The equatorial spread‐F (ESF) refers to the spread observed in return echoes of ionograms. Such spread in reflected echoes is found both in range and frequency, which can last for a few hours. These ...are due to large‐scale plasma irregularities occurring in the ionospheric heights on some nights. Under seemingly identical background conditions, ESF might occur on one night but remain absent on the other. These plasma irregularities adversely affect the trans‐ionospheric radio wave propagation but their occurrence on a given night continues to be one of the missing elements in our understanding of the equatorial ionospheric phenomena. In this work, the vertical propagation of gravity waves in daytime thermosphere has been shown to be a crucial parameter for the generation of ESF during post‐sunset hours. Using electron density profiles obtained from digisonde at Trivandrum, a dip‐equatorial location in India, we found that vertical propagation activity of gravity waves exists on 85% of the ESF days, whereas it is only 50% for the days without the occurrence of ESF during post‐sunset hours. Further, vertical propagation speeds of these gravity waves are higher on the ESF days than on the non‐ESF days. Also, ESF has been found to occur on 100% of the occasions, whenever the vertical speeds of these gravity waves are greater than 80 ms−1. This threshold value of vertical propagation speeds of gravity waves can be used to predict the ESF occurrence around 12–14 LT on that day, which is much in advance of the occurrence of ESF.
Plain Language Summary
The plasma in the Earth's upper atmosphere enables trans‐ionospheric radiowave communications. Therefore, unforeseen or sudden changes in this region can severely affect GPS and satellite communications. One such phenomenon that creates these ionospheric disturbances is the occurrence of plasma irregularities in the nighttime equatorial ionosphere. These are generated over the magnetic equator and expand to latitudes farther away, so the radio communications in all these regions get adversely affected. However, the prediction of their occurrence has been evading the scientific community globally. In this work, we have discussed the role of upward propagating atmospheric gravity waves in explaining the day‐to‐day variability in the occurrence of these plasma irregularities. We found that if the vertical speeds of these waves in the daytime are higher, they perturb the ionosphere more efficiently and thereby contribute to the occurrence of plasma irregularities on that night. On the days when the propagation speeds are not as high, plasma irregularity occurrence was not favored. Our work shows that the upward propagation speed of the gravity waves in the daytime thermosphere is one of the crucial factors that can be used to predict, well in advance, the occurrence of these plasma irregularities.
Key Points
Vertical propagation speeds of daytime gravity waves are higher on days with equatorial spread‐F (ESF) occurrence compared to those on days without ESF occurrence
The time periods of the daytime gravity waves match with those found in the variations in ionospheric base height
The occurrence of ESF can be predicted as early as 14 LT based on the gravity wave dynamics in the daytime thermosphere
Recent studies show that the vigorous seasonal cycle of the mixed layer modulates upper ocean submesoscale turbulence. Here we provide model‐based evidence that the seasonally changing upper ocean ...stratification in the Kuroshio Extension also modulates submesoscale (here 10–100 km) inertia‐gravity waves. Summertime restratification weakens submesoscale turbulence but enhances inertia‐gravity waves near the surface. Thus, submesoscale turbulence and inertia‐gravity waves undergo vigorous out‐of‐phase seasonal cycles. These results imply a strong seasonal modulation of the accuracy of geostrophic velocity diagnosed from submesoscale sea surface height delivered by the Surface Water and Ocean Topography satellite mission.
Key Points
Upper ocean submesoscale (here 10‐100 km) turbulence and inertia‐gravity waves undergo strong seasonal cycles that are out of phase
Submesoscale turbulence dominates the horizontal velocity and sea surface height variability in late winter/early spring
Submesoscale inertia‐gravity waves dominate the horizontal velocity and sea surface height variability in late summer/early fall
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 Yiğit et al. (). 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 anticorrelation 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.
Plain Language Summary
Atmospheric waves generated by meteorological processes in the lower atmosphere influence the state and evolution of the atmosphere at higher altitudes. Using a three‐dimensional model of the atmosphere, we study the effects of small‐scale gravity waves on the large‐scale tides. In addition to the study of the interaction of these waves in the mesosphere and lower thermosphere region of the atmosphere, we explore gravity wave effects even at much higher altitudes in the thermosphere (up to about 300 km). Our results show that gravity waves strengthen the tide in the lower thermosphere. In the upper thermosphere, gravity waves can either enhance or damp the tides. The underlying mechanisms are investigated.
Key Points
Gravity waves affect tides through modification of the mean flow and direct forcing wind variations
Broad spectra of gravity waves amplify DW1 tide in the low‐latitude MLT and damp at high latitudes
GWs can either enhance or reduce the tidal amplitude in the upper thermosphere
Dong et al. (2020, https://doi.org/10.1029/2019JD030691) employed a new compressible model to examine gravity wave (GW) self‐acceleration dynamics, instabilities, secondary gravity wave (SGW) ...generation, and mean forcing for GW packets localized in two dimensions (2D). This paper extends the exploration of self‐acceleration dynamics to a GW packet localized in three dimensions (3D) propagating into tidal winds in the mesosphere and thermosphere. As in the 2D packet responses, 3D GW self‐acceleration dynamics are found to be significant and include 3D GW phase distortions, stalled GW vertical propagation, local instabilities, and SGW and acoustic wave generation. Additional 3D responses described here include refraction by tidal winds, localized 3D instabilities, asymmetric SGW propagation, reduced SGW and acoustic wave responses at higher altitudes relative to 2D responses, and forcing of transient, large‐scale, 3D mean responses that may have implications for chemical and microphysical processes operating on longer time scales.
Key Points
3D gravity wave packets exhibit strong self‐acceleration, mean‐flow interactions, and instability dynamics
3D gravity wave packets yield strong local mean‐flow forcing and secondary gravity wave and acoustic wave generation
Secondary gravity waves are modulated by tidal winds and have large scales and large influences extending into the thermosphere
An exceptionally deep upper-air sounding launched from Kiruna
airport (67.82∘ N, 20.33∘ E) on 30 January 2016 stimulated
the current investigation of internal gravity waves excited during a minor
...sudden stratospheric warming (SSW) in the Arctic winter 2015/16. The analysis
of the radiosonde profile revealed large kinetic and potential energies in
the upper stratosphere without any simultaneous enhancement of upper
tropospheric and lower stratospheric values. Upward-propagating
inertia-gravity waves in the upper stratosphere and downward-propagating
modes in the lower stratosphere indicated a region of gravity wave generation
in the stratosphere. Two-dimensional wavelet analysis was applied to vertical
time series of temperature fluctuations in order to determine the vertical
propagation direction of the stratospheric gravity waves in 1-hourly
high-resolution meteorological analyses and short-term forecasts. The
separation of upward- and downward-propagating waves provided further evidence
for a stratospheric source of gravity waves. The scale-dependent
decomposition of the flow into a balanced component and inertia-gravity waves
showed that coherent wave packets preferentially occurred at the inner edge
of the Arctic polar vortex where a sub-vortex formed during the minor SSW.
Using a nonlinear global primitive equation model, spontaneous inertia‐gravity wave (IGW) emission is investigated in an idealized representation of the stratospheric polar night. It is shown that ...IGWs are spontaneously emitted in the interior of the fluid in a jet exit region that develops around a nonlinear Rossby wave critical layer. Two key ingredients for the generation are identified: the presence of a Rossby wave guide on the polar night jet; and a zero wind line on the jet flank that gives rise to nonlinear Rossby wave breaking and strong distortion of the flow. The emission of IGWs appears here as a quasi‐steady process that begins at a well‐defined time when the flow deformation becomes large enough. Part of the emitted IGWs undergoes wave capture by the cat's‐eye flow in a Rossby wave critical layer. Another part – in the form of a well‐defined IGW packet – escapes the wave capture limit, and propagates away into the far field. The propagating wave packet is numerically well‐converged to increases in both vertical and horizontal resolution and thus provides an ideal test bed for understanding IGW emission and informing non‐orographic gravity wave drag parametrization design.
We provide a first clean numerical example of spontaneous inertia‐gravity wave emission from the stratospheric polar vortex, providing a link with recent observations. We demonstrate both wave capture and wave packet emission in a single balanced flow configuration. We identify the jet exit region in a Rossby wave critical layer as a favourable dynamical situation for spontaneous emission.
Observations of the solar atmosphere show that internal gravity waves are generated by overshooting convection, but are suppressed at locations of magnetic flux, which is thought to be the result of ...mode conversion into magnetoacoustic waves. Here, we present a study of the acoustic-gravity wave spectrum emerging from a realistic, self-consistent simulation of solar (magneto)convection. A magnetic field free, hydrodynamic simulation and a magnetohydrodynamic (MHD) simulation with an initial, vertical, homogeneous field of 50 G flux density were carried out and compared with each other to highlight the effect of magnetic fields on the internal gravity wave propagation in the Sun's atmosphere. We find that the internal gravity waves are absent or partially reflected back into the lower layers in the presence of magnetic fields and argue that the suppression is due to the coupling of internal gravity waves to slow magnetoacoustic waves still within the high-β region of the upper photosphere. The conversion to Alfvén waves is highly unlikely in our model because there is no strongly inclined magnetic field present. We argue that the suppression of internal waves observed within magnetic flux concentrations may also be due to nonlinear breaking of internal waves due to vortex flows that are ubiquitously present in the upper photosphere and the chromosphere.
Abstract
The current standard version of the Whole Atmosphere Community Climate Model (WACCM) simulates Southern Hemisphere winter and spring temperatures that are too cold compared with ...observations. This “cold-pole bias” leads to unrealistically low ozone column amounts in Antarctic spring. Here, the cold-pole problem is addressed by introducing additional mechanical forcing of the circulation via parameterized gravity waves. Insofar as observational guidance is ambiguous regarding the gravity waves that might be important in the Southern Hemisphere stratosphere, the impact of increasing the forcing by orographic gravity waves was investigated. This reduces the strength of the Antarctic polar vortex in WACCM, bringing it into closer agreement with observations, and accelerates the Brewer–Dobson circulation in the polar stratosphere, which warms the polar cap and improves substantially the simulation of Antarctic temperature. These improvements are achieved without degrading the performance of the model in the Northern Hemisphere stratosphere or in the mesosphere and lower thermosphere of either hemisphere. It is shown, finally, that other approaches that enhance gravity wave forcing can also reduce the cold-pole bias such that careful examination of observational evidence and model performance will be required to establish which gravity wave sources are dominant in the real atmosphere. This is especially important because a “downward control” analysis of these results suggests that the improvement of the cold-pole bias itself is not very sensitive to the details of how gravity wave drag is altered.
Based on the theoretical and experimental facts that gravity waves (GWs) can be spontaneously emitted during the evolution of a near‐balanced flow, a stochastic parameterization of GWs linked to ...fronts and jets is proposed. Although the spontaneous adjustment theory used predicts “exponentially” small GW fields, it is shown that it is sufficient to produce realistic GW drag at mesospheric levels. Off‐line tests using reanalyzed meteorological fields are conducted and show that the GWs emitted present a strong annual cycle following that of the sources. Also, the GW momentum fluxes in the lower stratosphere are qualitatively realistic in terms of intermittency. Online tests in a middle atmosphere general circulation model show that the scheme can potentially perform as well as highly tuned existing GW schemes.
Key Points
Spontaneous adjustment mechanism gives required GW forcing for climate models
Source‐related GW annual cycle and realistic EP flux intermittency are produced
Gravity waves play an essential role in driving and maintaining global circulation. To understand their contribution in the atmosphere, the accurate reproduction of their distribution is important. ...Thus, a deep learning approach for the estimation of gravity wave momentum fluxes was proposed, and its performance at 100 hPa was tested using data from low‐resolution zonal and meridional winds, temperature, and specific humidity at 300, 700, and 850 hPa in the Hokkaido region (Japan). To this end, a deep convolutional neural network was trained on 29‐year reanalysis data sets (JRA‐55 and DSJRA‐55), and the final 5‐year data were reserved for evaluation. The results showed that the fine‐scale momentum flux distribution of the gravity waves could be estimated at a reasonable computational cost. Particularly, in winter, when gravity waves are stronger, the median root means square errors (RMSEs) of the maximum momentum flux and the characteristic zonal wavenumber were 0.06–0.13 mPa and 1.0 × 10−5, respectively.
Plain Language Summary
Deep learning has been proven to be a powerful tool in the atmospheric sciences and in weather and climate prediction applications. In this study, deep learning was used to obtain the physical parameters of fine‐scale orographic gravity waves in the lower stratosphere (~18 km), which propagate significant momentum in the middle atmosphere (10–100 km), based on large‐scale low‐level (1–9 km) atmospheric flows, temperature, and humidity. By training a convolutional neural network using a 29‐year atmospheric reanalysis data set, the large‐scale inputs were well down‐scaled into fine‐scale gravity wave parameters at a reasonable computational cost.
Key Points
A deep learning approach was proposed to estimate orographic gravity waves using 29‐year reanalysis data
Gravity wave momentum fluxes at 100 hPa were directly converted from lower atmospheric data with a spatial resolution of 60 km
Using the proposed method, wave structures of the strong momentum flux in the target area could be estimated quite well
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