Showing papers by "Sharon L. Vadas published in 2020"

Secondary Gravity Waves Generated by Breaking Mountain Waves Over Europe

Abstract: A strong mountain wave, observed over Central Europe on 12 January 2016, is simulated in 2D under two fixed background wind conditions representing opposite tidal phases. The aim of the simulation is to investigate the breaking of the mountain wave and subsequent generation of nonprimary waves in the upper atmosphere. The model results show that the mountain wave first breaks as it approaches a mesospheric critical level creating turbulence on horizontal scales of 8–30 km. These turbulence scales couple directly to horizontal secondary waves scales, but those scales are prevented from reaching the hermosphere by the tidal winds, which act like a filter. Initial secondary waves that can reach the thermosphere range from 60 to 120 km in horizontal scale and are influenced by the scales of the horizontal and vertical forcing associated with wave breaking at mountain wave zonal phase width, and horizontal wavelength scales. Large-scale nonprimary waves dominate over the whole duration of the simulation with horizontal scales of 107–300 km and periods of 11–22 minutes. The thermosphere winds heavily influence the time-averaged spatial distribution of wave forcing in the thermosphere, which peaks at 150 km altitude and occurs both westward and eastward of the source in the 2 UT background simulation and primarily eastward of the source in the 7 UT background simulation. The forcing amplitude is ∼2× that of the primary mountain wave breaking and dissipation. This suggests that nonprimary waves play a significant role in gravity waves dynamics and improved understanding of the thermospheric winds is crucial to understanding their forcing distribution.

Explicit global simulation of gravity waves in the thermosphere

Abstract: We present a new version of the high‐resolution Kühlungsborn Mechanistic general Circulation Model (KMCM) extended to z∼ 450 km. This model is called HIAMCM (HI Altitude Mechanistic general Circulation Model) and explicitly simulates gravity waves (GWs) down to horizontal wavelengths of λh ∼ 165 km. We find predominant tertiary GWs in the winter thermosphere at middle/high latitudes. These GWs typically have horizontal wavelengths λh∼ 300–1,100 km, ground‐based periods ∼ 25–90 min, and intrinsic horizontal phase speeds cIh∼ 250–350m s . Above z ∼ 200 km, the predominant GW horizontal propagation directions are roughly against the background winds from the diurnal tide; the GWs propagate mainly poleward at midnight, eastward at 6 local time (LT), equatorward at noon, and westward at 18 LT. Wintertime GWs at z ∼ 300 km having 165 km ≤ λh ≤ 330 km create a large hot spot over the Southern Andes/Antarctic Peninsula that agrees well with quiet time satellite measurements. Due to cancelation effects, the time‐averaged zonal mean Eliassen‐Palm flux divergence from the resolved GWs in the thermosphere is negligible compared to that of the tides and compared to the zonal component of the time‐averaged zonal mean ion drag. We also find that the thermospheric GWs dissipate mainly from macroturbulent diffusion and, above z ∼ 200 km, from molecular diffusion, whereas the tides dissipate mainly from ion drag. The averaged dissipative heating in the thermosphere due to tides is much stronger than that due to GWs.

First Direct Observational Evidence for Secondary Gravity Waves Generated by Mountain Waves over the Andes

Abstract: A mountain wave with a significant brightness temperature amplitude and ~500 km horizontal wavelength was observed over the Andes on 24–25 July 2017 in Atmospheric Infrared Sounder (AIRS)/Aqua satellite data. In the Modern‐Era Retrospective Analysis for Research and Applications, version 2 (MERRA‐2), reanalysis data, the intense eastward wind flowed over the Andes. Visible/Infrared Imaging Radiometer Suite (VIIRS)/Suomi‐NPP (National Polar‐orbiting Partnership) did not detect the mountain waves; however, it observed concentric ring‐like waves in the nightglow emissions at ~87 km with ~100 km wavelengths on the same night over and leeward of the Southern Andes. A ray tracing analysis showed that the mountain waves propagated to the east of the Andes, where concentric ring‐like waves appeared above a region of mountain wave breaking. Therefore, the concentric ring‐like waves were likely secondary waves generated by momentum deposition that accompanied mountain wave breaking. These results provide the first direct evidence for secondary gravity waves generated by momentum deposition. Plain Language Summary A recent model study (Vadas & Becker, 2019, https://doi.org/ 10.1029/2019JA026694) showed that mountain waves created over the Andes broke in the stratosphere and mesosphere, thereby depositing their momentum and creating “secondary” gravity waves. These waves then propagated into the lower thermosphere and created tertiary (or higher‐order) waves, some of which propagated to the upper thermosphere. This vertical multistep coupling mechanism is likely important for creating ionospheric disturbances in the F region. However, observational evidence supporting this mechanism is lacking. The purpose of this study is to show observational evidence using data from two satellite instruments: AIRS/Aqua and VIIRS/Suomi‐NPP. AIRS captured a mountain wave with a significant amplitude in the stratosphere over the Andes on 24–25 July 2017. VIIRS/Suomi‐NPP did not detect the mountain waves but instead observed concentric ring‐like gravity waves in the mesosphere on the leeward of the Andes. The concentric ring‐like structure is one of the features of secondary waves created frommomentum deposition that accompanies breaking gravity waves; thus, we conclude that the observed gravity waves were likely secondary gravity waves. These observational results provide the first direct evidence for secondary gravity waves generated by momentum deposition from breaking mountain waves and support the vertical multistep coupling mechanism.

Observations of Stratospheric Gravity Waves Over Europe on 12 January 2016: The Role of the Polar Night Jet

Abstract: Observations during 12 January 2016 revealed a series of events of significant gravity wave (GW) activity over Europe. Analysis of derived temperatures from the Atmospheric InfraRed Sounder (AIRS) provides insight into the sources of these GWs, and include a new observation of stratosphere polar night jet (PNJ) generated GWs. Mountain waves were present during this time as well over the French Alps and the Carpathian Mountains and had maximum temperature perturbations, T′, as large as 27 K over the French Alps. Further investigation of the mountain waves that demonstrated their presence in the stratosphere was determined not only by stratospheric conditions but also by strong winds in the troposphere and at the surface. GWs generated in the stratosphere by the PNJ hadmaximum T′ of 7 K. These observations demonstrate multiple sources of GWs during a dynamically active period and implicate the role of the PNJ in both the vertical propagation of GWs generated in the troposphere and the generation of GWs from the PNJ itself.