NG51A-01
Taylor's hypothesis in the atmosphere: a query on the underlying phenomenology
G.I. Taylor's transposition hypothesis for turbulent statistics from the spatial to the temporal domain (and vice- versa) is usually explained in terms of smaller features being advected by a large-scale transport velocity, while intrinsic temporal velocity fluctuations are slower than the corresponding inertial terms, and turbulent velocity fluctuations remain small in comparison with the transport velocity. Previous work by our team, in collaboration with Jose D. Fuentes, has shown that atmospheric boundary layer velocities show very similar behavior of their statistical moments in the time domain between periods of high transport velocities and periods of low activity, hereby casting a doubt on the above-mentioned explanation. Herein, new proof is being presented in favor of a space-time multifractal field-based phenomenology.
NG51A-02
Identifying Differences in Polar and Mid-latitude Turbulent Bursting Events in the Stably Stratified Atmospheric Boundary Layer Flows
Over land, stable conditions are usually characteristic of the nocturnal boundary layer, but can also persist for several months in polar regions during winter. Episodic development of turbulence (also known as ‘bursting') and coherent structures (patchy turbulence) are ubiquitous features of stably stratified flows. These seemingly random yet highly organized features pose great challenges in signal processing. Commonly used techniques (e.g., variable interval time averaging - VITA) are not always sufficient to characterize details in these flow patterns. In this study, we develop a rigorous statistical technique to detect the existence of intermittent high turbulence events which are not captured by standard stationary second-order models. The proposed methodology is based on the concept of an incoherent signal and does not make any assumption about the process spectrum. Observational data from the Cooperative Atmosphere-Surface Exchange Study - 1999 (CASES-99), in Kansas, and the Investigation of Sulfur Chemistry in the Antarctic Troposphere - 2000 (ISCAT00), in Antarctica, field campaigns are utilized to gain a better understanding of the differences between turbulent bursting episodes in polar and mid-latitude regions.
NG51A-03
Coupled seismic modes and earthquake hazard in Mexico City
Wave-to-wave coupling can arise when an acoustic pulse selects a Rayleigh mode of the same speed and both travel together swapping energy across an interface. A similar effect may cause severe damage at distances of several hundred kilometers when an Lg wavetrain incides upon a soft remote sedimentary waveguide, as in Mexico City. Energy at a single dominant frequency is then trapped in the waveguide. When the input power exceeds the damping losses, the trapped mode reverberates in the layer for up to five minutes, causing severe resonant damage to structures.
NG51A-04
Conservative fully discrete schemes for the shallow-water model
It is considered the classical nonlinear shallow-water model (SWM) of an ideal fluid. It is well known that the model conserves several integral characteristics such as the mass, total energy and potential enstrophy. It is extremely desirable to conserve the same characteristics in a fully discrete SWM (discrete both in space and time), because they guarantee the stability of calculations and correct description of the energy cascades in the discrete model. However, the full discretization usually destroys some, if not all, of the conservation laws, and therefore, the construction of conservative fully discrete SWMs is a non-trivial and actual scientific problem. For the last forty years there have been suggested various semi-discrete SWMs (discrete in space, but still continuous in time), which conserve one or all of the three above-mentioned integral characteristics. In particular, the model by Ringler and Randall (2002) uses rather complicated geodesic grids on a sphere, while that by Salmon (2004) applies a sophisticated stencil (containing 25 nodes) in a doubly periodic domain on the f-plane. Nevertheless, explicit time discretization used in both works resulted in the loss of all the conservation laws except the mass conservation. In this work, new fully discrete SWMs are suggested, which exactly conserve the mass and total energy. The splitting of the SWM operator in geometric coordinates provides substantial benefits in the computational cost of the solution, as well as in the applicability to a doubly periodic domain on the plane, in a periodic channel on a rotating sphere, and on the whole sphere. Each split one-dimensional fully discrete system conserves the mass and total energy, too. In fact, a family of finite-difference schemes of different approximation order is suggested, either linear or nonlinear, depending on the choice of certain parameters. Note that on a sphere and in a doubly periodic domain, our approach allows constructing various linear conservative schemes of arbitrary approximation order in space. Results of numerical experiments are discussed.