Aggregation of chemotactic organisms in a differential flow

We study the effect of advection on the aggregation and pattern formation in chemotactic systems described by Keller-Segel type models. The evolution of small perturbations is studied analytically in the linear regime complemented by numerical simulations. We show that a uniform differential flow can significantly alter the spatial structure and dynamics of the chemotactic system. The flow leads to the formation of anisotropic aggregates that move following the direction of the flow, even when the chemotactic organisms are not directly advected by the flow. Sufficiently strong advection can stop the aggregation and coarsening process that is then restricted to the direction perpendicular to the flow

The Gravity Wave Response Above Deep Convection in a Squall Line Simulation

High-frequency gravity waves generated by convective storms likely play an important role in the general circulation of the middle atmosphere. Yet little is known about waves from this source. This work utilizes a fully compressible, nonlinear, numerical, two-dimensional simulation of a midlatitude squall line to study vertically propagating waves generated by deep convection. The model includes a deep stratosphere layer with high enough resolution to characterize the wave motions at these altitudes. A spectral analysis of the stratospheric waves provides an understanding of the necessary characteristics of the spectrum for future studies of their effects on the middle atmosphere in realistic mean wind scenarios. The wave spectrum also displays specific characteristics that point to the physical mechanisms within the storm responsible for their forcing. Understanding these forcing mechanisms and the properties of the storm and atmosphere that control them are crucial first steps toward developing a parameterization of waves from this source. The simulation also provides a description of some observable signatures of convectively generated waves, which may promote observational verification of these results and help tie any such observations to their convective source

A hybrid bin scheme to solve the condensation/evaporation equation using a cubic distribution function

A computationally efficient method is proposed to replace the piecewise linear number distribution in a hybrid bin scheme with a piecewise cubic polynomial. When the linear distribution is replaced by a cubic, the errors generated in solutions to the condensation/evaporation equation are reduced by a factor of two to three. Alternatively, using the cubic distribution function allows reducing the number of bins by 20 ± 5% when solving the condensation/evaporation problem without sacrificing accuracy

Tails for the Einstein-Yang-Mills system

We study numerically the late-time behaviour of the coupled Einstein Yang-Mills system. We restrict ourselves to spherical symmetry and employ Bondi-like coordinates with radial compactification. Numerical results exhibit tails with exponents close to -4 at timelike infinity $i^+$ and -2 at future null infinity \Scri.Comment: 12 pages, 5 figure

Prevalence study of genetically defined skeletal muscle channelopathies in England.

To obtain minimum point prevalence rates for the skeletal muscle channelopathies and to evaluate the frequency distribution of mutations associated with these disorders

Multi-scale waves in sound-proof global simulations with EULAG

EULAG is a computational model for simulating flows across a wide range of scales and physical scenarios. A standard option employs an anelastic approximation to capture nonhydrostatic effects and simultaneously filter sound waves from the solution. In this study, we examine a localized gravity wave packet generated by instabilities in Held-Suarez climates. Although still simplified versus the Earth’s atmosphere, a rich set of planetary wave instabilities and ensuing radiated gravity waves can arise. Wave packets are observed that have lifetimes ≤ 2 days, are negligibly impacted by Coriolis force, and do not show the rotational effects of differential jet advection typical of inertia-gravity waves. Linear modal analysis shows that wavelength, period, and phase speed fit the dispersion equation to within a mean difference of ∼ 4%, suggesting an excellent fit. However, the group velocities match poorly even though a propagation of uncertainty analysis indicates that they should be predicted as well as the phase velocities. Theoretical arguments suggest the discrepancy is due to nonlinearity — a strong southerly flow leads to a critical surface forming to the southwest of the wave packet that prevents the expected propagation

Eddy diffusivities derived from a numerical model of the convective planetary boundary layer

Functional forms for the vertical eddy diffusivity K_z (z) are sought that optimize the performance of the K-theory diffusion equation. The method developed to determine the optimal diffusivity is first tested by applying it to analytic solution of the diffusion equation in which the functional form of the diffusivity is known precisely. In all test cases performed, the technique yields the correct K_z profile regardless of the initial estimate of K_z from which the technique’s search procedure begins. When applied to “observed” mean, cross-wind-integrated point source concentration fields derived from Lagrangian diffusion theory and data from a numerical turbulence model jointly, the technique yields optimal diffusivities that make the solution of the diffusion equation agree within ±20% of the “observed” values within the core of point source plumes. Expressed in terms of the convective-velocity scale w* and the mixed-layer depth z_i, the optimal diffusivity has a quasiuniversal form for atmospheric stabilities in the range z_i/L ⪯ -10 where L is the Monin-Obukhov length. The optimal diffusivity is found to be strongly dependent on the source hight. The K_z profiles derived for the two source heights z_s ∼ -0.025 z_i and z_s ∼ -0.25 z_i are of opposite shape, but they have comparable maximum values of K_z ∼ -0.25 w*z_i

Eddy diffusivities derived from a numerical model of the convective planetary boundary layer

Functional forms for the vertical eddy diffusivity K_z (z) are sought that optimize the performance of the K-theory diffusion equation. The method developed to determine the optimal diffusivity is first tested by applying it to analytic solution of the diffusion equation in which the functional form of the diffusivity is known precisely. In all test cases performed, the technique yields the correct K_z profile regardless of the initial estimate of K_z from which the technique’s search procedure begins. When applied to “observed” mean, cross-wind-integrated point source concentration fields derived from Lagrangian diffusion theory and data from a numerical turbulence model jointly, the technique yields optimal diffusivities that make the solution of the diffusion equation agree within ±20% of the “observed” values within the core of point source plumes. Expressed in terms of the convective-velocity scale w* and the mixed-layer depth z_i, the optimal diffusivity has a quasiuniversal form for atmospheric stabilities in the range z_i/L ⪯ -10 where L is the Monin-Obukhov length. The optimal diffusivity is found to be strongly dependent on the source hight. The K_z profiles derived for the two source heights z_s ∼ -0.025 z_i and z_s ∼ -0.25 z_i are of opposite shape, but they have comparable maximum values of K_z ∼ -0.25 w*z_i