514 research outputs found
Role of gravity waves in vertical coupling during sudden stratospheric warmings
Gravity waves are primarily generated in the lower atmosphere, and can reach
thermospheric heights in the course of their propagation. This paper reviews
the recent progress in understanding the role of gravity waves in vertical
coupling during sudden stratospheric warmings. Modeling of gravity wave effects
is briefly reviewed, and the recent developments in the field are presented.
Then, the impact of these waves on the general circulation of the upper
atmosphere is outlined. Finally, the role of gravity waves in vertical coupling
between the lower and the upper atmosphere is discussed in the context of
sudden stratospheric warmings.Comment: Accepted for publication in Geoscience Letter
Influence of parameterized small-scale gravity waves on the migrating diurnal tide in Earth's thermosphere
Effects of subgrid-scale gravity waves (GWs) on the diurnal migrating tides
are investigated from the mesosphere to the upper thermosphere for September
equinox conditions, using a general circulation model coupled with the extended
spectral nonlinear GW parameterization of Yi\u{g}it et al (2008). Simulations
with GW effects cut-off above the turbopause and included in the entire
thermosphere have been conducted. GWs appreciably impact the mean circulation
and cool the thermosphere down by up to 12-18%. GWs significantly affect the
winds modulated by the diurnal migrating tide, in particular in the
low-latitude mesosphere and lower thermosphere and in the high-latitude
thermosphere. These effects depend on the mutual correlation of the diurnal
phases of the GW forcing and tides: GWs can either enhance or reduce the tidal
amplitude. In the low-latitude MLT, the correlation between the direction of
the deposited GW momentum and the tidal phase is positive due to propagation of
a broad spectrum of GW harmonics through the alternating winds. In the Northern
Hemisphere high-latitude thermosphere, GWs act against the tide due to an
anti-correlation of tidal wind and GW momentum, while in the Southern
high-latitudes they weakly enhance the tidal amplitude via a combination of a
partial correlation of phases and GW-induced changes of the circulation. The
variable nature of GW effects on the thermal tide can be captured in GCMs
provided that a GW parameterization (1) considers a broad spectrum of
harmonics, (2) properly describes their propagation, and (3) correctly accounts
for the physics of wave breaking/saturation.Comment: Accepted for publication in Journal of Geophysical Research - Space
Physic
Gravity waves and high-altitude CO ice cloud formation in the Martian atmosphere
We present the first general circulation model simulations that quantify and
reproduce patches of extremely cold air required for CO condensation and
cloud formation in the Martian mesosphere. They are created by subgrid-scale
gravity waves (GWs) accounted for in the model with the interactively
implemented spectral parameterization. Distributions of GW-induced temperature
fluctuations and occurrences of supersaturation conditions are in a good
agreement with observations of high-altitude CO ice clouds. Our study
confirms the key role of GWs in facilitating CO cloud formation, discusses
their tidal modulation, and predicts clouds at altitudes higher than have been
observed to date.Comment: Accepted for publication in Geophysical Research Letters (GRL
Seasonal Water "Pump" in the Atmosphere of Mars: Vertical Transport to the Thermosphere
We present results of simulations with the Max Planck Institute general
circulation model (MPI-MGCM) implementing a hydrological cycle scheme. The
simulations reveal a seasonal water "pump" mechanism responsible for the upward
transport of water vapor. This mechanism occurs in high latitudes above
60 of the southern hemisphere at perihelion, when the upward branch of
the meridional circulation is particularly strong. A combination of the mean
vertical flux with variations induced by solar tides facilitates penetration of
water across the "bottleneck" at approximately 60 km. The meridional
circulation then transports water across the globe to the northern hemisphere.
Since the intensity of the meridional cell is tightly controlled by airborne
dust, the water abundance in the thermosphere strongly increases during dust
storms.Comment: 15 pages, 4 figure
Continuous Time-Delay Estimation From Sampled Measurements
An algorithm for continuous time-delay estimation from sampled output data
and known input of finite energy is presented. The continuous time-delay
modeling allows for the estimation of subsample delays. The proposed estimation
algorithm consists of two steps. First, the continuous Laguerre spectrum of the
output signal is estimated from discrete-time (sampled) noisy measurements.
Second, an estimate of the delay value is obtained in Laguerre domain given a
continuous-time description of the input. The second step of the algorithm is
shown to be intrinsically biased, the bias sources are established, and the
bias itself is modeled. The proposed delay estimation approach is compared in a
Monte-Carlo simulation with state-of-the-art methods implemented in time,
frequency, and Laguerre domain demonstrating comparable or higher accuracy for
the considered case
Robust One-Step Estimation of Impulsive Time Series
The paper deals with the estimation of a signal model in the form of the
output of a continuous linear time-invariant system driven by a sequence of
instantaneous impulses, i.e. an impulsive time series. This modeling concept
arises in, e.g., endocrinology when episodic hormone secretion events and
elimination rates are simultaneously estimated from sampled hormone
concentration measurements. The pulsatile secretion is modeled with a train of
Dirac impulses constituting the input to a linear plant, which represents
stimulated hormone secretion and elimination. A previously developed one-step
estimation algorithm effectively resolves the trade-off between data fit and
impulsive input sparsity. The present work improves the algorithm so that it
requires less manual tuning and produces more accurate results through the use
of an information criterion. It is also extended to handle outliers and unknown
basal levels that are commonly recognized issues in biomedical data. The
algorithm performance is evaluated both theoretically and experimentally on
synthetic and clinical data.Comment: 26 pages, 11 figure
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