Atmosphere In Middle Latitudes Research Articles

Dynamic stability of a jet near a transition in static stability

The vertical profile of the Earth's atmosphere in middle latitudes contains a sharp transition region between two roughly constant stability layers, termed the tropopause and also exhibits jet streams at nearly the same altitude, with the jet stream core possibly above or below the tropopause, depending on time and location. This proximity of the jet to the tropopause would be expected to greatly affect the dynamic stability of the jet, treated here with the jet modelled with the Bickley profile and the tropopause modelled as a smooth transition region with a tanh profile. Stability results are obtained numerically using a Chebyshev collocation spectral method. The results show that the jet becomes more unstable as it is moved further beneath the tropopause. Corresponding two- and three-dimensional nonlinear simulations of the flow confirm the initial growth, and indicate that when a jet is above the tropopause, the configuration is more stable and more likely to produce a strong single unstable mode. The simulations indicate that this instability will grow to form a solitary wave envelope pattern. Conversely, when a jet is below the tropopause, the jet is more likely to form a broad spectrum of motion.

Numerical solutions of the singular vortex problem

This study develops a finite-difference numerical formulation to describe the motion of a singular monopole in a quasigeostrophic β-channel model with scale-selective frictional damping, using parameter values typical for the middle-latitude atmosphere and a wide range of viscosities. In this model, the “theoretical” singular vortex is replaced by the equivalent nonsingular vortex of a finite amplitude, consistent with the finite spatial resolution of the model. Numerical experiments demonstrate that at initial stages of the singular-vortex (SV) evolution, this model accurately reproduces the behavior expected from the theoretical considerations of the inviscid case. The numerical model also approximately conserves several invariants of motion derived from the continuous equations and accurately represents their modifications in the presence of friction. The evolution of a singular cyclone in the Northern Hemisphere starts with the development of the dipolar β gyres in the regular component of the flow; these gyres induce initial northward displacement and subsequent westward bending of the cyclone trajectory. At larger times, the β gyres gradually disintegrate, and the singular cyclone in the Northern Hemisphere continues to move northwestward by forming a dipolelike system with the localized secondary regular-field anticyclone northeast of the singular-cyclone center resulting eventually in a friction-assisted steady-state regime. The SV trajectories tend to become more zonally elongated for large vortices and small values of viscosity. These results lay a foundation for numerical consideration of systems of multiple singular vortices, which could provide further insights into our fundamental understanding of the processes underlying the multiscale atmospheric and oceanic variability.

On the winter anomaly of the night-to-day ratio of ozone in the middle to upper mesosphere in middle to high latitudes

Long-term measurements of ozone by means of the microwave technique performed at Lindau (51.66°N, 10.13°E), Germany, revealed a winter anomaly of the night-to-day ratio (NDR) which is more clearly pronounced as the so-called tertiary nighttime ozone maximum. The domain of occurrence also differs somewhat from that of the nighttime ozone enhancement. The maximum winter-to-summer ratio amounts to a value of two to three in 70 km height. The annual variation of the NDR is modulated by oscillations of planetary time scale. 3D-calculations on the basis of the advanced GCM LIMA essentially reflect the observations but also show some typical differences which probably result from a somewhat too humid model atmosphere in middle latitudes. We analyzed the most important impacts on the middle mesospheric ozone. The strongest impacts are connected with the annual variation of water vapor and the so-called Doppler-Sonnemann effect considering the influence of the zonal wind on the chemistry due to the fact that ozone is subjected to an effective dissociation longer than molecular oxygen for an increasing solar zenith angle. Because of that the net odd oxygen production decreases faster than the formation of atomic oxygen from ozone which is involved in an odd oxygen destructing catalytic cycle. A shortening of the time of sunset by a west wind regime increases the nighttime ozone level relatively, whereas the daytime ozone is less influenced by the zonal wind in the domain considered.

A Model of Decadal Middle-Latitude Atmosphere–Ocean Coupled Modes

An analytical model of the mutual interaction of the middle-latitude atmosphere and ocean is formulated and studied. The model is found to support coupled modes in which oceanic baroclinic Rossby waves of decadal period grow through positive coupled feedback between the thermal forcing of the atmosphere induced by SST anomalies and the resulting wind stress forcing of the ocean. Growth only occurs if the atmospheric response to thermal forcing is equivalent barotropic, with a particular phase relationship with the underlying SST anomalies. The dependence of the growth rate and structure of the modes on the nature of the assumed physics of air–sea interaction is explored, and their possible relation to observed phenomena discussed.

Regional Models of the Atmosphere in Middle Latitudes

This review describes recent development in operational and research limited-area numerical weather prediction models in middle latitudes. The current skill of limited-area models is summarized through the use of conventional measures of verification such as S1 scores, root-mean-square errors and correlations between forecast and observed changes. Additional measures of verification, which measure the skill or realism of regional models in reproducing atmospheric structure on those scales, are discussed. Use of a uniform set of verification measures such as those discussed here would facilitate model comparisons and assessment of the impact of changes in model components on short-range (0–48 h) forecasts. Three major components of regional models are discussed. These include numerical aspects (e.g., the grid structure, boundary conditions and the approximations to the analytic differential equations), physical aspects (modeling surface and boundary layer processes, condensation and evaporation, and radiation) and the analysis and initialization procedure. The paper emphasizes the impact of these components on the forecast rather than the details of each component. The main conclusion of the paper is that further increases in the overall skill of operational regional forecasts are likely to occur through improvements in all the components of limited-area models. Improvements in various components developed and tested in research models are currently being incorporated in several operational models, and some modest but significant improvements in regional forecast skill are likely over the next five years.

On the General Circulation of the Atmosphere in Middle Latitudes: Southern and Northern Hemispheres Compared

On the General Circulation of the Atmosphere in Middle Latitudes: Southern and Northern Hemispheres Compared

On the General Circulation of the Atmosphere in Middle Latitudes

SUMMARY A synoptic, theoretical and experimental study of the general circulation of the atmosphere in middle latitudes and of the major perturbations superimposed on this circulation pattern was conducted by a group of research workers at the University of Chicago during the academic year 1946–1947. The principal results of these investigations are summarized below in a series of specific statements. Not all of these conclusions are new, and some require further verification, but it is hoped that they may be of some use as a starting point for further discussions and investigations of the general circulation of the atmosphere.