Zonal asymmetries in middle atmosphere temperatures

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doi: lO.l016/SO273-1177(03)00219-9

ZONAL ASYMMETRIES IN MIDDLE ATMOSPHERE TEMPERATURES D. Offermannl’,

M. Donnerl, K.U. Grossmannl, 0. Gusevl, M. Jarisch’, M. Kaufmannl, and A.I. Semenov3

J. Oberheide*

‘Physics Department, University of Wuppertal, Gauss-S&. SO, 42097 Wuppertal, Germany *High Altitude Observatory, National Center for Atmospheric Research, Boulder 80307, CO, USA 3 Obukhov Institute Atmospheric Physics, RAS, 109017 Moscow, Russia

ABSTRACT Zonal asymmetries are frequently seen in stratospheric temperature or trace gas fields as surf zones, streamers, filaments etc. They are also seen as very small-scale fluctuations, the intensity of which varies with longitude. Similar structures might be expected in the mesosphere as well, and several examples have recently been found. CRISTA 1 large-scale data are presented that indicate a surf zone in the middle mesosphere at the beginning of winter. Very small-scale data are shown from the CRISTA 2 mission. Mesospheric variability is found to be high at all altitudes, latitudes, and longitudes. There are considerable non-zonal structures in these fluctuations. (The duration of the Crista missions was about one week each.) Zonal asymmetries have been known for a long time from comparisons of ground stations measuring the same parameter in the mesosphere / lower thermosphere. As an example, upper mesosphere temperatures derived from OH* emissions are compared here for Wuppertal and Moscow (Zvenigorod), which are about 2000 km apart. A systematic and substantial difference in temperature is obtained, with higher temperatures at Wuppertal than at Moscow. The difference appears to follow the solar cycle: it is small at solar maximum and large (up to 28 K) at solar minimum. The reason for this surprising behavior is as yet unknown. The Moscow and Wuppertal temperatures have also been analyzed for long-term trends: a trend discrepancy between the two stations is not seen in the data interval common to the two stations. 0 2003 COSPAR. Published by Elsevier Ltd. All rights reserved. INTRODUCTION Deviations from zonal symmetry are ubiquitous in the middle atmosphere. Even though the stratosphere has a tendency to be zonally homogeneous, strong asymmetries are frequently observed on large scalessuch as: surf zones, streamers, filaments, etc. (e.g. Offermann et al., 1999; Riese et al., 1999; Riese et al., 2002). There are also many medium and small scale deviations from the zonal mean, which are interpreted for instance as vortex remnants, localized gravity wave sources, etc., (e.g. Preusse et al., 2001; Eidmann et al., 2002), not to mention polar stratospheric clouds in the lower regime and noctilucent clouds / PMCs above (Spang et al., 2001; Stevens et al., 2001; Grossmann et al., 2002a). Atmospheric fluctuations on small scales (deviations from a local 20” x 20” mean) have been analyzed by Eidmann et al. (2002), and were found to be quite substantial in many parts of the stratosphere. The fluctuation intensities were found to have a non-normal distribution. Deviations from zonal symmetry in the mesosphere have a similar appearance, but are even stronger. A few examples of this will be given below. The question arises, of course, whether waves (planetary waves, tides, etc.) are to be considered as non-zonal structures. Many of these are present in the stratosphere and - at increased amplitude - in the mesosphere and above.In the upper mesosphere planetary waves are - as opposed to the stratosphere - observed even in summer (Oberheide et al., 2000; P. Preusse, personal communication). Such structures on large scalesas well as on small scaleswill be discussed in the present paper. Space Res. Vol. 32, No. 9. pp. 1771-1780,2003 Q 2003 COSPAR. Published by Elsevier Ltd. All rights Printed in Great Britain 0273-l 177/$30.00 + 0.00

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MESOSPHERE: LARGE SCALE RESULTS FROM CRISTA Many - if not all - of the non-zonal structures of the stratosphere are expected also to be seen in the mesosphere. Additional disturbances are likely from the upper boundary, i.e. geomagnetic non-zonal influences. These structures occur on a variety of horizontal scales. Depending on the scale size they can mostly be detected by global measurements of satellites. Special and important structures can, however, also be detected by ground based measurements, for instance by networks of similar ground stations, rockets launched from local stations, or by pictures taken from the ground.

Temperature -3 203

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Fig. 1. Northern hemisphere temperatures and potential vorticities as derived from CRISTA measurements the stratosphere (30 km, left column) and mesosphere (70 km, right column) on 5 November 1994.

in

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The high data density of the CRISTA experiment (Offermann et al., 1999) allows high resolution temperature maps to be derived even at mesospheric altitudes. (CO, emissionsat 15pm wavelength are used here.) Daily maps of horizontally resolved geostrophic winds (zonal and meridional) in the altitude range 20 - 90 km have been derived by Oberheide et al. (2002). Comparisons to climatological and assimilated winds as well as nearly simultaneous measurements by rockets, balloons, and radar from the ground show good agreement. Furthermore daily maps have been derived for geopotential and for potential vorticity (from these wind fields). Temperature and geopotential during the first CR.ISTAmission (November 1994, Version 3 data) show a polar vortex that was displaced by the Aleutian high. A tongue-like structure in stratospheric temperature develops around 180’ E and westward of it, which is shown in Figure la. This structure almost exactly maps to mesospheric altitudes (Figure lb), but with opposite sign. A similar structure is seen in the potential vorticity maps at the two altitudes, as shown in Figure lc and d. Thus we have an indication of a surf zone in the mesosphere, which to our knowledge has not been observed before at these altitudes. The picture shows that there is strong coupling by planetary waves between the stratosphere and the mesosphere.

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Temperatures at even hiiher altitudes than shown ln Figure 1 have been measured by CRISTA (up to 100 km and beyond). The emissions have been inverted to yield temperatures, taking into account NLTE effects (Non-Local Thermodynamic Equilibrium effects, Grossmann et al., 2002b). A map of these data over Europe at 87 km is given in Figure 2. It shows mean temperatures from four days during the second CXISTA mission (in the time interval 8 to 15 August 1997). Due to the orbital geometry of this mission the data are daytime values only (later morning and afternoon). The data are still preliminary. The map shows strong local (non-zonal) structures. For comparison of horizontal structures the precision of the CRISTA measurements applies: It is estimated to be 1.5 K for a single measurement. This figure was derived from adjacent measurements in “quiet” atmospheric regions. Therefore it still contains some atmospheric variability. Considering the large atmospheric fluctuations typical of the altitude regime around 87 km, such precision is quite satisfactory and well suited to study, for instance, horizontal structures such as those seen in Figure 2.

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Fig. 3. Latitude / altitude distribution of temperature variability. Standard temperatures are given (in K, zonal averages). Contour interval is 2 K.

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The strong local variations seen in Figure 2 are typical of the global temperature field at these altitudes. To demonstrate this in detail the CRISTA 2 temperatures have been binned in 5”. latitude bands and averaged. (Altitude resolution is 2.5 km.) Standard deviations u from these,means (taken over an eight day time interval) have been calculated, and are used as a measure of atmospheric variability. Mean temperature distribution in latitude and height (Grossmann et. al., 2002a) shows the general mesospheric appearance with higher temperatures at high southern (winter) latitudes and high altitudes , and a pronounced cold spot at 80 - 90 km at high northern (summer) latitudes. The latitude distribution of the standard deviations is shown in Figure 3. In general the temperature variability strongly increases from the lower mesosphere (60 km) to the upper (100 km). The values in the lowermost part (not shown here) match very well the results of a comparable stratospheric analysis by Eidmann et al. (2002, their Figure 4). The lowest part of Figure 3 (up to about 75 km) shows the well-known summer / winter difference with low variability in summer and high variability in winter. However, at and above 80 km the picture changes significantly. High variability now extends northward beyond the equator and well into the summer hemisphere. The summer / winter difference disappears completely at the highest altitudes. The high variability at the upper altitudes results from the interplay of various types of waves. There are several islands of high u at or near the equator in Figure 3, and another high intensity spot at 30’ - 35’ S, 88 km. These features presumably belong to the tidal activity that has been analyzed by Oberheide (2000) and Hagan et al. (2002). Exceptionally strong planetary wave activity (waves 1 and 2) was observed in the upper stratosphere in the southern (winter) hemisphere by Riese et al. (2002). The wave activity extended (and was analyzed) up to and well beyond the middle mesosphere (0.01 hPa). Whether this activity spread into the northern (summer) hemisphere remains to be determined at this point. A planetary wave 2 in the southern hemisphere was found to propagate from the stratosphere upward into the mesosphere (80 km) and equatorwards (Ward et al., 2000). The non-LTE temperatures discussed here are presently being analyzed for planetary waves. A strong westward travelling wave 2 (up to 10 K amplitude) with about a 5-day period is indicated in the northern hemisphere (which is a frequent wave period at these altitudes in summer according to Bittner et al., 2000). This will be discussed in detail elsewhere. Finally, Smith et al. (2002) found considerable Kelvin wave activity in the lower mesosphere tropics and subtropics.

Zonal Assymetries in Middle Atmosphere Temperatures

Standard

Deviation

[K]

Fig. 4. Global distribution of temperature variability at 87 km altitude. Standard deviations CJ (in K) from mean temperatures in 20” x 20” longitude / latitude bins are given. Data have been detrended for latitudinal gradients (see text). The standard deviations m shown in Figure 3 are zonally averaged values. Hence zonal structures may have been suppressed in this picture. To analyze this Figure 4 shows the horizontal u distribution (nearly global) near the mesopause (at 87 km, i.e. at the altitude of the OH* layer, see below). In this picture data from the entire mission (d 220 to d 227) have been used. Spatial as well as time variations have been averaged, binning the data in 20” x 20” longitude / latitude areas and calculating the standard deviation from the mean temperature in each bin. Before this was done temperatures had been detrended for latitudinal gradients by means of zonal temperature averages (5” latitude bands). Even though Figure 4 was derived from a data interval longer than a week and thus includes some time averaging, considerable and fairly clear longitudinal and latitudinal structures are visible. The most pronounced features are the very high standard deviations in the tropics, which can already be seen in Figure 3. Beyond that the horizontal map now reveals that there are several maxima and minima along a latitudinal circle, suggesting some (quasi - stationary) wave structure. At moderate to higher latitudes the fluctuations are generally somewhat weaker. Nevertheless they are still large as compared to the stratosphere (Eidmann et al., 2002), even the smallest values being larger than 5 K! There are zonally regular structures also at these latitudes, which is indicative of the wave action already mentioned. Lowest variations in Figure 4 are found at middle to high northern latitudes. They are, nevertheless, high and quite variable in the zonal direction. In the European sector the standard deviations vary from 5 K to 12 K! The results in Figure. 3 and 4 show that it is difficult to validate temperature measurements above 80 km. This is because the atmospheric fluctuations are high at all altitudes, latitudes, and longitudes, and “quiet” areas with low variability are difficult to find. (At least special measurement precautions would need to be taken.) Figure 4 also gives hints as to the mesh width one has to use when designing a network to monitor the mesosphere at a given accuracy.

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Considerable small scale / non-zonal structures have occasionally been reported by ground stations close together. She and Lowe (1998) for instance, find 6 - 7 K temperature differences at the mesopause in North America. The two stations (Ft. Collins, 41” N, 105” W; Delaware Observatory, 43“ N, Slow) are 3’ apart in latitude and about 2000 km in longitude. A simiiar pair of stations measuring OH* temperatures are Moscow (Zvenigorod, 56” N, 37” E) and Wuppertal (51’ N, 7” E): the latitude difference is a little larger, and the longitudinal distance is about the same. Figure 2 shows a 5 K temperature difference between these two locations, measured by CHISTA during about one week in August 1997. The picture quite generally shows that such or even larger differences are not uncommon at these distances (see also Figure 4). To study thison a broader data basis, the OH* temperatures measured at Moscow (Zvenigorod) and Wuppertal have been compared as much as possible. Simultaneous nightly mean temperature values have been available since 1984. (Regrettably there are large gaps in this joint data set for various reasons.) Figure 5 shows two annual records at the two stations from two more recent years (a.) 1994, b.) 2000). In year 2000 there is a very close agreement of the two data sets: the mean temperature difference is only 1.4 K. By contrast in 1994 there are large and systematic differences, the Zvenigorod temperatures always being lower than the Wuppertal data. This difference of behavior in the two years is quite surprising, especially as the two stations are so close together. To study this feature further 240 230 220 210 200

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Fig. 6. Temperature differences (Moscow/Zvenigorod minus Wuppertal) during two solar cycles. a.) Daily pairs of data are shown whenever available. Solar fluxes F 10.7~~ in units of 1O-22 W/m2Hz are given for comparison. Flux data have been smoothed by a nine months running mean. b.) 3-months running means of the temperature differences. The two CRISTA mission intervals are also indicated (CRl, CR2).

we have calculated the Moscow - Wuppertal temperature differences for all days when simultaneous measurements were taken. The results are shown in Figure 6. The comparison covers eight years in the period from 1984 to 2001. Strong fluctuations of the daily differences are seen (Figure 6a). Nevertheless a clear tendency is found with differences varying from large (1984) to small (1990), and back to large (1994) and to small again (2000). As the time structure resembles that of a solar cycle, solar activity is also included in Figure 6 (in terms of F~o.,~~solar radio fluxes). Indeed, it appears as if the temperature differences follow the solar activity. The smoothed temperature data in Figure 6b show this even more clearly. Mean temperature differences of up to 28 K are seen. For the comparison of the Wuppertal and Moscow data only the relative stability of the two instruments applies. For the Wuppertal instrument no changes in measurement technique have been made in the time period discussed. The instrument stability is better than 1 - 2 K (Bittner et al., ZOOZ), 1. e. it is much better than the large variations discussed here. The Moscow instrument detection technique was changed in 1996 (and the measurement accuracy improved this way). It has been checked whether or not this change has introduced any shift in the data. No such shift has been detected (at an accuracy level of a few Kelvin).

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Fig. 7. Long-term and 2001 are given of the solar cycle, trend, dashed lines

trend estimate. Data of Figure 6b are repeated. Annual means of years 1984, 1990, 1994, as circled crosses. Pairs of years 1984/1994 and 1990/2001, respectively, are in similar phases and are referred to each other by dotted and dashed lines. Dotted lines show a -0.7 K/year show zero trend.

The temperature difference between Wuppertal and Moscow (Zvenigorod) cannot be attributed to the difference in geographical latitude (5”). Following reference atmospheres (as CIRA 86) this difference should be less than 5 K. Moreover it should not vary during the solar cycle. The apparent solar cycle effect is therefore quite surprising. It cannot be due to variations of solar light radiation, as the two stations are so close together. The magnetic latitudes are not very different, either (Wuppertal: 47.2” N, Moscow: 51,7” N), and hence a particle effect is not obvious. The mesopause temperature sensitivity to solar activity (in terms of Fic.rcm fluxes) has been determined for Moscow (e.g. Semenov and Shefov, 1999) and for Wuppertal (Offermann et al., 2002). The peak-to-peak variation of Wuppertal OH temperatures during a solar cycle is 4 - 5 K (Offermann et al., 2002). The respective variation at Moscow (Zvenigorod) is much larger (1 20 K). This difference is obviously the basis of the behavior shown in Fig. 6. It is, however, not an explanation! Two basic questions would seem to arise: 1. Why are the Moscow (Zvenigorod) temperatures so much lower than those at Wuppertal under geomagnetically quiet conditions? 2. Why is the increase in temperature so much stronger at Moscow than at Wuppertal during geomagnetic disturbances? This needs to be further studied in the future.

MESOPAUSE: TAL

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Mesopause temperatures have been measured at Zvenigorod since 1958 and at Wuppertal since 1980. A fairly strong negative trend has been reported for Zvenigorod (e.g. Lysenko et al., 1999; Semenov et al., 2002), whereas no significant trend was found at Wuppertal (Bittner et al., 2002; Offermann et al., 2002). This has been interpreted as a zonal asymmetry or even a discrepancy occasionally. That appears, however, to be a misinterpretation: The trend difference is rather a difference in the time intervals analyzed. The Zvenigorod results relied heavily on the early measurements, when Wuppertal data were not yet available. If we compare the simultaneous data since 1984 a discrepancy is not discernible: Before a trend analysis can be performed the data need to be corrected for geomagnetic effects. This is more difficult (Zvenigorod) than for Wuppertal because of the much greater sensitivity to geomagnetic disturbances at for Moscow Moscow. An alternative is to compare the temperatures during two solar cycles at the same phase of the cycle. Figure 6 offers such a comparison on two occasions: at solar maximum and near solar minimum. To study the solar

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maximum case, years 1990 and 2001 are compared as they are in about the same phase of the cycle. Mean data (mean temperature differences) for the two years are given as circled crosses in Figure 7, which otherwise repeats Figure 6b. The dotted line extending from the mean value of year 1990 to year 2001 has a gradient -0.7 K/year taken from Lysenko et al. (1999). If a zero trend of temperatures at Wuppertal is adopted, this dotted line predicts the trend of the Moscow (Zvenigorod) data. The dashed line gives a zero trend. Comparison of the two circled crosses in 1990 and 2001 shows that these data do not support the gradient of Lysenko et al. (1999). They would more likely be compatible with a zero trend. For solar minimum, mean values of years 1984 and 1994 are compared, which are in similar phases of the cycle. A similar result is obtained as for solar maximum. Admittedly thii analysis is relatively coarse. It is, however, about the best we can do with the simultaneous data available at present. The ongoing measurements should soon improve this picture. At present we must conclude, though, that there is no trend difference in the time interval of simultaneous measurements at Moscow and at Wuppertal.

ACKNOWLEDGEMENTS The CRISTA project is funded by Deutsches Zentrum fti Luft- und Raumfahrt e.V. (DLR), Bonn, under Grant No. 50 QV 9802 4. This work was also supported by GSF-Forschungszentrum fiir Umwelt und Gesundheit GmbH, Grant NO. 07 ATF 10. The National Center of Atmospheric Research is sponsored by NSF.

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She, CY and R.P. Lowe; Seasonal temperature variations in the mesopause region at mid-latitude: comparison of iidar and hydroxyl rotational temperatures using WINDII/UARS OH height profiles; AtmosSol. Terr.Phys., 60, 1573 1583, 1998. Smith, A.K., P. Preusse, and J. Oberheide; Middle atmosphere Kelvin waves observed in CRISTA 1 and 2 temperature and trace species; J.Gwphys.Res., 107, 10.1029 / 2001 JD000577, 2002. Spang, R., M. Riese, and D. Offermann; CRISTA-2 observations of the south polar vortex in winter 1997: A new dataset for polar process studies; Gwpyhs.Res.Lett., 28, 3159 - 3162,200l. Stevens, M.H., RR. Conway, C.R. Englert, M.E. Summers, K.U. Grossmann, and O.A. Gusev; PMCs and the water frost point in the Arctic summer mesosphere; Geophys.Res.Lett., 28, 4449-4452,200l. Ward, W.E., J. Oberheide, M. F&se, P. Preusse, and D. Offermann; Tidal signatures in temperature data from CRISTA 1 mission; J.Gwphys.Res. 104, 16,391 - 16,403, 1999. Ward, W.E., J. Oberheide, M. Riese, P. Preusse, and D. Offermann; Planetary wave two signatures in CRISTA 2 ozone and temperature data; GwphysMonogr. 123, AGU, 2000. Email address of D. Offermann [email protected] Manuscript received 18 October 2002; revised 20 December 2002; accepted 23 December 2002.