Seasonal variation of nocturnal temperature and sodium density in the mesopause region observed by a resonance scatter lidar over Uji, Japan

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115,, doi: /2009jd013799, 2010 Seasonal variation of nocturnal temperature and sodium density in the mesopause region observed by a resonance scatter lidar over Uji, Japan Mitsumu K. Ejiri, 1 Takuji Nakamura, 1 and Takuya D. Kawahara 2 Received 7 January 2010; revised 26 May 2010; accepted 2 June 2010; published 29 September [1] Temperature and sodium (Na) density profiles in the mesosphere and lower thermosphere region were obtained by a Na temperature lidar observation at Uji, Japan (35 N, 136 E) for 136 nights (more than 1200 h) from October 2007 to January The seasonal and height variations of Na density and temperature in a Northern Hemisphere midlatitude Asian sector are reported for the first time and compared with results obtained in the United States over Colorado (CO) and New Mexico (NM). Monthly composite nocturnal variation indicated significant amplitude of semidiurnal tide in winter and a different tidal mode in summer. An annual variation was dominant in the Na density seasonal variation, with a maximum in November and a minimum in May. An additional second peak was seen at high altitudes ( 94 km) in June July. This second peak was probably generated by the sporadic Na (Na s ) layer frequently observed over Japan. Temperature above 98 km showed an annual variation with a summer maximum. Temperature at km showed semiannual variation with peaks in September and April. This temperature variation was similar to that in NM, at the same latitude (35 N), but different from that in CO at 40 N, suggesting latitudinal difference. The summer temperature at 87 km agreed well among the three sites, but the winter temperature at Uji was significantly lower than at the other sites. This difference in winter could be due to an interannual variation and/or a longitudinal dependency possibly caused by nonmigrating tides. Citation: Ejiri, M. K., T. Nakamura, and T. D. Kawahara (2010), Seasonal variation of nocturnal temperature and sodium density in the mesopause region observed by a resonance scatter lidar over Uji, Japan, J. Geophys. Res., 115,, doi: /2009jd Department of Polar Science, National Institute of Polar Research, Graduate University for Advanced Studies, Tokyo, Japan. 2 Faculty of Engineering, Shinshu University, Nagano, Japan. Copyright 2010 by the American Geophysical Union /10/2009JD Introduction [2] For more than two decades, resonance scatter lidar systems have been used to profile temperature and wind velocities and the densities of scattering metallic atoms in the mesosphere and lower thermosphere (MLT) region. Temperature profiling is particularly important because other types of observation systems, such as ground based radars, cannot measure temperature profiles. Sodium lidars have been used to measure temperature (and wind) at various sites, including U.S. sites in Colorado, Illinois, New Mexico, and Hawaii [e.g., Fricke and von Zahn, 1985; She et al., 1990; Bills et al., 1991; Chu et al., 2005]. Among these, observations at Fort Collins, CO, by the Colorado State University (CSU) lidar began in 1990 [She et al., 1990] and have continued since, providing almost 20 years of observations [She et al., 2009; Krüeger and She, 2009]. The temperature climatology [She et al., 2000] observed at Fort Collins is useful as a typical temperature in the midlatitude region. Annual temperature variation and the double height of the mesopause in winter and summer have been indicated clearly. [3] The latitudinal distributions of temperature and metallic atom density were also studied using a resonance lidar [von Zahn et al., 1996]. However, longitudinal comparison of temperature and metallic atom densities by resonance lidar measurements has not been conducted because few lidars can observe both temperature and metallic atom densities. [4] On the other hand, some previous observations have shown longitudinal difference of the metallic layer in the midlatitude Northern Hemisphere. Nagasawa and Abo [1995] reported that sporadic sodium layers (Na s ) occur much more frequently over Japan (36 N, 139 E) than over Illinois in the United States (40 N, 88 W) and in France (44 N, 6 E). Observations in Wuhan, China (31 N, 114 E), also supported the frequent occurrence of Na s in the Asian sector [e.g., Gong et al., 2002]. Occurrence of sporadic E layers (E s ) is also known to be higher in Asian longitudes, 1of9

2 and correspondences with Na s have been pointed out in the above studies. Recently, Williams et al. [2007] reported that occurrence rate of Na s observed at Colorado in the United States (41 N, 105 W) in summer was as high as that in Asia. This is the only literature reporting high Na s occurrence in the United States, to our knowledge, but with very brief statements. Therefore, in this manuscript, we still treat that the occurrence of Na s in the United States is low. [5] More recent satellite observations have revealed strong zonal variability in the ionosphere and thermosphere. Ultraviolet (UV) images obtained by the far ultraviolet (FUV) instrument on the IMAGE satellite showed clear wave 4 structures in the longitudinal distribution of electron densities in the equatorial anomaly region [Sagawa et al., 2005], which are considered to be caused by diurnal tide with eastward propagating zonal wave number 3 (DE3 tide) propagating from the troposphere. Similar wave 4 structure has also been reported in the neutral atmosphere [Liu et al., 2009] and for the NO mixing ratio [Oberheide and Forbes, 2008]. [6] This paper reports temperature and sodium densities in the MLT region observed by a sodium temperature lidar over Uji, Japan (35 E, 136 N). The longitudinal difference between Japan and the western United States is about 120, and therefore, the wave 4 structure and DE 3 tides will have different phase if these two regions are compared at the same local time. Because many sodium temperature (and wind) lidar observations have been reported for the United States, comparison with the Japanese/Asian sector is of interest. Similarities and differences in the results will be discussed by comparing published results for observations over the United States, especially over Colorado and New Mexico, where the longitudinal difference from Japan is almost exactly Instrumentation and Observation [7] The Na temperature lidar was developed by the National Institute of Polar Research (NIPR) and Shinshu University for an Antarctic observation project in 1998 and completed a 3 year observation at Syowa station in Antarctica from 2000 to 2002 [Kawahara et al., 2002, 2004]. This lidar (hereafter referred to as the Na temperature lidar of Shinshu University) uses a two frequency technique [She et al., 1990] to measure Na density and temperature around km. The narrowband laser was tuned alternately to two wavelengths near the D2a peak and bottom. The lidar transmitter uses two injection seeded, pulsed Nd:YAG lasers with wavelengths of 1064 and 1319 nm. The sum frequency generator with a nonlinear crystal (beta barium borate, BBO) produces Na resonance line radiation at 589 nm, a technique originally suggested by Jeys et al. [1989]. The bandwidth of the output pulses is 0.06 pm ( 50 MHz) in full width at half maximum (FWHM), and the output frequency can be scanned across 589 nm by fine tuning the wavelength of the seeder lasers, which is monitored by a wavemeter with an uncertainty of ±0.05 pm (9 MHz at 1319 nm, 13 MHz at 1064 nm). The total uncertainty of the wavelength at 589 nm output is then 0.01 pm (9 MHz) in standard deviation. The laser pulse repetition was 10 Hz with output energy of 25 mj. Photon profiles were obtained alternately at each wavelength with an integration time of 2.5 min (1500 shots). Including the switching period of 30 s, the total length of one cycle of observation for two wavelengths was 6 min. The backscattered signal was received by a telescope with a diameter of 50 cm and an operational field of view (FOV) of 1 2 mrad. [8] The observations at Uji were carried out several nights per month starting on 1 October We tried to collect at least 50 h of data per month. Data were successfully collected for 136 nights (1223 h), as summarized in Table 1. Figure 1 shows the coverage in universal time (UT) of measurements on each night. The time difference between UT and the local time (LT) is 9 h, and 15 UT corresponds to midnight in Japan. The horizontal axis of Figure 1 gives the day of the year. Typical observations in summer and in winter were between 11 and 19 UT and 9 and 21 UT, respectively. [9] Figure 2 shows typical hourly profiles of Na density and temperature profiles in winter (Figures 2a and 2b) and in summer (Figures 2c 2f). Hourly profiles of Na density and temperature were calculated with vertical resolutions of 0.8 and 2.0 km, respectively. Maximum Na densities of hourly profiles in summer (a) were cm 3, and the error standard deviations were a few percentage points of the density. A peak of the Na layer moved downward from 93 km at 9 UT to 90 km at 19 UT. Measurement errors were smaller than 3 K at the peak altitudes of Na density (90 93 km) and larger than 5 K below 85 km and above 100 km. A local peak of the temperature profile moved downward from 93 km at 9 UT to 87 km at 20 UT, with a downward speed of 0.55 km/h. The temperature at a density peak altitude of 91 km varied between 140 K (12 UT) and 210 K (15 UT). Such downward progression of temperature structure with large amplitude was also frequently observed on other days in winter. Figures 2c and 2d show the nocturnal variations of Na density and temperature on 6 May 2008, respectively. In Figure 2c, Na density at peak altitudes of the Na layer was approximately 2000 cm 3 and smaller than in winter. The Na density decreased below 89 km and increased above 95 km over time. Figure 2d shows the temperature on the same day. The error standard deviations were smaller than 5 K at the peak altitudes of Na density and larger than 10 K below 85 km and at the upper part of the Na layer (above 95 km before midnight and above 100 km after midnight). The larger temperature errors in summer than in winter are due to the lower Na density as well as poorer visibility (or larger extinction) in the lower atmosphere in summer over Uji. In particular, the temperature at the upper part of the Na layer before midnight is quite difficult to measure with good precision, and temperature data are less frequently determined. Figures 2e and 2f show nocturnal variations on 6 June The sudden formation of a dense, thin Na layer, i.e., a sporadic sodium layer (Na s ), is superposed on variations of normal Na layers. The Na s at 95 km with a FWHM of 3 km lasted at least 3 h from 15 to 18 UT. The ratio of peak Na density (12,000 cm 3 ) to the normal Na density (2000 cm 3 ) was as large as 6. The temperature at 95 km increased from 165 K at 14 UT to 180 K at 16 UT and reached 200 K at 18 UT. This temperature enhancement of 9 K/h could be due to inertia gravity waves and/or tides. Although a relationship between the temperature enhancement and the Na s formation is still 2of9

3 Table 1. Summary of Numbers of Nights and Hours of Observation Year Total Month Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan 16 Nights (h) 9 (83) 7 (74) 8 (72) 9 (92) 10 (79) 9 (81) 10 (88) 10 (75) 6 (44) 9 (72) 8 (55) 6 (52) 11 (108) 8 (94) 10 (99) 6 (55) 136 (1223) under investigation, such Na s events are often seen in summer in our observations at Uji, Japan. 3. Results 3.1. Hourly Variations of MLT Temperature and Na Density [10] To characterize temperature and Na density, we created time height cross sections as a composite of every month. The monthly composites of temperature are shown in Figure 3. The composite data were computed using all temperature data in , excluding data with error larger than 50 K. Local peak temperatures in each panel are highlighted by white dotted lines. The monthly composites for January March and for October December are similar. That is, nighttime maximum temperatures were K at km around 20 UT. The high temperature region appeared to move downward with time from 95 km at 15 UT at a rate of 2 3 km/h. At around km, a small temperature increase ( 190 K) was also found in most months between October and March. Such small temperature increases, which occurred about h earlier than the major peak at around 20 UT, also moved downward. The monthly composites between May and August were similar to one another but were different from those of winter months. Maximum temperatures during nighttime were 200 K at km around 14 UT. The high temperature region seemed to move downward at a speed of 1.7 km/h, which is slower than in winter. Observation durations in summer were too short to determine the wave period. The downward progressing temperature pattern in winter was probably dominated by semidiurnal tide, but whether that pattern in summer was dominated by diurnal tide or by semidiurnal tide was unclear. However, the downward phase progress indicates that both wave structures were due to vertically propagating tides. Different tidal frequencies and modes could have caused difference in the downward phase speeds. The monthly composites for April and September show transition between summer and winter. [11] Figure 4 shows monthly composites of Na densities for 12 months. Data with an error bar larger than 100% were excluded. White dotted lines show the temperature maximum, the same as shown on the temperature composite plots in Figure 3. The major part of the Na layer is shown in the altitude range of km for all months, except at UT in May and UT in June. The peak altitudes before midnight in May and June were lower than 90 km and higher than 95 km, respectively. The route mean square (r.m.s.) width of the Na density profile was km throughout the year. Nighttime peak densities were located at km in winter (October March) and km in summer (June August), both at UT with a value of cm 3. The peak density in the composite cross section appears when the downward propagation of higher temperatures (shown by a white dotted line) intersects the altitudes of peak Na density in all the monthly composites Seasonal Variations of Na Density and MLT Temperature [12] The seasonal variations of Na densities and temperatures are illustrated in Figures 5 and 6, respectively. The altitude/month contour plots were computed from nightly Figure 1. Distribution of observation time by date and universal time. Each vertical line indicates the start and stop time of observation on each night. 3of9

4 Figure 2. Typical nocturnal variations of hourly profiles of temperature and Na density in (a, b) winter, (c, d) summer, and (e, f) on a night with a sporadic Na layer event. Each profile is shifted by 2000 cm 3 and 50 K for density and temperature, respectively. Dotted lines at both sides of each hourly profile show measurement errors. Temperature and Na density profiles were obtained with height resolutions of 2 and 0.8 km, respectively. 4of9

5 Figure 3. Monthly mean nocturnal variation of temperature as time height cross sections (monthly composites). The composite data were computed using all temperature data for Local peak temperatures in each frame are highlighted by white dotted lines. 5of9

6 Figure 4. As in Figure 3, but for Na density. The composite data were computed using all density data for White dotted lines show the temperature maximum, the same as shown in the temperature composite plots in Figure 3. 6of9

7 Figure 5. Seasonal variation of nocturnal Na density in the MLT region. The contours were computed from nightly mean profiles of the monthly composite data using a Hanning window with a FWHM of 2.0 km in altitude and 30 days of time. mean profiles of the monthly composite data using a Hanning window with a FWHM of 2.0 km in altitude and time period of 30 days. [13] Seasonal variation of Na density (Figure 5) clearly shows an annual variation. The maximum peak density was in November at 92 km ( 4200 cm 3 ), and the minimum peak density was in May at 92 km ( 2100 cm 3 ). Additionally, a second seasonal maximum ( 3000 cm 3 ) can be seen around 94 km in June July. [14] Seasonal variation of temperature (Figure 6) at an altitude range of km had two peaks, in September and in April, with peak temperatures of 200 K (89 km) and 190 K (93 km), respectively. Minimum temperature of 172 K (87 km) was observed in June July. Therefore, semiannual oscillation is dominant at this altitude region. On the other hand, the seasonal variation above 98 km showed mainly an annual oscillation with a maximum temperature of 190 K in June and a minimum temperature of less than 160 K in January February. The mesopause is considered to be located at km in June August (summer) and around 100 km in other months (mainly winter). 4. Discussion [15] Seasonal variations of Na density and temperature in the MLT region were obtained by Na lidar observations on 136 nights during at Uji, Japan (35 N, 136 E). The seasonal variation of Na density clearly showed annual oscillation with a maximum in October November and minimum in May. In addition, an enhancement of Na density at 95 km was observed in June July. The seasonal variation observed at Fort Collins (FC), CO (41 N, 105 W), in showed annual variation with a maximum in winter (November) and minimum in summer (June) [She et al., 2000]. However, the small enhancement in June July observed at Uji (Figure 5) was not observed at FC. At Uji, we observed Na s events (as shown in Figure 2(e)) on 11 of 15 nights of observations in June and July. Such a high occurrence of Na s agrees with a previous observation over Tokyo [Nagasawa and Abo, 1995]. The difference of summertime Na density found in the month height cross section between Japan and FC was probably caused by the frequent occurrence of Na s over Japan. [16] Recently, OSIRIS on the Odin satellite measured Na density profiles globally by limb scanning measurements of the Na radiance at 589 nm in the dayglow. Fan et al. [2007a] showed a seasonal variation of the zonally averaged Na density profile at 40 N from the OSIRIS measurements in (see Figure 3b in their paper). The seasonal variation with a maximum in October and minimum in May was similar to that at Uji, but the small enhancement in June July observed at Uji (Figure 5) was not shown in the satellite measurements. A global distribution of Na s measured by OSIRIS was reported by Fan et al. [2007b]. However, the occurrence rate of Na s was quite different from previous studies by ground based lidar observations because local time coverage of OSIRIS measurements was limited (only 0600 and 1800 LT). [17] In Figure 6, seasonal variation of temperature obtained at Uji shows an annual variation above 98 km and a semiannual variation at km. Previous observation at FC [She et al., 2000] revealed annual variations both below and above 98 km with opposite phases. The temperature above 98 km at FC had a maximum in summer and minimum in winter, the same as observed at Uji. However, the variation below 98 km was quite different between FC and Japan. Another previous observation at the Starfire Optical Range (SOR) in NM (35 N, 107 W) in [Chu et al., 2005] found an annual variation with a summer maximum above 98 km and a semiannual variation with maxima in March and November at km [see Chu et al., 2005, Figure 11]. The pattern of seasonal variation was very similar to that over Uji, at the same latitude (35 N). Semiannual variation can be seen in temperature of Figure 6 but not in Na density of Figure 5. According to previous studies of chemical reactions of Na at the mesosphere [Plane et al., 1999; Cox et al., 2001], there is a positive correlation between Na density and temperature below 96 km. However, concentration profiles of minor constituents are not invariant and probably show some seasonal dependency. It has been reported by radar observations that both gravity wave activity and turbulence intensity (eddy diffusivity) is the smallest in equinoctial months [Tsuda et al., 1990; Figure 6. Seasonal variations of nocturnal temperature in the MLT region. The contours were computed by the same process as in Figure 5. Average temperatures with errors smaller than 5 K are plotted. 7of9

8 Figure 7. Seasonal variation of monthly mean temperatures at 87 km observed at Uji in (asterisk), at SOR in (open circle) [Chu et al., 2005], and at FC in (diamond) [She et al., 2000]. Standard deviations (1s) of the temperature at Uji are calculated each month and are shown as error bars. Fukao et al., 1994] in the mesosphere at midlatitude over Japan. The difference of vertical eddy diffusivity can cause significant difference of minor constituents in the MLT region. So this could be a reason of warm temperature in the equinoctial months but no significant enhancement of Na density though more quantitative analysis is required. If we compare the temperature more quantitatively, the minimum temperature below 85 km (summer mesopause) at SOR was 180 K and similar to that at Uji. However, the temperatures below 98 km in winter at Uji were lower than those at both SOR and FC. For more detailed comparison of temperature, we examined temperature at 87 km; we chose this altitude because various previous studies have compared temperature at 87 km [e.g., She and Lowe, 1998]. [18] Figure 7 plots temperatures at 87 km for Uji, SOR, and FC. Little difference was observed among the temperatures in May September. The difference ranged from 2 to 6 K between Uji and SOR and from 1 to 8 K between Uji and FC. On the other hand, the temperatures in January April and October December at Uji were lower than those at the other two sites. In January April and October December, differences ranged from 10 to 25 K between Uji and SOR and from 3 to 31 K between Uji and FC, whereas the differences between SOR and FC were only 1 13 K. The large differences of winter temperature between Uji and the other two sites could be due to the lower temperatures in , when observation at Uji were carried out or to longitudinal variation or a colder nighttime mesopause region over Japan compared with the FC/SOR longitude. [19] In the middle atmosphere, year to year variability is large in winter because of the effects of sudden stratospheric warming and/or planetary wave activity, which show significant interannual variability [e.g., Manney et al., 2009]. In winters of both 2007/2008 and 2008/2009, large stratospheric warming was observed in the Northern Hemisphere [e.g., Manney et al., 2009]. Therefore, interannual variability is a possible reason for the lower temperature indicated here. Another possible cause of the differences is longitudinal difference. The temperature compared in this paper was the nighttime temperature, and aliasing of diurnal/semidiurnal tides by use of daytime only or nighttime only observations is a possible bias when discussing daily or monthly mean value [States and Gardner, 2000]. The migrating tide, which shows the same phase in LT at all longitudes, does not cause a longitudinal difference in the nocturnal mean value. However, nonmigrating tides can create a bias depending on the longitude. A DE3 tide has been reported to be significant in amplitude and to contribute to the creation of the zonal wave 4 structure [Forbes et al., 2008]. Both of these tides have a phase difference of 120 when the same local time is compared. For example, a tide with 10 K amplitude can create up to 17 K difference. Thus, longitudinal variation caused by tides is one candidate for explaining the large longitudinal difference in nocturnal average temperature. Stationary planetary waves could also contribute to the difference. 5. Summary [20] During the period from October 2007 to January 2009, temperature and Na density profiles were intensively observed on 136 nights at Uji, Japan (35 N, 136 E), resulting in more than 1200 h of observations. Monthly composites of nocturnal variations showed a significant semidiurnal tidal feature in winter and a different tidal mode in summer. Maximum Na densities occurred when high temperature regions associated with tides approached the peak height of the sodium density profile. Comparison of the seasonal variations at Uji with observations over the United States at Fort Collins (FC), CO (41 N, 105 W), and the Starfire Optical Range (SOR), NM (35 N, 107 W), revealed the following: [21] 1. Na density at Uji had similar seasonal variation to that at FC, except for an enhancement ( 95 km) in June July at Uji. Such enhancement is probably caused by the sporadic Na layer. [22] 2. Temperatures between 83 and 98 km at Uji and SOR showed semiannual variation, but annual variation was dominant at FC. The pattern of seasonal variation at Uji was quite similar to that at SOR. [23] 3. Minimum temperature at km (summer mesopause) at Uji was about 175 K and similar to minima at FC and SOR; however, winter temperatures at Uji were significantly lower than those at the other two stations. The differences of monthly temperature in winter could be caused by interannual variability of planetary wave activity and of sudden stratospheric warming and/or latitudinal difference in the phase of nonmigrating tide. [24] This is the first report of the characteristics of seasonal height variations of Na density and temperature observed by Na lidar in an Asian sector of the Northern Hemisphere midlatitude mesopause. The differences between other observations at FC and SOR in the United States should be more closely investigated in the future, considering both interannual variation and tidal effects, as well as other possible mechanisms. Differences in seasonal variation of temperature should also be considered when we interpret observational results obtained by ground based 8of9

9 measurements, especially in the case of nighttime only or daytime only observations. [25] Acknowledgments. M. K. Ejiri received support as a Japan Society for the Promotion of Science Postdoctoral Fellow (JSPS grant ). This study was also partially supported by the Grants in Aid for Scientific Research (B) ( , , and ) from the JSPS and by the Center for the Promotion of Integrated Sciences (CPIS) of Sokendai. The authors gratefully acknowledge the use of the Research Institute for Sustainable Humanosphere (Kyoto University) and the Na temperature lidar (Shinshu University) for this long term observation. References Bills, R. E., C. S. Gardner, and S. J. Franke (1991), Na Doppler/temperature lidar: Initial mesopause region observations and comparison with the Urbana medium frequency radar, J. Geophys. Res., 96, 22,701 22,707, doi: /91jd Chu, X., C. S. Gardner, and S. J. 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