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Upper Troposphere (upper + troposphere)
Selected AbstractsThe COST 723 ActionTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue S2 2007W. A. Lahoz Abstract An overview is provided of the COST 723 Action, ,Data Exploitation and Modelling of the Upper Troposphere and Lower Stratosphere'. The three working groups are introduced and a summary of Action activities within them is provided. The achievements of the Action are: three international workshops; the LAUTLOS humidity measurement campaign; dedicated meetings to discuss the quality of upper troposphere/lower stratosphere ozone and humidity measurements; two journal special issues; more than 90 papers in the peer-reviewed literature; one international summer school; and a successor COST Action which builds on COST 723. The recommendations made are: for COST to continue to support the short-term scientific missions instrument, as they are perceived to be value for money; to encourage the use of COST money to increase links between COST Actions and other scientific communities; and for the COST secretariat to recommend that Actions consider a summer school instead of a final workshop or meeting. Copyright © 2007 Royal Meteorological Society [source] The spatial pattern of summertime subtropical anticyclones over Asia and Africa: A climatological reviewINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 2 2010Azar Zarrin Abstract The National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) monthly mean reanalysis dataset has been used to analyze spatial variations of summertime subtropical anticyclones over the Asia,Africa region. The geopotential height and zonal wind components of 1000, 500, 200, and 100 hPa in a 30-year period (1971,2000) have been used to determine the spatial and temporal variations of the anticyclone centres, their monthly frequency and latitudinal axis variations during April,October. The results revealed that there is a clear difference in the location of the summer anticyclone centres in lower, middle and upper levels of the troposphere. In the lower levels, the Azores subtropical anticyclone is located at the east of North Atlantic. In the middle levels, the frequencies of anticyclone centre are concentrated over the northwest of Africa, Arabian Peninsula and Iranian Plateau. In the upper troposphere, the geographical location of the anticyclone centres and their frequencies in the summer season exhibit a scattered pattern from south of China up to western Iran at 200 hPa, and a bimodal pattern over the Tibetan and the Iranian Plateaus at 100 hPa. In fact, in the entire study domain, the Iranian Plateau is a preferable location of the middle and upper troposphere anticyclones. The highest observed latitude of the subtropical anticyclone at 100, 200 and 500 hPa levels have been seen over north of Tibetan plateau, a large area from east to west of Asia and Iran during August, July,August and July, respectively. The maximum monthly variation in the latitude of the ridgeline is seen at 500, 200, and 100 hPa from June to July which goes even up to 10 degrees at some longitudes. Copyright © 2009 Royal Meteorological Society [source] Simple radiative models for surface warming and upper-troposphere coolingINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 11 2009P. N. Keating Abstract A simple model of greenhouse-gas radiative processes intended to make the surface-warming effect of water-vapour and CO2 absorption more readily understandable leads to a conclusion that the greenhouse gases also cool the upper troposphere. The results from the simple model are compared with experimental observations, and a functional form for the decline of vertical convection and water-vapour radiation near the tropopause is derived from previously unexplained high-altitude cooling-trend data. A possible reason why global climate models do not show the observed upper-troposphere cooling trend is tentatively suggested. Copyright © 2008 Royal Meteorological Society [source] Long-term trends of synoptic-scale breaking Rossby waves in the Northern Hemisphere between 1958 and 2001INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 12 2008F. Isotta Abstract Breaking synoptic-scale Rossby waves are frequent features of the upper troposphere and lower stratosphere (UTLS) which affect both global- and regional-scale dynamics. Furthermore, they directly influence ozone distribution through meridional transport of ozone-rich air towards the south and ozone-poor air towards the north. Here, trends in the frequency of these breaking waves are assessed by analysing a 44-year climatology (1958,2002) of potential vorticity (PV) streamers on isentropic surfaces from 310 to 350 K. These streamers are viewed as breaking Rossby waves, whereby stratospheric (tropospheric) streamers indicate southward (northward) breaking waves. Two complementary techniques are used to analyse the trends. First, linear trends are computed using the least-squares regression technique. Statistically significant linear trends are found to vary in location and magnitude between isentropic levels and the four seasons. In winter significant trends are detected in the eastern Pacific between 340 and 350 K. A positive trend of stratospheric streamers in the Tropics is related to an increase of total column ozone, whereas the positive trend of tropospheric streamers in the mid-latitudes is associated with a decrease of total ozone. Secondly, a nonlinear trend analysis is performed using the seasonal-trend decomposition procedure based on Loess (STL). With this technique, the low-frequency variability of the time series is analysed during the 44-year period. For instance, over the eastern Atlantic on 350 K, a phase of decreasing PV streamer frequencies in the 1950s and 1960s is followed by a positive streamer tendency after the 1970s. Additionally, trends of the zonal wind are investigated. One prominent outcome of this analysis is the observation that equatorial easterlies over the Atlantic are weakening. A dynamically meaningful link exists between the trends observed in both wind velocity and PV streamers. Copyright © 2008 Royal Meteorological Society [source] An event-based jet-stream climatology and typologyINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 3 2006Patrick Koch Abstract A novel climatology is developed for upper-tropospheric jet streams, which is complementary to and an alternative for the traditional depictions of the time-mean jets. It entails identifying the occurrence of a jet event at a given location and then compiling the spatial frequency distribution of such events. The resulting climatology, derived using the ERA-15 reanalysis data set of the ECMWF for the period 1979,1993 indicates that (1) in both hemispheres the annual cycle of jet events takes the form of comparatively smooth transition from a quasi-annular structure in summer to a more spiral-like structure in winter with a temporally asymmetric return to the summer pattern; (2) the hemispheres differ primarily in the amplitude of the frequencies and the longitudinal overlap of the spiral portion of the pattern. In addition, the jet events are subdivided using a two-class typology comprising shallow and deep jets whose vertical shear (sic. baroclinicity) are/are not confined principally to the upper troposphere. This provides a conceptually simple and dynamically meaningful classification since deep jets are more likely to spawn tropospheric-spanning cyclones. The accompanying climatology displays important longitudinal variations and significant inter-hemispheric differences. A comparison is drawn between these new and conventional climatologies and typologies. Also, comments are proffered on the relationship between, on the one hand, the patterns of jet frequency including the differing distributions of the shallow and deep types and, on the other hand, the location of the time-mean jets and the downstream storm tracks. Copyright © 2006 Royal Meteorological Society. [source] Measurement of trace atmospheric species by chemical ionization mass spectrometry: Speciation of reactive nitrogen and future directionsMASS SPECTROMETRY REVIEWS, Issue 2 2007L. Gregory HueyArticle first published online: 22 JAN 200 Abstract Chemical ionization mass spectrometry (CIMS) has proven to be a powerful method for sensitive, fast time response (t,1 sec) measurements of various atmospheric compounds with limits of detection (LOD) of the order of tens of pptv and lower. The rapid time response of CIMS is particularly well suited for airborne measurements and its application has largely grown out of airborne measurements in the stratosphere and upper troposphere. This work reviews some of the advances in CIMS technology that have occurred in the past decade. In particular, CIMS methods for selective measurement of reactive nitrogen species (e.g., HNO3, HO2NO2, PAN, and NH3) in the lower atmosphere (altitudes ,0,8 km) are described. In addition, recent developments in CIMS technology for the selective measurement of gas-phase hydroperoxides and aerosol chemical composition are briefly described. © 2007 Wiley Periodicals, Inc., Mass Spec Rev 26:166,184, 2007 [source] The impact of soil moisture modifications on CBL characteristics in West Africa: A case-study from the AMMA campaignTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue S1 2010M. Kohler Abstract Within the framework of the AMMA campaign in 2006, the response of surface properties to precipitation and their effect on the state of the convective boundary layer (CBL) and on convective instability were analysed. The observation periods covered the pre-monsoon onset (SOP 1) and the mature monsoon phase (SOP 2) and were performed in southwest Burkina Faso. Precipitation caused a distinctive increase in the volumetric soil moisture content in the upper 20 cm of the soil. Coupled with the increase in soil moisture, a continuous decrease of surface and soil temperature with time was observed. Changes in surface temperature, albedo, and a higher availability of water affected the partitioning of the energy balance. Highest values of the Bowen ratio were found during SOP 1 when the surface was dry and vegetation sparse. In SOP 2, a higher vegetation cover made the albedo and Bowen ratio less sensitive to changes in soil moisture. Modifications of surface fluxes influenced the CBL conditions. The CBL height in SOP 1 was 1658 m and in SOP 2 877 m. The heat budget of the CBL was dominated by sensible heat flux convergence, whereas the moisture budget was controlled by both advection and latent heat flux convergence. It was confirmed by the measurements that the diurnal development of convective instability was dominated by the CBL evolution and controlled by changes in the mid- or upper troposphere to a minor degree only. Linear correlations were found between the near-surface equivalent potential temperature and both convective available potential energy and convection index. Copyright © 2009 Royal Meteorological Society [source] The diabatic heat budget of the upper troposphere and lower/mid stratosphere in ECMWF reanalysesTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 638 2009S. Fueglistaler Abstract We present an analysis of the diabatic terms in the thermodynamic energy equation from ERA-40 and the ECMWF reanalysis ERA-Interim. We analyse the clear-sky radiative heating, the cloud radiative effects, and the impact from latent heat exchange and mixing. The diabatic heat budget is closed with the calculation of the temperature assimilation increment. The previously noted excessive tropospheric circulation at low latitudes in ERA-40 is also reflected in the diabatic heat budget. The temperature increment acts to cool the excessive model heating. Conversely, ERA-Interim requires heating from the assimilation increment at low latitudes, suggesting too little convection. In the tropical tropopause layer (TTL), both reanalyses show a strong heating from the interaction of clouds with radiation, but lack of reliable independent estimates renders the role of clouds uncertain. Both reanalyses show cooling in the TTL by the assimilation increment, suggesting that the models may overestimate the cloud radiative heating, or that the convective parametrization scheme has difficulties in capturing the thermal effects of deep convection. In the stratosphere, ERA-40 shows unrealistic radiative heating due to problems in the temperature profile. The diabatic heat balance is dominated by the assimilation increment, and the residual circulation is much faster than in ERA-Interim. Conversely, ERA-Interim is better balanced and requires a substantially smaller temperature increment. Its structure and magnitude of radiative heating/cooling at low/high latitudes is quite realistic. Overall, ERA-Interim provides a much improved residual circulation, but uncertainties in the magnitude of terms in particular around the tropical tropopause remain large. Copyright © 2009 Royal Meteorological Society [source] Momentum transport processes in the stratiform regions of mesoscale convective systems over the western Pacific warm poolTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 616 2006David B. Mechem Abstract Momentum transport by the stratiform components of mesoscale convective systems (MCSs) during the Tropical Ocean,Global Atmosphere Coupled Ocean,Atmosphere Response Experiment in December 1992 is investigated using a cloud-resolving model. The mesoscale momentum transport by the stratiform regions of MCSs is examined in two distinct large-scale flow regimes associated with the intraseasonal oscillation over the western Pacific warm pool. Model simulations for 14 December 1992 characterize the ,westerly onset' period, which has relatively weak low-level westerlies with easterlies above. Simulations for 23,24 December represent the ,strong westerly' regime, when westerlies extend from the upper troposphere to the surface, with a jet 2,3 km above the surface. In the westerly onset simulation, the extensive stratiform region of a MCS contained a broad region of descent that transported easterly momentum associated with the mid-level easterly jet downward. Thus, the stratiform regions acted as a negative feedback to decrease the large-scale mean westerly momentum developing at low levels. In the strong westerly regime, the mesoscale downward air motion in the stratiform regions of large MCSs transported westerly momentum downward and thus acted as a positive feedback, strengthening the already strong westerly momentum at low levels. Momentum fluxes by the mesoscale stratiform region downdraughts are shown to have a systematic and measurable impact on the large-scale momentum budget. Copyright © 2006 Royal Meteorological Society. [source] Dynamics of jet streaks in a stratified quasi-geostrophic atmosphere: Steady-state representationsTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 600 2004Philip Cunningham Abstract The structure and dynamics of jet streaks in the extratropical upper troposphere are examined in the context of a continuously stratified quasi-geostrophic (QG) framework. It is hypothesized that jet streaks may result from the superposition of monopolar or dipolar vortices of mesoscale dimensions with the enhanced potential-vorticity gradients constituting the tropopause. Based on this hypothesis, steady-state monopolar and dipolar vortices in a uniform zonal background flow on an f -plane are investigated for their applicability as idealized dynamical representations of jet streaks. The representations of jet streaks satisfy the nonlinear governing equations of the continuously stratified QG framework: the monopolar vortex is specified in terms of axisymmetric distributions of QG potential vorticity in the interior of the domain and perturbation potential temperature on upper (tropopause) and lower (surface) boundaries, whereas the dipolar vortex is adapted from a closed-form analytical solution for the geostrophic stream function. Through the incorporation of vertical structure and divergent circulations, these representations of jet streaks extend those presented previously by the authors using a non-divergent barotropic model. It is shown that these vortex representations display characteristic signatures similar to those observed in atmospheric jet streaks. In particular, both the monopole and the dipole exhibit an ageostrophic wind directed towards lower geopotential height in the entrance region of the streak and towards higher geopotential height in the exit region. For the monopole, this ageostrophic wind is entirely rotational and there is no vertical motion. For the dipole, the rotational part of the ageostrophic wind dominates the divergent part; the latter is associated with a four-cell pattern of vertical velocity similar to that described in conceptual models of straight jet streaks. For both the monopole and the dipole, the jet streak is induced by the vortex structure such that the wind speed maximum translates at the same speed as the individual vortices; this translation speed is slower than the maximum wind speed in the core of the speed maximum, consistent with observations of jet streaks. It is proposed that the above representations provide a formal theoretical foundation for the conceptual models of jet streaks prevalent in the literature; these conceptual models typically are based on heuristic kinematic or parcel arguments and not on consistent solutions to a physically plausible set of equations. The representations also provide a foundation upon which to explore the unsteady behaviour of jet streaks in terms of the superposition of monopolar and dipolar vortices with non-uniform zonal background flows. Copyright © 2004 Royal Meteorological Society. [source] Southern hemisphere winter ozone fluctuationsTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 572 2001P. K. Vigliarolo Abstract In this paper the relationship between ozone and atmospheric variability is explored over the southern hemisphere during the austral winter season, with special emphasis on synoptic transient fluctuations. The analysis of ozone tracks (or high-frequency ozone variability) shows that they have a significant correspondence with storm tracks at middle and high latitudes. Moreover, ozone tracks maximize over the Indian Ocean slightly downstream of the storm-track maximum, while over the Pacific region both ozone and storm tracks show decreased amplitudes. In particular, over southern South America (a region of climatological winter ozone minima and moderate to high ozone variability) the leading winter synoptic-scale variability mode was identified through a rotated extended empirical orthogonal function analysis applied to the meridional-wind perturbation at 300 hPa. The resulting mode is characterized by a baroclinic wave travelling eastward along subpolar latitudes, which maximizes near the tropopause level. Composite ozone fields based on this mode confirm, from a statistical point of view, the classical relationship between ridges (troughs) and minimum (maximum) ozone content. Furthermore, it is shown that dynamical processes in the upper troposphere and lower stratosphere associated with subpolar waves are responsible for the observed ozone distribution. This happens due to the barotropic equivalent vertical structure of the wave, together with the fact that ozone partial pressure maximizes near the level where the waves attain maximum amplitudes. [source] Use of GPS/MET refraction angles in three-dimensional variational analysisTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 570 2000X. Zou Abstract The Spectral Statistical Interpolation (SSI) analysis system of the National Centers for Environmental Prediction (NCEP) is modified to include GPS/MET data (meteorological data from the Global Positioning Satellite system) using a GPS ray-tracing operator. The new system is tested by incorporating 30 actual GPS/MET observations of refraction angles obtained during the GPS/MET experiment. This is the first time that real radio occupation refraction angles and refractivities have been incorporated into a three-dimensional variational analysis system. We examine the magnitude and the vertical distribution of the analysis adjustments that result from using refraction-angle observations in the NCEP SSI analysis system. The average magnitudes of the adjustments in the temperature and specific-humidity fields are approximately 0.4 degC and 0.6 g kg,1, respectively. Individual changes can be as large as 4 degC and 4g kg,1, respectively. The greatest adjustments to the temperature occur in the middle and upper troposphere and stratosphere, while the major changes in specific humidity occur in the lower troposphere. An assessment of the impact of the GPS/MET observations on the analysis, verified by conventional (mostly radiosonde) data, is difficult because of the small number of GPS/MET data used. Nevertheless, it is found that, even over data-rich regions (regions containing many radiosonde observations), and even when the verification data were the radiosonde data themselves, the use of GPS/MET refraction angles makes a slight improvement, overall, to the analysed temperatures and winds. The impact on the water-vapour analyses, again as measured against radiosonde data, is mixed, with improvements shown in some layers and degradation in others. Compared with the background field, the use of refraction angles from one occultation results in an analysis whose simulated refraction angles are much closer to the withheld GPS/MET refraction angles at the two nearby occultation locations, and whose temperature and moisture profiles are also closer to those resulting from the direct assimilation of the two withheld occultations. Although the forward model used in this study, with the ray tracing being carried out in a two-dimensional plane, is much cheaper than a more accurate three-dimensional forward model, it is still quite expensive. In order to further reduce the computational requirement for the assimilation of GPS/MET data, we test a scheme in which the GPS/MET-retrieved refractivities (instead of refraction angles) are used above a selected height for each occupation. These heights are determined objectively based on the departures from spherical symmetry of the model field. It is shown that the mixed use of GPS/MET refraction angles and refractivities produces an analysis result similar to the one using refraction angles alone, while the computational cost is reduced by more than 30%. [source] | ||||||||||