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 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 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 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 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.

scholarly journals Wave fluxes of equatorial Kelvin waves and QBO zonal wind forcing derived from SABER and ECMWF temperature space-time spectra

2009 ◽  
Vol 9 (2) ◽  
pp. 5623-5677 ◽  
Author(s):  
M. Ern ◽  
P. Preusse
Keyword(s):  
Gravity Waves ◽  
Zonal Wind ◽  
Wind Shear ◽  
Space Time ◽  
Kelvin Waves ◽  
Wind Forcing ◽  
Westerly Wind ◽  
Kelvin Wave ◽  
Wave Forcing ◽  
Total Wave

Abstract. The quasi-biennial oscillation (QBO) of the zonal mean zonal wind is one of the most important processes in the dynamics of the middle atmosphere in the tropics. Influences of the QBO can even be found at mid and high latitudes. It is widely accepted that the phase descent of alternating tropical easterlies and westerlies is driven by atmospheric waves of both global scale (equatorial wave modes like Kelvin, equatorial Rossby, Rossby-gravity, or inertia-gravity waves), as well as mesoscale gravity waves. However, the relative distribution of the different types of waves to the forcing of the QBO winds is highly uncertain. This is the case because until recently there were no high resolution long-term global measurements in the stratosphere. In our study we estimate Kelvin wave momentum flux and the contribution of zonal wind forcing by Kelvin waves based on space-time spectra determined from both Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) temperature measurements as well as temperatures from European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses. Peak values of total Kelvin wave zonal wind forcing found are about 0.2 m/s/day. There is good agreement between SABER and ECMWF results. Global distributions are shown and the results are compared to the total wave forcing required to balance the background atmosphere. Sometimes Kelvin wave forcing is sufficient to explain almost the whole total wave forcing required for the momentum balance during the transition from QBO easterly to westerly winds. This is especially the case during the later parts of the periods of westerly wind shear at the equator between 20 and 35 km altitude. During other phases of the westerly wind shear periods, however, the contribution of Kelvin waves can be comparably low and the missing wave forcing, which is often attributed to mesoscale gravity waves or intermediate scale waves, can be the by far dominant contribution of the QBO forcing.

scholarly journals An Object-Based Approach to Assessing the Organization of Tropical Convection

2012 ◽  
Vol 69 (8) ◽  
pp. 2488-2504 ◽  
Author(s):  
Juliana Dias ◽  
Stefan N. Tulich ◽  
George N. Kiladis
Keyword(s):  
Dimensional Space ◽  
Three Dimensional ◽  
Physical Space ◽  
Tropical Convection ◽  
Minimum Size ◽  
Spectral Space ◽  
Background Spectrum ◽  
Planetary Scale ◽  
Object Based ◽  
Spectral Peaks

Abstract The organization of tropical convection is assessed through an object-based analysis of satellite brightness temperature data Tb, a proxy for convective activity. The analysis involves the detection of contiguous cloud regions (CCRs) in the three-dimensional space of latitude, longitude, and time where Tb falls below a given threshold. A range of thresholds is considered and only CCRs that satisfy a minimum size constraint are retained in the analysis. Various statistical properties of CCRs are documented including their zonal speed of propagation, which is estimated using a Radon transformation technique. Consistent with previous studies, a majority of CCRs are found to propagate westward, typically at speeds of around 15 m s−1, regardless of underlying Tb threshold. Most of these zonally propagating CCRs have lifetimes less than 2 days and zonal widths less than 800 km, implying aggregation of just a few individual mesoscale convective systems. This object-based perspective is somewhat different than that obtained in previous Fourier-based analyses, which primarily emphasize the organization of convection on synoptic and planetary scales via wave–convection coupling. To reconcile these contrasting views, an object-based data reconstruction is developed that objectively demonstrates how the spectral peaks of synoptic- to planetary-scale waves can be attributed to the organization of CCRs into larger-scale wave envelopes. A novel method based on the randomization of CCRs in physical space leads to an empirical background spectrum for organized tropical convection that does not rely on any smoothing in spectral space. Normalization by this background reveals spectral peaks associated with synoptic- and planetary-scale waves that are consistent with previous studies.

scholarly journals Equatorial Waves in the Upper Troposphere and Lower Stratosphere Forced by Latent Heating Estimated from TRMM Rain Rates

2011 ◽  
Vol 68 (10) ◽  
pp. 2321-2342 ◽  
Author(s):  
Jung-Hee Ryu ◽  
M. Joan Alexander ◽  
David A. Ortland
Keyword(s):  
Rain Rate ◽  
Lower Stratosphere ◽  
Equatorial Waves ◽  
Latent Heating ◽  
Upper Troposphere ◽  
Equivalent Depth ◽  
Equatorial Wave ◽  
Spectral Peaks ◽  
Heating Profiles ◽  
Rain Rates

Abstract Equatorial atmospheric waves in the upper troposphere and lower stratosphere (UTLS), excited by latent heating, are investigated by using a global spectral model. The latent heating profiles are derived from the 3-hourly Tropical Rainfall Measuring Mission (TRMM) rain rates, which include both convective- and stratiform-type profiles. The type of heating profile is determined based on an intensity of the surface rain rate. Latent heating profiles over stratiform rain regions, estimated from the TRMM Precipitation Radar (PR) product, are applied to derive the stratiform-type latent heating profiles from the gridded rain rate data. Monthly zonal-mean latent heating profiles derived from the rain rates appear to be reasonably comparable with the TRMM convective/stratiform heating product. A broad spectrum of Kelvin, mixed Rossby–gravity (MRG), equatorial Rossby (ER), and inertia–gravity waves are generated in the model. Particularly, equatorial waves (Kelvin, ER, and MRG waves) of zonal wavenumbers 1–5 appear to be dominant in the UTLS. In the wavenumber–frequency domain, the equatorial waves have prominent spectral peaks in the range of 12–200 m of the equivalent depth, while the spectral peaks of the equatorial waves having shallower equivalent depth (<50 m) increase in the case where stratiform-type heating is included. These results imply that the stratiform-type heating might be relevant for the shallower equivalent depth of the observed convectively coupled equatorial waves. The horizontal and vertical structures of the simulated equatorial waves (Kelvin, ER, and MRG waves) are in a good agreement with the equatorial wave theory and observed wave structure. In particular, comparisons of the simulated Kelvin waves and the High Resolution Dynamics Limb Sounder (HIRDLS) satellite observation are discussed.

scholarly journals Real-time identification of equatorial waves and evaluation of waves in global forecasts

2020 ◽  
Author(s):  
Gui-Ying Yang ◽  
Samantha Ferrett ◽  
Steve Woolnough ◽  
John Methven ◽  
Chris Holloway
Keyword(s):  
Real Time ◽  
Time Window ◽  
Broad Band ◽  
Kelvin Waves ◽  
Equatorial Waves ◽  
Frequency Filter ◽  
Central Pacific ◽  
Band Frequency ◽  
Equatorial Wave ◽  
The Impact

AbstractA novel technique is developed to identify equatorial waves in analyses and forecasts. In a real-time operational context, it is not possible to apply a frequency filter based on a wide centred time-window due to the lack of future data. Therefore, equatorial wave identification is performed based primarily on spatial projection onto wave mode horizontal structures. Spatial projection alone cannot distinguish eastward from westward-moving waves, so a broad-band frequency filter is also applied. The novelty in the real-time technique is to off-centre the time-window needed for frequency filtering, using forecasts to extend the window beyond the current analysis. The quality of this equatorial wave diagnosis is evaluated. Firstly, the “edge effect” arising because the analysis is near the end of the filter time-window is assessed. Secondly, the impact of using forecasts to extend the window beyond the current date is quantified. Both impacts are shown to be small referenced to wave diagnosis based on a centred time-window of re-analysis data. The technique is used to evaluate the skill of the Met Office forecast system in 2015-2018. Global forecasts exhibit substantial skill (correlation > 0.6) in equatorial waves, to at least day 4 for Kelvin waves and day 6 for Westward Mixed Rossby-Gravity (WMRG), and meridional mode number n=1 and n=2 Rossby waves. A local wave phase diagram is introduced that is useful to visualise and validate wave forecasts. It shows that in the model Kelvin waves systematically propagate too fast and there is a 25% underestimate of amplitude in Kelvin and WMRG waves over the Central Pacific.

scholarly journals Wave fluxes of equatorial Kelvin waves and QBO zonal wind forcing derived from SABER and ECMWF temperature space-time spectra

2009 ◽  
Vol 9 (12) ◽  
pp. 3957-3986 ◽  
Author(s):  
M. Ern ◽  
P. Preusse
Keyword(s):  
Gravity Waves ◽  
Zonal Wind ◽  
Wind Shear ◽  
Space Time ◽  
Kelvin Waves ◽  
Wind Forcing ◽  
Westerly Wind ◽  
Kelvin Wave ◽  
Wave Forcing ◽  
Total Wave

Abstract. The quasi-biennial oscillation (QBO) of the zonal mean zonal wind is a dynamical phenomenon of the tropical middle atmosphere. Influences of the QBO can even be found at mid and high latitudes. It is widely accepted that the phase descent of alternating tropical easterlies and westerlies is driven by atmospheric waves of both global scale (equatorial wave modes like Kelvin, equatorial Rossby, Rossby-gravity, or inertia-gravity waves), as well as mesoscale gravity waves. However, the relative distribution of the different types of waves to the forcing of the QBO winds is highly uncertain. This is the case because until recently there were no high resolution long-term global measurements in the stratosphere. In our study we estimate Kelvin wave momentum flux and the contribution of zonal wind forcing by Kelvin waves based on space-time spectra determined from both Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) temperature measurements as well as temperatures from European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses. Peak values of total Kelvin wave zonal wind forcing found are about 0.2 m/s/day. There is good agreement between SABER and ECMWF results. Altitude-time cross sections are shown and the results are compared to the total wave forcing required to balance the background atmosphere. Sometimes Kelvin wave forcing is sufficient to explain almost the whole total wave forcing required for the momentum balance during the transition from QBO easterly to westerly winds. This is especially the case during the periods of strong westerly wind shear when the zonal wind is between −20 and 10 m/s at the equator in the altitude range 20 to 35 km. During other parts of the phases of strong westerly wind shear, however, the contribution of Kelvin waves can be comparably low and the missing wave forcing, which is often attributed to mesoscale gravity waves or intermediate scale waves, can be the by far dominant contribution of the QBO forcing. It is also found that seasonal variations of Kelvin wave accelerations could play an important role for the maintenance of the QBO westerly wind jets in the lower stratosphere.

scholarly journals Differences between Faster versus Slower Components of Convectively Coupled Equatorial Waves

2013 ◽  
Vol 71 (1) ◽  
pp. 98-111 ◽  
Author(s):  
Kazuaki Yasunaga ◽  
Brian Mapes
Keyword(s):  
Gravity Waves ◽  
Tropical Rainfall Measuring Mission ◽  
Space Time ◽  
Equatorial Waves ◽  
Convectively Coupled Equatorial Waves ◽  
Spectral Signal ◽  
Equatorial Wave ◽  
Significant Difference ◽  
Signal Peak ◽  
Inertia Gravity Waves

Abstract This paper describes an analysis of multiyear satellite datasets that subdivide two halves (faster and slower) of the space–time spectral signal peaks corresponding to convectively coupled equatorial waves such as Kelvin and inertia–gravity waves [n = 0 eastward inertia–gravity wave (EIGn0 wave), and n = 1 and n = 2 westward inertia–gravity waves (WIGn1 and WIGn2 waves, respectively)]. The faster (slower) component of an equatorial wave is defined as that which has a spectral signal peak in the regions with deeper (shallower) equivalent depths. The data obtained from the Tropical Rainfall Measuring Mission (TRMM) precipitation radar (TRMM-PR) are composited around space–time-filtered equatorial-belt data from the TRMM-3B42 rainfall product to separately estimate the convective and stratiform rainfall modulations. Results indicate that the faster components of WIGn1 and WIGn2 waves modulate convective rain relatively more (and stratiform rain relatively less) than their slower counterparts. For Kelvin and EIGn0 waves, however, there is no significant difference in the rainfall modulation between their faster and slower components. A space–time cospectral analysis of the satellite-retrieved rainfall and moisture shows that in the spectral regions corresponding to WIGn1 and WIGn2 waves, precipitation is significantly correlated with low-level moisture but not with midlevel moisture. In contrast, significant coherence between rainfall and moisture at these levels is found in the spectral regions corresponding to the Kelvin and EIGn0 waves. These results may bear on different convection–wave coupling mechanisms for these “divergent” waves (stratiform instability versus moisture–stratiform instability).

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