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SCIENCE DIRECT*

doi: lO.l016/SO273-1177(03)00228-X

TEMPERATURE AND WIND TIDES AROUND THE SUMMER MESOPAUSE AT MIDDLE AND ARCTIC

LATITUDES

W. Singer’, J. Bremer’, WK. Hocking’, J. Weiss’, R. Latteck’, and M. Zecha’

’ Leibniz-Institute of Atmospheric Physics, I8225 Kiihlungsborn, Germany ’ University of Western Ontario, Department of Physics and Astronomy, 1 I.51 Richmond Street North,

London, Ontario, N6A 3K7, Canada

ABSTRACT

Temperatures and winds have been determined with a meteor radar at middle and arctic latitudes. Tem- peratures are estimated as daily averages around 90 km and winds as hourly means between about 82 km and 98 km. Tides are extracted by a superposed epoch analysis using data from periods of typically 10 or 20 days. The variability of meteor radar temperatures and winds obtained at mid-latitudes during summer in 2000 and 2001 as well as at arctic latitudes in summer 2002 is discussed. The observed low temperatures in early summer (150- 170K at 54ON and 120- 130K at 69”N) are correlated with the appearance of strong mesospheric radar echoes in the VHF range and of noctilucent clouds at arctic latitudes. At mid-latitudes the amplitudes of the diurnal and semidiurnal temperature tide are in the order of 5 K during summer. The tidal amplitudes at arctic latitudes are smaller with about 4 K for the diurnal tide and 2 K for the semidiurnal tide. The steep temperature decrease from spring to summer at mid-latitudes is accompanied by an enhanced semi-diurnal temperature tide (7-10 K) between middle of May and middle of June. 0 2003 COSPAR. Published by Elsevier Ltd. All rights reserved.

INTRODUCTION Breaking of gravity waves in the mesosphere modifies the global scale circulation and results in a cool-

ing of the mesopause region at high latitudes during the summer months (Liibken and von Zahn, 1991; Liibken, 1999). The low temperatures in combination with the available mesospheric water vapor lead to the formation of ice particles and heavy water clusters. The ice particles are detected visually or by lidar backscatter as noctilucent clouds (NLC). Temperatures below about 150K are necessary for the formation of NLC ice particles. Another phenomenon related with the cold summer mesosphere is the appearance of strong radar echoes in the VHF range from the mesopause region. These echoes are called polar meso- sphere summer ethos (PMSE) or mesosphere summer echoes (MSE) at mid-latitudes. They are a dominant phenomenon at polar latitudes (Bremer et al., 2003) but rare at mid-latitudes (Latteck et al., 1999, Zecha et al., 2003). Radio waves in the VHF range are backscattered by electron density structures with spatial scales of about half the radar wave length (3 m at a radar frequency of 50 MHz). These small irregularities are normally in the viscous subrange of turbulence and destroyed by molecular diffusion, but charged ice particles or charged aerosols can reduce the electron diffusion and allow the existence of such small radio wave scatterers (see review by Cho and RGttger, 1997). PMSE/MSE like NLC are therefore bound to the existence of low mesospheric temperatures necessary for the formation of charged aerosols. Observations of both phenomena at arctic latitudes have shown that a tidal-like modulation of their appearance exists (Bar-abash et al., 1998; Hoffmann et al., 1999; Huaman et al., 2001). Meteor radar observations provide continuous measurements of temperature and winds and their tidal variation in the upper mesosphere. In this paper the results of observations at mid-latitudes and arctic latitudes will be presented and the relation to observations of MSE/PMSE and NLC will be discussed.

Adv. Space Res. Vol. 31, No. 9, pp. 2055-2060,2003 0 2003 COSPAR. Published by Elsevier Ltd. All rights reserved Printed in Great Britain 0273-I 177103 $30.00 + 0.00

2056 W. Singer et al.

Fig. 1. Seasonal variation of temperatures from meteor decay times at Juliusruh (55”N) and of OH rotational temperatures at Wuppertal (52”N).

i I I I I b I II I I 11 I I ,mm;,,, lJtn 1Feblwu 1ppcmy ‘hpoz;-’ lP&glsep lob

Fig. 2. Seasonal variation of temperatures from meteor decay times and of climatological mean tem- peratures obtained from in-situ rocket measurements and lidar observations at Andenes (69”N).

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t-w Jux,

Fig. 3. Daily mean temperatures from meteor decay times obtained at Kiihlungsborn (54”N, 53.5MHz) and at Andenes (69’N, 32.55MHz) during summer 2002. The appearance of the first VHF radar echoes at mid-latitudes (MSE) and arctic latitudes (PMSE) as well as of NLCs at 69’N are depicted.

MEASUREMENTS AND DATA ANALYSIS The all-sky meteor radars used in this study apply

crossed antenna elements to ensure a near uniform az- imuthal sensitivity to meteor echoes. A 5-antenna in- terferometer on reception (Hocking et al., 2001) results in a range accuracy of 2 km and angular accuracy of about 2 degrees in meteor location. Observations have been made at Juliusruh, Germany (54.6”N, 13.4”E) from November 1999 to August 2001, and at An- denes, Norway (69.3”N, 16.0’E) from September 2001 onward at 32.55 MHz. Meteor observations are per- formed with a pulse repetition frequency of 2144Hz, a pulse width of 13~s (Gaussian shape) and a peak power of 12 kW. Total meteor count rates of unam- bigious detections vary throughout the year between 2500 and 8000 meteors per day. The diurnal varia- tion of meteor count rates under summer conditions amounts to about 100 to 500 meteors per hour. In addition, the same meteor detection capabilities have been added to the MST radar operated at 53.5 MHz in Kiihlungsborn (54.1”N, 11.8’E; Latteck et al., (1999)) from April 2002 onward, and an interleaved operation between MST and meteor mode was applied.

Meteor radars measure the decay time of under- dense meteor echoes which is inversely proportional to the ambipolar diffusion coefficient. Daily mean tem- peratures are estimated on the basis of the variation of the ambipolar diffusion coefficient with height in com- bination with an empirical model of the mean tem- perature gradient at the peak altitude of the meteor layer (Hocking, 1999). The gradient model for mid- latitudes has been derived from Potassium lidar tem- peratures in combination with OH rotational temper- atures obtained close to the meteor radar site. The estimated temperature gradient amounts about -3.2 K/km in early May, increases to 0.5 K/km at the end of June and decreases again to about -2.7 K/km at the end of August. The gradient model for arctic latitudes is nearly the same in that time period with differences in the order of a few tenth K/km (for details see Singer et al., 2003). Several thousand meteor detections are preferred for the derivation of daily mean tempera- tures. These are averages over the width of the me- teor layer (8-10 km) which peaks between 88 and 91 km (90-91 km at 32.5MHz, 88-89km at 53.5MHz). The absolute temperatures are determined by the gradient model (a change of the gradient by 1 K/km results in a temperature change of about 10-15 K).

Temperature tides are analyzed on the basis of a composite day formed by 10 or 20 days under summer conditions. Hourly bins with at least several hundred, preferably several thousand, meteors are required for the determination of reliable temperature values. A first guess of the tidal components is obtained by

Wind and Temperature Tides around the Summer Mesopause 2057

205

195

165

175

165

155

145

135 lOMay JOMay 1QJun QJUI 29Jul 16Aug 1OMay 30tday 19Jun QJUI 29JUl 16Aug

2000 2001

Fig, 4. Diurnal variation of meteor temperatures around 90 km in Juliusruh during summers 2000 and 2001 (bottom panels) and of occurrence rate of mesospheric summer echoes at Kiihlungsborn during summers 2000 and 2001 (top panels).

harmonic analysis of the hourly means but the diurnal variation of the temperature gradient due to tides is still present in the data. Improved estimates of amplitudes and phases of the diurnal and semidiurnal temperature tides can be obtained using the vertical wave lengths of the 24h/12h wind tides from simulta- neously measured meteor winds (Hocking and Hocking, 2002). The corrected tidal components depend only weakly from the applied gradient model in contrast to the mean temperatures.

RESULTS Mean Temperatures

Daily mean temperatures based on 24 hours meteor observations centered on mid-night have been analyzed at altitudes of about 90 km at 54”N and 69”N. The seasonal variation of meteor temperatures at mid-latitudes is shown in Fig. 1 together with night-time OH rotational temperatures observed in Wuppertal (Bittner et al., 2002). The OH temperatures are representative for the peak altitude of the OH emission layer at about 87 km which has a width of about 8 km (comparable to that of the meteor layer). Daily mean meteor temperatures for arctic latitudes are presented in Fig. 2 together with climatological mean temperatures estimated from in-situ rocket measurements and lidar observations at Andenes (Liibken and von Zahn, 1991; Liibken, 1999). The meteor temperatures are in good agreement with both data sets regarding their seasonal evolution and absolute values with about 30K lower summer temperatures in the Arctic. In early summer, the temperature varies at mid-latitudes between 150K and 170K and at the arctic location between about 120 K and 150 K. The seasonal variation at mid-latitudes is characterized by a strong temperature decrease during the spring/summer transition (last decade of April - last decade of May) and a moderate temperature increase during the summer/autumn transition (last decade of June - end of September). We observe the opposite behavior at arctic latitudes with a moderate spring/summer transition (end of March - end of May and a fast summer/autumn transition (last decade of August - end of September).

A similar situation has also been observed in 2002 (Fig. 3). The cooling of the mesopause region starts at 69”N in the first decade of April. Temperatures below 140K appear at the beginning of June when the first NLC was observed by the ALOMAR RMR lidar and continue until the second decade of August. At mid-latitudes temperatures below 150K are confined to the first half of June. In summer 2002 we observe the interesting feature that strong mesospheric radar echoes appeared at first at mid-latitudes on 29 May followed by the first polar mesosphere summer echo (PMSE) at Andenes on 31 May (1st MSE, 1st PMSF in Fig. 3). This observation is well supported by the meteor temperatures. At arctic latitudes temperatures around 160 K are observed well above the temperature necessary for the formation of ice particles allowing VHF radar backscatter whereas the temperatures at mid-latitudes are below I50 K.

2058 W. Singer et al.

90 120 160 180 210 240 90 1-a 150 180 210 240 iI 120 150 Daynu* 0svnrf-f & 210 240

Fig. 5. Mean values as well as diurnal and semi-diurnal tides of temperature, zonal and meridional winds in Juliusruh (20-d composite day analysis) during summer 2000 (full lines) and 2001 (dashed lines). Tidal phases (P24, P12) are given in UT. The wind tidal phases of 2000 marked by dots (zonal wind) and crosses (meridional wind) are additionally presented together with the temperature tidal phases in the left panel. The appearance of MSE is indicated by the hatched area.

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40

L 0 20

24 185

20 175

16 165

155

8 145

4 135

0 125 30Apr 2OMay QiJn 2QJun IQJul

2002 ~4l =Aue

Fig. 6. Diurnal variation of meteor temperatures around 91 km (bottom panel) and of occurrence rate of polar mesospheric summer echoes (top panel) in Andenes during summer 2002.

Temperature and Wind Tides Temperature and wind tides are analyzed on the ba-

sis of 10/20-day composite days shifted by 5/10 days for summer conditions. The diurnal temperature vari- ations estimated for mid-latitudes in summers 2000 and 2001 (based on 10-d composite days) are pre- sented in Fig. 4 together with the observed occurrence rates of mesospheric summer echoes observed by the Kiihlungsborn MST radar in the altitude range 82- 90 km. In both years the first MSE appear soon after the temperature drops below 150 K, but the morpholo- gies of the temperatures and the MSE thereafter are not strongly correlated. At that time between mid- dle of May and middle of June an enhanced semi- diurnal tide with a peak amplitude of about 10 K is ob- served (Fig. 5; based on 20-d composite days). Wind and temperature tides (except the diurnal tempera- ture tide) show stable phases during summer with a high degree of persistence from year to year. The semi- diurnal wind tide reaches amplitudes between lOm/s and 25 m/s and is circular polarized as shown by the 3-hour phase difference between zonal and meridional

winds (dots and crosses in left panel of Fig. 5). The semi-diurnal tide of temperature and meridional wind (Fig. 5, crosses in the corresponding temperature panel) are in phase in May and June. At arctic latitudes the temperature variation observed in summer 2002 is dominated by a diurnal variation (Figs. 6 and 7) with amplitudes between about 3 and 8 K. The phases of the semi-diurnal temperature and wind tides are more variable at arctic latitudes. Diurnal and semi-diurnal wind tides are circularly polarized as demonstrated by a 6-hour/3-hour difference between the meridional and zonal wind tidal phases (dots and crosses in left panel of Fig. 7). Also at arctic latitudes the semi-diurnal tides of temperature and meridional wind are in

Wind and Temperature Tides around the Summer Mesopause 2059

Fig. 7. Mean values as well as diurnal and semi-diurnal tides of temperature, zonal and meridional winds in

o- %.I 120 150 180 210 240

Days Gw

Andenes during summer 2002 (20-d composite day analysis). Tidal phases (P24, P12) are given in UT. The wind tidal phases marked by dots (zonal wind) and crosses (meridional wind) are additionally presented together with the temperature tidal phases in the left panel. The appearance of PMSE is indicated by the hatched area.

phase during May and June and reach their maximum at about 03 UT to 04 UT (see crosses in the left panel of Fig. 7).

A first comparison of monthly mean tides of temperature and winds with results of the Global Scale Wave Model, version GSWM-00 (Hagan, 2002) shows good agreement between the tidal phases for summer and winter at mid-latitudes (Fig. 8). The model temperature amplitudes for both seasons and the wind amplitudes for summer are considerably smaller than the observations and could be related to the restriction of GSWM-00 to migrating tides only.

24h Temperature MN) 12h Terrperature(55N) 1M !~~~~"~~~'~~'1!."".'~"~"":

12h Meridional M-d WN)

Fig. 8. Comparison of GSWM-00 results (full lines) with monthly mean temperature and wind tides from meteor radar observations (dots, thin lines) at mid-latitudes for summer and winter.

CONCLUSIONS Radar-meteor observations in the VHF frequency range provide continuous meteor data from the altitude

range 80 km to 100 km to estimate horizontal winds and temperatures. Daily mean temperatures derived for the peak of the meteor layer (90-91 km) at middle and arctic latitudes agree well with OH rotational temperatures, and temperature determinations from rocket-borne instruments and lidar. The meteor tem- peratures at 90 km are characterized by low summer temperatures between 150 and 170 K at mid-latitudes

2060 W. Singer et al.

and by about 30K lower temperatures at arctic latitudes. Here, the appearance of PMSE is confined to a period from the last decade of May to the last decade of August when the mean temperatures at 90 km are 140 K or lower. A further correlation with specific characteristics of the temperature tides is not evident. At mid-latitudes the fast spring/summer transition and the formation of the low summer temperatures seems to be influenced by tides. The semi-diurnal tidal amplitude peaks as the mean temperature reaches its lowest value and the first MSE appears. In addition, the semi-diurnal temperature and meridional wind tides are in phase during the spring/summer transition at middle and arctic latitudes resulting in a further temper- ature decrease during southward directed tidal winds. The southward directed mean winds are intensified supporting the transport of cold air from higher to lower latitudes in summer.

ACKNOWLEDGEMENT We would like to thank the staff of the Andoya Rocket Range for assistance in operating the meteor radar.

We are grateful to U. von Zahn for providing the ALOMAR RMR lidar data contained in Fig. 3. This study was in part supported by grant BR 2023/1-l of the Deutsche Forschungsgemeinschaft, Bonn, Germany, the radar operation at Andenes was in part supported by grant HPRI-CT-1999-00002 (Transnational Access to Major Research Infrastructures) of the European Union. The 32.55 MHz meteor radar system was designed and manufactured by Genesis Software, Adelaide, Australia and Mardoc Inc., London, Ontario, Canada.

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radar observations at Andenes, Norway, J. Geophys. Res., 108(D8), 8438, doi:l0.1029/2002JD002369, 2003.

Bittner, M., D. Offermann, H.-H. Graef, M. Donner and K. Hamilton, An l&year time series of OH rotational temperatures and middle atmosphere decadal variations, J. Atmos. Solar-Terr. Phys., 64, 1147, 2002.

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Latteck, R., W. Singer, and J. Hoffner, Mesosphere summer echoes as observed by VHF radar at Kiihlungs- born (54’N), Geophys. Res. Lett., 26, 1533-1536, 1999.

Lubken, F.-J, and U. von Zahn, Thermal structure of mesopause region at polar latitudes, J. Geophys. Res., 96, 20,841-20,857, 1991.

Liibken, F.-J., The thermal structure of the Arctic summer mesosphere, J. Geophys. Res., 104, 9135, 1999. Singer, W., J. Bremer, J. Wei, W.K. Hocking, J. Hoffner, M. Donner and P. Espy, Meteor radar observations

ad middle and arctic latitudes, Part 1: Mean temperatures, J. Atmos. Solar-Terr. Phys., submitted, 2003. Zecha, M., J. Bremer, R. Latteck, W. Singer, P. Hoffmann, Properties of mid-latitude mesosphere sum-

mer echoes after three seasons of VHF radar observations at 54”N, J. Geophys. Res., 108(D8), 8439, doi:l0.1029/2002JD002442,‘2003.

E-mail address of W. Singer singer@iap-kborn.de Manuscript received 10 December 2002; revised 28 March 2003, accepted 28 March 2003