16,435 research outputs found
Fast and slow Kelvin waves in the Madden-Julian Oscillation of a GCM
The structure of the Madden-Julian Oscillation (MJO) in an 1800-day integration of the Hadley Centre Unified Model was analysed, and interpreted within a Kelvin wave framework. The model was forced with constant equinoctial (March) boundary conditions so that a ``clean'' MJO signal could be separated from the effects of the seasonal cycle and forced interannual variability. The simulated MJO was fairly realistic in terms of its large-scale spatial structure and propagation characteristics, although its period of 30 days (corresponding to an average phase speed of 15 \mps) was shorter than that observed. The signal in deep convection was less coherent than in observations, and appeared to move eastward as a sequence of discrete convective anomalies, rather than by a smooth eastward propagation. Both ``fast'' and ``slow'' equatorial Kelvin waves appeared to play an important role in the eastward propagation of the simulated MJO. Enhanced convection over the Indian Ocean was associated with a ``fast'' equatorial Kelvin wave that propagated eastward at 55 m s-1 over the Pacific. On reaching the west coast of South America, a component of this Kelvin wave propagated northward and southward as a trapped wave along the mountain ranges of Central America and the Andes, in agreement with observations. The anomalous surface easterlies over the tropical eastern Pacific associated with this fast Kelvin wave enhanced the climatological mean easterlies and led to positive convective anomalies over the eastern Pacific consistent with the WISHE mechanism. However, WISHE was not able to account for the eastward development of the convective anomalies over the Indian Ocean/western Pacific region. By splitting the equatorial divergence anomalies of the simulated MJO into their du/dx and dv/dy components, the role of Kelvin wave dynamics in the ``slow'' (15 m s-1) average eastward propagation of the simulated MJO was examined. Although the two components were of comparable magnitude, the \dudx\ component exhibited a pronounced eastward propagation which tended to be disrupted by the \dvdy\ component, thus supporting the paradigm of an underlying, but strongly modified, Kelvin wave mechanism
Influence of aerosols, clouds, and sunglint on polarization spectra of Earthshine
Ground-based observations of the Earthshine, i.e., the light scattered by
Earth to the Moon, and then reflected back to Earth, simulate space
observations of our planet and represent a powerful benchmark for the studies
of Earth-like planets. Earthshine spectra are strongly linearly polarized,
owing to scattering by molecules and small particles in the atmosphere of the
Earth and surface reflection, and may allow us to measure global atmospheric
and surface properties of planet Earth. Aims. We aim to interpret already
published spectropolarimetric observations of the Earthshine by comparing them
with new radiative transfer model simulations including a fully realistic
three-dimensional (3D) surface-atmosphere model for planet Earth. We used the
highly advanced Monte Carlo radiative transfer model MYSTIC to simulate
polarized radiative transfer in the atmosphere of the Earth without
approximations regarding the geometry, taking into account the polarization
from surface reflection and multiple scattering by molecules, aerosol
particles, cloud droplets, and ice crystals. We have shown that Earth
spectropolarimetry is highly sensitive to all these input parameters, and we
have presented simulations of a fully realistic Earth atmosphere-surface model
including 3D cloud fields and two-dimensional (2D) surface property maps. Our
modeling results show that scattering in high ice water clouds and reflection
from the ocean surface are crucial to explain the continuum polarization at
longer wavelengths as has been reported in Earthshine observations taken at the
Very Large Telescope in 2011 (3.8 % and 6.6 % at 800 nm, depending on which
part of Earth was visible from the Moon at the time of the observations). We
found that the relatively high degree of polarization of 6.6 % can be
attributed to light reflected by the ocean surface in the sunglint region
The Doppler Spectra of Medium Grazing Angle Sea Clutter; Part 1: Characterisation
This paper is concerned with the characterisation of Doppler spectra from high range resolution X-band radar sea clutter observed from an airborne platform over the range of grazing angles, 15° to 45°. It is observed that when looking up or down wind there is a strong correlation between mean Doppler shift and local spectrum intensity. When combined with random fluctuations of spectrum width, these characteristics give the spectra a temporal and spatial variability. This behaviour has previously been observed in low grazing angle data and these results confirm the wider applicability of the models developed using that data. The modelling method is also extended here to capture the bimodal behaviour observed with high intensity returns from breaking waves looking up or down-wind
Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments with Dynamics 1.0: A General Circulation Model for Simulating the Climates of Rocky Planets
Resolving Orbital and Climate Keys of Earth and Extraterrestrial Environments
with Dynamics (ROCKE-3D) is a 3-Dimensional General Circulation Model (GCM)
developed at the NASA Goddard Institute for Space Studies for the modeling of
atmospheres of Solar System and exoplanetary terrestrial planets. Its parent
model, known as ModelE2 (Schmidt et al. 2014), is used to simulate modern and
21st Century Earth and near-term paleo-Earth climates. ROCKE-3D is an ongoing
effort to expand the capabilities of ModelE2 to handle a broader range of
atmospheric conditions including higher and lower atmospheric pressures, more
diverse chemistries and compositions, larger and smaller planet radii and
gravity, different rotation rates (slowly rotating to more rapidly rotating
than modern Earth, including synchronous rotation), diverse ocean and land
distributions and topographies, and potential basic biosphere functions. The
first aim of ROCKE-3D is to model planetary atmospheres on terrestrial worlds
within the Solar System such as paleo-Earth, modern and paleo-Mars,
paleo-Venus, and Saturn's moon Titan. By validating the model for a broad range
of temperatures, pressures, and atmospheric constituents we can then expand its
capabilities further to those exoplanetary rocky worlds that have been
discovered in the past and those to be discovered in the future. We discuss the
current and near-future capabilities of ROCKE-3D as a community model for
studying planetary and exoplanetary atmospheres.Comment: Revisions since previous draft. Now submitted to Astrophysical
Journal Supplement Serie
Semi-empirical dissipation source functions for ocean waves: Part I, definition, calibration and validation
New parameterizations for the spectra dissipation of wind-generated waves are
proposed. The rates of dissipation have no predetermined spectral shapes and
are functions of the wave spectrum and wind speed and direction, in a way
consistent with observation of wave breaking and swell dissipation properties.
Namely, the swell dissipation is nonlinear and proportional to the swell
steepness, and dissipation due to wave breaking is non-zero only when a
non-dimensional spectrum exceeds the threshold at which waves are observed to
start breaking. An additional source of short wave dissipation due to long wave
breaking is introduced to represent the dissipation of short waves due to
longer breaking waves. Several degrees of freedom are introduced in the wave
breaking and the wind-wave generation term of Janssen (J. Phys. Oceanogr.
1991). These parameterizations are combined and calibrated with the Discrete
Interaction Approximation of Hasselmann et al. (J. Phys. Oceangr. 1985) for the
nonlinear interactions. Parameters are adjusted to reproduce observed shapes of
directional wave spectra, and the variability of spectral moments with wind
speed and wave height. The wave energy balance is verified in a wide range of
conditions and scales, from gentle swells to major hurricanes, from the global
ocean to coastal settings. Wave height, peak and mean periods, and spectral
data are validated using in situ and remote sensing data. Some systematic
defects are still present, but the parameterizations yield the best overall
results to date. Perspectives for further improvement are also given.Comment: revised version for Journal of Physical Oceanograph
High Frequency Radar Wind Turbine Interference Community Working Group Report
Land-based High Frequency (HF) Radars provide critically important observations of the coastal ocean that will be adversely affected by the spinning blades of utility-scale wind turbines. Pathways to mitigate the interference of turbines on HF radar observations exist for small number of turbines; however, a greatly increased pace of research is required to understand how
to minimize the complex interference patterns that will be caused by the large arrays of turbines planned for the U.S. outer continental shelf. To support the U.S.’s operational and scientific needs, HF radars must be able to collect high-quality measurements of the ocean’s surface inand around areas with significant numbers of wind turbines. This is a solvable problem, but given the rapid pace of wind energy development, immediate action is needed to ensure that HF radar wind turbine interference mitigation efforts keep pace with the planned build out of turbines
- …