On-orbit structural dynamic performance of a 15-meter microwave radiometer antenna

The on-orbit structural dynamic performance of a microwave radiometer antenna for Earth science applications is addressed. The radiometer is one of the Earth-observing instruments aboard a proposed geostationary platform as part of the Mission to the Planet Earth. A sequential approach is presented for assessing the ability of an antenna structure to retain its geometric shape subject to a representative onboard disturbance. This approach includes establishing the structural requirements of the antenna, developing the structural and disturbance models, performing modal and forced response analyses, and evaluating the resulting distortions in terms of the antenna's ability to meet stringent structural performance requirements. Two antenna configurations are discussed: free-flying and platform-mounted. These configurations are analyzed for a representative disturbance function which simulates rotation of the subreflector in order to perform a raster-type scan of the Earth disk. Results show that the scanning maneuver modeled would not induce antenna structural errors outside the specified limits

Crystallization of random trigonometric polynomials

We give a precise measure of the rate at which repeated differentiation of a random trigonometric polynomial causes the roots of the function to approach equal spacing. This can be viewed as a toy model of crystallization in one dimension. In particular we determine the asymptotics of the distribution of the roots around the crystalline configuration and find that the distribution is not Gaussian.Comment: 10 pages, 3 figure

Instabilities in an Internal Solitary-like Wave on the Oregon Shelf

Observations of internal solitary-like waves (ISWs) on the Oregon Shelf suggest the presence of Kelvin–Helmholtz billows in the pycnocline and larger-scale overturns at the back of the wave above the pycnocline. Numerical simulations designed to explore the mechanisms responsible for these features in one particular wave reveal that shear instabilities occur when (i) the minimum Richardson number Ri in the pycnocline is less than about 0.1; (ii) Lx/λ \u3e 0.8, where Lx is the length of the unstable region with Ri \u3c 0.25 and λ is a half wavelength of the wave; and (iii) a linear spatial stability analysis predicts that ln(af/ai) \u3e≈ 4, where ai and af are the amplitudes of perturbations entering and leaving the unstable region. The maximum energy loss rate in our simulations is 50 W m−1, occurring at a frequency 8% below that with the maximum spatial growth rate. The observations revealed the presence of anomalously light fluid in the center of the wave above the pycnocline. Simulations of a wave encountering a patch of light surface water were used to model this effect. In the presence of a background current with near-surface shear, the simulated ISW has a trapped surface core. As this wave encounters a patch of lighter surface water, the light surface water at first passes beneath the core. Convective instabilities set in and the light fluid is entrained into the core. This results in the formation of overturning features, which exhibit some similarities with the observed overturns

Cooling processes in deep, temperate lakes: A review with examples from two lakes in British Columbia

The cooling period in deep, temperate lakes includes the breakdown of the seasonal thermocline, isothermal overturn as the lake cools through 4°C, and winter restratification. The first part of this paper is taken up with a review of the relatively few studies on the seasonal cooling of lakes...

Observations of the influence of diurnal convection on upper ocean dissolved gas measurements

An important example of the interaction between biological productivity and near-surface oceanography is the role of nocturnal convection and diurnal restratification in modifying the environment in which photosynthetic activity takes place. In situ time series measurements of dissolved oxygen reveal the effects of photosynthetic activity, respiration and redistribution by mixing. Moored thermistor time series and frequent CTD casts show that restratification during the day is confined to a warmer shallow surface layer where most of the biological production is expected to occur. The depth and rate of mixing is measured with neutrally buoyant floats which track the vertical excursions of convecting water parcels. Early in the evening, at the onset of night time convection, this warm oxygenated water is mixed down and diluted by deeper less oxygenated water. The interpretation of oxygen time series at specified depths (here 21 m and 30 m) requires knowledge of this mixing process. Use is made of in situ dissolved nitrogen time series to infer that gas transfer at the surface is of secondary importance in determining the diurnal dissolved oxygen budget. A qualitative coupled biological/oceanographic model of the data is presented and discussed. It is concluded that a serious overestimate of daily oxygen production can result from excluding diurnal convection from the interpretation of oxygen time series

Tidal fronts and their role in air-sea gas exchange

Author Posting. © Sears Foundation for Marine Research, 2006. This article is posted here by permission of Sears Foundation for Marine Research for personal use, not for redistribution. The definitive version was published in Journal of Marine Research 64 (2006): 483-515, doi:10.1357/002224006778715766.Tidal fronts are a common feature of many coastal environments. They are characterized by a surface convergence zone that enhances wave breaking and the generation of gas bubbles due to wave-current interaction. The associated downwelling currents carry bubbles to depths of up to 160 m and increase the amount of air that dissolves from them. An energetic tidal front is formed at the entrance to the Strait of Georgia, BC, Canada, by a hydraulically controlled sill flow with vertical velocities of up to 0.75 m s−1. Extensive ship-board measurements during two cruises are interpreted with models of wave-current interaction and gas bubble behavior. The observations suggest that tidal fronts may contribute significantly to the aeration of the subsurface waters in the Fraser Estuary. This process may be also of importance for other coastal environments with plunging sill flows of dense water that deliver aerated surface water to intermediate depths

Random polynomials, random matrices, and LL-functions

We show that the Circular Orthogonal Ensemble of random matrices arises naturally from a family of random polynomials. This sheds light on the appearance of random matrix statistics in the zeros of the Riemann zeta-function.Comment: Added background material. Final version. To appear in Nonlinearit

Transonic shock-induced dynamics of a flexible wing with a thick circular-arc airfoil

Transonic shock boundary layer oscillations occur on rigid models over a small range of Mach numbers on thick circular-arc airfoils. Extensive tests and analyses of this phenomena have been made in the past but essentially all of them were for rigid models. A simple flexible wing model with an 18 pct. circular arc airfoil was constructed and tested in the Langley Transonic Dynamics Tunnel to study the dynamic characteristics that a wing might have under these circumstances. In the region of shock boundary layer oscillations, buffeting of the first bending mode was obtained. This mode was well separated in frequency from the shock boundary layer oscillations. A limit cycle oscillation was also measured in a third bending like mode, involving wind vertical bending and splitter plate motion, which was in the frequency range of the shock boundary layer oscillations. Several model configurations were tested, and a few potential fixes were investigated