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Static Stability (static + stability)
Selected AbstractsPrecipitation characteristics of the Eurasian Arctic drainage systemINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 11 2003Mark C. Serreze Abstract This study examines characteristics of precipitation over the major watersheds of the Eurasian Arctic drainage system over the period 1960,92. In addition to the Ob, Yenisey and Lena (the three largest drainage systems), we examine the combined Kolyma,Indigirka in eastern Eurasia. Each basin exhibits approximately symmetric mean annual cycles of monthly total precipitation and daily event size, with winter minima and July maxima. These are strikingly similar to the annual cycles of total column water vapour (precipitable water), which fundamentally reflects the control on saturation vapour pressure by temperature. Effective precipitation mechanisms exist in all seasons. However, because of the long distance from strong moisture sources (continentality), precipitation tends to follow the seasonality in column water vapour. An effective contrast is presented for the Iceland sector. Here, the annual cycle of precipitation is tied not to the seasonality in column water vapour, but to the stronger precipitation-generating mechanisms in winter. Hence, the annual cycles of precipitation and column water vapour in this region oppose each other. Mean winter precipitation over the Eurasian watersheds is primarily driven by a modest convergence of water vapour. Whereas precipitation peaks in summer, the mean flux convergence exhibits a general minimum (negative in the Ob). Summer precipitation is hence primarily associated with surface evaporation. A strong role of convection is supported from consideration of static stability, the fairly weak spatial organization of precipitation totals and results from prior studies. On daily time scales, the largest basin-averaged precipitation events, for both summer and winter, are allied with synoptic-scale forcing. This is seen in relationships with cyclone frequency, and patterns of 500 hPa height, vertical motion and the 700 hPa vapour flux. The relative frequency of four 500 hPa synoptic types captures the basic time series structures of precipitation. Copyright © 2003 Royal Meteorological Society [source] Internal wave drag in stratified flow over mountains on a beta planeTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 630 2008M. A. C. Teixeira Abstract The impact of the variation of the Coriolis parameter f on the drag exerted by internal Rossby-gravity waves on elliptical mountains is evaluated using linear theory, assuming constant wind and static stability and a beta-plane approximation. Previous calculations of inertia-gravity wave drag are thus extended in an attempt to establish a connection with existing studies on planetary wave drag, developed primarily for fluids topped by a rigid lid. It is found that the internal wave drag for zonal westerly flow strongly increases relative to that given by the calculation where f is assumed to be a constant, particularly at high latitudes and for mountains aligned meridionally. Drag increases with mountain width for sufficiently wide mountains, reaching values much larger than those valid in the non-rotating limit. This occurs because the drag receives contributions from a low wavenumber range, controlled by the beta effect, which accounts for the drag amplification found here. This drag amplification is shown to be considerable for idealized analogues of real mountain ranges, such as the Himalayas and the Rocky mountains, and comparable to the barotropic Rossby wave drag addressed in previous studies. Copyright © 2008 Royal Meteorological Society [source] Lessons on orographic precipitation from the Mesoscale Alpine ProgrammeTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 625 2007Richard Rotunno Abstract Although moisture-laden airflow towards a mountain is a necessary ingredient, the results from the Mesoscale Alpine Programme (MAP) demonstrate that detailed knowledge of the orographically modified flow is crucial for predicting the intensity, location and duration of orographic precipitation. Understanding the orographically modified flow as it occurs in the Alps is difficult since it depends on the static stability of the flow at low levels, which is heavily influenced by synoptic conditions, the complex effects of latent heating, and the mountain shape, which has important and complicated variations on scales ranging from a few to hundreds of kilometres. Central themes in all of the precipitation-related MAP studies are the ways in which the complex Alpine orography influences the moist, stratified airflow to produce the observed precipitation patterns, by determining the location and rate of upward air motion and triggering fine-scale motions and microphysical processes that locally enhance the growth and fallout of precipitation. In this paper we review the major findings from the MAP observations and describe some new research directions that have been stimulated by MAP results. Copyright © 2007 Royal Meteorological Society [source] Effects of shear sheltering in a stable atmospheric boundary layer with strong shearTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 596 2004Ann-Sofi Smedman Abstract Data from two marine field experiments in the Baltic Sea with stable stratification have been analysed. The purpose was to test the concept of the ,detached' or ,top-down' eddies and the ,shear-sheltering' mechanism in the presence of a low-level wind speed maximum in the atmosphere. Data used include turbulence and profile measurements on two 30 m towers and concurrent wind profiles throughout the boundary layer obtained from pilot-balloon soundings. Measurements show that large eddies are being suppressed when there is a low-level wind speed maximum present somewhere in the layer 40,300 m above the water surface and when the stratification is slightly stable. The effect is seen both in normalized standard deviations of the velocity components and in corresponding component spectra. In previous work it was argued that the relatively large eddies, which dominate the low wave number spectra in measurements in the surface layer, are detached or top-down eddies generated higher up in the boundary layer, that interact with the surface layer. The low-level wind maximum introduces a distinct layer with strong vorticity which, according to the shear-sheltering hypothesis, prevents these eddies from penetrating downwards. In the limit of the wind maximum occurring at a very low height (less than about 100 m), usual turbulence statistics characteristic of the ,canonical' boundary layer are found. Combining all the statistics, it is demonstrated that the wavelength of maximum spectral energy is locally related to a turbulence length-scale, which shows that for values of the Richardson number of unity or less the effect of the local wind gradient is greater than that of static stability. The reduction of length-scale with the strength of a low-level wind maximum, explains the observed reduction (by a factor of two) of the turbulent flux of sensible heat at the surface. This result indicates that the shear-sheltering mechanism is likely to play an important role in the turbulent exchange process at the surface in sea areas where low-level wind maxima are a frequently occurring phenomenon, such as the Baltic and other large water bodies surrounded by landmasses. Copyright © 2004 Royal Meteorological Society [source] Diagnostic and prognostic equations for the depth of the stably stratified Ekman boundary layerTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 579 2002Sergej Zilitinkevich Abstract Refined diagnostic and prognostic equations for the depth of the stably stratified barotropic Ekman boundary later (SBL) are derived employing a recently developed non-local formulation for the eddy viscosity. In well-studied cases of the thoroughly neutral SBL, the nocturnal atmospheric SBL and the oceanic SBL dominantly affected by the static stability in the thermocline, the proposed diagnostic equation reduces to the Rossby,Montgomery, Zilitinkevich and Pollard,Rhines,Thompson equations, respectively. In its general form it is applicable to a range of regimes including long-lived atmospheric SBLs affected by the near-surface buoyancy flux and the static stability in the free atmosphere. Both diagnostic and prognostic SBL depth equations are validated against recent data from atmospheric measurements. Copyright © 2002 Royal Meteorological Society. [source] Thermal versus dynamical tropopause in upper-tropospheric balanced flow anomaliesTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 562 2000V. Wirth Abstract This paper systematically investigates differences between the thermal and the dynamical tropopause for upper-tropospheric balanced flow anomalies. Idealized cyclonic and anticyclonic anomalies are considered, which are either axisymmetric or plane symmetric. Given a distribution of potential vorticity (PV), the inversion problem is solved numerically to obtain the corresponding balanced flow (i.e. wind and temperature). The control parameter is the aspect ratio of the PV anomaly, which governs the partitioning into a thermal and a dynamical anomaly. For PV anomalies of intermediate and tall aspect ratios, the location of the thermal tropopause differs significantly from the location of the dynamical tropopause. The thermal tropopause is rather indistinct for intermediate aspect ratios, while it is sharp and well defined for both tall and shallow anomalies. A barotropic deformation flow field superimposed on a plane symmetric anomaly induces an ageostrophic wind which modifies the static stability throughout the PV anomaly such that the thermal and dynamical tropopauses evolve differently. Recent observations concerning the correlation between the thermal and ozone tropopauses can be interpreted consistently in terms of the present results. [source] | ||||||||||