Search results for "init"

H-2, H-3(+) and the age of molecular clouds and prestellar cores

2012

Measuring the age of molecular clouds and prestellar cores is a difficult task that has not yet been successfully accomplished although the information is of paramount importance to help in understanding and discriminating between different formation scenarios. Most chemical clocks suffer from unknown initial conditions and are therefore difficult to use. We propose a new approach based on a subset of deuterium chemistry that takes place in the gas phase and for which initial conditions are relatively well known. It relies primarily on the conversion of H 3 + into H 2D + to initiate deuterium enrichment of the molecular gas. This conversion is controlled by the ortho/para ratio of H2 that i…

Discontinuous Gradient Constraints and the Infinity Laplacian

2012

Motivated by tug-of-war games and asymptotic analysis of certain variational problems, we consider a gradient constraint problem involving the infinity Laplace operator. We prove that this problem always has a solution that is unique if a certain regularity condition on the constraint is satisfied. If this regularity condition fails, then solutions obtained from game theory and $L^p$-approximation need not coincide.

Multiplicity results for asymptotically linear equations, using the rotation number approach

2007

By using a topological approach and the relation between rotation numbers and weighted eigenvalues, we give some multiplicity results for the boundary value problem u′′ + f(t, u) = 0, u(0) = u(T) = 0, under suitable assumptions on f(t, x)/x at zero and infinity. Solutions are characterized by their nodal properties.

Sociedades fragiles

2001

Dynamics in the extratropical tropopause region: A case of transition between dynamically active and passive tracer advection?

2005

It is argued that certain aspects of tracer patterns and related stirring by the flow in the extratropical tropopause region can be understood in terms of a transition between dynamically active and passive tracer advection, called ‘active-to-passive tracer transition’. In the framework of surface quasi-geostrophic dynamics, a specific initial-value problem is defined and investigated. It features a gradual transition between the two paradigms of tracer advection and allows a clear interpretation thanks to the idealized nature of the model setup. Physical reasoning and anecdotal evidence from previous studies suggest that, at least in specific cases, this interpretation is relevant for the …

Evidence of small-scale quasi-isentropic mixing in ridges of extratropical baroclinic waves

2019

Abstract. Stratosphere–troposphere exchange within extratropical cyclones provides the potential for anthropogenic and natural surface emissions to rapidly reach the stratosphere as well as for ozone from the stratosphere to penetrate deep into the troposphere, even down into the boundary layer. The efficiency of this process directly influences the surface climate, the chemistry in the stratosphere, the chemical composition of the extratropical transition layer, and surface pollution levels. Here, we present evidence for a mixing process within extratropical cyclones which has gained only a small amount of attention so far and which fosters the transport of tropospheric air masses into the…

The tropopause inversion layer in baroclinic life-cycle experiments: the role of diabatic processes

2016

Abstract. Recent studies on the formation of a quasi-permanent layer of enhanced static stability above the thermal tropopause revealed the contributions of dynamical and radiative processes. Dry dynamics leads to the evolution of a tropopause inversion layer (TIL), which is, however, too weak compared to observations and thus diabatic contributions are required. In this study we aim to assess the importance of diabatic processes in the understanding of TIL formation at midlatitudes. The non-hydrostatic model COSMO (COnsortium for Small-scale MOdelling) is applied in an idealized midlatitude channel configuration to simulate baroclinic life cycles. The effect of individual diabatic processe…

Ground-based off-line aerosol measurements at Praia, Cape Verde, during the Saharan Mineral Dust Experiment: microphysical properties and mineralogy

2011

A large field experiment of the Saharan Mineral Dust Experiment (SAMUM) was performed in Praia, Cape Verde, in January and February 2008. This work reports on the aerosol mass concentrations, size distributions and mineralogical composition of the aerosol arriving at Praia. Three dust periods were recorded during the measurements, divided by transitional periods and embedded in maritime-influenced situations. The total suspended particle mass/PM 10 /PM 2.5 were 250/180/74 μg/m 3 on average for the first dust period (17–21 January) and 250/230/83 μg/m 3 for the second (24–26 January). The third period (28 January to 2 February) was the most intensive with 410/340/130 μg/m 3 . Four modes were…

Freshening of the Alaska Coastal Current recorded by coralline algal Ba/Ca ratios

2011

Arctic Ocean freshening can exert a controlling influence on global climate, triggering strong feedbacks on ocean‐atmospheric processes and affecting the global cycling of the world’s oceans. Glacier‐fed ocean currents such as the Alaska Coastal Current are important sources of freshwater for the Bering Sea shelf, and may also influence the Arctic Ocean freshwater budget. Instrumental data indicate a multiyear freshening episode of the Alaska Coastal Current in the early 21st century. It is uncertain whether this freshening is part of natural multidecadal climate variability or a unique feature of anthropogenically induced warming. In order to answer this, a better understanding of past var…

Interaction of a tropical cyclone with a high-amplitude, midlatitude wave pattern: Waviness analysis, trough deformation and track bifurcation

2013

An idealized scenario of extratropical transition (ET) is investigated, in which a tropical cyclone interacts with a high-amplitude, upper-level wave pattern and well-developed surface cyclones. Early during the interaction, the external forcing of the upper-level wave by the ET system is quantified based on a metric for the waviness of the midlatitude flow. Local amplification of the wave pattern is diagnosed, associated prominently with the trough downstream of ET. This amplified trough, however, exhibits pronounced anticyclonic breaking and thus, in contrast to many previous ET studies, it is not clear that the amplification of the upper-level wave propagates into the farther downstream …