186,956 research outputs found

    First imaging of corotating interaction regions using the STEREO spacecraft

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    Plasma parcels are observed propagating from the Sun out to the large coronal heights monitored by the Heliospheric Imagers (HI) instruments onboard the NASA STEREO spacecraft during September 2007. The source region of these out-flowing parcels is found to corotate with the Sun and to be rooted near the western boundary of an equatorial coronal hole. These plasma enhancements evolve during their propagation through the HI cameras’ fields of view and only becoming fully developed in the outer camera field of view. We provide evidence that HI is observing the formation of a Corotating Interaction Region(CIR) where fast solar wind from the equatorial coronal hole is interacting with the slow solar wind of the streamer belt located on the western edge of that coronal hole. A dense plasma parcel is also observed near the footpoint of the observed CIR at a distance less than 0.1AU from the Sun where fast wind would have not had time to catch up slow wind. We suggest that this low-lying plasma enhancement is a plasma parcel which has been disconnected from a helmet streamer and subsequently becomes embedded inside the corotating interaction region

    Eulerian Derivation of the Coriolis Force

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    In textbooks of geophysical fluid dynamics, the Coriolis force and the centrifugal force in a rotating fluid system are derived by making use of the fluid parcel concept. In contrast to this intuitive derivation to the apparent forces, more rigorous derivation would be useful not only for the pedagogical purpose, but also for the applications to other kinds of rotating geophysical systems rather than the fluid. The purpose of this paper is to show a general procedure to derive the transformed equations in the rotating frame of reference based on the local Galilean transformation and rotational coordinate transformation of field quantities. The generality and usefulness of this Eulerian approach is demonstrated in the derivation of apparent forces in rotating fluids as well as the transformed electromagnetic field equation in the rotating system.Comment: Added references. Corrected typo

    Effective buoyancy and CAPE: Some implications for tropical cyclones

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    We review the widely used concepts of buoyancy and convective available potential energy (CAPE) in relation to deep convection in tropical cyclones and discuss their limitations. A fact easily forgotten in applying these concepts is that the buoyancy force of an air parcel, as often defined, is non-unique because it depends on the arbitrary definition of a reference density field. However, when calculating CAPE, the buoyancy of a lifted air parcel is related to the specific reference density field along a vertical column passing through that parcel. Both concepts can be generalized for a vortical flow and to slantwise ascent of a lifted air parcel in such a flow. In all cases, the air parcel is assumed to have infinitely small dimensions. In this article, we explore the consequences of generalizing buoyancy and CAPE for buoyant regions of finite size that perturb the pressure field in their immediate environment. Quantitative calculations of effective buoyancy, defined as the sum of the conventional buoyancy and the static vertical perturbation pressure gradient force induced by it, are shown for buoyant regions of finite width. For a judicious choice of reference density, the effective buoyancy per unit mass is essentially a unique force, independent of the reference density, but its distribution depends on the horizontal scale of the buoyant region. A corresponding concept of effective CAPE is introduced and its relevance to deep convection in tropical cyclones is discussed. The study is conceived as a first step to understanding the decreasing ability of inner-core deep convection in tropical cyclones to ventilate the mass of air converging in the frictional boundary layer as the vortex matures and decays

    Euler's fluid equations: Optimal Control vs Optimization

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    An optimization method used in image-processing (metamorphosis) is found to imply Euler's equations for incompressible flow of an inviscid fluid, without requiring that the Lagrangian particle labels exactly follow the flow lines of the Eulerian velocity vector field. Thus, an optimal control problem and an optimization problem for incompressible ideal fluid flow both yield the \emph {same} Euler fluid equations, although their Lagrangian parcel dynamics are \emph{different}. This is a result of the \emph{gauge freedom} in the definition of the fluid pressure for an incompressible flow, in combination with the symmetry of fluid dynamics under relabeling of their Lagrangian coordinates. Similar ideas are also illustrated for SO(N) rigid body motion.Comment: 12 page

    The proximity of a field plot and land-use choice: implications for land consolidation

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    Traditional methods in agricultural economics and agricultural engineering have yielded mixed results when specifying the costs of an unfavourable parcel structure. Concepts related to travel costs and the production function are frequently applied when the costs of farming distant parcels are examined. However, farmers’ perspective regarding preferences for land use is ignored or partly overlapped by predictions made by researchers. Based on applied econometric models fitted to stated preference data, we revealed that the proximity of a field plot is a relevant factor affecting land-use decisions. One-fourth of landowners would change the use of a field plot if the condition of distance was changed. Landowners would continue farming a field plot if its distance from the farm compound was reduced, being willing to accept on average €79 less in net income per ha per year. The effect of a greater proximity of field plots to the farm compound following land consolidation was heterogeneous, particularly depending on the farm size and its location.land use options, distance factor, land consolidation, choice experiment, multinomial logit model, random parameters model., Land Economics/Use,

    Dehydration in the TTL estimated from the water vapor match

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    The match method is applied to the quantification of the dehydration process in the tropical tropopause layer (TTL) over the western Pacific. The match pairs are sought from the Soundings of Ozone and Water in the Equatorial Region (SOWER) campaign network observations with the use of isentropic trajectories. For those pairs identified, extensive screening procedures are performed to verify the representativeness of the air parcel and the validity of the isentropic treatment and to check possible water injection by deep convection, consistency between the sonde data and analysis field, and conservation of the ozone content. Among those pairs remaining, we found some cases corresponding to the first quantitative value of dehydration associated with horizontal advection in the TTL. The statistical features on the dehydration for the air parcels advected in the lower TTL are derived from the match pairs. Match analysis indicates that ice nucleation starts before the relative humidity with respect to ice (RHice) reaches the value of 207 ± 81% (1σ) and that the air mass is dehydrated until the RHice reaches 83 ± 30% (1σ). The efficiency of dehydration is estimated as the relaxation time of the relative humidity for the supersaturated air parcel to approach the saturation state. This is empirically estimated from the match pairs as the quantity that reproduces the second water vapor observation given the first observed water vapor amount and the sequence of the saturation mixing ratio of the match air mass exposed during the advection. The relaxation time is found to range from 2 to 3 hours, which agrees with those reported from previous studies