Evidence for Rossby-gravity waves in tropical data fields produced by the European Center for Medium Range Weather Forecasts (ECMWF) was recently reported. Similar features are observable in fields of total column ozone from the Total Ozone Mapping Spectrometer (TOMS) satellite instrument. The observed features are episodic, have zonal (east-west) wavelengths of 6,000-10,000 km, and oscillate with periods of 5-10 days. In accord with simple linear theory, the modes exhibit westward phase progression and eastward group velocity. The significance of finding Rossby-gravity waves in total ozone fields is that (1) the report of similar features in ECMWF tropical fields is corroborated with an independent data set and (2) the TOMS data set is demonstrated to possess surprising versatility and sensitivity to relatively smaller scale tropical phenomena

Stanford, J. L.

Ziemke, J. R.

English

NASA Technical Reports Server

NASA-CR-192983
ROSSBY-GRAVITY WAVES
IN TROPICAL TOTAL OZONE DATA
J. L. Stanford and J. R. Ziemke
Department of Physics &: Astronomy, Iowa State University
Ames, Iowa 50011
Abstract. Randel (1992) has recently reported evidence for Rossby-gravity waves in
tropical data fields produced by the European Centre for Medium Range Weather Forecasts
(ECMWF). The purpose of this paper is to show that similar features are observable in
fields of total column ozone from the Total Ozone Mapping Spectrometer (TOMS) satellite
instrument. The observed features are episodic, have zonal (east-west) wavelengths of ._
6,000-10,000 km and oscillate with periods of 5-20 days. In accord with simple linear
theory, the modes exhibit westward ph.ase progression and eastward group velocity. The
significance of finding Rossby-gravity waves in total ozone fields is that (1) the report of
similar features in ECMWF tropical fields is corroborated with an independent data set
and (2) the TOMS data set is demonstrated to possess surprising versatility and sensitivity
to relatively smaller scale tropical phenomena.
Submitted to Geophysical Research ZeH.ers, March 1993
(NASA-CR-192983) ROSSBY-GRAVITY
WAVES IN TROPICAL TOTAL OZONE DATA
(Iowa State Univ. of Science and
Technology) 10 p
N93-25z_32
Unclas
G3/_5 0160Z66
https://ntrs.nasa.gov/search.jsp?R=19930016243 2020-03-17T06:50:23+00:00Z
Introduction
When one looks at small scale features found in tropical model output a question
that comes to mind is the extent to which such features reflect the real atmosphere versus
reflecting the dynamics built into the model. A recent study by Randel (1992) was mo-
tivated, in part, to obtain "truth" for validating tropical model results from the NCAR
CCM (National Center for Atmospheric Research Community Climate Model). To do this,
Randel used European Centre for Medium Range Weather Forecasts (ECMWF) tropical
data. The ECMWF data themselves are global analyses resulting from a four-dimensional
assimilation of available data using first-guess fields genera.ted by the ECMWF forecast
model, and later initialized to damp spurious fast gravity v_aves. Tropical data fields may
be inaccurate due to sparse observational data and uncertainties in diabatic effects and
mass-wind balance effects. Thus it is important to substantiate fields from tropical models
and from combined model-observations (such as ECMWF). Randel (1992) found evidence
for episodic Rossby-gravity wave signatures in the tropical ECMWF data, and was able
to further corroborate the signals with tropical rawinsonde observations. The dynamical
significance of Rossby-gravity waves is that they are believed to play a crucial role in the
general circulation of the middle atmosphere through momentum transport and deposition,
forcing the westerly phase of the well-known quasi-biennial oscillation.
In the present paper we show that features which compare closely with Randel's
ECMWF Rossby-gravity signals also can be found in tropical total ozone measurements
from the TOMS (Total Ozone Mapping Spectrometer) satellite instrument.
Randel (1990) used space and time spectral decomposition to demonstrate significant
differences between data retrieval schemes used with LIMS (Limb Infrared Monitor of the
Stratosphere). Such techniques can provide very sensitive tests of both model output and
data retrieval schemes. Our investigation utilizes similar techniques with TOMS measure-
ments representing total column ozone, integrated over a thick vertical column throughout
the stratosphere; such data might be thought insensitive to waves with relatively short
vertical wavelengths (_ 5 km) such as Rossby-gravity waves. The results presented here
point to surprising utility of TOMS data sets: we find it is possible to observe several types
of tropical wave signatures in total column ozone and in the next section show one such
example, viz. Rossby-gravity waves.
Since TOMS is a totally independent instrument and data set from those used in
producing ECMWF fields, the analyses presented here further corroborate the validity of
the wave activity discussed by Randel (1992), and strengthen confidence in the ECMWF
tropical data sets.
Data and analysis
The TOMS instrument aboard the Nimbus 7 sun-synchronous polar orbiting satellite
has provided daily data since launch in October 1978. Bowman and Krueger (1985) ana-
lyzed the first four years of 5 ° x 5°gridded TOMS data from an earlier version and gave an
account of missing data and global ozone characteristics. The long time series available for
TOMS (we use 13 years of data, starting in 1979), plus its excellent horizontal resolution,
make it useful for model validation studies.
2
This study usesTOMS version 6 daily data from the NASA/Goddard Space Flight
Center made available on CD-ROM disk in a format of l°latitude by 1.25°longitude bins,
covering 1 November 1978to 31January 1992. We regridded the data onto a 5°x 5°mesh
extending from 85°N to 85°S in latitude and 180°to 175°W in longitude. Zonally averaged
monthly mean percentagesof missing 5°x 5°data were found to be less than about 2%
at all latitudes (excluding polar night) and all months from June 1980to January 1992.
The worst month for missing data is June 1979for which _30-40% of data is missing at
latitudes not in polar night. Similarly, July 1979to May 1980have roughly 5-10% missing
data. All missing data were replacedusing linear interpolation in time.
Space-time spectra were computed using either 180, 90, or 60-day window lengths.
During each calculation, time seriesaveragesand possible low frequency trends were re-
moved using the filtering technique discussedby Stanford et al. (1993); the chosenfilter
responsehad half-amplitude at a period of 250 days. This was followed by applying a
10%-10% cosine taper window to reduce leakage. These power spectra calculations also
included a decomposition in space-timevariablesinto eastward and westward components
(see, for example' the Appendix of Ziemke and Stanford, .1990);no effort was made to
remove standing-wave variability in the data. A 0.25-0.5-0.25 running mean was applied
once to the raw power ordinates as_.spectral estimator. Raw power spectra ordinates are
defined in this study by (2A f) -1 [A2(k, _) + B2(k, _)], where Af is the unit bandwidth (1
day -1 here). Spectral amplitude used in this study is the square root of twice the power.
A(k,ca) and B(k,w) are calculated via Fast Fourier Transform (FFT) in both time and
space from the following zonal space-time harmonic definition for total column ozone fl:
(1)
Here, t (x) denotes time (zonal distance), At (Az) is the temporal (spatial) sampling
interval, k is the zonal wavevector, w is the circular frequency, and + (-) denotes the
westward (eastward) propagating component of f_. Spectra involving more than one zonal
wavenumber were derived by combining the spectra for each individual wavenumber.
All plots in this study, other than spectra, do not incorporate an eastward/westward
separation. The bandpass filter response function given by Murakami (1979) was applied
in frequency space (using an inverse FFT) to reconstruct bandpass filtered time series.
Rossby-gravity waves in TOMS
The study by Randel (1992) used ECMWF tropical wind fields for 1980-1987. He
found waves with 6-10 day periods in the meridional wind, propagating westward and
concentrated primarily in zonal waves 4-7 (wavelengths of ,-_ 6,000-10,000 km). One
lengthy episode extending from the latter half of July 1985 into October 1985 was partic-
ularly strong, with meridional wind amplitudes maximizing in the upper troposphere near
200 hPa pressure level (,-- 10 km altitude). The wave signatures resembled Rossby-gravity
waves trapped near the equator, although not exactly symmetric about the equator. The
spectral characteristics for this 1985 episode in RandeI's 200 hPa meridional wind at 2°S are
captured by waves 3-8 with periods 4-15 days. To compare with his study, we searched for
3
this episodein the tropical TOMS data after retaining only zonal waves3-8 and employing
a bandpassfilter with half amplitudes at 4 and 15-dayperiods.
Figure 1 presents the TOMS spectra during July-October 1985, at the time of the
Rossby-gravity wavesfound by Randel. The spectraare computed with a 90-day window
beginning 16July 1985. As highlighted by the arrows,there i§ a clearwestward propagating
disturbance in zonal waves5-6 with a period near 6 days. Randel (1992) finds a similar 6-
day wavein zonal waves5-6 (his Fig. llb). Figure 2 showsa HofmSller (time vs longitude)
diagram during the time coveredby Fig. 1, for TOMS data evaluated at 5°N. Also shown
for comparison is Randel's analysis (his Fig. lla) of unfiltered ECMWF meridional winds.
Figure 2 revealsclear evidenceof westwardphasepropagation and eastwardpacket (group
velocity) propagation. As discussednext, such behavior is consistent with linear tropical
Rossby-gravity wave dynamics.
Comparison with theory
The usual quasi-geostrophicapproximation is invalid for wave dynamics in low lati-
tudes becausethe apparent Coriolis force is small near the equator. Starting instead with
the primitive equations, in the form of an equatorial beta plane model linearized about a
state of rest, the equations of motion yield solutionsof the form (Andrews et al., 1987):
Here x, y, z are eastward, northward, and vertical coordinates; u, v and w are zonal, merid-
ional and vertical winds; _ is geopotential; w is angular frequency and H is scale height
(about 7 km for the lower stratosphere). The wavenumbers in the x and z directions are
k and m. Primes are perturbation quantities and carets 0 refer to their amplitudes as a
function of y. The solutions are those which occur in quantum harmonic oscillator theory,
having the form of a Gaussian times Hermite polynomials. The gravest solution contains
the Rossby-gravity mode, for which the zonal, meridional and vertical wind perturbation
components are
(_, _, _ ) = do. [i[mlwyN -1 , 1, iwy]exp(-_3[m[(2N)-ty2). (3)
The vertical wavenumber is given by m = -sgn(w)Nw-2(fl + oak), where fl is the Coriolis
parameter 2_2a-lcos(y/a), a is the earth's radius, and N is a measure of the static stability,
,,_ 2 × 10 -2 for the stratosphere. In the absence of a background wind, the Rossby-gravity
wave zonal phase speed is given by
vp= = (2m)-' {N + IN + '/2} (4)
For Rossby-gravity waves whose zonal phase speed is westward (as observed in Fig.
2), w is negative and the negative sign must be chosen in. (4). For this case, the zonal
group velocity is given by
vg = Ow/Ok = (2m) -1 __Y - Y2k[.V_k 2 + 4Nflm] -'/2 } (5)
which is positive, viz. eastward, opposite to the direction of phase movement in (4). These
vp and vg are speeds with respect to mean background winds. The winds in the lower
stratosphere over Singapore during this time are ,,, 10 ms -1 (B. Naujokat, Free University
of Berlin, personal communication), but this is considerably to the west of the maximum
Rossby-gravity wave amplitudes in the eastern Pacific (Fig. 2). For the latter region,
Randel's Fig. 12b shows that the 200 hPa mean wind is near zero. The observed phase
speed of wave 6, 6-day period waves is --, 12 ms -1 westward with respect to the earth.
With respect to the mean background wind, the phase of the waves in Fig. 2 is thus
likely moving slowly westward, while that of the packet envelope (Vg) is eastward, opposite
that of the mean wind. These observations are thus consistent with Rossby-gravity wave
dynamics.
Expression (3), for zero background wind, predicts pert .urbati°ns localized in the vicin-
ity of the equator (y = 0), decaying in amplitude by 1/e in distance L = _/2N/_3m. Randel
(1992) estimated m = 7.9 x 10 -4 meter -1, so that L - 13 ° latitude. For variance, the
1/e distance is half this value. Figure 3 shows both Randel's meridional ECMWF wind
(variance) and our TOMS (amplitude) analyses near the equator. The horizontal phase
structure of the waves is shown in Fig. 4. The ozone perturbations were examined by
calculating covariances, N -1. _]_--N 12,.,f(t). f_(t), after filtering and removing all time
means. For this two-month case study, N = 60. Figure 4b shows the results. Note that
the observed perturbations indeed are trapped near the equator, although the 1/e distance
may be larger than the zero background wind prediction. The presence of near-by midlat-
itude disturbances makes it difficult to accurately estimate the 1/e distance. As noted by
Randel, the background zonal wind field exhibits latitudinal asymmetry about the equator
for this case study and a marked reduction in meridional trapping can occur with wind
shear (Zhang and Webster, 1989). The fact that the disturbances are not symmetric about
the equator is perhaps also related to background wind shear.
Finally, we note that the TOMS perturbation envelope (lower stratosphere) is some-
what to the east of the upper tropospheric ECMWF signal in Fig. 2. This eastward packet
displacement with height is consistent with Holton's (1972) modeling of Rossby-gravity
wave excitation from upper tropospheric sources: Holton's .perturbations propagated up-
ward and eastward into the lower stratosphere.
Summary
1. The features described here in TOMS total ozone, an independent data set, cor-
roborate the Rossby-gravity waves found recently by Randel (1992) in ECMWF tropical
analysis fields.
2. The original TOMS design objective was to measure ozone integrated throughout
the depth of the stratosphere. The total ozone analyses presented here demonstrate that
TOMS data are considerably more versatile and can be utilized for investigations of relative
short-vertical-scale tropical features such as Rossby-gravity waves.
The long time series (,-_ 15 years), near global coverage and high quality characteristics
of TOMS data suggest that they are suitable for validating model results and other data
sets. They may be particularly valuable in the relatively data poor and dynamically
difficult tropical region.
Acknowledgments. We thank Ms. Patricia T. Guimaraes, Dr. Richard D. McPeters,
Dr. Arlin J. Krueger, and the members of the NASA TOMS NIMBUS Experiment and
Information Processing Teams, and the National Space Science Data Center for providing
the TOMS data on CD-ROM disc. We have enjoyed several helpful conversations with Dr.
W. J. Randel. This work was supported by National Aeronautics and Space Administration
Grant NAG 5-1519.
References
Andrews, D. G., J. R. Holton, and C. B. Leovy, 1987: Middle Atmosphere Dynamics,
Academic Press, Inc., New York. pp. 489.
Bowman, K. P., and A. J. Krueger, 1985: A global climatology of total ozone from
the Nimbus 7 Total Ozone Mapping Spectrometer. 3". Geophys. Res., 90, 7967-7976.
Holton, ,1. R., 1972: Waves in the equatorial stratosphere generated by tropospheric
heat sources. ]. Atmos. Sci., 29, 368-375.
Murakami, M., 1979: Large-scale aspects of deep convective activity over the GATE
area. Mon. Wen. Rev., I07, 994-1013.
Randel, W. J., 1990: Kelvin wave-induced trace Constituent oscillations in the equa-
torial stratosphere. J. Geophys. Res., 95, 18641-18652.
Randel, W. ,1., 1992: Upper tropospheric equatorial waves in ECMWF analyses. Q.
J. Roy. Meteor. Soc., 118, 365-394.
Stanford, J. L., J. R. Ziemke, and S. Y. Gao, 1993: Stratospheric circulation features
deduced from SAMS constituent data. J. Atmos. Sci., 50, 226-246.
Zhang, C. D., and P. J. Webster, 1989: Effects of zonal flow on equatorially trapped
waves, J. Atmos. Sci., 46, 3632-3652.
Ziemke, J. R., and ,1. L. Stanford, 1990: One-to-two month oscillations in the strato-
sphere during Southern winter. 3". Atmos. Sci., 47, 1778-1793.
6
or-
L_
m
D
Z
LaJ
>
_.J
Z
C)
N
9
8
7
6
5
4
3
2
1
2
PERIODCdays)
5 N
I I
8 3 2
PERIODCdays)
FIGUP_ 1. Zonal wavenumber vs. frequency westward/eastward spectral amplitudes
for TOMS version 6 data at 5°N. Dates: 90 days beginning 16 July 1985. Units: DU.
Dashed contours are 0.15. Solid contours begin at 0.3 and increment by 0.15. Periods in
days shown. Arrows point to 6-day feature discussed in text.
v .I I 3, • ._'i_ II "' i_I. _ flt II*II _'i" '_'" _,lll,'
0 _Ok/ - ,._ ,, ...... ',
I _'l_,'_ "-_ _ '%.,'_ Y ._ "L_ .'.'o_,',,.',.,.o_0-
r(J_ ,_'d:-- &,:£,e'?" •
August September
43
0'3
P_
r,4
rm
_--
CO
,--
Z
0
_J
O"
3O
20
_ -10
-20
-30
(a) _ _.. i! "::,.';".(
- i. _ _.,,. i - /
I -- . I : _' .': ,:" I , I L .':
0 90 180 270 368
LONGITUDE
K=4-7 10d,5d WESTWARD AUG-SEP 1986
30N ......................
](b),_ :: /_( _ _-_
!:ili...,, :i i
_Lx'l "/." _
_ ":i'" "
E(_U I
I--4 ."i
b--
15S
RMS AMPL(,DU} , ,30S
0 90 180 270 360
LONGITUDE
FIGURE 3. (a) Latitude-longitude map of the meridional wind variance for westward-
propagating zonal Waves 4-7 with periods 5-10 days, calculated from data over August-
September 1985. Contour interval is 2 m2s -2. Dots denote rawinsonde stations. [This is
Fig.12a of Randel, 1992] (b) Similar to (a), but for RMS amplitudes of westward propagat-
ing components in TOMS total column ozone. Calculation is over August-September 1985
for zonal waves 4-7, including bandpass filtering for periods 5-10 days (filter response
one-half at 5 and 10 days). Units: DU. Solid (dashed) contours begin at 1.0 (0.8) and
increment (decrement) by 0.2.
10
0
,_ -10
.J
-20
-30
. . , ;
. . i / ""1/ " It "' _ _, /"" _ ' '-,
, , ! / . \ L/ _. \ i . -._.._
• r tti _,Jt_._, _,
',' r"t ''7" t .r-I....LL
• ,,, . - I _, - 1 _ " ,_, ..--.. -"
• - "'.l i%, "_' / I " 1 \ " ' :
50 180 210 240 270 300 330 360
LONG [ TUOE
WESTWARD COV K=4-7 AUG-SEP85
180 270 360
LONGITUDE
FIGURE 4. (a) Latitude-longitude diagram of the horizontal structure of westward-
propagating fluctuations in 200 hPa filtered ECMWF meridional wind data used in Fig.
3a. Components of each vector are calculated as regressionsupon the reference time series
of meridional wind at 260°E, 7°S. The longest vector represents 4.5 ms -1. The heavy
(lashed line indicates the approximate axis of symmetry for these waves. [This is Fig.13 of
Randel, 1992] (b) Similar to (a), but for calculated covariance of the ozone data in Fig.3b.
Reference point used is 260"E, 5°S (marked by "X"). Units: DU 2. Solid (dashed) contours
begin at 0.3 (-0.3) and increment (decrement) by 0.1.


Rossby-gravity waves in tropical total ozone data

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server