543 research outputs found
Dynamics of Atmospheric Regression Patterns: Regional Mountain Torque Events
No full textThe regression of atmospheric fields against a parameter P with lag τ is a standard procedure in meteorology. Here, the torque exerted by a mountain massif is chosen as a parameter in order to study the interaction of weather systems with orography on a statistical basis. It is normally found that the amplitudes of the correlation patterns increase with τ → 0 and decrease for increasing positive lag. It is proposed to explain this ubiquitous feature in the orographic case on the basis of the covariance equations that govern these regressions. Two examples are discussed. First, a version of the low-order Charney–DeVore model of β-plane flow over a mountain is considered where stochastic forcing stirs a Rossby wave mode. It is found that the general increase of covariance amplitudes for τ → 0 (if it occurs) is mainly due to the forcing, but triple covariances of mountain torque and vorticity advection are important as well. A new covariance energy equation is derived to demonstrate that the frictional decay for τ > 0 is supported by these triple covariances while the stationary wave acts as a source for τ > 0. A dynamical interpretation of the triple terms is given. Next, data from the ECMWF 40-yr Re-Analysis (ERA-40) set are used to study mountain torque events in winter near Greenland, where the covariances of all standard variables with the torque P exhibit a rapid quasi-barotropic increase with τ → 0 near Greenland. This amplification process is investigated by looking at the barotropic vorticity equation adapted to this statistical problem. This equation captures the evolution of the regression patterns reasonably well in the range -2 ≤ τ ≤ 2 days. The triple covariances of torque and nonlinear vorticity advection play the key role in the amplification process. In particular, covariance enstrophy is generated and destroyed by these terms, a process without counterpart in the standard vorticity equation. Stochastic forcing is presumably unimportant. The interpretation of the triple terms is difficult in contrast to that of the other “linear” terms of the vorticity equation. The angular momentum in the Greenland domain decreases during events of positive torque
Stratosphere-troposphere exchange: An angular momentum perspective
No full textThe exchange of axial angular momentum (AM) between stratosphere and troposphere forms an important part of stratosphere–troposphere exchange (STE). Data are used to quantify the related fluxes through an idealized tropopause for specific flux events both for the global tropopause and regional segments. Time mean fluxes are considered as well. These are largest in the tropics and closely tied to mass fluxes. The data accuracy is found to be insufficient for a closure of the momentum budget of the Brewer–Dobson circulation. Positive regional flux events at the tropical tropopause are partly due to a short lived intensification of the Hadley circulation, but are also embedded in a rather long lived circulation anomaly which leads to a gain of positive AM in the lower tropical stratosphere. Moreover, there are large fluxes through the ‘tropopause breaks’. Flux events at midlatitude segments involve also strong horizontal fluxes through the ‘tropopause breaks’, which override an otherwise downward flux from the stratosphere. These events are closely linked to tropical anomalies of AM, so that all ‘regional’ events turn out to be of global character. Data accuracy becomes a problem when fluxes through a global tropopause are investigated where substantial uncertainties are encountered near the ‘tropopause breaks’.
All cases discussed exhibit a distinct similarity with deep anomalies of AM in the tropics sandwiched between anomalous AM columns of opposite sign to the north and south. There is always a center of flux convergence or divergence in the lower stratosphere. This suggests that all the events of large scale-angular momentum exchange between stratosphere and troposphere as analysed here follow a distinct dynamic pattern, which appears not to have been described before
Iberian thermal lows in a changed climate
No full textRecent results from enhanced greenhouse-gas scenarios over southern Europe suggest that climate change implies a summertime mean surface warming of up to 4 K over the Iberian Peninsula. It is suggested that thermal lows, the most prominent Iberian summer weather, will undergo significant modifications in a changed climate. This is studied in the present paper by determining statistical features of the Iberian thermal low, at first for today's climate using data provided by ERA-40 and by the regional climate model PROMES. In general the analysed and simulated climatic structures compare well. However, the ERA-40 data provide conditions more favourable for thermal lows than simulated with the model. Statistics in a changed climate are obtained by performing two 30-year model simulations for 1961-1990 and 2071-2100. It is found that thermal lows strengthen by decreasing the central Iberian surface pressure from 1012.2 to 1010.5 hPa and increasing its daily variation by about 7% in magnitude. Associated with this is an increase in the number of thermal low days by more than 60%. A second finding is an enhancement of the sea breezes over the Iberian westerly and easterly coasts and a particularly significant strengthening of the sea breeze between Cadiz and Huelva. The westerly flow over the Strait of Gibraltar, observed during thermal low events, decreases. The increase in occurrence of the thermal low is associated with a significant increase in the number of dry spells of longer period, and could involve considerable impact on desertification, water resources and wildfire hazards around the Mediterranean basin. The results suggest that there is some evidence that the subtropical character of the Mediterranean climate strengthens
Downward Control from Lower Stratosphere?
No full textThe concept of downward control proposes a mechanism for the impact of the stratospheric circulation on the troposphere. Momentum forcing at upper-stratospheric levels induces a meridional circulation that eventually reaches the surface. So far, a lack of sufficiently accurate data hindered an observational test of this downward propagation. The concept is extended in this paper by looking at the effect of angular momentum forcing in prescribed regions in the lower stratosphere on the tropospheric circulation. In that
case, the European Centre for Medium-Range Weather Forecasts Reanalysis Project (ERA) data can be used to investigate the atmospheric response to forcing in a prescribed domain. It is found that these forcing events are quite short lived and that angular momentum flux convergence in the prescribed domain is highly correlated with convergence outside this forcing area. Typically, these fields of convergence and also divergence extend to the surface in a quasibarotropic manner outside the Tropics. This structure of the forcing is not compatible with the assumptions of the downward control concept. The observed related meridional circulation therefore differs widely from that predicted. In particular, there is no obvious descent of the circulation to the ground. Even so, such forcing events are accompanied by an intensive exchange of angular momentum between stratosphere and troposphere. The confinement of the forcing to the selected forcing domain is reasonably strict in the Tropics. A relatively narrow tongue of angular momentum is growing at the equator underneath the forcing area. Frictional torques play a role in this development.
Altogether, the forcing events as selected involve a strong angular momentum exchange between stratosphere and troposphere but are not suited for a test of the downward control concept. Alternatives are discussed
Quantum Anomaly and 2D-3D Crossover in Strongly Interacting Fermi Gases
No full textWe present an experimental investigation of collective oscillations in harmonically trapped Fermi gases through the crossover from two to three dimensions. Specifically, we measure the frequency of the radial monopole oscillation or breathing mode in highly oblate gases with tunable interactions. The breathing mode frequency is set by the adiabatic compressibility and probes the thermodynamic equation of state. In 2D, a dynamical scaling symmetry for atoms interacting via a δ potential predicts the breathing mode to occur at exactly twice the harmonic confinement frequency. However, a renormalized quantum treatment introduces a new length scale which breaks this classical scale invariance resulting in a so-called quantum anomaly. Our measurements deep in the 2D regime lie above the scale-invariant prediction for a range of interaction strengths providing evidence for the quantum anomaly and signifying the breakdown of an elementary δ-potential model of atomic interactions. By varying the atom number we can tune the chemical potential and see the breathing mode frequency evolve smoothly between the 2D to 3D thermodynamic limits.T. Peppler, P. Dyke, M. Zamorano, I. Herrera, S. Hoinka, and C. J. Val
Cirren und Kondensstreifen am 18.10.89 (ICE '89): Ermittlung von Höhe und optischer Dicke mit ALEX, AVHRR und HIRS
No full textMountain torques and synoptic systems in the Mediterranean
No full textThe mountains surrounding the Mediterranean exert torques T during the passage of North Atlantic systems which affect the angular momentum of the airflow passing over and around the massifs. The Alps, the Atlas range and the
orographic block of Asia Minor are selected to investigate the typical flow conditions during torque events. These mountain ranges are small enough to justify a local angular momentum analysis. Both the zonal and the meridional components of a mountain’s torque (Tλ and Tϕ) are used as stratification parameters in a statistical investigation of the interaction of large-scale perturbations with this mountain. How are these flows affected by the obstacle? A simple scheme is tested which attempts to interpret results.
The torque analysis singles out eastward-moving large-scale systems. Their isobars are oriented from southwest (northwest) to northeast (southeast) near the mountain in zonal torque Tλ (Tϕ) cases. The massifs tend to generate a low-level distortion of the pressure field such that the angular momentum of the flow over the mountain is reduced. These results can be explained within the framework of the scheme. The influence of the mountains on the pressure field is seen only at heights ≤4000 m. The low-level distortions of the pressure field contribute positively to the total torque for lags τ ≤ 0 in the Alps and for all lags −2 ≤ τ ≤ 2 days in Asia Minor, where only Tλ is evaluated. The impact of the Atlas mountains is seen only at τ = 0
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