Naval Postgraduate School

About: Naval Postgraduate School is a education organization based out in Monterey, California, United States. It is known for research contribution in the topics: Tropical cyclone & Nonlinear system. The organization has 5246 authors who have published 11614 publications receiving 298300 citations. The organization is also known as: NPS & U.S. Naval Postgraduate School.

The Physics of Sediment Transport Initiation, Cessation, and Entrainment Across Aeolian and Fluvial Environments

Abstract: Predicting the morphodynamics of sedimentary landscapes due to fluvial and aeolian flows requires answering the following questions: is the flow strong enough to initiate sediment transport, is the flow strong enough to sustain sediment transport once initiated, and how much sediment is transported by the flow in the saturated state (i.e., what is the transport capacity)? In the geomorphological and related literature, the widespread consensus has been that the initiation, cessation, and capacity of fluvial transport, and the initiation of aeolian transport, are controlled by fluid entrainment of bed sediment caused by flow forces overcoming local resisting forces, whereas aeolian transport cessation and capacity are controlled by impact entrainment caused by the impacts of transported particles with the bed. Here the physics of sediment transport initiation, cessation, and capacity is reviewed with emphasis on recent consensus‐challenging developments in sediment transport experiments, two‐phase flow modeling, and the incorporation of granular physics' concepts. Highlighted are the similarities between dense granular flows and sediment transport, such as a superslow granular motion known as creeping (which occurs for arbitrarily weak driving flows) and system‐spanning force networks that resist bed sediment entrainment; the roles of the magnitude and duration of turbulent fluctuation events in fluid entrainment; the traditionally overlooked role of particle‐bed impacts in triggering entrainment events in fluvial transport; and the common physical underpinning of transport thresholds across aeolian and fluvial environments. This sheds a new light on the well‐known Shields diagram, where measurements of fluid‐entrainment thresholds could actually correspond to entrainment‐independent cessation thresholds.

Evaluation of physical processes in an idealized extratropical cyclone using adjoint sensitivity

Abstract: An adjoint model is used to examine the sensitivity of an idealized dry extratropical cyclogenesis simulation to perturbations of predictive variables and parameters during the cyclone life cycle. the adjoint sensitivity indicates how small perturbations of model variables or parameters anywhere in the model domain can influence cyclone central pressure. Largest sensitivity for both temperature and wind perturbations is located between 600 and 900 hPa in the baroclinic zone above the developing cyclone. Perturbations of a given size have more influence on cyclone intensity when located in high-sensitivity regions (the middle and lower troposphere in this simulation). the effects of physical processes can be interpreted with adjoint sensitivity by considering perturbations that are proportional to temperature and wind tendencies in the basic state (nonlinear forecast). In the early phase of the cyclone life cycle, temperature advection near the steering level in the lower troposphere (about 800 hPa) is strongly cyclogenetic and resembles a Charney mode of baroclinic instability. During the phase of most rapid deepening, temperature advection in the lower troposphere remains important, while interpretation of sensitivity to wind perturbations suggests that increased vorticity in the middle and upper troposphere above the surface low-pressure centre may also be significant for cyclone intensification. Adjoint techniques can provide insight into spatial and temporal sensitivity not easily obtained from other methods. Higher sea surface temperature (SST) has a cyclogenetic effect mainly in a localized region corresponding to the cyclone warm sector. Outside the areas of high sensitivity, small perturbations of SST have very little effect on central pressure of the forecast cyclone. When strong upward sensible-heat flux, Fs, exists, it can have a cyclogenetic (preconditioning) influence early in the cyclone life cycle, although downward Fs in the cyclone warm sector is anticyclogenetic during the phase of most rapid deepening. the sensitivity indicates that Fs can be cyclogenetic in one location and anticyclogenetic at the same time in another location, so that Fs effects on cyclone intensity are partially self-cancelling. Surface momentum stress is anticyclogenetic, with sensitivity highly localized in the cyclone warm sector.

Essential Dynamics of Secondary Eyewall Formation

Abstract: The authors conduct an analysis of the dynamics of secondary eyewall formation in two modeling frameworks to obtain a more complete understanding of the phenomenon. The first is a full-physics, three-dimensional mesoscale model in which the authors examine an idealized hurricane simulation that undergoes a canonical eyewall replacement cycle. Analysis of the mesoscale simulation shows that secondary eyewall formation occurs in a conditionally unstable environment, questioning the applicability of moist-neutral viewpoints and related mathematical formulations thereto for studying this process of tropical cyclone intensity change. The analysis offers also new evidence in support of a recent hypothesis that secondary eyewalls form via a progressive boundary layer control of the vortex dynamics in response to a radial broadening of the tangential wind field.The second analysis framework is an axisymmetric, nonlinear, time-dependent, slab boundary layer model with radial diffusion. When this boundary l...

The impact of ship-produced aerosols on the microstructure and albedo of warm marine stratocumulus clouds: A test of MAST hypotheses 1i and 1ii

Abstract: Anomalously high reflectivity tracks in stratus and stratocumulus sheets associated with ships (known as ship tracks) are commonly seen in visible and near-infrared satellite imagery. Until now there have been only a limited number of in situ measurements made in ship tracks. The Monterey Area Ship Track (MAST) experiment, which was conducted off the coast of California in June 1994, provided a substantial dataset on ship emissions and their effects on boundary layer clouds. Several platforms, including the University of Washington C-131A aircraft, the Meteorological Research Flight C-130 aircraft, the National Aeronautics and Space Administration ER-2 aircraft, the Naval Research Laboratory airship, the Research Vessel Glorita, and dedicated U.S. Navy ships, participated in MAST in order to study processes governing the formation and maintenance of ship tracks. This paper tests the hypotheses that the cloud microphysical changes that produce ship tracks are due to (a) particulate emission from the ship’s stack and/or (b) sea-salt particles from the ship’s wake. It was found that ships powered by diesel propulsion units that emitted high concentrations of aerosols in the accumulation mode produced ship tracks. Ships that produced few particles (such as nuclear ships), or ships that produced high concentrations of particles but at sizes too small to be activated as cloud drops in typical stratocumulus (such as gas turbine and some steam-powered ships), did not produce ship tracks. Statistics and case studies, combined with model simulations, show that provided a cloud layer is susceptible to an aerosol perturbation, and the atmospheric stability enables aerosol to be mixed throughout the boundary layer, the direct emissions of cloud condensation nuclei from the stack of a diesel-powered ship is the most likely, if not the only, cause of the formation of ship tracks. There was no evidence that salt particles from ship wakes cause ship tracks.

Surf zone surface retention on a rip-channeled beach

Abstract: The retention of floating matter within the surf zone on a rip-channeled beach is examined with a combination of detailed field observations obtained during the Rip Current Experiment and a three-dimensional (3-D) wave and flow model. The acoustic Doppler current profiler–observed hourly vertical cross-shore velocity structure variability over a period of 3 days with normally incident swell is well reproduced by the computations, although the strong vertical attenuation of the subsurface rip current velocities at the most offshore location outside the surf zone in 4 m water depth is not well predicted. Corresponding mean alongshore velocities are less well predicted with errors on the order of 10 cm/s for the most offshore sensors. Model calculations of very low frequency motions (VLFs) with O(10) min timescales typically explain over 60% of the observed variability, both inside and outside of the surf zone. The model calculations also match the mean rip-current surface flow field inferred from GPS-equipped drifter trajectories. Seeding the surf zone with a large number of equally spaced virtual drifters, the computed instantaneous surface velocity fields are used to calculate the hourly drifter trajectories. Collecting the hourly drifter exits, good agreement with the observed surf zone retention is obtained provided that both Stokes drift and VLF motions are accounted for in the modeling of the computed drifter trajectories. Without Stokes drift, the estimated number of virtual drifter exits is O(80)%, almost an order of magnitude larger than the O(20)% of observed exits during the drifter deployments. Conversely, when excluding the VLF motions instead, the number of calculated drifter exits is less than 5%, thus significantly underestimating the number of observed exits.