Atmospheric Sciences II
Presiding: M A Geller, Stony Brook University; M Alexander, NorthWest Research Associates
A12A-01 10:30h
Middle Atmosphere Nonlinear Hadley Circulation Features Inferred From Physical Considerations and Scaling of the Equations
There has been renewed interest in the middle atmosphere nonlinear Hadley circulation, given the signature of such a circulation in the tropical upwelling circulation diagnosed from both thermodynamics and the evolution of satellite-observed patterns of long-lived chemical tracers. The numerical solution for the middle atmosphere nonlinear Hadley circulation is quite complex, but it is possible to derive the main features of this circulation from the essential physics and relatively simple scaling considerations. Here, we derive the approximate magnitude, latitudinal width, and the nature of the latitudinal tilt in this manner.
A12A-02 10:45h
Evaluation of the Mean Climate as Simulated by Coupled Model Simulations Performed for the Fourth Assessment of the IPCC
Evaluation of the C20C and control runs in this new data set will provide an important foundation for the analysis of the various scenario simulations. We intend to apply simulation summary statistics developed at PCMDI (including already published and new metrics) to examine global and large scale climatological features of the coupled control runs being run for AR4. While emphasis will be on the simulated atmosphere, basic characteristices of the ocean will also be examined. A diverse collection of observationally-based reference data will be utilized. Taylor and Portrait Plots, both used in the TAR, will be revisited and expanded, comparing models with each other and available observations. These simulations will also be compared with those in the CMIP2+ database, and the newly submitted AMIP simulations. Virtually all of the monthly mean database for which we have observational estimates will be utilized.
A12A-03 11:00h
The Gravity Wave Response to Observed Fine-Scale Latent Heating Structure
Gravity waves generated by convection are important to the global circulation in the stratosphere and above, and may also play a role in initiation of subsequent remote convective events. We are studying wave emission from convective rain cells by forcing a mesoscale cloud resolving model with observed precipitation patterns from radar. The wave forcing is known to be sensitive to the details of the latent heating, and the radar data provide the most realistic possible description of these details. Our model has been previously validated with other observations from the Darwin Area Wave Experiment (DAWEX). The waves generated in our model are analyzed and compared to linear models of wave generation and wave propagation. Parameterizations of convectively generated waves are based on linear models of heat sources in different shear environments. We compare the mesoscale model to the Beres et al. (2004) source parameterization for convectively generated waves. We find descrepancies that suggest improvements to the parameterization, namely inclusion of a forcing term due to the so-called obstacle effect, where the heating centers act as obstacle to shear flow in the upper troposphere. We also find interesting wave reflection effects in the upper troposphere which are less significant to the momentum budget and parameterization problem, but which lead to trapped waves that may be relavent to remote convective initiation. We study the wave reflection by comparison of the mesoscale model to linear wave propagation and tunneling models that suggest more than half of the wave energy is trapped in the troposphere at certain wave phase speeds.
A12A-04 11:15h
Observational Evidence of Critical-Level Filtering of Mountain Waves
Falling sphere wind and temperature data from the winter campaign (January 2003) of NASA's MaCWAVE (Mountain and Convective Waves Ascending Vertically) project are analyzed to investigate gravity wave characteristics in the stratosphereand mesosphere. We focus on the data from two salvos of rockets launched over Kiruna, Sweden (67.9 N, 21.1 E), which is located at the lee side of the major mountain ranges in the Scandinavia Peninsula. It is found that gravity wave amplitudes maximize at about 48-50 km and 60 km for salvo 1 and salvo 2, respectively, whereas the waves' vertical scales minimize at the above mentioned altitudes, respectively. Also, the altitudes of the wave amplitude maxima and verticalscale minima correspond roughly to where the background winds are close to zero for both salvos. Such a coherence of the locations of wave amplitude maxima, wave vertical scale minima and zero background winds agrees with the theoretical expectation of the behavior of a mountain wave approaching its critical level(i.e., the altitude at which the background winds are zero), suggesting that the waves are most likely mountain waves. Also, the wave's horizontal propagation directions are analyzed using the S-transform method in this study. Compared to the conventional Stokes method, the new approach is found to give better estimate of gravity wave propagation directions, especially when there is a mixture of waves.
A12A-05 11:30h
Wave driven quasi-biennial and annual equatorial oscillations in the zonal circulation as potential amplifiers of UV solar cycle influence on the lower atmosphere
We discuss a 3D global modeling study that describes the gravity wave (GW) driven quasi-biennial and annual equatorial oscillations under the influence of solar cycle (SC) UV variations. For a SC period of 10 years, the amplitude of the variations of radiative forcing is taken to vary from 0.2% at the surface to 2% at 50 km to 20% at 100 km and above. Applying spectral analysis to filter out and identify the SC signatures, this model produces two distinct dynamical phenomena. (1) A relatively large modulation is generated in the Quasi-biennial Oscillation (QBO) of the lower stratospheric zonal circulation, which is in qualitative agreement with the results obtained by Salby and Callaghan (2000) who analyzed zonal wind observations covering more than 40 years. The modeled SC modulation of the QBO extends to high latitudes where it produces temperature variations of < 1 K in the troposphere. (2) Modulated by the SC, a hemispherically symmetric Annual Oscillation (AO) is generated in the zonal winds, which is largely confined to low latitudes. Under the influence of the GWs, this AO propagates down into the lower stratosphere like the QBO. As is the case for the QBO, the energy of this tropical AO is partially redistributed by the meridional circulation and planetary waves, presumably, to generate measurable SC signatures in the tropospheric temperature of the polar regions, which may be related to the so called Arctic Oscillation (Thompson and Wallace, 1998). Because of the symmetry of this AO, and the QBO, the resulting SC signatures in the northern and southern hemispheres are significantly different. We tentatively conclude that the wave mean flow interactions at equatorial latitudes appear to pull down, through the QBO and "symmetric" AO, the UV solar cycle influence of the middle atmosphere and thereby amplify effectively the SC effect at lower altitudes.
A12A-06 11:45h
Exploring the Relationships Between Snowpack and Modes of Atmospheric Circulation Over North America
The relationships between snowpack, the Pacific North American pattern (PNA), North Atlantic Oscillation (NAO) and El-Nino Southern Oscillation (ENSO) are examined over the course of North American winters from 1980-1997, utilizing a gridded SWE dataset developed by Brown and Brasnet, of the Canadian Cryospheric Network. The present research examines the statistical significance of the relationships between these three patterns of atmospheric variability and SWE (snow water equivalent) from 35° N to 55° N over entire extent of the North American continent. Seasonal (JFM) SWE values are correlated to seasonal mean teleconnection indices. Regions of significant negative correlations between the indices and SWE are found over the Pacific Northwest, Southern Plains of Canada and Great Lakes region. The NAO shows some locally significant positive correlations over the Northern Plains and Rocky Mountains. ENSO is significantly correlated to SWE over the Southwestern U.S. PNA shows significant negative correlations over Western New England. Other areas of significance exist but must be treated cautiously due to small numbers of observation stations in these regions. All correlations were performed using Spearman's Ranked method. Composite analysis is employed to better understand the relationships between indices and the winter (JFM) snowpack. PNA positive(negative), NAO positive(negative) and ENSO positive(negative) years were tested for significance, using the t-test, against all other years to determine a relationship between highly positive(negative) events and SWE. Multiple cutoffs were used in determining positive(negative) years. Resulting areas of significance generally agreed well with regions from the correlation analysis. Composite analysis results also suggest that there may be some potential for predictive skill over certain regions for years above(below) a certain index threshold. SWE has a significant impact on spring snow-melt runoff, water resource availability and may be an indicator for summer temperature variability. Understanding the relationships explored herein will aid in water resources planning and research, improve our understanding of climate variability and the treatment of teleconnections in climate and hydrological models, and may contribute to climate prediction efforts.