Home About us Contact
|
Home About us Contact | ||
|
|
||
Potential Temperature (potential + temperature)
Selected AbstractsAn improved PDF cloud scheme for climate simulationsTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 651 2010Akira Kuwano-Yoshida Abstract An efficient grid-scale cloud scheme for climate simulation is implemented in the atmospheric general circulation model for the Earth Simulator (AFES). The new cloud scheme uses statistical partial condensation using joint-Gaussian probability distribution functions (PDFs) of the liquid water potential temperature and total water content, with standard deviations estimated by the moist Mellor,Yamada level-2 turbulence scheme. It also adopts improved closure parameters based on large-eddy simulations and a revised mixing length that varies with the stability and turbulent kinetic energy. These changes not only enable better representation of low-level boundary layer clouds, but also improve the atmospheric boundary layer structure. Sensitivity experiments for vertical resolution suggest that O(100,200 m) intervals are adequate to represent well-mixed boundary layers with the new scheme. The new scheme performs well at relatively low horizontal resolution (about 150 km), although inversion layers near the coast become more intense at a higher horizontal resolution (about 50 km). Copyright © 2010 Royal Meteorological Society [source] Diagnostic study of generalized moist potential vorticity in a non-uniformly saturated atmosphere with heavy precipitationTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 650 2010Zhaoming Liang Abstract A Meiyu front accompanied by a band of heavy precipitation in East Asia is typically characterized by a much larger moisture gradient than temperature gradient. Many previous studies have suggested use of equivalent potential temperature as a thermodynamic variable under this circumstance. However, dynamic variables coupled with such a thermodynamic variable, e.g. a derived moist potential vorticity (MPV) based on equivalent potential temperature, does not provide useful dynamic insight into these systems. In this study, generalized moist potential vorticity (GMPV) is derived based on a generalized form of potential temperature. Diagnoses of numerical simulations for three typical Meiyu rainfall events show that GMPV provides remarkably accurate tracking of rainfall location, suggesting its potential use as a dynamic tracer for heavy rainfall events such as Meiyu rain bands. Copyright © 2010 Royal Meteorological Society [source] The 11 August 2006 squall-line system as observed from MIT Doppler radar during the AMMA SOPTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue S1 2010Michel Chong Abstract On the evening of 9 August 2006, a mesoscale convective system (MCS) having a north-south oriented squall-line organization formed over the border between Chad and Nigeria. It propagated westward, intensified over Nigeria on 10 August, and reached Niamey (Niger) at 0320 UTC on 11 August. Its passage over Niamey was accompanied by dust lifting and was well tracked by the Massachusetts Institute of Technology (MIT) Doppler radar. The three-dimensional structure of the airflow and precipitation pattern is investigated from regular radar volume scans performed every ten minutes between 0200 and 0321 UTC. The 3D wind components are deduced from the multiple-Doppler synthesis and continuity adjustment technique (MUSCAT) applied to a set of three volume scans obtained over a time period of one hour, which are equivalent to a three-radar observation of the squall line when considering a reference frame moving with the system and the hypothesis of a stationary field. Results of the wind synthesis reveal several features commonly observed in tropical squall lines, such as the deep convective cells in front of the system, fed by the monsoon air and extending up to 15 km altitude, and the well-marked stratiform rain region at the rear, associated with mesoscale vertical motions. Forward and trailing anvils are clearly identified as resulting from the outflow of air reaching the tropopause and transported to this level by the sloping convective updraughts occurring in a sheared environment. In the northern part, a deeper and stronger front-to-rear flow at mid-levels is found to contribute to the rearward deflection of the leading line and to promote a broader (over 300 km) stratiform cloud region. Eddy vertical transports of the cross-line momentum mainly accounts for the mid-level flow acceleration due to a momentum redistribution from low to higher levels. The height distribution of hydrometeors and their associated production terms derived from a one-dimensional microphysical retrieval model indicate the distinct roles of the convective and stratiform regions in the formation of graupel and rain, and the respective contributions of cold (riming) and warm (coalescence, melting) processes. Cooling from melting, and heating/cooling from condensation/evaporation processes yield a net decrease and increase of the potential temperature at low and mid-to-upper levels, respectively, with respect to an environmental thermodynamic profile taken three hours ahead of the analysis. Finally, the upper-level rearward flow could convey the non-negligible proportion of ice particles farther from the leading deep convection to the trailing stratiform region, thereby favouring the extent of this region. Copyright © 2009 Royal Meteorological Society [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] Role of nocturnal turbulence and advection in the formation of shallow cumulus over landTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 628 2007Jordi Vilà-Guerau de Arellano Abstract Shallow cumuli over land are normally studied from a diurnal perspective. However, the thermodynamic vertical profiles of the morning transition may play an important role in setting up favourable conditions for the formation of shallow cumuli. In turn, these profiles are highly dependent on the evolution of the nocturnal boundary characteristics and of their layer aloft. By analysing thermodynamic profiles measured by radiosondes launched every three hours at four different stations, we are able to determine how horizontal advection and turbulent mixing modify the atmospheric stability and the differences in potential temperature and specific humidity at the interface between the atmospheric boundary layer and the layer above it. Two consecutive nights are studied. They show very similar boundary-layer development; but variations in the layer aloft by a low-level-jet advection event during the second night, and intense turbulent mechanical mixing, lead to the development of two diurnal boundary layers with very different characteristics: the first one clear, the second cloudy. To complete the observational study, we perform a sensitivity analysis using a mixed-layer model to examine the role of the morning initial conditions in the formation of shallow cumuli over land. The complexity and subtlety of the observed situation,namely, the interaction of a strongly-mixed nocturnal boundary layer and horizontal advection,make this case suitable for testing the capacity of mesoscale models to reproduce cloudy boundary layers that are largely dependent on conditions during the previous night. Copyright © 2007 Royal Meteorological Society [source] The cumulus-capped boundary layer.THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 618 2006I: Modelling transports in the cloud layer Abstract Scalar-flux budgets have been obtained from large-eddy simulations (LESs) of the cumulus-capped boundary layer. Parametrizations of the terms in the budgets are discussed, and two parametrizations for the transport term in the cloud layer are proposed. It is shown that these lead to two models for scalar transports by shallow cumulus convection. One is equivalent to the subsidence detrainment form of convective tendencies obtained from mass-flux parametrizations of cumulus convection. The second is a flux-gradient relationship that is similar in form to the non-local parametrizations of turbulent transports in the dry-convective boundary layer. Using the fluxes of liquid-water potential temperature and total water content from the LES, it is shown that both models are reasonable diagnostic relations between fluxes and the vertical gradients of the mean fields. The LESs used in this study are for steady-state convection and it is possible to treat the fluxes of conserved thermodynamic variables as independent, and ignore the effects of condensation. It is argued that a parametrization of cumulus transports in a model of the cumulus-capped boundary layer should also include an explicit representation of condensation. A simple parametrization of the liquid-water flux in terms of conserved variables is also derived. © Crown Copyright, 2006 [source] The cumulus-capped boundary layer.THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 618 2006II: Interface fluxes Abstract This paper considers the relationship between the mean temperature and humidity profiles and the fluxes of heat and moisture at cloud base and the base of the inversion in the cumulus-capped boundary layer. The relationships derived are based on an approximate form of the scalar-flux budget and the scaling properties of the turbulent kinetic energy (TKE) budget. The scalar-flux budget gives a relationship between the change in the virtual potential temperature across either the cloud base transition zone or the inversion and the flux at the base of the layer. The scaling properties of the TKE budget lead to a relationship between the heat and moisture fluxes and the mean subsaturation through the liquid-water flux. The ,jump relation' for the virtual potential temperature at cloud base shows the close connection between the cumulus mass flux in the cumulus-capped boundary layer and the entrainment velocity in the dry-convective boundary layer. Gravity waves are shown to be an important feature of the inversion. © Crown copyright. 2006 [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] Isentropic zonal average formalism and the near-surface circulationTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 600 2004T.-Y. Koh Abstract The isentropic zonal average formalism is extended to include a rigorous treatment of the bottom boundary of the atmosphere. We define a ,surface zone', where isentropes in the latitudinal plane are interrupted by the earth's surface. The zonal average equations of motion and their time average in isentropic coordinates are rederived in the presence of the surface zone. Applying the extended formalism to a baroclinic wave model, we show that near-surface equatorward mean flow is driven by eastward surface form drag in isentropic coordinates, which in turn is related to poleward geostrophic potential temperature flux at the surface. A potential vorticity,potential temperature picture of extratropical general circulation dynamics above and within the surface zone is presented. We highlight the importance of poleward mean flow in the upper region of the surface zone and investigate the antisymmetric distribution of mean meridional mass flow about the median potential temperature of surface air. Copyright © 2004 Royal Meteorological Society. [source] Density currents in shear flows: Effects of rigid lid and cold-pool internal circulation, and application to squall line dynamicsTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 579 2002Ming Xue Abstract Idealized density current models that contain a rigid lid, developed in previous studies, are evaluated. It is shown, through numerical experiments, that a realistic stable layer placed at the level of the rigid lid acts very much like a rigid lid, in confining the environmental flow to the levels below and in controlling the behaviour of density currents that propagate in the flow. When this stable layer is replaced by a neutrally stable layer, the behaviour of the density current is very different. These results support the hypothesis that in the atmosphere, the tropopause or a tropospheric inversion layer acts much like a rigid lid in the idealized density current models, and the solutions of these models are applicable to organized quasi-two-dimensional convective systems such as squall lines in the atmosphere. The effects of cold-pool internal circulations on density current behaviour are also examined. When a single circulation is present initially inside the cold pool, the direction of circulation that gives rise to a smaller shear across the density interface leads to a smoother interface and a much more steady density current head. Large-amplitude eddies develop along the interface when the direction of circulation is reversed. When two layers of shear are initially present inside the cold pool, the flow pattern inside the cold pool tends to be dominated by the circulation of the lower layer. In this case, the behaviour of the density interface and density current head depends on more than just the initial cross-interface shear. The overall flow pattern in the density current, in which the cold-pool circulation contains rearward flows at the ground level, bears a close similarity to that found in mature squall line systems, as shown by comparisons with a simulated squall line. When the cold pool in a squall line is defined in terms of the equivalent potential temperature, the solutions of idealized density currents in sheared flows become more readily applicable to squall lines. Copyright © 2002 Royal Meteorological Society. [source] Hamiltonian particle-mesh simulations for a non-hydrostatic vertical slice modelATMOSPHERIC SCIENCE LETTERS, Issue 4 2009Seoleun Shin Abstract A Lagrangian particle method is developed for the simulation of atmospheric flows in a non-hydrostatic vertical slice model. The proposed particle method is an extension of the Hamiltonian particle mesh (HPM) [Frank J, Gottwald G, Reich S. 2002. The Hamiltonian particle-mesh method. In Meshfree Methods for Partial Differential Equations, Lecture Notes in Computational Science and Engineering, Vol. 26, Griebel M, Schweitzer M (eds). Springer-Verlag: Berlin Heidelberg; 131,142] and provides preservation of mass, momentum, and energy. We tested the method for the gravity wave test in Skamarock W, Klemp J. 1994. Efficiency and accuracy of the Klemp-Wilhelmson time-splitting technique. Monthly Weather Review 122: 2623,2630 and the bubble experiments in Robert A. 1993. Bubble convection experiments with a semi-implicit formulation of the Euler equations. Journal of the Atmospheric Sciences 50: 1865,1873. The accuracy of the solutions from the HPM simulation is comparable to those reported in these references. A particularly appealing aspect of the method is in its non-diffusive transport of potential temperature. The solutions are maintained smooth largely due to a ,regularization' of pressure, which is controlled carefully to preserve the total energy and the time-reversibility of the model. In case of the bubble experiments, one also needs to regularize the buoyancy contributions. The simulations demonstrate that particle methods are potentially applicable to non-hydrostatic atmospheric flow regimes and that they lead to a highly accurate transport of materially conserved quantities such as potential temperature under adiabatic flow regimes. Copyright © 2009 Royal Meteorological Society [source] Föhn as a response to changing upstream and downstream air massesTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 635 2008Georg J. Mayr Abstract Observations of föhn from the field phase of the Mesoscale Alpine Programme (MAP) are used to study how differences between the air masses upstream and downstream of the central Alpine crest determine whether the flow can descend to the lee as either shallow föhn, when it passes through passes in the mountains, or deep föhn, when it overflows the Alpine crest. First, the föhn case of 30 October 1999 is examined using ECMWF analyses and radiosonde data at various upstream and downstream locations. Additional measurements from aircraft, dropsondes, an instrumented car and automatic weather stations are then used for a detailed study of the föhn flow across the Brenner Pass. Advection of cold air around the eastern edges of the Alps and warm air around the western edge of the Alps ahead of a synoptic ridge set up a reservoir of colder air on the south side of the Alps and a reservoir of warmer air to the north. The depth to where the air was colder on the southern side was sufficient for a shallow föhn to flow through the pass. After the passage of the ridge axis, synoptic cold air advection provided another source of colder air, this time from the southwest, growing deeper with time and having a synoptically imposed cross-barrier flow component. The maximum depth to where the air upstream was colder than downstream extended just above the peaks of the highest mountains. An analysis of the detailed measurements across the Brenner Pass showed that this depth was also the top of the layer that descended and accelerated down the lee slopes of the Wipp Valley. Upstream, air above the föhn layer had an even stronger cross-barrier component yet did not descend because it did not have lower potential temperatures than the downstream side at that level. Deep föhn never developed. An examination of other well-documented MAP föhn cases confirmed the conclusion from the 30 October event that shallow and deep föhns , at least for the central Alps , are mostly a response to differences in air masses between the upstream and downstream side. A cross-barrier component of the flow was only a modification but in itself not sufficient to cause the flow to both descend and accelerate down the lee slope, unless potential temperatures on the upstream side were lower in this layer than on the downstream side. Copyright © 2008 Royal Meteorological Society [source] | ||||||||||