced weekly SST makes a more realistic representation of the surface structure of intraseasonal oscillation over the western Pacific warm pool.

Impacts of sea surface temperature in the tropical Pacific on interannual variability of Madden-Julian Oscillation in precipitation

The Madden-Julian Oscillation (MJO) is investigated in two sets of 11-year records of observed precipitation, the daily mean Microwave Sounding Units (MSU) oceanic rainfall (Spencer, 1993) data and the pentad Climate Prediction Center Merged Analysis of Precipitation (CMAP) data (Xie and Arkin, 1997). Obvious interannual variability is found in the MJO in the tropical Pacific. MJO is limited to the west of dateline in normal years while extends more east during the year of warm sea surface temperature (SST) appeared in the eastern Pacific (i.e., El Nino years of 1982–1983, 1986–1988, 1991–1992) and manifested in the central-eastern Pacific for several months. The most significant correlation between interannual variability of MJO in the central-eastern Pacific and SST was found in the vicinity of the Nino3 region. Forced by observed SST, CCM3 presents a realistic trend of interannual variability to MJO in the 11 years, with a smaller magnitude than that from the observation. Comparison between the two realizations of the CCM3 simulation, which are forced by weekly and monthly mean SST respectively, showed that the MJO activities resemble each other in central-eastern Pacific while there is discrepancy in the western Pacific. It is suggested that the interannual variability of MJO is controlled, to certain extent bythe powerful interannual variability of SST in the central-eastern Pacific. In the western Pacific, however, there were remarkable impacts of the intraseasonal oscillation of SST on the MJO, where there was active MJO around the year. The notable disagreement between simulated and observed MJO in the western Pacific may come from the lack of high frequency variation of SST force, or from the shortage of air sea interaction for the intraseasonal time scale. It might be of importance to the MJO which is unable to be represented in the atmospheric model.

The Excitation of Equatorial Waves by Deep Convection in the NCAR Community Climate Model (CCM3)

The forcing of equatorial waves by convective heating in the National Center for Atmospheric Research Community Climate Model (CCM3) is investigated and compared with the forcing deduced from observations of convective clouds. The analysis is performed on two different simulations, wherein convection is represented by the Zhang‐McFarlane and the Hack parameterization schemes, respectively. Spectra of equatorial waves excited by convective heating (Rossby, Kelvin, and gravity waves) are obtained by projecting the heating field onto Hough modes; the dynamical response to the heating is then calculated in terms of the vertical component of the Eliassen‐Palm flux, Fz, focusing on waves that are able to propagate into the middle atmosphere. The same analysis is repeated using observations of outgoing longwave radiation as a proxy for tropical convection. Comparison of CCM3 results with those derived from observations indicates that high-frequency heating variability is underestimated in both CCM3 simulations, despite the fact that time-mean values of convective heating are well represented. Moreover, the two convective parameterization schemes differ substantially from each other: Compared to observations, Fz is severely underestimated at most frequencies when CCM3 is run with the Zhang‐McFarlane scheme. When the Hack scheme is used, Fz at frequencies |v| , 0.5 cycles per day is comparable to the observations, but it is underestimated at higher frequencies. Misrepresentation of the variability of convective heating is likely to have important consequences for the dynamical simulation of the middle atmosphere and even the troposphere.

ENSO Surface Winds in CCM3 Simulation: Diagnosis of Errors

Abstract The structure of surface-wind anomalies associated with ENSO variability is extracted from ComprehensiveOcean–Atmosphere Dataset observations and European Centre for Medium-Range Forecasts (ECMWF) and National Centers for Environmental Prediction (NCEP) reanalyses, along with estimates of uncertainty. The targets are used to evaluate ENSO surface winds produced by the National Center for Atmospheric Research’s atmospheric GCM known as the Community Climate Model, version 3 (CCM3), when integrated in the climate-simulation mode. Simulated anomalies have stronger easterlies in the off-equatorial Tropics and stronger equatorward flow in the Pacific than any of the observational estimates do. CCM3’s wind departures are found to be large when compared with the difference of the reanalysis anomalies and should thus be considered to be errors. In a companion paper, the authors make a compelling case for the presence of robust errors in CCM3’s ENSO heating distribution, based on comparisons with the resi...

ENSO Diabatic Heating in ECMWF and NCEP–NCAR Reanalyses, and NCAR CCM3 Simulation

Abstract Diabatic heating associated with El Nino–Southern Oscillation (ENSO) variability is residually diagnosed from the European Centre for Medium-Range Forecasts (ECMWF) and National Centers for Environmental Prediction (NCEP)–National Center for Atmospheric Research (NCAR) atmospheric reanalysis datasets during the overlapping 1979–93 period. Quantitative characterization of the horizontal and vertical structure of ENSO heating anomalies, including estimates of uncertainty, provides observationally constrained validation targets for GCM physical parameterizations. The diagnosed ENSO heating anomalies have similar horizontal structure, but the vertically averaged ECMWF heating is stronger and in better agreement with the Xie–Arkin precipitation anomalies, particularly with respect to precipitation reduction over the western tropical Pacific. Comparison of heating vertical structures in the central equatorial Pacific shows ECMWF heating to be considerably stronger in the lower troposphere, where it exh...

Sensitivity of equilibrium surface temperature of CCM3 to systematic changes in atmospheric CO2

We have redone our computations of the equilibrium response of surface temperature to atmospheric CO2 concentrations using the latest version of the NCAR Community Climate Model (version 3 rather than version 1). Eight GCM simulations with CO2 concentrations varying from 180 to 3000 ppmv were conducted with CCM3 compared to six CO2 concentrations with CCM1. A preliminary examination of the CCM3 simulations showed the same basic non‐linear behaviour of temperature to CO2 concentrations obtained previously with CCM1. The magnitude of the sensitivities, however, were much lower in the new CCM3 runs than in the older CCM1 runs. Four possible reasons for the reduced sensitivity in CCM3 are discussed.