scholarly journals Equatorial wave analysis from SABER and ECMWF temperatures

2007 ◽  
Vol 7 (4) ◽  
pp. 11685-11723 ◽  
Author(s):  
M. Ern ◽  
P. Preusse ◽  
M. Krebsbach ◽  
M. G. Mlynczak ◽  
J. M. Russell III
Keyword(s):  
Gravity Waves ◽  
Lower Stratosphere ◽  
Spectral Power ◽  
Space Time ◽  
Kelvin Waves ◽  
Special Focus ◽  
Quasi Biennial Oscillation ◽  
Planetary Scale ◽  
Equatorial Wave ◽  
Wave Modes

Abstract. Equatorial planetary scale wave modes such as Kelvin waves or Rossby-gravity waves are excited by convective processes in the troposphere. In this paper an analysis for these and other equatorial wave modes is carried out with special focus on the stratosphere using temperature data from the SABER instrument as well as ECMWF temperatures. Space-time spectra of symmetric and antisymmetric spectral power are derived to separate the different equatorial wave types and the contribution of gravity waves is determined from the spectral background of the space-time spectra. Both gravity waves and equatorial planetary scale wave modes are main drivers of the quasi-biennial oscillation (QBO) in the stratosphere. Temperature variances attributed to the different wave types are calculated for the period from February 2002 until March 2006 and compared to previous findings. A comparison between SABER and ECMWF wave analyses shows that in the lower stratosphere SABER and ECMWF spectra and temperature variances agree remarkably well while in the upper stratosphere ECMWF tends to overestimate Kelvin wave components. Gravity wave variances are partly reproduced by ECMWF but have a significant low-bias. A case study for the time period of the SCOUT-O3 tropical aircraft measurement campaign in Darwin/Australia (in November and December 2005) is performed and we find that in the lower stratosphere also the longitude-time distribution of the Kelvin waves is correctly reproduced by ECMWF.

scholarly journals Equatorial wave analysis from SABER and ECMWF temperatures

2008 ◽  
Vol 8 (4) ◽  
pp. 845-869 ◽  
Author(s):  
M. Ern ◽  
P. Preusse ◽  
M. Krebsbach ◽  
M. G. Mlynczak ◽  
J. M. Russell
Keyword(s):  
Gravity Waves ◽  
Lower Stratosphere ◽  
Spectral Power ◽  
Space Time ◽  
Special Focus ◽  
Kelvin Wave ◽  
Quasi Biennial Oscillation ◽  
Planetary Scale ◽  
Equatorial Wave ◽  
Wave Modes

Abstract. Equatorial planetary scale wave modes such as Kelvin waves or Rossby-gravity waves are excited by convective processes in the troposphere. In this paper an analysis for these and other equatorial wave modes is carried out with special focus on the stratosphere using temperature data from the SABER satellite instrument as well as ECMWF temperatures. Space-time spectra of symmetric and antisymmetric spectral power are derived to separate the different equatorial wave types and the contribution of gravity waves is determined from the spectral background of the space-time spectra. Both gravity waves and equatorial planetary scale wave modes are main drivers of the quasi-biennial oscillation (QBO) in the stratosphere. Temperature variances attributed to the different wave types are calculated for the period from February 2002 until March 2006 and compared to previous findings. A comparison between SABER and ECMWF wave analyses shows that in the lower stratosphere SABER and ECMWF spectra and temperature variances agree remarkably well while in the upper stratosphere ECMWF tends to overestimate Kelvin wave components. Gravity wave variances are partly reproduced by ECMWF but have a significant low-bias. For the examples of a QBO westerly phase (October–December 2004) and a QBO easterly phase (November/December 2005, period of the SCOUT-O3 tropical aircraft campaign in Darwin/Australia) in the lower stratosphere we find qualitatively good agreement between SABER and ECMWF in the longitude-time distribution of Kelvin, Rossby (n=1), and Rossby-gravity waves.

scholarly journals Some Space–Time Spectral Analyses of Tropical Convection and Planetary-Scale Waves

2008 ◽  
Vol 65 (9) ◽  
pp. 2936-2948 ◽  
Author(s):  
Harry H. Hendon ◽  
Matthew C. Wheeler
Keyword(s):  
Zonal Wind ◽  
Space Time ◽  
Tropical Convection ◽  
Kelvin Waves ◽  
Equatorial Waves ◽  
Background Spectrum ◽  
Noise Process ◽  
Equatorial Wave ◽  
Spectral Peaks ◽  
Wave Modes

Abstract Three aspects of space–time spectral analysis are explored for diagnosis of the organization of tropical convection by the Madden–Julian oscillation (MJO) and other equatorial wave modes: 1) definition of the background spectrum upon which spectral peaks are assessed, 2) alternate variance preserving display of the spectra, and 3) the space–time coherence spectrum. Here the background spectrum at each zonal wavenumber is assumed to result from a red noise process. The associated decorrelation time for the red noise process for tropical convection is found to be half as long as for zonal wind, reflecting the different physical processes controlling each field. The significance of spectral peaks associated with equatorial wave modes for outgoing longwave radiation (OLR), which is a proxy for precipitating deep convection, and zonal winds that stand out above the red background spectrum is similar to that identified using a background spectrum resulting from ad hoc smoothing of the original spectrum. A variance-preserving display of the space–time power spectrum with a logarithmic frequency axis is useful for directly detecting Kelvin waves (periods 5–15 days for eastward zonal wavenumbers 1–5) and for highlighting their distinction from the MJO. The space–time coherence of OLR and zonal wind is predominantly associated with the MJO and other equatorial waves. The space–time coherence is independent of estimating the background spectrum and is quantifiable; thus, it is suggested as a useful metric for the MJO and other equatorial waves in observations and simulations. The space–time coherence is also used to quantify the association of Kelvin waves in the stratosphere with convective variability in the troposphere and for detection of barotropic Rossby–Haurwitz waves.

scholarly journals On the presence of equatorial waves in the lower stratosphere of a general circulation model

2013 ◽  
Vol 13 (8) ◽  
pp. 22607-22637 ◽  
Author(s):  
P. Maury ◽  
F. Lott
Keyword(s):  
Gravity Waves ◽  
General Circulation ◽  
Lower Stratosphere ◽  
Circulation Model ◽  
Atmospheric Model ◽  
Kelvin Waves ◽  
Flux Analysis ◽  
Equatorial Waves ◽  
Quasi Biennial Oscillation ◽  
The Waves

Abstract. To challenge the hypothesis that equatorial waves in the lower stratosphere are essentially forced by convection, we use the LMDz atmospheric model extended to the stratosphere and compare two versions having very different convection schemes but no quasi biennial oscillation (QBO). The two versions have realistic time mean precipitation climatologies but very different precipitation variabilities. Despite these differences, the equatorial stratospheric Kelvin waves at 50 hPa are almost identical in the two versions and quite realistic. The Rossby-gravity waves are also very close but significantly weaker than in observations. We demonstrate that this bias on the Rossby-gravity waves is essentially due to a dynamical filtering occurring because the model zonal wind is systematically westward: during a westward phase of the QBO, the Rossby-gravity waves in ERA-Interim compare well with those in the model. These results suggest that in the model the effect of the convection scheme on the waves is in part hidden by the dynamical filtering and the waves are produced by other sources than equatorial convection. For the Kelvin waves, this last point is illustrated by an Eliassen and Palm flux analysis, showing that in the model they come more from the subtropics and mid-latitude regions whereas in the ERA-Interim reanalysis the sources are more equatorial. We also show that non-equatorial sources are significant in reanalysis data, and we consider the case of the Rossby-gravity waves. We identify situations in the reanalysis where here are large Rossby-gravity waves in the middle stratosphere, and for dates when the stratosphere is dynamically separated from the equatorial troposphere. We refer to this process as a "stratospheric reloading".

scholarly journals On the presence of equatorial waves in the lower stratosphere of a general circulation model

2014 ◽  
Vol 14 (4) ◽  
pp. 1869-1880 ◽  
Author(s):  
P. Maury ◽  
F. Lott
Keyword(s):  
Gravity Waves ◽  
General Circulation ◽  
Lower Stratosphere ◽  
Circulation Model ◽  
Atmospheric Model ◽  
Kelvin Waves ◽  
Flux Analysis ◽  
Equatorial Waves ◽  
Quasi Biennial Oscillation ◽  
The Waves

Abstract. To challenge the hypothesis that equatorial waves in the lower stratosphere are essentially forced by convection, we use the LMDz atmospheric model extended to the stratosphere and compare two versions having very different convection schemes but no quasi-biennial oscillation (QBO). The two versions have realistic time mean precipitation climatologies but very different precipitation variabilities. Despite these differences, the equatorial stratospheric Kelvin waves at 50 hPa are almost identical in the two versions and quite realistic. The Rossby gravity waves are also very similar but significantly weaker than in observations. We demonstrate that this bias on the Rossby gravity waves is essentially due to a dynamical filtering occurring because the model zonal wind is systematically westward. During a westward phase of the QBO, the ERA-Interim Rossby gravity waves compare well with those in the model. These results suggest that (i) in the model the effect of the convection scheme on the waves is in part hidden by the dynamical filtering, and (ii) the waves are produced by other sources than equatorial convection. For the Kelvin waves, this last point is illustrated by an Eliassen and Palm flux analysis, showing that in the model they come more from the subtropics and mid-latitude regions, whereas in the ERA-Interim reanalysis the sources are more equatorial. We show that non-equatorial sources are also significant in reanalysis data sets as they explain the presence of the Rossby gravity waves in the stratosphere. To illustrate this point, we identify situations with large Rossby gravity waves in the reanalysis middle stratosphere for dates selected when the stratosphere is dynamically separated from the equatorial troposphere. We refer to this process as a stratospheric reloading.

scholarly journals Contributions of equatorial planetary waves and small-scale convective gravity waves to the 2019/20 QBO disruption

2021 ◽  
Author(s):  
Min-Jee Kang ◽  
Hye-Yeong Chun
Keyword(s):  
Gravity Waves ◽  
Rossby Wave ◽  
Rossby Waves ◽  
Reanalysis Data ◽  
Small Scale ◽  
Negative Wave ◽  
Quasi Biennial Oscillation ◽  
Wave Forcing ◽  
Equatorial Wave ◽  
Convective Gravity Waves

Abstract. In January 2020, unexpected easterly winds developed in the downward-propagating westerly quasi-biennial oscillation (QBO) phase. This event corresponds to the second QBO disruption in history, and it occurred four years after the first disruption that occurred in 2015/16. According to several previous studies, strong midlatitude Rossby waves propagating from the Southern Hemisphere (SH) during the SH winter likely initiated the disruption; nevertheless, the wave forcing that finally led to the disruption has not been investigated. In this study, we examine the role of equatorial waves and small-scale convective gravity waves (CGWs) in the 2019/20 QBO disruption using MERRA-2 global reanalysis data. In June–September 2019, unusually strong Rossby wave forcing originating from the SH decelerated the westerly QBO at 0°–5° N at ~50 hPa. In October–November 2019, vertically (horizontally) propagating Rossby waves and mixed Rossby–gravity (MRG) waves began to increase (decrease). From December 2019, contribution of the MRG wave forcing to the zonal wind deceleration was the largest, followed by the Rossby wave forcing originating from the Northern Hemisphere and the equatorial troposphere. In January 2020, CGWs provided 11 % of the total negative wave forcing at ~43 hPa. Inertia–gravity (IG) waves exhibited a moderate contribution to the negative forcing throughout. Although the zonal-mean precipitation was not significantly larger than the climatology, convectively coupled equatorial wave activities were increased during the 2019/20 disruption. As in the 2015/16 QBO disruption, the increased barotropic instability at the QBO edges generated more MRG waves at 70–90 hPa, and westerly anomalies in the upper troposphere allowed more westward IG waves and CGWs to propagate to the stratosphere. Combining the 2015/16 and 2019/20 disruption cases, Rossby waves and MRG waves can be considered the key factors inducing QBO disruption.

scholarly journals Observation and a numerical study of gravity waves during tropical cyclone Ivan~(2008)

2013 ◽  
Vol 13 (4) ◽  
pp. 10757-10807 ◽  
Author(s):  
F. Chane Ming ◽  
C. Ibrahim ◽  
S. Jolivet ◽  
P. Keckhut ◽  
Y.-A. Liou ◽  
...  
Keyword(s):  
Tropical Cyclone ◽  
Gravity Waves ◽  
High Frequency ◽  
Lower Stratosphere ◽  
Numerical Study ◽  
Wide Spectrum ◽  
Spectral Characteristics ◽  
Background Wind ◽  
Quasi Biennial Oscillation ◽  
North East

Abstract. Activity and spectral characteristics of gravity-waves (GWs) are analyzed during tropical cyclone (TC) Ivan (2008) in the troposphere and lower stratosphere using radiosonde and GPS radio occultation data, ECMWF outputs and simulations of French numerical model Meso-NH with vertical resolution varying between 150 m near the surface and 500 m in the lower stratosphere. Conventional methods for GW analysis and signal and image processing tools provide information on a wide spectrum of GWs with horizontal wavelengths of 40–1800 km and short vertical wavelengths of 0.6–10 km respectively and periods of 20 min–2 days. MesoNH model, initialized with Aladin-Réunion analyses, produces realistic and detailed description of TC dynamics, GWs, variability of the tropospheric and stratospheric background wind and TC rainband characteristics at different stages of TC Ivan. In particular a dominant eastward propagating TC-related quasi-inertia GW is present during intensification of TC Ivan with horizontal and vertical wavelengths of 400–600 km and 1.5–3.5 km respectively during intensification. A wavenumber-1 vortex Rossby wave is identified as a source of this medium-scale mode while short-scale modes located at north-east and south-east of the TC could be attributed to strong localized convection in spiral bands resulting from wavenumber-2 vortex Rossby waves. Meso-NH simulations also reveal high-frequency GWs with horizontal wavelengths of 20–80 km near the TC eye and high-frequency GWs-related clouds behind TC Ivan. In addition, GWs produced during landfall are likely to strongly contribute to background wind in the middle and upper troposphere as well as the stratospheric quasi-biennial oscillation.

scholarly journals Equatorial Waves in Opposite QBO Phases

2011 ◽  
Vol 68 (4) ◽  
pp. 839-862 ◽  
Author(s):  
Gui-Ying Yang ◽  
Brian J. Hoskins ◽  
Julia M. Slingo
Keyword(s):  
Group Velocity ◽  
Lower Stratosphere ◽  
Phase Speed ◽  
Kelvin Waves ◽  
Western Hemisphere ◽  
Equatorial Waves ◽  
Vertical Wavelength ◽  
Quasi Biennial Oscillation ◽  
Zonal Wavenumber ◽  
The Waves

Abstract A methodology for identifying equatorial waves is used to analyze the multilevel 40-yr ECMWF Re-Analysis (ERA-40) data for two different years (1992 and 1993) to investigate the behavior of the equatorial waves under opposite phases of the quasi-biennial oscillation (QBO). A comprehensive view of 3D structures and of zonal and vertical propagation of equatorial Kelvin, westward-moving mixed Rossby–gravity (WMRG), and n = 1 Rossby (R1) waves in different QBO phases is presented. Consistent with expectation based on theory, upward-propagating Kelvin waves occur more frequently during the easterly QBO phase than during the westerly QBO phase. However, the westward-moving WMRG and R1 waves show the opposite behavior. The presence of vertically propagating equatorial waves in the stratosphere also depends on the upper tropospheric winds and tropospheric forcing. Typical propagation parameters such as the zonal wavenumber, zonal phase speed, period, vertical wavelength, and vertical group velocity are found. In general, waves in the lower stratosphere have a smaller zonal wavenumber, shorter period, faster phase speed, and shorter vertical wavelength than those in the upper troposphere. All of the waves in the lower stratosphere show an upward group velocity and downward phase speed. When the phase of the QBO is not favorable for waves to propagate, their phase speed in the lower stratosphere is larger and their period is shorter than in the favorable phase, suggesting Doppler shifting by the ambient flow and a filtering of the slow waves. Tropospheric WMRG and R1 waves in the Western Hemisphere also show upward phase speed and downward group velocity, with an indication of their forcing from middle latitudes. Although the waves observed in the lower stratosphere are dominated by “free” waves, there is evidence of some connection with previous tropical convection in the favorable year for the Kelvin waves in the warm water hemisphere and WMRG and R1 waves in the Western Hemisphere, which is suggestive of the importance of convective forcing for the existence of propagating coupled Kelvin waves and midlatitude forcing for the existence of coupled WMRG and R1 waves.

scholarly journals Momentum forcing of the quasi-biennial oscillation by equatorial waves in recent reanalyses

2015 ◽  
Vol 15 (12) ◽  
pp. 6577-6587 ◽  
Author(s):  
Y.-H. Kim ◽  
H.-Y. Chun
Keyword(s):  
Gravity Wave ◽  
Transition Phase ◽  
Equatorial Waves ◽  
Quasi Biennial Oscillation ◽  
Wave Forcing ◽  
Opposite Phase ◽  
Level Data ◽  
Equatorial Wave ◽  
Biennial Oscillation ◽  
Wave Modes

Abstract. The momentum forcing of the QBO (quasi-biennial oscillation) by equatorial waves is estimated using recent reanalyses. Based on the estimation using the conventional pressure-level data sets, the forcing by the Kelvin waves (3–9 m s−1 month−1) dominates the net forcing by all equatorial wave modes (3–11 m s−1 month−1) in the easterly-to-westerly transition phase at 30 hPa. In the opposite phase, the net forcing by equatorial wave modes is small (1–5 m s−1 month−1). By comparing the results with those from the native model-level data set of the ERA-Interim reanalysis, it is suggested that the use of conventional-level data causes the Kelvin wave forcing to be underestimated by 2–4 m s−1 month−1. The momentum forcing by mesoscale gravity waves, which are unresolved in the reanalyses, is deduced from the residual of the zonal wind tendency equation. In the easterly-to-westerly transition phase at 30 hPa, the mesoscale gravity wave forcing is found to be smaller than the resolved wave forcing, whereas the gravity wave forcing dominates over the resolved wave forcing in the opposite phase. Finally, we discuss the uncertainties in the wave forcing estimates using the reanalyses.

scholarly journals Momentum forcing of the QBO by equatorial waves in recent reanalyses

2015 ◽  
Vol 15 (4) ◽  
pp. 5175-5202
Author(s):  
Y.-H. Kim ◽  
H.-Y. Chun
Keyword(s):  
Gravity Wave ◽  
Gravity Waves ◽  
Pressure Level ◽  
Equatorial Waves ◽  
Kelvin Wave ◽  
Wave Forcing ◽  
Opposite Phase ◽  
Level Data ◽  
Equatorial Wave ◽  
Wave Modes

Abstract. The momentum forcing by equatorial waves to the QBO is estimated using recent reanalyses. Based on the estimation using the conventional pressure level datasets, the forcing by the Kelvin waves (3–9 m s−1 month−1) dominates the net forcing by all equatorial wave modes in the easterly-to-westerly transition phase at 30 hPa (3–11 m s−1 month−1). In the opposite phase, the net forcing by equatorial wave modes is small (1–5 m s−1 month−1). By comparing the results with those from the native model-level dataset of the ERA-Interim reanalysis, it is suggested that the use of conventional-level data causes the Kelvin wave forcing to be underestimated by 2–4 m s−1 month−1. The momentum forcing by mesoscale gravity waves, which are unresolved in the reanalyses, is deduced from the residual of the zonal wind tendency equation. In the easterly-to-westerly phase at 30 hPa, the mesoscale gravity wave forcing is found to be smaller than the resolved wave forcing, whereas the gravity wave forcing dominates over the resolved wave forcing in the opposite phase. Finally, we discuss the uncertainties in the wave forcing estimates using the reanalyses.

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