scholarly journals Mean winds, temperatures and the 16- and 5-day planetary waves in the mesosphere and lower thermosphere over Bear Lake Observatory (42° N, 111° W)

2012 ◽  
Vol 12 (3) ◽  
pp. 1571-1585 ◽  
Author(s):  
K. A. Day ◽  
M. J. Taylor ◽  
N. J. Mitchell
Keyword(s):  
Zonal Wind ◽  
Lower Thermosphere ◽  
Planetary Waves ◽  
Zonal Winds ◽  
Annual Variability ◽  
Bear Lake ◽  
Meridional Winds ◽  
The Mean ◽  
Mesosphere And Lower Thermosphere ◽  
Wind Shears

Abstract. Atmospheric temperatures and winds in the mesosphere and lower thermosphere have been measured simultaneously using the Aura satellite and a meteor radar at Bear Lake Observatory (42° N, 111° W), respectively. The data presented in this study is from the interval March 2008 to July 2011. The mean winds observed in the summer-time over Bear Lake Observatory show the meridional winds to be equatorward at meteor heights during April−August and to reach monthly-mean velocities of −12 m s−1. The mean winds are closely related to temperatures in this region of the atmosphere and in the summer the coldest mesospheric temperatures occur about the same time as the strongest equatorward meridional winds. The zonal winds are eastward through most of the year and in the summer strong eastward zonal wind shears of up to ~4.5 m s−1 km−1 are present. However, westward winds are observed at the upper heights in winter and sometimes during the equinoxes. Considerable inter-annual variability is observed in the mean winds and temperatures. Comparisons of the observed winds with URAP and HWM-07 reveal some large differences. Our radar zonal wind observations are generally more eastward than predicted by the URAP model zonal winds. Considering the radar meridional winds, in comparison to HWM-07 our observations reveal equatorward flow at all meteor heights in the summer whereas HWM-07 suggests that only weakly equatorward, or even poleward flows occur at the lower heights. However, the zonal winds observed by the radar and modelled by HWM-07 are generally similar in structure and strength. Signatures of the 16- and 5-day planetary waves are clearly evident in both the radar-wind data and Aura-temperature data. Short-lived wave events can reach large amplitudes of up to ~15 m s−1 and 8 K and 20 m s−1 and 10 K for the 16- and 5-day waves, respectively. A clear seasonal and short-term variability are observed in the 16- and 5-day planetary wave amplitudes. The 16-day wave reaches largest amplitude in winter and is also present in summer, but with smaller amplitudes. The 5-day wave reaches largest amplitude in winter and in late summer. An inter-annual variability in the amplitude of the planetary waves is evident in the four years of observations. Some 41 episodes of large-amplitude wave occurrence are identified. Temperature and wind amplitudes for these episodes, AT and AW, that passed the Student T-test were found to be related by, AT = 0.34 AW and AT = 0.62 AW for the 16- and 5-day wave, respectively.

scholarly journals Mean winds, temperatures and the 16- and 5-day planetary waves in the mesosphere and lower thermosphere over Bear Lake Observatory (42° N 111° W)

2011 ◽  
Vol 11 (11) ◽  
pp. 30381-30418
Author(s):  
K. A. Day ◽  
M. J. Taylor ◽  
N. J. Mitchell
Keyword(s):  
Zonal Wind ◽  
Lower Thermosphere ◽  
Planetary Waves ◽  
Zonal Winds ◽  
Annual Variability ◽  
Bear Lake ◽  
Meridional Winds ◽  
The Mean ◽  
Mesosphere And Lower Thermosphere ◽  
Wind Shears

Abstract. Atmospheric temperatures and winds in the mesosphere and lower thermosphere have been measured simultaneously using the Aura satellite and a meteor radar at Bear Lake Observatory (42° N, 111° W). The data presented in this study is from the interval March 2008 to July 2011. The mean winds observed in the summer-time over Bear Lake Observatory show the meridional winds to be equatorward at all heights during April-August and to reach monthly-mean speeds of −12 ms−1. The mean winds are closely related to temperatures in this region of the atmosphere and in the summer the coldest mesospheric temperatures occur about two weeks after the strongest equatorward meridional winds. In other seasons the meridional winds are poleward, reaching monthly-mean values of up to 12 ms−1. The zonal winds are eastward through most of the year and in the summer strong eastward zonal wind shears of up to ~4.5 ms−1 km−1 are present. However, westward winds are observed at the upper heights in winter and sometimes during the equinoxes. Considerable inter-annual variability is observed in the mean winds and temperatures. Comparisons of the observed winds with URAP and HWM-07 reveal some significant differences. Our radar zonal wind observations are generally more weakly eastward than these predicted by the URAP model zonal winds. Considering the radar meridional winds, in comparison to the HWM-07 our observations reveal equatorward flow at all heights in the summer whereas HWM-07 suggests that only weakly equatorward, or even poleward, flows occur at the lower heights. However, the zonal winds observed by the radar and modelled by HWM-07 are generally similar in structure and strength. Signatures of the 16- and 5-day planetary waves are clearly evident in both the radar-wind data and Aura-temperature. Short-lived wave events can reach large amplitudes of up to ~15 ms−1 and 8 K and 20 ms−1 and 10 K for the 16- and 5-day wave, respectively. A clear seasonal and short-term variability are observed in the 16- and 5-day planetary wave amplitudes. The 16-day wave reaches largest amplitude in winter and is also present in summer, but with smaller amplitudes. The 5-day wave reaches largest amplitude in winter and in late summer. An inter-annual variability of the amplitude of the planetary waves are evident in the four years of observations. Some 32 episodes of large-amplitude wave occurrence are investigated and the temperature and wind amplitudes, AT and AW, are found to be related by, AT=0.49 AW and AT=0.58 AW for the 16- and 5-day wave, respectively.

scholarly journals Dynamics of the Antarctic and Arctic mesosphere and lower thermosphere – Part 1: Mean winds

2010 ◽  
Vol 10 (21) ◽  
pp. 10273-10289 ◽  
Author(s):  
D. J. Sandford ◽  
C. L. Beldon ◽  
R. E. Hibbins ◽  
N. J. Mitchell
Keyword(s):  
Lower Thermosphere ◽  
The Arctic ◽  
Data Sets ◽  
Zonal Winds ◽  
Arctic Regions ◽  
Strong Shear ◽  
Meridional Winds ◽  
The Mean ◽  
Mesosphere And Lower Thermosphere ◽  
The Antarctic

Abstract. Zonal and meridional winds have been measured in the upper mesosphere and lower thermosphere at polar latitudes using two ground-based meteor radars. One radar is located at Rothera (68° S, 68° W) in the Antarctic and has been operational since February 2005. The second radar is located at Esrange (68° N, 21° E) in the Arctic and has been operational since October 1999. Both radars have produced relatively continuous measurements. Here we consider measurements made up to the end of 2009. Both radars are of similar design and at conjugate geographical latitudes, making the results directly comparable and thus allowing investigation of the differences in the mean winds of the Antarctic and Arctic regions. The data from each radar have been used to construct climatologies of monthly-mean zonal and meridional winds at heights between 80 and 100 km. Both Antarctic and Arctic data sets reveal seasonally varying zonal and meridional winds in which the broad pattern repeats from year to year. In particular, the zonal winds display a strong shear in summer associated with the upper part of the westward summertime zonal jet. The winds generally reverse to eastward flow at heights of ~90 km. The zonal winds are eastward throughout the rest of the year. The meridional winds are generally equatorward over both sites, although brief episodes of poleward flow are often evident near the equinoxes and during winter. The strongest equatorward flows occur at heights of ~90 km during summer. There are significant differences between the mean winds observed in the Antarctic and Arctic. In particular, the westward winds in summer are stronger and occur earlier in the season in the Antarctic compared with the Arctic. The eastward winds evident above the summertime zonal wind reversal are significantly stronger in the Arctic. The summertime equatorward flow in the Antarctic is slightly weaker, but occurs over a greater depth than is the case in the Arctic. Comparisons of these observations with those of the URAP and HWM-07 empirical models reveal a number of significant differences. In particular, the zonal winds observed in the Antarctic during wintertime are significantly weaker than those of URAP. However, the URAP zonal winds are a good match to the observations of the Arctic. Significant differences are evident between the observations and HWM-07. In particular, the strong wintertime zonal winds of the Arctic in HWM-07 are not evident in the observations and the summertime zonal winds in HWM-07 are systematically stronger than observed. The agreement with meridional winds is generally poor. There is a significant amount of inter-annual variability in the observed zonal and meridional winds. Particularly high variability is observed in the Arctic zonal winds in spring and is probably associated with stratospheric warmings.

scholarly journals Dynamics of the Antarctic and Arctic mesosphere and lower thermosphere – Part 1: Mean winds

2010 ◽  
Vol 10 (7) ◽  
pp. 17527-17567 ◽  
Author(s):  
D. J. Sandford ◽  
C. L. Beldon ◽  
R. E. Hibbins ◽  
N. J. Mitchell
Keyword(s):  
Lower Thermosphere ◽  
The Arctic ◽  
Data Sets ◽  
Zonal Winds ◽  
Arctic Regions ◽  
Strong Shear ◽  
Meridional Winds ◽  
The Mean ◽  
Mesosphere And Lower Thermosphere ◽  
The Antarctic

Abstract. Zonal and meridional winds have been measured in the upper mesosphere and lower thermosphere at polar latitudes using two ground-based meteor radars. One radar is located at Rothera (68° S, 68° W) in the Antarctic and has been operational since February 2005. The second radar is located at Esrange (68° N, 21° E) in the Arctic and has been operational since October 1999. Both radars have produced relatively continuous measurements. Here we consider measurements made up to the end of 2009. Both radars are of similar design and at conjugate geographical latitudes, making the results directly comparable and thus allowing investigation of the differences in the mean winds of the Antarctic and Arctic regions. The data from each radar have been used to construct climatologies of monthly-mean zonal and meridional winds at heights between 80 and 100 km. Both Antarctic and Arctic data sets reveal seasonally varying zonal and meridional winds in which the broad pattern repeats from year to year. In particular, the zonal winds display a strong shear in summer associated with the upper part of the westward summertime zonal jet. The winds generally reverse to eastward flow at heights of ~90 km. The zonal winds are eastward throughout the rest of the year. The meridional winds are generally equatorward throughout the year over both sites, although brief episodes of poleward flow are often evident near the equinoxes. The strongest equatorward flows occur at heights of ~90 km during summer. There are significant differences between the mean winds observed in the Antarctic and Arctic. In particular, the westward winds in summer are stronger and occur earlier in the season in the Antarctic compared with the Arctic. The eastward winds evident above the summertime zonal wind reversal are significantly stronger in the Arctic. The summertime equatorward flow in the Antarctic is slightly weaker, but occurs over a greater depth than is the case in the Arctic. Comparisons of these observations with those of the URAP and HWM-07 empirical models reveal a number of significant differences. In particular, the zonal winds observed in the Antarctic during wintertime are significantly weaker than those of URAP. However, the URAP zonal winds are a good match to the observations of the Arctic. Significant differences are evident between the observations and HWM-07. In particular, the strong wintertime zonal winds of the Arctic in HWM-07 are not evident in the observations and the summertime zonal winds in HWM-07 are systematically stronger than observed. The agreement with meridional winds is generally poor. There is a significant amount of inter-annual variability in the observed zonal and meridional winds. Particularly high variability is observed in the Arctic zonal winds in spring and is probably associated the stratospheric warmings.

scholarly journals Seasonal Variations of High-Frequency Gravity Wave Momentum Fluxes and Their Forcing toward Zonal Winds in the Mesosphere and Lower Thermosphere over Langfang, China (39.4° N, 116.7° E)

Atmosphere ◽  
2020 ◽  
Vol 11 (11) ◽  
pp. 1253
Author(s):  
Caixia Tian ◽  
Xiong Hu ◽  
Yurong Liu ◽  
Xuan Cheng ◽  
Zhaoai Yan ◽  
...  
Keyword(s):  
Seasonal Variations ◽  
High Frequency ◽  
Momentum Flux ◽  
Lower Thermosphere ◽  
Mean Flow ◽  
Zonal Winds ◽  
Short Period ◽  
Momentum Fluxes ◽  
The Mean ◽  
Mesosphere And Lower Thermosphere

Meteor radar data collected over Langfang, China (39.4° N, 116.7° E) were used to estimate the momentum flux of short-period (less than 2 h) gravity waves (GWs) in the mesosphere and lower thermosphere (MLT), using the Hocking (2005) analysis technique. Seasonal variations in GW momentum flux exhibited annual oscillation (AO), semiannual oscillation (SAO), and quasi-4-month oscillation. Quantitative estimations of GW forcing toward the mean zonal flow were provided using the determined GW momentum flux. The mean flow acceleration estimated from the divergence of this flux was compared with the observed acceleration of zonal winds displaying SAO and quasi-4-month oscillations. These comparisons were used to analyze the contribution of zonal momentum fluxes of SAO and quasi-4-month oscillations to zonal winds. The estimated acceleration from high-frequency GWs was in the same direction as the observed acceleration of zonal winds for quasi-4-month oscillation winds, with GWs contributing more than 69%. The estimated acceleration due to Coriolis forces to the zonal wind was studied; the findings were opposite to the estimated acceleration of high-frequency GWs for quasi-4-month oscillation winds. The significance of this study lies in estimating and quantifying the contribution of the GW momentum fluxes to zonal winds with quasi-4-month periods over mid-latitude regions for the first time.

Influence of geomagnetic disturbances on midlatitude mesosphere/lower thermosphere mean winds and tides 

2021 ◽  
Author(s):  
Christoph Jacobi ◽  
Friederike Lilienthal ◽  
Dmitry Korotyshkin ◽  
Evgeny Merzlyakov ◽  
Gunter Stober
Keyword(s):  
Lower Thermosphere ◽  
Meridional Wind ◽  
Vertical Profiles ◽  
Semidiurnal Tide ◽  
Geomagnetic Disturbances ◽  
Noticeable Effect ◽  
Zonal Winds ◽  
The Mean ◽  
Mesosphere And Lower Thermosphere ◽  
Midlatitude Mesosphere

<p>Observations of upper mesosphere/lower thermosphere (MLT) wind have been performed at Collm (51°N, 13°E) and Kazan (56°N, 49°E), using two SKiYMET all-sky meteor radars with similar configuration. Daily vertical profiles of mean winds and tidal amplitudes have been constructed from hourly horizontal winds. We analyze the response of mean winds and tidal amplitudes to geomagnetic disturbances. To this end we compare winds and amplitudes for very quiet (Ap ≤ 5) and unsettled/disturbed (Ap ≥ 20) geomagnetic conditions. Zonal winds in both the mesosphere and lower thermosphere are weaker during disturbed conditions for both summer and winter. The summer equatorward meridional wind jet is weaker for disturbed geomagnetic conditions. Tendencies over Collm and Kazan for geomagnetic effects on mean winds qualitatively agree during most of the year. For the diurnal tide, amplitudes in summer are smaller in the mesosphere but greater in the lower thermosphere, but no clear tendency is seen for winter. Semidiurnal tidal amplitudes increase during geomagnetic active days in summer and winter. Terdiurnal amplitudes are slightly reduced in the mesosphere during disturbed days, but no clear effect is visible for the lower thermosphere. Overall, while there is a noticeable effect of geomagnetic variability on the mean wind, the effect on tidal amplitudes, except for the semidiurnal tide, is relatively small and partly different over Collm and Kazan.</p>

scholarly journals Influence of geomagnetic disturbances on mean winds and tides in the mesosphere/lower thermosphere at midlatitudes

2021 ◽  
Vol 19 ◽  
pp. 185-193
Author(s):  
Christoph Jacobi ◽  
Friederike Lilienthal ◽  
Dmitry Korotyshkin ◽  
Evgeny Merzlyakov ◽  
Gunter Stober
Keyword(s):  
Lower Thermosphere ◽  
Meridional Wind ◽  
Vertical Profiles ◽  
Semidiurnal Tide ◽  
Geomagnetic Disturbances ◽  
Noticeable Effect ◽  
Clear Tendency ◽  
Zonal Winds ◽  
The Mean ◽  
Mesosphere And Lower Thermosphere

Abstract. Observations of upper mesosphere/lower thermosphere (MLT) wind have been performed at Collm (51.3∘ N, 13.0∘ E) and Kazan (56∘ N, 49∘ E), using two SKiYMET all-sky meteor radars with similar configuration. Daily vertical profiles of mean winds and tidal amplitudes have been constructed from hourly horizontal winds. We analyse the response of mean winds and tidal amplitudes to geomagnetic disturbances. To this end, we compare winds and amplitudes for very quiet (Ap ≤ 5) and unsettled/disturbed (Ap ≥ 20) geomagnetic conditions. Zonal winds in both the mesosphere and lower thermosphere are weaker during disturbed conditions for both summer and winter. The summer equatorward meridional wind jet is weaker for disturbed geomagnetic conditions. Tendencies for geomagnetic effects on mean winds over Collm and Kazan qualitatively agree during most of the year. For the diurnal tide, amplitudes in summer are smaller in the mesosphere and greater in the lower thermosphere, but no clear tendency is seen for winter. Semidiurnal tidal amplitudes increase during geomagnetic active days in summer and winter. Terdiurnal amplitudes are slightly reduced in the mesosphere during disturbed days, but no clear effect is visible for the lower thermosphere. Overall, while there is a noticeable effect of geomagnetic variability on the mean wind, the effect on tidal amplitudes, except for the semidiurnal tide, is relatively small and partly different over Collm and Kazan.

scholarly journals Connection between the length of day and wind measurements in the mesosphere and lower thermosphere at mid- and high latitudes

2019 ◽  
Vol 37 (1) ◽  
pp. 1-14
Author(s):  
Sven Wilhelm ◽  
Gunter Stober ◽  
Vivien Matthias ◽  
Christoph Jacobi ◽  
Damian J. Murphy
Keyword(s):  
Solar Radiation ◽  
Zonal Wind ◽  
Large Scale ◽  
Lower Thermosphere ◽  
Earth’S Rotation ◽  
The Sun ◽  
Earth's Rotation ◽  
The Earth ◽  
The Mean ◽  
Mesosphere And Lower Thermosphere

Abstract. This work presents a connection between the density variation within the mesosphere and lower thermosphere (MLT) and changes in the intensity of solar radiation. On a seasonal timescale, these changes take place due to the revolution of the Earth around the Sun. While the Earth, during the northern-hemispheric (NH) winter, is closer to the Sun, the upper mesosphere expands due to an increased radiation intensity, which results in changes in density at these heights. These density variations, i.e., a vertical redistribution of atmospheric mass, have an effect on the rotation rate of Earth's upper atmosphere owing to angular momentum conservation. In order to test this effect, we applied a theoretical model, which shows a decrease in the atmospheric rotation speed of about ∼4 m s−1 at a latitude of 45∘ in the case of a density change of 1 % between 70 and 100 km. To support this statement, we compare the wind variability obtained from meteor radar (MR) and Microwave Limb Sounder (MLS) satellite observations with fluctuations in the length of a day (LOD). Changes in the LOD on timescales of a year and less are primarily driven by tropospheric large-scale geophysical processes and their impact on the Earth's rotation. A global increase in lower-atmospheric eastward-directed winds leads, due to friction with the Earth's surface, to an acceleration of the Earth's rotation by up to a few milliseconds per rotation. The LOD shows an increase during northern winter and decreases during summer, which corresponds to changes in the MLT density due to the Earth–Sun movement. Within the MLT the mean zonal wind shows similar fluctuations to the LOD on annual scales as well as longer time series, which are connected to the seasonal wind regime as well as to density changes excited by variations in the solar radiation. A direct correlation between the local measured winds and the LOD on shorter timescales cannot clearly be identified, due to stronger influences of other natural oscillations on the wind. Further, we show that, even after removing the seasonal and 11-year solar cycle variations, the mean zonal wind and the LOD are connected by analyzing long-term tendencies for the years 2005–2016.

scholarly journals Mean winds, SAO and QBO in the stratosphere, mesosphere and lower thermosphere over Ascension Island (8° S 14° W)

2013 ◽  
Vol 13 (3) ◽  
pp. 6779-6805
Author(s):  
K. A. Day ◽  
N. J. Mitchell
Keyword(s):  
Lower Thermosphere ◽  
Strong Support ◽  
Phase Relationship ◽  
Phase Speed ◽  
Zonal Flow ◽  
Quasi Biennial Oscillation ◽  
Zonal Winds ◽  
Ascension Island ◽  
Meridional Winds ◽  
Mesosphere And Lower Thermosphere

Abstract. Mean winds in the mesosphere and lower thermosphere (MLT) over Ascension Island (8° S and 14° W) have been investigated using meteor radar wind observations. The results presented in this study are from the interval October 2001 to December 2011. There is a clear annual oscillation in the monthly-mean meridional winds. The monthly-mean meridional winds observed over Ascension Island at meteor heights are found to be southward during April–October, reaching velocities up to about −23 m s−1 and northward the rest of the year, reaching velocities up to about 16 m s−1. The monthly-mean zonal winds are generally westward through most of the year, reaching velocities up to about −46 m s−1. However, there are eastward winds in May–August and again in December in the lower heights that the radar observes. These winds maximises at heights of about 86 km reaching velocities up to about 36 m s−1 and decays quickly above and below. The Mesospheric Semi-Annual Oscillation (MSAO) is clearly observed in the monthly-mean zonal winds. The first westward phase of the winds is much stronger than the second. The first westward phase of the MSAO was found to maximise at heights of about 84 km and to in general reach amplitudes of about −35 m s−1. We have compared the HWM-07 model to our observations. Our observed meridional winds are generally more southward than those of the model at meteor heights in the southern hemispheric winter, whereas HWM-07 suggests that in this season only weakly southward, or even northward flows occur at the lower heights. The zonal monthly-mean winds are in general agreement but somewhat less westward than observed by the radar. In one of the eight events in which the first westward phase of the MSAO was observed, the strongest westward winds reached about −75 m s−1, compared to the mean of about −35 m s−1 for other events. We explain this observation in terms of a mechanism which has been previously proposed by others. In this the relative phasing of the Stratospheric Quasi-Biennial Oscillation (SQBO) and the MSAO allow an unusually large flux of gravity waves with westward phase speed to reach the mesosphere. The dissipation of these waves then drives the MLT winds to large westward velocities. We demonstrate that the necessary phase relationship existed during the event we observed in 2002 and not during other times. This provides strong support for the suggestion that those extremes in zonal flow are a~result of modulated gravity-wave fluxes.

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