A51F-01
Analysis of the interactions of planetary waves with the mean flow of the stratosphere
During the winter period, large scale waves (planetary waves) are observed to propagate from the troposphere into the stratosphere. Such wave events have been recognized since the 1950s. The very largest wave events result in major stratospheric warmings. These large scale wave events have typical durations of a few days to 2 weeks. The wave events deposit easterly momentum in the stratosphere, decelerating the polar night jet and warming the polar region. In this presentation we show the typical characteristics of these events via a compositing analysis. We will show the typical periods and scales of motion and the associated decelerations and warmings. We will illustrate some of the differences between major and minor warming wave events. We will further illustrate the feedback by the mean flow on subsequent wave events.
A51F-02
Simulations of Madden-Julian Oscillations with the Reduce Acceleration in the VErtical (RAVE) approach
The Madden-Julian Oscillation (MJO) is a dominant mode of tropical intra-seasonal variability and has far- reaching influences within and beyond the tropics. Despite numerous studies, its mechanism is still not well understood and General Circulation Models in general do not simulate MJO well. In this talk, we present results from near-global simulations using the Reduced Acceleration in the VErtical (RAVE) approach (Kuang et al. 2005). The Weather Research and Forecasting (WRF) model is modified for this purpose and is forced with realistic surface boundary conditions and run with a horizontal resolution of ~80km. The simulated precipitation patterns in general agree with the observed ones, except that during northern hemisphere summer, there is excessive precipitation over western Pacific associated with the Southeast Asian monsoon, similar to that seen in simulations using the superparameterization approach (Khairoutdinov et al., 2005). The simulated spectra compare favorably with the observations, so do the spatial distribution of the MJO-filtered variance and the MJO composite structures. A simulation at the same resolution but without using the RAVE approach did not produce realistic MJOs. On the other hand, reasonable simulations of MJO were obtained with a 160-km horizontal resolution using the RAVE approach, which appears to be a relatively inexpensive way to simulate and study MJOs. Reference: Kuang, Z, P. N. Blossey, C. S. Bretherton, A new approach for 3D cloud resolving simulations of large scale atmospheric circulation, Geophys. Res. Letts., Vol. 32, No. 2, L02809, 10.1029/2004GL021024, (2005). Khairoutdinov M., D. Randall, C. DeMott, Simulations of the atmospheric general circulation using a cloud- resolving model as a superparameterization of physical processes, J. Atmos. Sci., 62, 2136-2154, (2005).
A51F-03
The Meridional and Equatorial Modes of Tropical Atlantic Variability: Related?
The leading interannual modes of Tropical Atlantic SST variability in Spring and Summer – the meridional and equatorial modes, respectively – are shown to be related, with the spring meridional mode leading into equatorial mode with a corresponding phase in summer. For example, if the meridional mode has warm SSTAs in southern tropical basin, the equatorial mode has warm SSTAs along the equator in summer; and vice-versa. It is shown that this modal linkage is not orchestrated by ENSO variability. The relationship between these modes and the variability of June-August African rainfall along the Coast of Guinea is also investigated. The mode relationship enhances the prospects for prediction of boreal summer rainfall over the Coast of Guinea.
A51F-04
Tropical Large-Scale Circulations : Asymptotically Nondivergent ?
The large-scale tropical atmospheric flows are overall in balance between the diabatic heating associated with deep convection and the vertical advection to the leading order in the thermodynamic equation (cf., Sobel et al., 2001). Moreover, a simple scale analysis implies, under this thermodynamic balance condition, that the tropical large-scale circulations are more dominated by vorticity than divergence. The present work considers the tendency of the tropical large-scale circulations to asymptotic non divergence by analyzing data from the TOGA COARE (Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment). We consider the evolution of divergence and vorticity during the TOGA COARE period with regard to the passage of a Madden Jullian oscillation (MJO). We quantify the non divergence tendency by a statistical analysis comparing divergence and vorticity. A scatter plot between divergence and vorticity shows that, at 850mb, divergence is less important than vorticity. Finally, we study the space and time scale dependence of the asymptotic tendency for non divergence.
A51F-05
Present-day climatic simulations run with two GCMs: a comparative evaluation against ERA- 40 reanalysis data
Global circulation models (GCMs) are the best currentently available tools to describe the complexity of atmospheric variability and climate evolution on a global scale. However, the large number of existing models shows a wide spectrum of approaches and results, as shown by the intercomparison project of the Intergovernmental Panel on Climate Change (IPCC); or, e.g. Garcia-Herrera et al. (2006), Schmidt at al. (2006), Hansen et al. (2007) and Garcia et al. (2007), among others. Therefore, this work compares side-by-side results from the ModelE GCM version of the NASA Goddard Institute for Space Studies (GISS) at 2º x 2.5º horizontal resolution and 20 vertical layers and the NCAR Whole-Atmosphere Community Climate Model (WACCM) at 2º x 2.5º resolution against reanalysis products of the European Centre for Medium-Range Weather Forecasts (ERA-40) available at 2.5º x 2.5º horizontal resolution and 23 pressure levels (obtained by the ECMWF three-dimensional assimilation system based on satellite, radiosondes and other conventional observations). Both GISS ModelE and WACCM were implemented in a parallel high- performance computing infrastructure, the Marenostrum supercomputer. Model outputs are available for different decades (GISS ModelE simulations were run from 1950 to 2050 while results from WACCM cover the period 1950-2003), but comparison focuses on the period 1957-2002, conditioned by the availability of the ERA-40 re- analysis. The main aim of this study is the definition of the degree of reliability of two specific models for their use in climate prediction and to analyze their seasonal behavior in several regions. Statistical comparisons are performed for global and regional averaged distribution maps of sea level pressure, 2m-temperature, geopotential heights and precipitation at several time scales. The Root Mean Square Error (RMSE) and bias with respect to ERA-40 data have also been estimated and diagrammed for the years 1957- 2002 evolution. Averaged values are then analyzed for different seasons and regions. Moreover, through the use of Taylor diagrams, we quantify and discuss the different performances for each model simulated patterns in terms of standard deviation, centered RMSE and their correlations. Discrepancies between reanalysis and models emerge widely through the study, especially for total cloud cover and precipitation. The global behavior of both models is accurate when compared to ERA-40 re-analysis in terms of low bias and RMSE for 2m-temperature computed by both models, and very high correlations appear (greater than 0.95). Given that the WACCM and the GISS simulations were run at specified sea surface temperatures (SST), in the oceanic regions this result must be expected. However, in terms of sea-level pressure larger differences emerge, with RMSE as large as 0.45 (normalized value) and correlation coefficients in the range between 0.8 and 0.9. In extremely complex topography regions such as the Himalaya range, large differences in error and tendencies appear for pressure and geopotential heights for both models. Despite such problems, a detailed analysis of the results coming shows that both GISS ModelE and WACCM results are within the range of error estimates found in the revised scientific literature.
A51F-06
A coupled atmosphere-ocean general circulation model based on the Modular Earth Submodel System (MESSy)
The ocean general circulation model MPI-OM has been coupled to the atmospheric general circulation model ECHAM5, following the coding standard and the philosophy of the MESSy project (www.messy-interface.org). Physically the two GCM are coupled through the exchange of heat and momentum via the MESSy interface. Thanks to this approach, the coupled model provides a high flexibility and a fast interpolating/coupling procedure. The feedback can be activated/deactivated in both directions separately. The time step of the coupling (i.e the information exchange between atmosphere and ocean) can be selected by the user. First results of a long term simulation ( ~eq 100 years) will be presented, with a discussion on the performance and the technical aspect of the coupling.
A51F-07
The modelled impact of changes in the CO2 concentration on the middle and upper atmosphere: sensitivity to gravity wave activity and parameterization
Cooling of the middle and upper atmosphere by CO2 is dependent on the temperature of the ambient atmosphere. Since the temperature structure is partly controlled by the residual circulation, driven mostly by the dissipation of gravity waves, CO2 cooling is expected to be dependent on the strength of the residual circulation, and thereby on the gravity wave activity. Also, the dissipation process of gravity waves is dependent on the temperature of the atmosphere they are propagating through, and this dependence is reflected differently in different gravity wave parameterization schemes. This may have important consequences for temperature trends due to changes in the CO2 concentration as calculated by middle and upper atmospheric GCMs. Therefore we will show, using the Coupled Middle Atmosphere and Thermosphere model 2 (CMAT2), how such trends depend on the gravity wave parameterization that is used and the gravity wave activity that is assumed by the model.
A51F-08
Coupled Chemistry Climate Model Simulations of Stratospheric Temperature for the Recent Past
Temperature results for the recent past from multi-decadal simulations of eleven coupled chemistry climate models are analysed using multi-linear regression including a trend, solar cycle and volcanic aerosol terms. The climatology of the models since 1980 is in good agreement with observations for the troposphere but diverge from each other and from observations in the stratosphere. Overall, the models agree better with observations than previous assessments. As a function of latitude and pressure, the simulated trends vary substantially from model to model, although all models show several consistent features. These include statistically significant cooling trends from about the lower stratosphere upwards in the low and middle latitudes. Several models have statistically significant cooling in the lower stratosphere over the polar region. The temporal variation in the global average temperature in the lower stratosphere indicates a clear increase during volcanic eruptions, superimposed on an overall cooling. The model responses to the volcanic aerosol varies by about a factor of two with several models substantially overpredicting the observed response during the 1980s and 1990s. The globally averaged temperature simulated by the models is generally in agreement with corrected satellite observations over much of their range. Model trend comparisons are also shown for the polar spring and illlustrate even larger inter-model differences. These differences are caused by different simulations of trends in planetary waves and ozone amounts, and illustrate the challenge of predicting ozone recovery in polar regions.
A51F-09
Tidal Waves in the Upper Stratosphere and Lower Mesosphere as Inferred From Coupled Chemistry-Climate Model Simulations
Atmospheric tides are mainly forced by diurnal variations of the heating due to absorption of solar radiation by ozone and water vapor. Features of atmospheric tides in the region from the upper stratosphere to lower mesosphere are examined by the use of coupled chemistry-climate models (CCMs) with sophisticated stratospheric chemistry developed at the Center for Climate System Research of the University of Tokyo (CCSR) / National Institute for Environmental Studies (NIES) and MRI (Meteorological Research Institute). Results show that in both simulations diurnal tides are clearly seen in summer hemispheres of the stratosphere and lower mesosphere. The global structure is identifiable with the gravest external mode of diurnal tides based on the classical tidal theory: The maxima appear around 0.1 hPa at 40° in both summer and winter hemispheres, which travel westward behind the sun by about 6 hours. The maximum amplitude is larger in summer hemispheres than in winter hemispheres by a factor of 2. As regards the ozone field around the forcing region of tides, corresponding diurnal components are also predominant, having two maxima around 1 hPa and 5 hPa at 70° in summer hemispheres; they travel westward behind the sun by about 13 hours and 5 hours, respectively. The formation mechanism of the distinctive structure in the ozone field will be discussed in the presentation.
A51F-10
Aerosol Properties Over the Eastern US in July 2002: Comparison of Long-Term Measurements With Results From a Coupled Meteorology-Chemistry Model
Elemental or black carbon aerosols absorb solar radiation and have direct and indirect effect on radiative forcing. Unfortunately, global EC data are rather sparse. Therefore, for planetary radiative forcing calculations atmospheric EC burdens are obtained from model estimates based on energy consumptions. In this work we have attempted to evaluate a multiscale integrated model for aerosol physics, chemistry and radiative effects using measurements of EC and sulfate at two sites in New York State. The EC and sulfate concentrations were measured for January, February, July and August 2002 at Mayville, and Whiteface Mountain. Our Mayville site is located near Lake Chautauqua, ~ 100 km southwest of Buffalo and ~ 530 km upwind of Whiteface Mountain. Our observatory at Whiteface Mountain is located at an altitude of 1.5 km above mean sea level. Whereas at Mayville samples were collected daily, the duration of sampling at Whiteface Mountain varied from 6 to 48 h. The samples were analyzed for EC using the thermal optical method and sulfate by ion chromatography. The observations of elemental carbon and sulfate from Whiteface and Mayville for the summer of 2002 have been used along with other network measurements from the IMPROVE and CASTNet networks to evaluate METCHEM, a tightly coupled meteorology-chemistry model in nested simulations over the U.S. to examine the radiative impacts of absorbing and scattering aerosols. The model is being driven by a high-quality bottom-up inventory of emissions compiled by the Regional Planning Organizations for their 2002 visibility assessments, and the National Emissions Inventories compiled previously by the U.S. EPA in the years 2002 and 1999. The model was used to compare both the effects of evolving emissions, and the successive improvements in characterizing wildfire emissions in each of these inventories. Results of aerosol concentrations and aerosol optical depths are compared for a 3-week test period of simulation using the RPO and EPA 2002 inventories.
A51F-11
On the Lagrangian Dynamics of Atmospheric Zonal Jets and the Permeability of the Stratospheric Polar Vortex
The Lagrangian dynamics of zonal jets in the atmosphere are considered, with particular attention paid to explaining why, under commonly encountered conditions, zonal jets serve as barriers to meridional transport. The velocity field is assumed to be two dimensional and incompressible, and composed of a steady zonal flow with an isolated maximum (a zonal jet) on which two or more traveling Rossby waves are superimposed. The associated Lagrangian motion is studied with the aid of the Kolmogorov–Arnold–Moser (KAM) theory, including nontrivial extensions of well-known results. These extensions include applicability of the theory when the usual statements of nondegeneracy are violated, and applicability of the theory to multiply periodic systems, including the absence of Arnold diffusion in such systems. These results, together with numerical simulations based on a model system, provide an explanation of the mechanism by which zonal jets serve as barriers to the meridional transport of passive tracers under commonly encountered conditions. Causes for the breakdown of such a barrier are discussed. It is argued that a barrier of this type accounts for the sharp boundary of the Antarctic ozone hole at the perimeter of the stratospheric polar vortex in the austral spring.
A51F-12
Implications of the Cyclostratigraphy of Jurassic Lacustrine Strata of the Hartford Rift Basin (CT and MA, USA) for the Time Scale of the Early Mesozoic
The Hartford basin of CT and MA contains at least 7 km of continental deposits formed during the Late Triassic and Early Jurassic. About 3 km of the middle part of the section beginning at the oldest flood basalt sequence of Jurassic age, part of the CAMP LIP, are characterized by fine-grained lacustrine strata that based on fourier analysis exhibit a hierarchy of sedimentary cycles characteristic of Milankovitch climate forcing. Cycles with periods of ~20, ~100, and 405 ky are present and a larger scale cyclicity with a period of 1.75 m.y. carries on from the Triassic pattern seen in continuous core from the 100-km distant cyclical Newark basin sequence. Six virtually complete 405 ky cycles are present, with the oldest one beginning close to the Triassic-Jurassic boundary. Based on paleomagnetic polarity stratigraphic correlation with the Moncornet core of the marine Paris basin (Yang et al., 1996 J.G.R. 101:8025), the youngest of these 400 ky cycles is of early Sinemurian or very latest Hettangian age, giving a duration for the Hettangian of approximately 2 m.y. Assuming an age of approximately 201.3 Ma for the basal lava flow (from Schoene et al, 2006, Geochem. Cosmochim. Acta 70:426), this gives a Hettangian-Sinemurian boundary age of about 199 Ma, with the precision limited by the biostratigraphy in the Moncornet core. This age is in agreement with the new high-precision U-Pb dates from marine sections (Pálfy & Mudil, 2007, Volumina Jurassica, IV:294), providing independent confirmation of the orbital forcing interpretation.