Seasonal and source variations of migrating and nonmigrating tides are studied using Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics‐Sounding of the Atmosphere using Broadband Emission ...Radiometry temperature data at 10°N (5–15°N) for the year 2009. The migrating DW1 shows equinoctial maximum and summer minimum at low latitudes. It shows equinoctial asymmetry with larger amplitudes during spring equinox than fall equinox. The migrating semidiurnal tidal amplitude (SW2) shows larger amplitudes (~20 K) during March–October at 30–60°S. Its seasonal variation resembles stratospheric (10 hPa) ozone variations at southern midlatitudes. During the sudden stratospheric warming of 2009, the SW1 shows larger amplitudes over the equator and it is generated due to nonlinear interaction between SW2 and planetary wave of zonal wave number 1. The eastward nonmigrating DE4 and DE3 tides enhance in summer. The DE3 and DE4 appear to be generated due to latent heat release in the troposphere, as their amplitudes in the National Center for Environmental Prediction (NCEP)'s Precipitable water vapor (proxy for latent heat release) enhance at similar times as in mesosphere. The DW2 and DW0 tides are likely to be generated due to nonlinear interaction between DW1 and planetary wave of zonal wave number 1. The SW3 enhancement during the early winter (November‐December) may be due to nonlinear interaction between DW1 and the large‐amplitude DW2. The nonlinear interactions of DW1 with planetary wave and nonmigrating tides explain the summer minimum and equinoctial asymmetry of DW1.
Key Points
The migrating DW1 tide shows semiannual variation with larger amplitudes during equinox months
DE3 and DE4 tides maximize in summer, when DW1 shows less amplitude and they are generated due to latent heat release in tropical convection
The SW2 shows larger amplitudes during March‐October and its seasonal variation resembles that of 10‐hPa ozone at 30–60°S
The Global Positioning System deduced total electron content (TEC) data at 15°N (geomagnetic), which is the crest region of equatorial ionization anomaly, are used to study tidal variabilities during ...the 2013 sudden stratospheric warming (SSW) event. The results from space‐time spectral analysis reveal that the amplitudes of migrating diurnal (DW1) and semidiurnal (SW2) tides are larger than those of nonmigrating tides. After the SSW onset, the amplitudes of DW1, SW2, SW1, and DS0 increase. Moreover, they show 16 day variations similar to the periodicity of the high‐latitude stratospheric planetary wave (PW), suggesting that the nonmigrating tides (SW1 and DS0) are possibly generated due to nonlinear interaction of migrating tides with PW. Similar spectral analysis on temperature at 10°N obtained from the Sounding of Atmosphere by Broadband Emission Radiometry (SABER) shows that the SW2 enhances at stratospheric heights and the SW2 is more dominant at 80–90 km, but its amplitude decreases around 100 km. The amplitudes of nonmigrating tides become comparable to those of SW2 around 100 km, and their contribution becomes increasingly important at higher heights. This suggests that the nonlinear interaction between migrating tides and PW occurs at low‐latitude upper mesospheric heights, as SW2 exhibits 16 day periodicity in SABER temperature at 100 km as observed in TEC. Besides, it is observed that the eastward propagating tides are less dominant than westward propagating tides in both TEC and SABER temperatures.
Key Points
Migrating tides in 15°N (geomagnetic) GPS‐TEC and 10°N SABER temperatures show 16 day variability indicating nonlinear PW tidal interactions
Migrating semidiurnal tide (SW2) enhances largely during the SSW and dominates nonmigrating tides in stratosphere and lower mesosphere.
Nonmigrating tides enhance at 100 km, revealing that the interaction occurs at low‐latitude upper mesospheric heights
Plain Language Summary
Though there is an enhancement of semidiurnal tide during the sudden stratospheric warming events, it is not known whether the enhancement is due to migrating tide or nonmigrating tide. If it is due to nonmigrating tides, how and where are they generated? The results from this paper show that though migrating semidiurnal tide is so dominant at stratosphere and lower mesospheric heights, its amplitude decreases at heights around 100 km, where nonmigrating tidal amplitudes become significant. This suggests that the nonlinear interaction between PW and tides occurs at upper mesospheric heights and they contribute significantly to the low‐latitude ionospheric variabilities, as migrating tide.
The Global Positioning System (GPS) deduced total electron content (TEC) data at 15°N (geomagnetic), which is the northern crest region of equatorial ionization anomaly, are used to study solar and ...lunar tidal variabilities during the years 2008 and 2009 and also during the 2009–2010 winter, when a major sudden stratospheric warming (SSW) event has occurred. The diurnal and semidiurnal tidal amplitudes show semiannual variation with maximum amplitudes during February–March and September–November, whereas terdiurnal tide is larger during April–September. They show significant longitudinal variability with larger (smaller) amplitudes over 250°E–150°E (200°E–250°E). Lunar semidiurnal tidal amplitudes show sporadic enhancements during northern winter months and negligible amplitudes during northern summer months. They also show notable longitudinal variabilities. The solar migrating tides DW1 and SW2 show semiannual variation with larger amplitudes during spring equinox months, whereas TW3 maximizes during northern summer. DW2 shows larger amplitudes during summer months. During the SSW, except TW3, the migrating tides DW1 and SW2 show considerable enhancements. Among solar nonmigrating tides, SW1, TW2, and DS0 show larger enhancements. Solar tides in TEC and equatorial electrojet strength over Tirunelveli vary with the time scale of 60 days during October 2009–March 2010 similar to ozone mass mixing ratio at 10 hPa, and this confirms the vital role of ozone in tidal variabilities in ionospheric parameters. Lunar tidal amplitudes in changes in horizontal component of geomagnetic field (ΔH) are larger over Tirunelveli, a station near dip equator. Solar semidiurnal tides in ΔH have larger amplitudes than lunar tides over polar stations, Mawson and Godhavn.
Plain Language Summary
In this paper, the variations of solar and lunar tides in a few ionospheric parameters during the years 2008 and 2009 and during a disturbed winter are presented. We found that the migrating tides show semiannual variation, where as a nonmigrating diurnal tide DW2 shows maximum during summer. This explains the additional summer maximum observed in the seasonal variation of mesospheric tides over low‐latitude stations. Besides, the semidiurnal tidal variation shows clearly 60 day variability as shown by the stratospheric ozone. This suggests the dominant role of stratospheric ozone in the variations of upper atmospheric tides.
Key Points
Migrating (DW1 and SW2) and nonmigrating (SW1 and DS0) tidal amplitudes in TEC at 15°N vary semiannually and TW3 and DW2 maximize in summer
Lunar tides in TEC show sporadic winter enhancements and summer minimum and they are less than solar tides in high latitude and ΔH
Solar tidal amplitudes show 60 day periodicity in EEJ, TEC, and ozone mass mixing ratio at 10 hPa during 2009–2010 winter
The present study investigates wavenumber‐4 (wave‐4) structure in the longitude variation of zonal and meridional winds observed by the Michelson Interferometer for Global High‐resolution ...Thermospheric Imaging (MIGHTI) instrument onboard the Ionospheric Connection Explorer (ICON) satellite. The amplitude of the wave‐4 pattern in meridional wind displays semi‐annual variation with equinoctial maxima whereas its seasonal variation in zonal wind shows maxima during August–October at the equatorial and low latitudes. The wave‐4 longitude variation maximizes at lower thermospheric heights (below 130 km) in zonal and meridional winds. It is considered primarily driven by the non‐migrating eastward propagating diurnal tide with zonal wavenumber‐3 (DE3) in the zonal wind. However, the amplitude of DE3 tide in the meridional wind does not show any enhancement during September–October. The seasonal variations of the wave‐4 amplitude and the DE3 tide are not similar in the zonal and meridional winds. The migrating ter‐diurnal tide (TW3) exhibits significant amplitudes during March–April and September–November in the meridional wind. In addition, the latitude variation of non‐migrating TE1 tide shows maximum amplitude during September–October. These results suggest that the non‐linear interaction between the TW3 and TE1 tides can serve as a potential source for the wave‐4 longitude variation in the meridional wind at lower thermospheric altitudes.
Plain Language Summary
The four peaked longitudinal structure observed in ionosphere and thermosphere has long been investigated to determine its seasonal and inter‐annual variability in many parameters, as well as its sources, which can be of lower atmospheric origin. Earlier studies have shown that wave‐4 structure in the longitude variation of thermospheric zonal wind has large amplitudes between June and October. The investigation of ICON‐MIGHTI zonal wind confirms this. The wave‐4 pattern in the longitude variation of the meridional wind reveals that it has maxima during equinox different from the zonal wind. The DE3 tide is the primary contributor to the wave‐4 pattern in zonal wind. The DE3 shows large amplitudes during March–April however, it is weak during September–October in meridional wind. The source of wave‐4 pattern in the meridional wind is investigated. In meridional wind, the seasonal variation of migrating ter‐diurnal TW3 tide shows large amplitudes (40 m/s) during March–April and September–November. The non‐migrating ter‐diurnal TE1 tide shows enhanced amplitudes during September–October which suggests that the non‐linear interaction between the TW3 and TE1 tides can generate wave‐4 longitude structure in meridional wind at lower thermospheric altitudes. This study indicates the existence of other possible sources for the wave‐4 pattern observed in thermosphere.
Key Points
Wave‐4 longitude structure in thermospheric meridional wind shows semi‐annual variation with equinoctial maxima at 100–110 km
Though DE3 tide does not show any enhancement during September–October, TW3 tide is found to be dominant in the meridional wind
It is suggested that the non‐linear interaction between TW3 and TE1 tides can generate wave‐4 structure in thermospheric meridional wind
The meteor radar wind observations over Esrange (67.9°N, 21.10°E) and Rothera (67.5°S, 68.1°W), located respectively in the northern and southern polar latitudes show seasonal variation in the upper ...mesosphere and lower thermosphere (UMLT) winds with strong westward flow in the zonal wind and equatorward flow in the meridional wind during April‐September over Esrange and October‐February over Rothera. The semi‐diurnal tide shows larger amplitudes over Esrange than over Rothera. However, the diurnal tide (DT) shows comparable amplitudes in both stations. The DT amplitude decreases with height over both stations. Significant correlations between the DT amplitude and mean zonal wind (−0.74 for Esrange and −0.54 and −0.77 for Rothera) indicate the possible role of DT in driving the westward winds. The space‐time spectral analysis of the temperature obtained from the radiance observations of the SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) instrument on board the TIMED (Thermosphere Ionosphere Mesosphere Energetics Dynamics) satellite reveals the summer maximum of migrating diurnal tide of zonal wavenumber 1 (DW1) over both 60°N and 60°S, indicating that the DT is mostly composed of DW1. These results suggest that the westward winds in the polar UMLT region are largely driven by the westward momentum contributed by the DW1 tide due to its interaction with the background wind.
Plain Language Summary
The study provides an observational evidence for the influence of tides on the background wind flow at the polar upper mesospheric heights. The meteor radar observations over two polar latitude stations are used to study the seasonal and interannual variation of winds and investigate whether there is any relation between winds and tides. It is observed that stronger westward wind flows at the upper mesosphere lower thermospheric heights when the amplitude of diurnal tide is larger. Moreover, the amplitude of the diurnal tides is found to decrease with height, unlike the semi‐diurnal tidal amplitude, indicating the interaction of diurnal tide with background flow. Due to the tidal mean flow interaction, the diurnal tide which is mostly the westward propagating migrating diurnal tide (DW1), can drive the large westward wind through its westward momentum deposition into the background flow.
Key Points
Strong westward winds are observed when the amplitude of westward propagating diurnal tide is larger
The semi‐diurnal tidal amplitude increases with height, while the diurnal tidal amplitude decreases with height
Diurnal tide being the major source of westward momentum possibly drives the zonal wind flow due to its interaction with background flow
The Equatorial Atmosphere Radar observations at Kototabang (0.2°S, 100.3°E) are used to study the possible semidiurnal tidal influence on the occurrence of postmidnight echoes from the field‐aligned ...irregularities (FAIs) due to spread F. It is found that the postmidnight FAI echoes show high percentage of occurrence (PO) during June–July and low PO in December–January of low solar activity years. As solar activity approaches minimum, the PO increase is extended to May, August, and September. The space‐time Fourier analysis on the temperature information obtained from the Sounding of Atmosphere by Broadband Emission Radiometry instrument onboard Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite reveals that at the low‐latitude upper mesospheric heights, though the migrating diurnal tide propagating westward with zonal wavenumber (k) 1 (DW1) dominates the migrating semidiurnal tide propagating westward with k = 2 (SW2) particularly during the equinox months, the SW2 tidal amplitudes are larger than DW1 during June–July. The postmidnight FAI echoes appear to be more frequent during the sudden stratospheric warming of the Solar Minimum Years 2018–2019 and 2008–2009 but not in the Solar Maximum Year 2013. The dominance of SW2 over DW1 is also noted during these years. This indicates that the SW2 plays a major role in the occurrence of postmidnight spread F. It is suggested that the dominant semidiurnal variation of zonal electric field can become eastward and lift the F layer to higher heights during midnight hours, which can favor the growth of the Rayleigh‐Taylor instability to cause delayed spread F around midnight.
Key Points
The postmidnight FAI echo occurrence shows high percentage during June–July and SSW times and low percentage in December–January
Upper mesospheric migrating semidiurnal tide (SW2) shows larger amplitudes than migrating diurnal tide (DW1) in June–July and in SSW times
As SW2 is dominant than DW1 both during June–July and SSW times, it plays a major role in the occurrence of postmidnight FAI radar echoes
In this study, the boreal sudden stratospheric warming (SSW) event of 2013 and the austral SSW event of 2019 are considered to investigate the influence of the SSW events on the polar and antipodal ...upper mesosphere and lower thermosphere (UMLT) regions using ground‐based, space‐borne, reanalysis and model data sets. During the SSW events, the solar semi‐diurnal tidal (SDT) amplitudes are much larger than the lunar amplitudes in both UMLT regions. Besides, the solar SDT shows an increase in its amplitude in both the polar UMLT heights during the SSW events. However, the lunar tides show enhancement in its amplitude in the boreal polar UMLT region during both the SSW events. In the antipodal UMLT region, a peak enhancement in solar SDT amplitude is observed a few days after the onset of the boreal SSW and near to the onset time of the austral SSW. No concurrent stratospheric ozone volume mixing ratio (vmr) increase is observed which indicates that the SDT peak can be unlikely due to the underlying stratospheric ozone vmr changes. However, similar periodicity in the UMLT zonal winds of both poles indicates the possibility of cross equatorial propagation of planetary wave (PW). As the SDT amplitude also reveal similar planetary wave periodicity as observed in zonal wind, it is suggested that the PW modulation of the SDT could be the reason for the enhancement of SDT in the opposite polar UMLT region.
Plain Language Summary
This work provides an observational evidence of the cross equatorial propagation of PWs associated with the SSW events that affect the UMLT region extending up to the next pole. Using ground‐based, space‐borne, reanalysis and model outputs, the tidal and PW activity during two SSWs (2013 and 2019) are studied simultaneously over both the poles. The meteor radar observation reveals the dominance of solar SDT in the polar UMLT region compared to the lunar tides and its amplitude increases around the SSW onset days. A peak in the antipodal SDT is observed in the UMLT region with no simultaneous increase in the stratospheric ozone. It is suggested that a cross equatorial propagation of PW periodicity associated with the SSW events may modulate the solar SDT in the antipodal UMLT region and result in its increased amplitude after the SSW events.
Key Points
Solar semi‐diurnal tide enhances in both polar mesosphere during sudden stratospheric warming but at no relation with stratospheric ozone
Solar tides are relatively stronger than lunar tides with the enhancement of the latter observed only in the boreal polar mesosphere
Similar planetary waves in both polar mesosphere and their modulation of solar tides in antipode show their cross equatorial propagation
Analysis of the meteor radar wind observations in the upper mesosphere and lower thermosphere (UMLT) region (82–98 km) over Rothera (67.5°S, 68.1°W) reveals that the monthly mean zonal winds show a ...large interannual variability during November-December, when compared to other months. Large westward winds are observed during November-December of the years 2006–2011, 2015, 2018 and 2020, when the amount of ozone loss is more in the southern high-latitude stratosphere (57.5°S–77.5°S). The ERA-5 reanalysis winds at 30 hPa over 67.5°S also show an interannual variability with large eastward winds at stratospheric heights during October-November of those years. The presence of high meridional heat flux due primarily to the planetary wave (PW) of zonal wavenumber 1 in the other years increases the polar stratospheric temperature and subsequently reduces the catalytic destruction of ozone by preventing the formation of polar stratospheric clouds (PSCs). Further, it is also found that the ozone loss influences the migrating semi-diurnal (SW2) tidal variabilities at upper mesospheric heights with low activity corresponding to the years having more ozone loss. These results indicate the impact of planetary waves on the catalytic destruction of Antarctic ozone leading to the circulation and tidal amplitude changes extending up to the UMLT region.
The monthly 1°×1° global snow cover (SC) data taken from version three of the twentieth-century reanalysis (20CR) project and the high-resolution gridded rainfall data provided by the India ...Meteorological Department (IMD) for the years 1957–2015 are used to study the relation between southwest monsoon rainfall (SWMR) over India and the SC over Eurasia (35°–65°N; 40°–80°E) and Himalayas (20°–50°N; 50°–110°E) for the four sesquidecades 1957–1970, 1971–1985, 1986–2000 and 2001–2015. It is observed that the SC over Eurasia in April–May shows generally the well-known negative correlation with the all India averaged SWMR. However, the correlation became positive in the recent sesquidecade 2001–2015. Though the correlation between Himalayan SC and SWMR is increasingly positive during the first three sesquidecades, it becomes negative during 2001–2015 over the majority of the grid points. This drastic change in the relationship is attributed to the decreasing trend in the area of SC and the increasing trend in the North Atlantic Sea surface temperature (SST) during the last decade.
In this paper are presented PIV measurements of turbulent pipe flow at bulk Reynolds numbers Re
D
between 3.4
×
10
5
and 6.9
×
10
5
. So-called single-pixel correlation is applied that yields a ...superior spatial resolution that is slightly larger than the equivalent size of a pixel in the flow. The location and shape of the averaged correlation peak give the mean velocity and normal and Reynolds stresses. A novel aspect of the single-pixel correlation approach is the extension to determine the 2-point spatial correlation of the velocity fluctuations and the spectrum of the longitudinal velocity fluctuations. Detailed results are presented for Re
D
= 4.98
×
10
5
, corresponding to a shear Reynolds number Re
τ
= 10.3
×
10
3
, with a spatial resolution in wall units of
Δ
y
+
= 19.
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