Halogen Occultation Experiment (HALOE) and balloon-borne in situ measurements of methane in stratosphere and their relation to the quasi-biennial oscillation (QBO)

<jats:p>Abstract. Measurements of methane have been made from various observational platforms in the atmosphere. In this article, we have compared four high precision balloon-borne measurements from Hyderabad (17.5°N), India in the period of 1987 and 1998 with a part of HALOE/UARS global observations available since 1991. All the balloon measurements correspond to boreal spring (March and April) but belong to different years. A comparison shows fairly good agreement with each other. The gradient in CH4 profiles in the troposphere is controlled by the variation in vertical transport. The strongest effect of dynamical influence on methane vertical profiles is shown to be resulting from the dynamical quasi-biennial oscillation in the stratosphere, and that has been consistently derived from both the measurement techniques and chemistry-transport model simulations. It is observed that the QBO signal in CH4 anomaly exhibits interhemispheric asymmetry caused by the tropics to midlatitude circulation in the stratosphere. A mechanism is suggested to explain how and to what extent the methane vertical profiles over Hyderabad and higher latitudes could be modulated by the prevailing QBO winds in the tropics. We have also discussed how the same mechanism would affect ozone distribution in the stratosphere quite differently.                     </jats:p>

section_content: Methane (CH 4 ) plays important role in chemistry and radiative balance of the lower and middle atmosphere. It has been estimated that typically about 97% of CH 4 oxidation by OH takes place in the troposphere (Lawrence et al., 2001), and this gas generates 20 times more radiative forcing compared to that of CO 2 on a per molecule basis (IPCC, 1996). In the stratosphere chemical reactions with O 1 D and Cl dominate CH 4 destruction. Its abundance has increased rapidly since Correspondence to: P. K. Patra (prabir@jamstec. go. jp) the 1950s to the present day (Etheridge et al., 1998). However, from the beginning of 1990s a dramatic decline in CH 4 increase rate has been observed globally (Dlugokencky et al., 2001). Since its emission rate is changing very slowly and its total photochemical loss rate in the lower/middle stratosphere is quite small, anomalies in its stratospheric distribution bear footprints of dynamics, such as quasi-biennial, annual and semi-annual oscillations etc. (see Baldwin et al., 2001, for a review). It is established that the QBO constitutes a major part of the variabilities in CH 4 and O 3 anomaly with a time period of about 27-30 months in the stratosphere (Tung and Yang, 1994; Randel et al., 1998). The QBO phase in O 3 anomaly changes at around 15 • on either side of the equator and the higher latitude anomaly extends as deep as 60 • latitude (Randel and Wu, 1996). However, the latitudinal extent and interhemispheric differences of QBO influence on CH 4 anomaly have not been very well understood due to shorter temporal coverage of data on the record. Only recently, extensive discussions on observations of the QBO in HALOE CH 4 have been made (Ruth et al., 1997; Randel et al., 1998; Dunkerton, 2001). Two-dimensional (2-D) chemistry-transport-radiation models of the stratosphere based on analysed meteorological parameters (e. g. winds, temperature) are being used to simulate the derived interannual variations from the satellite measurements of CH 4, O 3, H 2 O etc. (e. g Fleming et al., 2002; McCormak and Siskind, 2002). Their sensitivity tests are useful to visualize the effects of dynamical QBO and temperature QBO on trace gas distribution, or to study the role planetary wave amplitude on QBO periodicity. Here we take advantage of some of these understandings in the explanation of observed features in CH 4 distributions over a tropical site in India. 

The measurements of CH 4 in the stratosphere have been started with the advent of appropriate technology, and records are available since the early 1970s by using balloonborne cryogenic collection of air samples (Ehhalt, 1980; Fabian, 1981), and from the satellite platforms only during c European Geosciences Union 2003 1052 P. K. Patra et al. : Interannual variability of CH 4 vertical profiles the past two decades (Jones and Pyle, 1984; Russell et al., 1993). But quality CH 4 data from a tropical latitude using balloons, and more frequently by the satellite are available since about past 15 and 10 years, respectively (Borchers et al., 1989; Russell et al., 1993; Lal et al., 1994 Lal et al.,, 1996)). These observations provide us a unique opportunity to compare two different kinds of dataset spreading over a decade, coverring about 4 complete QBO cycles. The satellite data sets are also being utilized extensively to study the influence of various dynamical, and chemical composition changes in the stratosphere. However, no significant usage of satellite observations together with in situ measurements in lower and middle stratosphere could be found in the literature. It could also be noted that though there have been some discussion on the differences in behaviour of vertical distribution of long-lived gases over Hyderabad, India (Lal et al., 1996; Patra et al., 1997 Patra et al.,, 2000)), no detailed study has been carried out so far. 

Here we have attempted to find possible cause for the differences in the vertical profiles observed over the tropics using balloon-borne techniques during March-April period of four different years that are well separated by an interval of about 4 years. The satellite observations and 2-D model simulations over this location and its dynamical neighbourhood are used to support our findings. In the next section details about the data sources and analysis procedure are given, followed by the results and discussion section. The major conclusions of this study are listed in Sect. 4.
section_name: Introduction

section_content: In the period of 1987 to 1998, four balloon flights have been conducted from the National Balloon-launching Facility (NBF) at Hyderabad (17. 5 • N, 78. 6 • E), India on (1) 27 March 1987, (2) 9 April 1990, (3) 16 April 1994, and (4) 18 April 1998. The first two balloon flights were conducted jointly by Max Planck Institute for Aeronomy (MPAE), Lindau and Physical Research Laboratory (PRL), and the latter two flights by PRL only. The payload (balloon gondola) containing a 16-tube cyro-sampler along with its control unit were flown with the help of large volume (∼150 000 m 3 ) Pyrex rubber balloon filled with hydrogen gas. The ambient air samples were collected at 15 separate altitudes in the upper troposphere and stratosphere during both ascent and descent of the balloon. Typically, a ceiling altitude of about 35 km was attained during the experiments. The sampling tubes were then brought to the laboratory and analysed for several chemical constituents using GCs coupled with flame ionization detector (for CH 4 and CO), electron capture detector (SF 6, N 2 O, chlofluorocarbons, Halons etc. ) and mass spectroscopy (for hydrogenated halocarbons). The calibration for absolute concentration of CH 4 was done with respect to the standard gases supplied by Linde gas, UK and NIST, USA. The precision of GC analyses is estimated to be within 5 ppbv (1σ ), and the errors in absolute concentration are es-timated to be in the range of 2-3% (1σ ). The results from these balloon flights have been published earlier (Borchers et al., 1989; Lal et al., 1996; Patra et al., 2000; Lal and Sheel, 2000), but no systematic comparison has been made so far in terms of interannual variations and with satellite data. 

The vertical profiles of CH 4, constructed from 4 different balloon flight experiments during boreal spring at an interval of about four years, are shown in Fig. 1a. As argued using the stratospheric measurements of SF 6 over India in a previous study (Patra et al., 1997), the dynamical condition on the measurement day of 16 April 1994 represents a normal day at 17. 5 • N, that is typical of the tropical stratosphere (hereinafter referred to as "normal" profiles). The 1994 profiles of SF 6, N 2 O, CF 2 Cl 2 etc. also match fairly well with two-dimensional model simulations (Patra et al., 1997 (Patra et al.,, 2000)). The location of the measurement station is subtle in a sense that the tropical upwelling region in stratosphere is located over Hyderabad during northern hemisphere (NH) summer (May-June-July), and in NH winter (December-January-February) the upwelling region moves to the southern hemisphere (SH). Thus sometimes the balloon travelled close to the stratospheric "surf zone", and measured vertical distributions often deviate from the normal profile. The observations on 27 March 1987 and 9 April 1990 are the examples of disturbed profiles from the "normal" profile. It can be noted that the magnitude of deviation as well as the altitude of occurrence could vary largely with time. The characteristics of the profile observed on 18 April 1998 exhibit a relatively better match with the 1994 profile compared to the other two. This also suggests that the transition from winter to summer in the stratosphere takes place during late March to early April period at 17. 5 • N. We have checked the horizontal distributions of monthly mean geopotential heights (z) from the NCEP/NCAR reanalysis datasets (Kistler et al., 2001), which commonly show that the gradient in z starts around 30 • N in April at 50 mb height. This behaviour of z was quite similar at 10 mb pressure layer during April. However, in March the location of z gradient region shifts over to lower latitudes and lies over Hyderabad. 

Figure 1b shows the zonal wind component as obtained from the NCEP/NCAR reanalysis at Hyderabad. It is wellknown that the distribution of CH 4 in the free troposphere (above the planetary boundary layer) is rather well-mixed and does not depend on local dynamics to a large extent. Thus the mixing ratios of CH 4 in the middle troposphere region (∼6-12 km) do not deviate significantly (less than 10%) from one another with time. More discussions on tropospheric data are given in a latter section. The main features in this plot are observed at the stratospheric heights: 1) The 1994 profile is uniformly higher in methane above 20 km than other profiles -from the wind profiles it is seen that magnitude of easterly winds (also the easterly wind shear) was maximum in 1994 compared to other years of measurements. 2) This also readily shows that the mixing ratio minimum in 1990 around 25-33 km layer is related to 1990, 1994 and 1998 (a, left panel). The 1987 and 1990 profiles are obtained from the PRL-MPAE collaborative programme (Borchers et al., 1989 (Borchers et al., ), and 1994 (Borchers et al., and 1998 profiles are from the PRL conducted experiments (Lal et al., 1996). On the right panel (b) vertical profiles of zonal winds from the NCEP/NCAR reanalysis, corresponding to Hyderabad (17. 5 • N, 77. 5 • E), are depicted. 

the westerly wind shear and downwelling in the deep tropics and subtropics as CH 4 concentration decreases upward. The increase in CH 4 above 32 km in 1990 is probably related to lofting in QBO easterly wind shear regime; indication of strong easterly shear in the wind profile is appearing above 27 km. 3) In 1987 the minimum in methane near 20-26 km should be related to a westerly wind shear regime and downwelling, but is not very clear from the zonal wind diagram. 

Then there appears to be lofting in the easterly shear zone and the reversal in wind shear above 30 km seem to be causing the very low CH 4 concentration at 33 km. In addition to the changes in vertical transport in different zonal wind regimes, the secondary meridional circulation (Choi et al., 2002) can also contribute to the low CH 4 values in the vertical profiles. 

At the same time, global coverage of several chemical constituents are available since the launch of HALogen Occultation Experiment (HALOE) onboard the Upper Atmosphere Research Satellite (UARS) in September 1991. Though the UARS mission itself began with an expected lifetime of about 2-3 years, this satellite and many of the instruments onboard continues to make equally good quality measurements (as of April 2003). The datasets are being used for various other kinds of study; e. g. deriving trends in CH 4 in the stratosphere, timeseries correlation of CH 4 with other constituents such as ClO x, and also to create four dimensional regular grid tracer distributions (Froidevaux et al., 2000; Patra and Santhanam, 2001). For comparison with the balloonborne measurements of CH 4, we have created the represen- ( Tung and Yang, 1994; Hitchman et al., 1994; Randel et al., 1998; Jones et al., 1998). Some studies also highlighted the interaction between seasonal, subennial and quasi-biennial oscillations in the meteorological parameters to explain the observed variations in CH 4 (Ruth et al., 1997; Dunkerton, 2001). In Fig. 3, we show the altitude-time variation of zonal winds anomaly in the period of 1985 and 2002 as revealed from the NCEP/NCAR reanalysis data (Huesmann and Hitchman, 2001). This diagram shows the temporal extent and altitude distribution of QBO phase and intensity at the equator. As it is discussed above only two out of four balloon-borne observations of CH 4 have overlapping measurements using HALOE. To compare the balloon data with satellite observations, we have identified the QBO phases from this diagram that year 2001 and 1997 closely match with 1987 and 1990 boreal spring conditions, respectively. This search was needed because the QBO is an irregular oscillation, and its period has averaged about 30 months in the period of 1980-1992 and averaged about 27 months in the earlier decades (Tung and Yang, 1994). In addition the QBO and annual cycle are known to have interplay that influence the constituent transport and distribution due to upwelling motion in the equatorial lower stratosphere (Mote et al., 1996; Hinsta et al., 1998). During this selection process, the observed vertical profiles of CH 4 from Hyderbad were also kept in mind to ensure the best possible suitability.
section_name: Data and Analysis

section_content: In Fig. Since the HALOE observations were nonexistent before 1991, we have used 2001 and 1997 HALOE profiles, respectively, corresponding to the balloon observations in 1987 and 1990 following the discussion in the previous section. We choose to show the best possible matches between the disturbed in situ measurements of CH 4 from Hyderabad on March 1987 and April 1990, and the profiles from HALOE data. For the simultaneous measurement periods in 1994 and 1998, the profiles of HALOE CH 4 in the latitude band of 17. 5±1. 5 • are presented. The CH 4 concentrations in the middle troposphere are found to be about 1. 65, 1. 67, 1. 68, 1. 68 ppmv, from the balloon measurements, respectively, for the years 1987, 1990, 1994, and 1998 (ref. Fig. 1) (ref. Fig. 1). On a few occasions similar concentrations are also found in the upper troposphere and lower stratosphere (UT/LS) region due to intense upwelling in the upper troposphere and across the tropical tropopause. It can be noticed here that the CH 4 increase rate in middle troposphere has been fairly stabilized in the period of 1990-1998, which agrees well with the observations on the ground (Dlugokencky et al., 2001). Further analysis suggest that the CH 4 concentrations across the tropopause (at ∼17 km or 100 mb) generally do not exhibit detectable 1 for details). The anti-correlation between the slopes in CH 4 mixing ratios from middle troposphere to the tropopause with relative humidity at 300 mb is very compact (correlation coefficient, R 2 =0. 99). Similar correlations with Omega also produce compact linear relationships. 

The magnitude of CH 4 mixing ratio variation are much larger in the middle stratospheric altitudes. The CH 4 concentrations at around 30-32 km altitude in 1990 and at 34 km in 1987 are only about 60% of those observed in 1994 and 1998 in the same altitude regions. Such large changes cannot be explained by the changes in atmospheric CH 4 burden, solar irradiance variations, or shifts in the abundances of reactive shown; because earlier studies have suggested that the monsoon structures in the upper troposphere could impose zonal asymmetries in lower 15-20 km of the stratosphere, while above that level quasi-stationary planetary waves can cause zonal asymmetries in the meteorological fields (e. g. Quiroz et al., 1975). As seen from the legends, the HALOE observations had large zonal coverage within a day in a narrow latitude band. However, no gross feature could be extracted in order to find any zonal asymmetry in the CH 4 distributions. 

In general, the balloon measurement closely matches with the HALOE observations, except in 1994, when the balloon observed CH 4 mixing ratios are higher by 10-15% than an average HALOE profile at all altitudes. Even though for 1987 and 1990 balloon measurements there are no contemporaneous HALOE observations, the matches between two compared datasets for different years are unexpectedly good. The HALOE observations in 2001 and 1997 reproduce the CH 4 concentrations at most altitudes and successfully capture the low values at 34 km in 1987 and around 30-32 km in 1990 balloon data, respectively. The HALOE CH 4 observations in 2001 also have a tendency to create a kink at 25 km, similar to that could be seen more prominently in 1987 balloon profile. 

The obvious next step is to relate the observed CH 4 anomalies with the known dynamical oscillations of the tropical stratosphere, the QBO being the most dominant one. It should be made clear that the rest of the study uses the 4dimensional CH 4 distribution produced by Patra and Santhanam (2001). In Fig. 5, the time-height crosssections of CH 4 anomalies are plotted at four latitudes (5 •, 15 •, 25 •, 35 • ) in both the hemispheres. An oscillation in the CH 4 anomalies with similar time period like that of the QBO (∼27 months) can be seen in the whole height range of tropical stratosphere. The most surprising finding is the latitudinal extent of the QBO features in CH 4 anomalies, which is in contrast to the presence of in-phase QBO signals in the zonal winds as well as in ozone distribution; that generally is restricted to about 15 • on either side of the equator (Swinbank and O'Neill, 1994; Baldwin et al., 2001). The panels in Fig. 5 suggest that the height distribution of phase propagation of CH 4 anomaly (ascending in nature) to be opposite in phase with the zonal wind anomaly (descending). Another possible explanation for this is the difference in QBO winds effect on transport of long-lived trace constituents from the tropical stratosphere to the higher latitudes through the secondary meridional transport (Choi et al., 2002). It has also been shown in a previous study that the descending westerlies in the tropics enhance transport out of the tropics in the lower stratosphere, whereas the descending easterly winds enhance such transport in the middle stratosphere (Jones et al., 1998). Because the descending limbs of zonal wind anomalies are already out-of-phase to a great extent by about 14 months between the lower (e. g. 50 mb or ∼20 km) and middle (e. g. 5 mb or ∼35 km) stratosphere near the equator, and the "out of the tropics" transport acts oppositely in the two QBO wind regimes. It could be possible to draw a changeover line at around 8 mb for this regime shift from the time-height plots of CH 4 anomalies (see Fig. 5 ). It may be relevant to point out here that the Fig. 5 is quite similar to the time-height plot of water vapour anomaly in the stratosphere, depicting the "tape recorder effect" (Mote et al., 1996) ; except there is an important difference that the variation of water vapour in stratosphere is closely linked to the entrainment of dry and wet tropospheric air through the tropical tropopause, and has a periodicity of about a year. The modelling studies illustrate the differences between the QBO in CH 4 and H 2 O distri-butions in the lower stratosphere (McCormak and Siskind, 2002). 

The CH 4 anomaly distribution with QBO periodicity is far more prominent in the case of NH than that in the SH (compare left and right panels on Fig. 5 ), which could be caused by the asymmetry of the low to middle latitude circulation anomaly (Kinnersley, 1999). He also suggested that strong cross-equatorial flow occurs near the shear zone if the zonal wind anomaly is easterly. From a plot (not shown here) of zonal winds from the NCEP/NCAR reanalysis (Kistler et al., 2001) as a function of time and latitude at 10 mb, it can be seen clearly that the latitude extent of QBO easterlies are larger in the NH compared to that in the SH. Thus the CH 4 anomalies in the NH are also larger compared to the anomalies corresponding to SH stratosphere. The strength of CH 4 concentration change with height is in good agreement with that which has been observed for the zonal winds, forming a maximum in the middle stratosphere and diminishing to weaker speeds towards the lower and upper stratosphere (see Figs. 5 and 3 ). Though the NCEP/NCAR reanalysis winds as well as the model assimilated winds of the UKMO (Swinbank and O'Neill, 1994) show that the QBO signal in the zonal winds does not generally exist beyond the tropics, but the CH 4 anomalies in the NH stratosphere do bear the QBO signal well into the midlatitudes as shown in Fig. 5. The latitude-time plot of CH 4 anomaly does not decipher opposite QBO phases between the tropics and extratropics (or midlatitudes) (see also Randel et al., 1998; Dunkerton, 2001), which is contrary to observations of the O 3 distribution (Randel and Wu, 1996). 

The difference between the CH 4 and ozone anomaly distribution is mainly caused by the dissimilarity in their source/sink regions, and hence the influence of stratospheric transport mechanisms. The source of CH 4 in the stratosphere is located at the tropical tropopause; whereas ozone is produced largely in the middle to upper stratosphere region, spreading over a greater latitude region around the equator. The gross picture of latitude-altitude cross sections of CH 4 distribution is relatively simple in the stratosphere; the mixing ratio decreases with the increase in both latitude and height. On the other hand the ozone mixing ratio decreases on either side of the equator only at the middle stratosphere, forming a peak around 10 mb height over the tropics. While the ozone mixing ratio increases with latitude at the base as well as in the top layers of the stratosphere (Fig. 6 ). Thus transport of O 3, in the lower and upper stratosphere (below 30 mb and above 3 mb, respectively), due to horizontal mixing (from low to high mixing ratio) and cross-equatorial advection (tropics to extratropics) counter each other. This mechanism does not allow the in-phase QBO signal in O 3 distribution to propagate much across the latitudes. The 2-D model study shows that QBO in transport and QBO in temperature dominates ozone interannual variability in the lower stratosphere and upper stratosphere, respectively (Fleming et al., 2002). The equator to midlatitude transport in the stratosphere works quite differently for CH 4 since the effects of meridional advection and horizontal mixing are additive. The distribution of aerosols adds another dimension to constrain the understanding of stratospheric transport. Hitchman and coworkers (e. g. Hitchman et al., 1994, and references therein) diagnosed tropical reservoir regions with an "upper" and "lower" transport regime, which divide the stratosphere around 22 km in the tropics. 

The processes of stratospheric transport around the tropical region has been schematically represented in Fig. 7 during both phases of the QBO, based on the two distinct trace species, i. e. CH 4 and O 3, as far as their sources, sinks and distributions in the stratosphere are concerned. For this diagram we have assumed the tropical upwelling, associated with the diabatic processes (see for a review Shepherd, 2002), at the base of the stratosphere to be constant over a QBO cycle as that signal in CH 4 distribution is not very strong in the lower stratosphere (ref. Fig. 5 ). However, the strength of the meridional transport due to advection is assumed to be variable, which is believed to be forcing the QBO signal in tracer distribution. The downwelling (lofting) during the westerly (easterly) phase of QBO wind shear helps to produce the prominent "rabbit ears" in the CH 4 distribution in the upper stratosphere. In addition, the "return arm" of tropical QBO circulation over midlatitude, which is where the air descends (ascends) in the extratropics during the easterly (westerly) equatorial shear (Baldwin et al., 2001), is considered to produce the mid-and high latitude O 3 anomaly. Thus the resulting anomaly in the subtropics is out of phase with that in the tropics. Because the CH 4 mixing ratio decreases sharply with an increase in both latitude and altitude, the effect of the return arm of the equatorial QBO is not very significant (ref. Fig. 6 ). These transport processes in the tropics and subtropics could explain the differences in altitude and latitude variations in CH 4 and O 3 anomaly. This discussion is not only restricted to the trace gases alone, but is in compliance with the transport models derived from the aerosol distribution in the stratosphere (Hitchman et al., 1994). Lastly, if an analogy is drawn from the aerosol distribution in the tropics and on basis of the CH 4 and O 3 distributions in the stratosphere, it could be suggested qualitatively that the major part of O 3 transport into the subtropics should be occurring in the middle/upper stratosphere, whereas the "out of tropics" transport of CH 4 would be far more stronger in the lower part of stratosphere.
section_name: Results and Discussion

section_content: The comparisons between balloon-borne measurements from Hyderabad (17. 2. In the troposphere the gradients in CH 4 mixing ratio show tight anti-correlation with relative humidity at 300 mb layer, suggesting that the vertical velocity ( ) controls the CH 4 transport from the lower troposphere to upper troposphere. 

3. We show that the interannual CH 4 variability in the middle stratosphere over Hyderabad during 1987, 1990, 1994 and 1998 are caused by the stratospheric dynamics, primarily governed by the quasi-biennial oscillations. In general, this relationship would provide useful information for planning the future balloon flight experiments from Hyderabad. 

4. Our analysis of HALOE CH 4 data suggests that the effect of QBO on the transport of tracers in the stratosphere are asymmetric across the equator and this signal could penetrate as deep as the midlatitudes, particularly in the northern hemisphere. 

5. A mechanism for opposite QBO influence on CH 4 and O 3 mixing ratios beyond ≈15 • latitude has been described qualitatively, which is primarily arising due to the differences in their stratospheric distribution.
section_name: Conclusions

section_content: Acknowledgement. We appreciate the help of Y. B. Acharya, P. Rajaratnam, and G. N. Dutta for making the balloon measurement programme a success. B. H. Subbaraya has been the integral part of balloon flight programme for trace gas studies in India. We sincerely acknowledge invaluable help of the NBF personnel during the flights. The 1994 and 1998 balloon flights were funded by the ISRO-GBP, Bangalore. The HALOE/UARS CH 4 data are downloaded from http://haloedata. larc. nasa. gov,and we thank all the team members for keeping the data public for scientific use. We are grateful to Charles Jackman and Eric Fleming for providing us the GSFC 2-D model results and useful discussions during the revision.