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Intercomparison of the seasonal cycle in 200 hPa kinetic energy in AMIP GCM simulations

Description: The 200 hPa kinetic energy is represented by means of the spherical harmonic components for the Atmospheric Model Intercomparison Project (AMIP) simulations, the National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis and the European Centre for Medium Range Weather Forecast Reanalysis (ERA). The data used are the monthly mean wind fields from 1979 to 1988. The kinetic energy is decomposed into the divergent (DKE) and rotational (RKE) components and emphasis is placed on examining the former. The two reanalysis data sets show reasonable agreement that is best for the rotational kinetic energy. The largest difference in the divergent kinetic energy occurs during the northern summer. As might be expected, the two analyses are closet in regions where there are sufficient observations such that the effect of the model used in the assimilation cycle are minimized. The observed RKE show only a slight seasonal cycle with a maximum occuring during the northern winter. The DKE, on the other hand, has a very pronounced seasonal cycle with maxima at the solsticial seasons and minima during the equinoctial seasons. The model results show a very large spread in the magnitudes of the RKE and DKE although the models all evince a seasonal variation in phase with that observed. The median values of the seasonal cycle of RKE and DKE for the models are usually superior to those of any individual model. Results are also presented for simulation following the AMIP protocol but using updated versions of the original AMIP entries. In most cases these new integrations show better agreement with the observations.
Date: October 1, 1996
Creator: Boyle, J.S.
Partner: UNT Libraries Government Documents Department

Comparison of CCM3 simulations using two climatological ozone data sets

Description: A comparison of two six year simulations with the CCM3 using different monthly mean, zonally symmetric ozone climatologies is presented. Each run was identical except for the ozone specification. The climatological SSTs supplied with CCM3 were cycled for the extent of the simulation. The ozone data sets were used were the data distributed with the CCM3 code and that compiled at SUNY Albany. The SUNYA data set reflects contemporary ozone measurements extensively using remote sensing data. The CCM3 data were produced from measurements prior to 1974. A brief comparison of the two ozone climatologies is presented. The monthly mean difference fields were computed for the six years of the simulations. A t-test was applied to the monthly mean difference to judge if the changes between the integrations were significant. The significant changes in temperature were for the most part confined to the levels above 200 hPa. In the zonal mean the patterns of differences were largely consistent with regions of the ozone variations, deeper tropospheric penetration of temperature difference occurred in October near the South Pole in the region of the `ozone hole`. The significant temperature changes at the lowest model level (approximately 992 hPa) were confined to very small areas. The 200 hPa zonal wind differences demonstrated that the stationary wave structure was evidently altered by the ozone difference. Although the ozone specifications were zonally symmetric, the zonal wind differences were zonally asymmetric at 200 hPa.
Date: February 1, 1997
Creator: Boyle, J.S.
Partner: UNT Libraries Government Documents Department

Intercomparison of low-frequency variability of the global 200 hPa circulation for AMIP simulations

Description: In the Atmospheric Model Intercomparison Project (AMIP) a number of GCMs are integrated for a 10 year period, 1979-1988, all using the same monthly mean sea surface temperature (SST). This permits a useful intercomparison of the response of the models to the imposed SST. The variables used here for the intercomparison are the 200 hPa divergence and streamfunction. The data used are in the form of monthly averages and are filtered to a spatial resolution of T10, although the actual spatial resolution of the models varies from R15 to T42. The data are manipulated in this manner to concentrate on the low frequency, large scale response. The tools of the analysis are principal components analysis (PCA) and common principal components (CPC). These analyses are carried out on the 120 months of data with the seasonal cycle removed and in the case of the streamfunction with the zonal average also removed. The 1979-1988 period encompasses two El Nino/Southern Oscillation (ENSO) events (1982/83 and 1986/87), and as could be expected the ENSO characteristic response has a prominent impact in the model simulations.
Date: March 1, 1996
Creator: Boyle, J.S.
Partner: UNT Libraries Government Documents Department

Estimates of zonally averaged tropical diabatic heating in AMIP GCM simulations. PCMDI report No. 25

Description: An understanding of the processess that generate the atmospheric diabatic heating rates is basic to an understanding of the time averaged general circulation of the atmosphere and also circulation anomalies. Knowledge of the sources and sinks of atmospheric heating enables a fuller understanding of the nature of the atmospheric circulation. An actual assesment of the diabatic heating rates in the atmosphere is a difficult problem that has been approached in a number of ways. One way is to estimate the total diabatic heating by estimating individual components associated with the radiative fluxes, the latent heat release, and sensible heat fluxes. An example of this approach is provided by Newell. Another approach is to estimate the net heating rates from consideration of the balance required of the mass and wind variables as routinely observed and analyzed. This budget computation has been done using the thermodynamic equation and more recently done by using the vorticity and thermodynamic equations. Schaak and Johnson compute the heating rates through the integration of the isentropic mass continuity equation. The estimates of heating arrived at all these methods are severely handicapped by the uncertainties in the observational data and analyses. In addition the estimates of the individual heating components suffer an additional source of error from the parameterizations used to approximate these quantities.
Date: July 1, 1995
Creator: Boyle, J.S.
Partner: UNT Libraries Government Documents Department

Intercomparison of the spectral characteristics of 200 hPa kinetic energy in some AMIP simulations. PCMDI report No. 23

Description: The 200 hPa kinetic energy budget terms are represented by means of their spherical harmonic components for a number of AMIP simulations. The data used are the monthly mean wind fields for 1979 to 1988 decadal simulations. The budget terms are decomposed into the divergent and rotational components. The comparison is limited to the lower wavenumbers so as to be within the nominal limits imposed by the models with the coarsest spatial resolution. The results show considerable differences among the models. The comparison is best for the rotational wind and decades for the divergent components, especially the conversion term. There is some ambiguity among the models as to the sign of some terms at certain wavenumbers. The models tend to overestimate the Walker type (east-west) divergent circulations compared to the Hadley type (north-south) compared to recent NMC and ECMWF operational analyses. The inter-model differences are substantial when viewed in light of the differences in the observational analyses and ensemble differences for the ECMWF model. However, the median values of all the models taken together are usually in fair agreement with observational values.
Date: July 1, 1995
Creator: Boyle, J.S.
Partner: UNT Libraries Government Documents Department

Seasonal characteristics of precipitation over the United States in AMIP simulations

Description: The monthly mean precipitation patterns of the Atmospheric Model Intercomparison Project (AMIP) decadal simulations over the US and adjoining oceans are intercompared. A simple harmonic analysis of the 12 month seasonal mean precipitation values and a principal component (PC) analysis of the 120 monthly values were carried out. Emphasis is placed on the basic seasonal variation for three subregions, the Eastern US, Central US and West Coast US.
Date: February 1, 1996
Creator: Boyle, J.S.
Partner: UNT Libraries Government Documents Department

Comparison of the 200 hPa circulation in CSM and CCM3 simulations and NCEP and ERA reanalysis: seasonal cycle and interannual variation

Description: In this paper the monthly mean vorticity and divergence at 200 hPa are compared from four data sources: The NCEP/NCAR reanalyses 1958 through 1994, the ECMWF (ERA) reanalyses, 1979 through 1994, a NCAR CCM3 integration using prescribed SSTs from 1979 through 1993, and the NCAR CSM 300 year integration. The NCEP, ERA and CCM3 all provide data for the period 1979 through1993. The timescales examined are the annual cycle and interannual variations. The annual mean vorticity of the ERA and NCEP match very closely. The annual cycle is likewise close except in the eastern equatorial Pacific and Indian Ocean. Compared to the reanalyses, the models have adequate annual means but suffer in the depiction of the annual cycle in the regions of the jet maxima and in some regions of the Tropics. The CSM appears to inherit errors from the CCM3 and apparently add some new ones. The annual mean divergence shows a much larger difference between the reanalyses. This is most pronounced in the Tropics especially over the African and South American land masses. The models also show large differences, with the CSM being an outlier in the tropical Pacific. For many tropical and extratropical locations even the annual cycle is not well defined between the NCEP and ERA reanalysis. The NCEP, ERA, CCM3 and CSM agree with respect to the variance of the monthly mean vorticity. The variance for low pass filtered data is too large in the ENSO regions for the CCM3, but too small for the CSM. Both models tend to underestimate the low frequency variance in midlatitudes. The ERA has substantially more monthly variance in the divergence than the NCEP data, especially over the tropical South America and Africa and the dateline. Both models have variance more on the order of the ERA, and have ...
Date: October 8, 1998
Creator: Boyle, J.S.
Partner: UNT Libraries Government Documents Department

The northern wintertime divergence extrema at 200 hPa and surface cyclones as simulated in the AMIP integration of the ECMWF general circulation model

Description: Divergence and convergence centers at 200 hPa and mean sea level pressure (MSLP) cyclones were located every 6 hr for a 10-yr general circulation model (GCM) simulation with the ECMWF (Cycle 36) for the boreal winters from 1980 to 1988. The simulation used the observed monthly mean sea surface temperature (SST) for the decade. Analysis of the frequency, location, and strength of these centers and cyclones gives insight into the dynamical response of the model to the varying SST. The results indicate that (1) the model produces reasonable climatologies of upper-level divergence and MSLP cyclones; (2) the model distribution of anomalies of divergence and convergence centers and MSLP cyclones is consistent with observations for the 1982-83 and 1986-87 El Nifio events; (3) the tropical Indian Ocean is the region of greatest divergence activity and interannual variability in the model; (4) the variability of the divergence centers is greater than that of the convergence centers; (5) strong divergence centers occur chiefly over the ocean in the midlatitudes but are more land-based in the tropics, except in the Indian Ocean; and (6) locations of divergence and convergence centers can be a useful tool for the intercomparison of global atmospheric simulations.
Date: November 1, 1994
Creator: Boyle, J. S.
Partner: UNT Libraries Government Documents Department

Comparison of Atmospheric Water Vapor in Observational and Model Data Sets

Description: The global water vapor distribution for five observational based data sets and three GCM integrations are compared. The variables considered are the mean and standard deviation values of the precipitable water for the entire atmospheric column and the 500 to 300 hPa layer for January and July. The observationally based sets are the radiosonde data of Ross and Elliott, the ERA and NCEP reanalyses, and the NVAP blend of sonde and satellite data. The three GCM simulations all use the NCAR CCM3 as the atmospheric model. They include: a AMIP type simulation using observed SSTs for the period 1979 to 1993, the NCAR CSM 300 year coupled ocean--atmosphere integration, and a CSM integration with a 1% CO2 increase per year. The observational data exhibit some serious inconsistencies. There are geographical patterns of differences related to interannual variations and national instrument biases. It is clear that the proper characterization of water vapor is somewhat uncertain. Some conclusions about these data appear to be robust even given the discrepancies. The ERA data are too dry especially in the upper levels. The observational data evince much better agreement in the data rich Northern Hemisphere compared to the Southern. Distinct biases are quite pronounced over the Southern Ocean. The mean values and particularly the standard deviations of the three reanalyses are very dependent upon the GCM used as the assimilation vehicle for the analyses. This is made clear by the much enhanced tropical variability in the NCEP/DOE/ AMIP reanalyses compared the initial NCEP/NCAR Reanalysis. The NCAR CCM3 shows consistent evidence of a dry bias. The 1% CO2 experiment shows a very similar pattern of disagreement with the sonde data as the other integrations, once account is taken of the warming trend. No new modes of difference are evident in the 1% CO2 experiment. All ...
Date: March 1, 2000
Creator: Boyle, J. S.
Partner: UNT Libraries Government Documents Department

Comparison of the 200 hPa circulation in CSM and CCM3 simulations and NCEP and ERA reanalysis: principal and common principa

Description: In this paper the interannual variation of monthly mean vorticity and divergence at 200 hPa are compared from four data sources: The NCEP/NCAR reanalyses 1958 through 1994, the ECMWF (ERA) reanalyses, 1979 through 1994, a NCAR CCM3 integration using prescribed SSTs from 1979 through 1993, and the NCAR CSM 300 year integration. Four twenty year periods were taken from the 300 year simulation for analysis. The NCEP, ERA and CCM3 all provide data for the period 1979 through1993. The techniques used are principal and common principal component analyses on the fields transformed to spherical harmonics. The seasonal cycle is removed. For the common time period, 1979 through 1994, the ERA, NCEP and CCM3 display a close correspondence for the leading PC of the 200 hPa vorticity.This mode is closely related to the ENSO variations of the period but the agreement extends to the extratropics. All four CSM periods have similar leading modes which are dominated by a PNA type pattern and lack any Equatorial Pacific ENSO patterns. The agreement between the leading PC for the 200 hPa divergence was somewhat less than that of the vorticity. The CCM3 and ERA indicate a larger magnitude center in the Equatorial Pacific about the dateline than NCEP. The CSM has an intense center a 150E. There are indications in the vorticity and divergence fields that this center is at the source for waves propagating to the midlatitudes. Two twenty year periods of the 1958 to 1996 NCEP reanalyses show a distinct difference between the two periods. The variations are comparable in magnitude if not nature to the variations seen amongst the time sections of the CSM run examined. A CPC analysis of the NCEP, ERA and CCM3 show a common ENS0 type response as the leading common component. The models depart from the ...
Date: October 20, 1998
Creator: Boyle, J. S.
Partner: UNT Libraries Government Documents Department

Comparison of Variability of the Monthly Mean Temperature of the ECMWF and NCEP Reanalyses and CCM3 and DSM Simulations

Description: The low frequency variation in the three dimensional air temperature fields of two reanalyses and two model simulations are described. The data sets used are the monthly mean temperature fields for the NCAR Climate Simulation Model (CSM, Boville and Gent, 1998) 300 year run, a NCAR Community Climate Model version 3 (CCM3, Kiehl et al., 1998) AMIP type simulation, and the NCEPLNCAR and ECMWF (ERA) reanalysis data sets. The variances and correlations are computed for the anomalies from the annual cycle for each data set. In general the reanalyses and models agree fairly well on the structure of the temperature variance. The models tend to have too much variance at the surface compared to the reanalyses. The CSM's poor simulation of the SST in the eastern Pacific leads to a much reduced variance in the Nino3 region. The enhanced variability over land appears to affect the midlatitude simulation of the CSM in that the higher surface variability extends off the east coast of continents. This is not evident in CCM3 and reanalyses where the SSTs are prescribed. At 200 hPa the CCM3 and reanalyses all evince the dumb bell pattern straddling the Equator in the eastern Pacific attributed by Yulaeva and Wallace (1994) to ENS0 variations. The CSM shows no such pattern. A CCM3 integration using climatological SSTs displays more variance that the CSM in this region, apparently the CSM suppresses variability in this locale. The correlations of the temperature fields with the surface air temperature show that the regions of subtropical subsidence are virtually uncorrelated to the surface at the 700 hPa level. The regions of the cold water off the west coast of continents evince decoupling with the surface at 850 hPa. In the region from 30s to 30N the zonal mean correlation falls to about 0.7 below ...
Date: February 16, 2000
Creator: Boyle, J.S.
Partner: UNT Libraries Government Documents Department

Simulation of the Indian and East-Asian summer monsoon in the ECMWF model: Sensitivity to horizontal resolution

Description: The ability of the ECMWF model (Cycle 33) to simulate the Indian and East Asian summer monsoon is evaluated at four different horizontal resolutions: T21, T42, T63, and T106. Generally, with respect to the large scale features of the circulation, the largest differences among the simulations occur at T42 relative to T21. However, on regional scales, important differences among the high frequency temporal variabilitY serve as a further critical test of the model`s ability to simulate the monsoon. More generally, the results indicate the importance of evaluating high frequency time scales as a component of the climate system. T106 best captures both the spatial and temporal characteristics of the Indian and East Asian Monsoon, while T42 fails to correctly simulate the sequence and development of synoptic scale milestones that characterize the monsoon flow. In particular, T106 is superior at simulating the development and migration of the monsoon trough over the Bay of Bengal. In the T42 simulation, the development of the monsoon occurs one month earlier than typically observed. At this time the trough is incorrectly located adjacent to the east coast of India which results in an underestimate of precipitation over the Burma/Thailand region. This early establishment of the monsoon trough affects the evolution of the East-Asian monsoon and yields excessive preseason rainfall over the Mei-yu region. EOF analysis of precipitation over China indicates that T106 best simulates the Mei-yu mode of variability associated with an oscillation of the rainband that gives rise to periods of enhanced rainfall over the Yangize River Valley. The coarse resolution of T21 precludes simulation of the aforementioned regional scale monsoon flows.
Date: November 1, 1993
Creator: Sperber, K. R.; Potter, G. L.; Boyle, J. S. & Hameed, S.
Partner: UNT Libraries Government Documents Department

Causes of Ocean Surface temperature Changes in Atlantic andPacific Topical Cyclogenesis Regions

Description: Previous research has identified links between changes in sea surface temperature (SST) and hurricane intensity. We use climate models to study the possible causes of SST changes in Atlantic and Pacific tropical cyclogenesis regions. The observed SST increases in these regions range from 0.32 to 0.67 C over the 20th century. The 22 climate models examined here suggest that century-timescale SST changes of this magnitude cannot be explained solely by unforced variability of the climate system, even under conservative assumptions regarding the magnitude of this variability. Model simulations that include external forcing by combined anthropogenic and natural factors are generally capable of replicating observed SST changes in both tropical cyclogenesis regions.
Date: January 31, 2006
Creator: Santer, B.D.; Wigley, T.M.L.; Gleckler, P.J.; Bonfils, C.; Wehner, M.F.; AchutaRao, K. et al.
Partner: UNT Libraries Government Documents Department

Amplification of surface temperature trends and variability in thetropical atmosphere

Description: The month-to-month variability of tropical temperatures is larger in the troposphere than at the Earth's surface. This amplification behavior is similar in a range of observations and climate model simulations, and is consistent with basic theory. On multi-decadal timescales, tropospheric amplification of surface warming is a robust feature of model simulations, but occurs in only one observational dataset. Other observations show weak or even negative amplification. These results suggest that either different physical mechanisms control amplification processes on monthly and decadal timescales, and models fail to capture such behavior, or (more plausibly) that residual errors in several observational datasets used here affect their representation of long-term trends.
Date: August 11, 2005
Creator: Santer, B.D.; Wigley, T.M.L.; Mears, C.; Wentz, F.J.; Klein,S.A.; Seidel, D.J. et al.
Partner: UNT Libraries Government Documents Department