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Middle Atmosphere (middle + atmosphere)
Selected AbstractsLatitudinal height couplings between single tropopause and 500 and 100 hPa within the Southern HemisphereINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 4 2010Adrián E. Yuchechen Abstract In order to provide further insights into the relationships between the tropopause and different mandatory levels, this paper discusses the coupling between standardized tropopause height anomalies (STHAs) and standardized 500-hPa and 100-hPa height anomalies (S5HAs and S1HAs, respectively) within the ,climatic year' for three sets of upper-air stations located approximately along 20°S, 30°S and 45°S. Data used in this research consists in a radiosonde database spanning the period 1973,2007. The mandatory levels are supposed to be included in each radiosonde profile. The tropopause, on the other hand, is calculated from the significant levels available for each sounding using the lapse rate definition. After applying a selection procedure, a basic statistical analysis combined with Fourier analysis is carried out in order to build up the standardized variables. Empirical orthogonal functions (EOFs) in S-mode are used to get the normal modes of oscillation as well as their time evolution, for STHA/S5HA as well as for STHA/S1HA coupling, separately, within the aforementioned latitudes. Overall, there are definite cycles in the time evolution associated with each EOF structure at all three latitudes, the semi-annual wave playing the most important role in most of the cases. Nevertheless, 20°S seems to be the only latitude driven by diabatic heating cycles in the middle atmosphere. Certainly, EOF1 at this latitude has a semi-annual behaviour and seems to be strongly influenced by the tropical convection seasonality. Apparently, the convectively driven release of latent heat in the middle troposphere affects the time evolution of the EOF1 structure. By contrast, the vertical propagation of planetary waves is raised as a possible explanation for the EOF1 and EOF2 behaviour at latitudes beyond 20°S, in view of the close connection existent between the semi-annual oscillation (SAO) and the reversion in the direction of the zonal wind. Copyright © 2009 Royal Meteorological Society [source] Recent developments in gravity-wave effects in climate models and the global distribution of gravity-wave momentum flux from observations and modelsTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 650 2010M. J. Alexander Abstract Recent observational and theoretical studies of the global properties of small-scale atmospheric gravity waves have highlighted the global effects of these waves on the circulation from the surface to the middle atmosphere. The effects of gravity waves on the large-scale circulation have long been treated via parametrizations in both climate and weather-forecasting applications. In these parametrizations, key parameters describe the global distributions of gravity-wave momentum flux, wavelengths and frequencies. Until recently, global observations could not define the required parameters because the waves are small in scale and intermittent in occurrence. Recent satellite and other global datasets with improved resolution, along with innovative analysis methods, are now providing constraints for the parametrizations that can improve the treatment of these waves in climate-prediction models. Research using very-high-resolution global models has also recently demonstrated the capability to resolve gravity waves and their circulation effects, and when tested against observations these models show some very realistic properties. Here we review recent studies on gravity-wave effects in stratosphere-resolving climate models, recent observations and analysis methods that reveal global patterns in gravity-wave momentum fluxes and results of very-high-resolution model studies, and we outline some future research requirements to improve the treatment of these waves in climate simulations. Copyright © 2010 Royal Meteorological Society and Crown in the right of Canada [source] Gravity wave drag estimation from global analyses using variational data assimilation principles.THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 609 2005I: Theory, implementation Abstract A novel technique to estimate gravity wave drag from global-scale analyses is presented. It is based on the principles of four-dimensional variational data assimilation, using a dynamical model of the middle atmosphere and its adjoint. The global analyses are treated as observations. A cost function that measures the mismatch between the model state and observations is defined. The control variables are the components of the three-dimensional gravity wave drag field, so that minimization of the cost function gives the optimal gravity wave drag field. The minimization is performed using a conjugate gradient method, with the adjoint model used to calculate the gradient of the cost function. In this work, we present the theory behind the new technique and evaluate extensively the ability of the technique to estimate the gravity wave drag using so-called twin experiments, in which the ,observations' are given by the evolution of the dynamical model with a prescribed gravity wave drag. The results show that the technique can estimate accurately the prescribed gravity wave drag. When the cost function is suitably defined, there is good convergence of the minimization scheme under realistic atmospheric conditions. We also show that the cost function gradient is well approximated taking into account only adiabatic processes. We note some limitations of the technique for estimating gravity wave drag in tropical regions if satellite temperature measurements are the only observational information available. Copyright © 2005 Royal Meteorological Society. [source] The parametrization of drag induced by stratified flow over anisotropic orographyTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 568 2000J. F. Scinocca Abstract A new parametrization of drag arising from the flow over unresolved topography (UT) in a general-circulation model (GCM) is presented. It is comprised of three principle components: a parametrization of the source spectrum and drag associated with freely propagating hydrostatic gravity waves in the absence of rotation, a parametrization of the drag associated with low-level wave breaking, and a parametrization of low-level drag associated with upstream blocking and lee-vortex dynamics. Novel features of the scheme include: a new procedure for defining the UT in each GCM grid cell which takes account of the GCM resolution and includes only the scales represented by the parametrization scheme, a new method of representing the azimuthal distribution of vertical momentum flux by two waves whose direction and magnitude systematically vary with the flow direction and with the anisotropy of the UT in each GCM grid cell, and a new application of form drag in the lowest levels which can change the direction of the low-level flow so that it is more parallel to unresolved two-dimensional topographic ridges. The new scheme is tested in the Canadian Centre for Climate Modelling and Analysis third generation atmospheric GCM at horizontal resolutions of T47 and T63. Five-year seasonal means of present-day climate show that the new scheme improves mean sea level pressures (or mass distribution) and improves the tropospheric circulation when compared with the gravity-wave drag scheme used currently in the GCM. The benefits are most pronounced during northern hemisphere winter. It is also found that representing the azimuthal distribution of the momentum flux of the freely propagating gravity-wave field with two waves rather than just one allows 30-50% more gravity-wave momentum flux up into the middle atmosphere, depending on the season. The additional momentum flux into the middle atmosphere is expected to have a beneficial impact on GCMs that employ a more realistic representation of the stratosphere. [source] | ||||||||||