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Mesopause region temperature structure observed by sodium resonance lidar
ARTICLE IN PRESS Journal of Atmospheric and Solar-Terrestrial Physics 72 (2010) 740–744
Contents lists available at ScienceDirect
Journal of Atmospheric and Solar-Terrestrial Physics journal homepage: www.elsevier.com/locate/jastp
Mesopause region temperature structure observed by sodium resonance lidar Barclay Clemesha n, Dale Simonich, Paulo Batista Instituto Nacional de Pesquisas Espaciais, Avenida dos Astronautas, 1758, 12227-010 Sa~ o Jose´ dos Campos, SP, Brazil
a r t i c l e in f o
a b s t r a c t
Article history: Received 31 October 2009 Received in revised form 9 March 2010 Accepted 20 March 2010 Available online 27 March 2010
Mesopause region temperature measurements made with a sodium resonance lidar show two unexpected features: (1) Strong positive temperature gradients are often associated with strong gradients in the sodium concentration and (2) positive temperature gradients are generally much stronger than negative ones. Although the structures we see frequently appear to be associated with gravity waves or tides, the asymmetrical temperature oscillations cannot be explained as the result of simple wave propagation. We suggest that strong positive temperature gradients correspond to regions of high atmospheric stability, where eddy diffusion is inhibited, permitting the build-up of strong gradients in temperature and minor constituent mixing ratio. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Mesopause region Temperature Lidar
1. Introduction It is well-known that gravity waves, tides and planetary waves play a major role in the dynamics of the mesopause region. Generated in the troposphere and lower stratosphere, such waves increase in amplitude as they propagate upwards, depositing energy and momentum in the middle atmosphere. It is generally believed that the cold summer mesopause is the result of wave momentum deposition which provides the major forcing for the meridional circulation. The behaviour of such waves and their interactions with the atmospheric constituents has been the subject of many studies, both experimental and theoretical (Chiu and Ching, 1978; Shelton et al., 1980; Gardner and Shelton, 1985; Batista et al., 1985; Yang et al., 2008b). Lidar measurements of the vertical distribution of meteor metals such as sodium have been used to study tides (Batista et al., 1985; Fricke-Begemann and ¨ Hoffner, 2005) and gravity waves (Shelton et al., 1980; Yang et al., 2008a) in the 80–100 km region. Simultaneous measurements of temperature and sodium concentration (Gardner et al., 2005) have shown, as expected, a high degree of correlation between the time variations of these two parameters, the correlation being positive on the bottomside of the layer and negative on the topside. Although this correlation has been ascribed by some workers to temperature dependent chemistry, mainly involving ions on the topside of the layer and neutrals on the bottom (Gardner et al., 2005), it seems far more likely that it is mainly the result of the
n
Corresponding author. Tel.: + 55 12 3945 6953; fax: + 55 12 3945 6952. E-mail addresses: [email protected] (B. Clemesha), [email protected] (D. Simonich), [email protected] (P. Batista). 1364-6826/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2010.03.017
wave-associated vertical motions in the atmosphere. On the bottomside of the layer, where the mixing ratio of sodium increases with height, upward motion of an air parcel leads to adiabatic cooling and a decrease in sodium concentration relative to the unperturbed concentration at the new height. On the topside of the layer, where the mixing ratio decreases with height, upward motion still produces cooling, but in association with an increase in concentration relative to the background density. This mechanism will inevitably lead to a positive correlation between sodium density and temperature on the bottomside of the layer, and a negative correlation on the top. A detailed analysis by Plane et al. (1999) has shown that only about 20% of the correlated density fluctuations on the bottomside of the layer can be ascribed to chemistry. It is, of course, this relative weakness of short term chemical effects that has made it possible to use sodium as a tracer for gravity wave and tidal studies. The simultaneous temperature and sodium concentration measurements reported in the present paper show the expected correlations, as described above, but also suggest that turbulent vertical mixing has a major effect on the gradients in both temperature and sodium concentration. This latter effect, which has not been reported previously, is the main topic to be explored in this paper.
2. Observations At Sa~ o Jose´ dos Campos we have been making mesopause region temperature measurements with the aid of a sodium lidar since March 2007. Our lidar mixes the outputs from two NdYag oscillators, one operating at 1064 nm and the other at 1319, to generate the 589 nm emission required to probe the sodium layer.
ARTICLE IN PRESS
3. Results Fig. 1 shows typical temperature and Na concentration profiles, averaged over 900 m in height and 15 min in time. The main mesopause is at 101 km, with a temperature of 162 K, but a secondary minimum appears at 92 km with a temperature of 183 K. The appearance of inversion layers of this sort is a very common occurrence in our data; in fact, profiles showing a monotonic temperature decrease from 80 km up to the mesopause are rare. It seems probable that the structure we see in the layer is caused mainly by tides and gravity waves. Fig. 2 shows a typical time history of sodium concentration and atmospheric temperature for a little more than 7 h at a fixed 9
-3
Na Concentration (10 m ) 0
1
2
3
4
105 Temperature
Height (km)
100 95 90 85
Concentration
80 160
180 200 220 Temperature (K)
240
Fig. 1. Temperature and Na concentration profiles, 22:25–22:40 LT on May 13, 2009.
-3
9
12 10 8 6 4 2 0
Temperature (K)
741
Na Concentration at 89 km
270
Na Temperature at 89 km
240 210 180 150 18
19
20
21
22 23 Local Time
24
25
26
Fig. 2. Time histories of the Na concentration and temperature at 89 km for the night of September 14, 2007.
1
Correlation Coefficient
The oscillators are seeded by single frequency CW NdYag lasers in order to achieve a final bandwidth of less than 100 MHz. The 1064 nm seeder is temperature tuned to switch the final output between the D2a peak and the cross-over wavelength. Our normal data taking routine involves taking 250 shots on-line followed by 750 shots at the cross-over wavelength in a continuous sequence which takes 3 min per cycle, including the time required for frequency shifting the 1064 seeder. The lidar receiver uses a 75 cm telescope coupled to two thermoelectrically cooled H7442 Hamamatsu photomultipliers which have a quantum efficiency of nearly 40% at 589 nm. Two pmts are used in order to extend the dynamic range, and a rotating shutter protects the pmts from the strong signal scattered by the troposphere. The photon pulse outputs from the pmts are counted in Ortec multi-channel scalers with a dwell time of 2 ms, corresponding to a range resolution of 300 m. Under good seeing conditions we typically get profiles with one to two thousand counts per channel at the peak of the sodium layer. To reduce measurement noise we typically average over 5 height intervals (1500 m) and 5 time intervals (15 min). The data analysis takes into account the extinction of the lidar beam in the sodium layer and provides us with estimates of the sodium concentration and temperature profiles. We estimate the absolute error in our temperature measurement in regions of high sodium concentration to be 75 K. The absolute sodium concentration is determined by comparing the resonant scattering from the sodium layer with the Rayleigh scattering from around 40 km. We estimate the absolute accuracy of the measured sodium concentration to be about 710%, and the relative error of the time/height-averaged profiles is about 2% at the peak of the layer.
Na Density (10 m )
B. Clemesha et al. / Journal of Atmospheric and Solar-Terrestrial Physics 72 (2010) 740–744
0
-1 80
85
90 Height (km)
95
100
Fig. 3. Correlation between time variations in Na density and temperature as a function of height for the night of September 14, 2007.
height (89 km), below the peak of the layer. A strong positive correlation between temperature and sodium concentration is obvious. In Fig. 3 we show how the correlation varies with height for the same time period, showing a maximum positive correlation of about 0.8 in the region of 89 km, and maximum negative correlation of similar magnitude around 97 km. The correlations shown in Fig. 3 are expected, and are quite similar to those found by Gardner et al. (2005) for data obtained at the South Pole. Since tides and gravity waves are a major source of density and temperature variations in the mesopause region, a close relationship between the temperature profile and the sodium concentration is not unexpected. However, some aspects of the correlated density and temperature variations observed in our data do not appear to simply represent the density and temperature fields of a propagating wave. In Fig. 4 we show a 2-h average temperature profile and the corresponding sodium concentration distribution. The first thing to notice about the temperature profile is that the positive gradient from about 88 to 90 km is about 4 times steeper than the negative gradient above 90 km. Note that the mesopause on this night was above 103 km, outside the range of our measurement. The second point of interest in Fig. 4 is the presence of a strong positive gradient in the sodium concentration at precisely the same height as the positive temperature gradient. Fig. 4 illustrates 2 common
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features of our observations: (1) positive gradients in temperature are generally much stronger than negative ones; (2) strong positive gradients in temperature are frequently associated with strong gradients in sodium density. With respect to the latter feature, in a total of 145 nights with more than 3 h of continuous data, involving more than 1000 h of observations, we observed well-correlated positive temperature and sodium concentration gradients on 106 nights, with the correlation frequently extending over a period of several hours. Below the peak of the sodium layer, typically around 92 km, the correlated concentration gradients are almost always positive, as illustrated by Fig. 4. On the topside of the sodium layer or, more precisely, above the height at which the mixing ratio gradient becomes negative, the correlated concentration gradients are negative, as illustrated in Fig. 5. Note that in Fig. 5 we have plotted sodium mixing ratio (on an arbitrary scale) rather than concentration. Also note that the statistic given above (106 nights in 145) does not include the negatively correlated gradients seen on the topside of the layer. On many occasions the correlated temperature and concentration gradients are seen to propagate downwards, as would be expected for a tidal or gravity wave oscillation. In Fig. 6 we show a sequence of 4 temperature and Na mixing ratio profiles observed over a period of about 2 h on September 14, 2007. It is immediately obvious from this figure that the relationship is maintained from profile to profile. The temperature structure is followed by the Na mixing ratio structure from profile to profile,
with a coincidence in height within less than one km. Particularly convincing is the fact that unequally spaced differences in height between consecutive temperature profiles are exactly reproduced in the Na mixing ratio profiles. The occurrence of sequences such as that shown in Fig. 6, together with the fact that we see strong positive coincident temperature and concentration gradients on 106 out of a total of 145 nights of data, make it quite clear that we are dealing with a consistent phenomenon and not just an occasional coincidence. It should also be noted that, on the majority of the nights when correlated gradients were not seen, the temperature gradients present were small. The existence of sporadic layers of enhanced sodium concentration has been known for many years. Such layers are seen frequently at our location and others, and their characteristics have been reported in a large number of papers (see, for example, Clemesha et al., 1978; Gardner et al., 1995; Quian et al., 1998; Sridharan et al., 2009). At least one mechanism has been suggested for the production of sporadic layers via atmospheric heating (Zhou et al., 1993, Zhou and Mathews, 1995), so it is interesting to see if the temperature within sporadic layers is enhanced. We have examined our data from this point of view with the result that although on some occasions sporadic layers are accompanied by clear temperature enhancements, as shown in Fig. 7, on others there is no effect, as shown in Fig. 8. It is always possible, of course, that the sporadic layer was originally associated with a layer of enhanced temperature which had dissipated before our lidar observed the sporadic sodium layer. Perhaps surprisingly, in view of this inconsistent relationship,
110 100
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90 23.2 23.9 24.5 25.15
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80
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180 200 220 Temperature (K)
2
4
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Fig. 4. Temperatures (left) and Na concentrations (right) seen on Aug 13, 2007, averaged over 2 h.
160 180 200 220 240 260 0 Temperature (K)
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4 6 8 10 12 Na Mixing Ratio
Fig. 6. Temperatures (left) and Na mixing ratio (right) seen on September 14, 2007.
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Fig. 5. Temperatures (left) and Na mixing ratio (right) seen on July 18, 2008. Mixing ratio is in arbitrary units.
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Fig. 7. Temperature perturbation associated with a sporadic layer: 2027 LT, September 10, 2008.
ARTICLE IN PRESS B. Clemesha et al. / Journal of Atmospheric and Solar-Terrestrial Physics 72 (2010) 740–744
there are occasions when a long lived sporadic layer has been tracked by a temperature enhancement for many hours. An example of this is shown in Fig. 9, where enhanced temperatures accompany a downward propagating sporadic layer for more than 6 h. Note that it is a positive gradient in temperature that coincides with the lower edge of the sporadic layer. This is consistent with the relationship between temperature and concentration gradients observed in the main sodium layer. Other workers have also found inconsistent effects with respect to sporadic layers and temperature structure (Gardner et al., 1995), although Qian et al. (1998) did find an average temperature enhancement of 12.9 K in sporadic sodium layers observed in the ALOHA/ANLC-93 campaigns.
4. Discussion At least one other set of measurements has indicated that positive temperature gradients are stronger than negative ones. Fritts et al. (2004), using lidar measurements made at ALOMAR and falling sphere measurements at Andoya, both at around 691N, found temperature gradients up to nearly 100 K/km, with positive gradients generally larger than negative ones. These workers suggested that this asymmetry was caused by the positive mean background temperature gradient above the mesopause. This explanation does not work for our observations because our mesopause is typically at about 100 km, so the mean background temperature gradient is negative, rather than positive. With
Height (km)
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90
80 160
20 40 60 80 Na Mixing Ratio
180 200 220 Temperature (K)
100
Fig. 8. Sporadic layer with no accompanying temperature perturbation: 2403 LT, November 10, 2007.
-3
Na Concentration (cm ) 140
0 1000
95 Height (km)
respect to the ALOMAR measurements it is interesting to note that Fritts and his co-workers considered the temperature gradients observed by them to be extreme, and suggested that they resulted from an unusual mesopause circulation and structure. Note, however, that we frequently see positive gradients up to nearly 100 K/km, and at 231S we would not expect to see the same sort of unusual circulation as observed close to the Arctic Circle. At first sight the observations suggest that the correlation between temperature gradients and Na concentration might simply be the result of a propagating tide or gravity wave. If this is the case then modulation of the sodium profile would mainly result from the wave-related vertical wind, in association with vertical gradients in the sodium mixing ratio, and the temperature gradients would be the direct result of the wave field. Supporting this suggestion is the fact that above the peak of the background Na layer, positive temperature gradients correlate with negative Na gradients. As pointed out in the introduction, this phase reversal is to be expected and would result from the reversal in the vertical gradient of Na mixing ratio above the layer peak. Although such wave modulation does, indeed, appear to occur (and has been pointed out in much earlier studies, see, for example, Shelton et al., 1980; Gardner and Shelton, 1985; Batista et al., 1985), it does not fully explain our observations. In particular, we note that only positive temperature gradients are correlated with the gradient in Na concentration. In regions where the temperature gradient is negative, there appears to be no special relationship between the gradients in temperature and Na concentration (although the temperatures and concentrations themselves are correlated). Furthermore, as already pointed out, the observed positive temperature gradients are much stronger than the negative ones. It is our belief that the explanation of the observed behaviour of the temperature structure and the sodium distribution lies in the effects of the temperature gradient on atmospheric stability. Vertical transport in the atmospheric sodium layer is almost entirely due to eddy diffusion, and the magnitude of such diffusion depends on the stability of the atmosphere. In principle, the atmosphere will exhibit static stability provided the lapse rate is less than the adiabatic lapse rate of around 9 K/km. Dynamic stability will obtain for Richardson’s nos. greater than 0.25. In general, dynamic instability will occur in regions of large wind shear. We do not have wind measurements for the lidar site, so we cannot say when conditions for dynamic instability occur, but we can determine the lapse rate, which is invariably less than the adiabatic rate. Since we know that eddy diffusion occurs in the mesopause region it appears that strong dynamic instability must
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4000 5000 6000 7000 7500
220
80 20
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24 Local Time
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Fig. 9. Long-lived sporadic layer accompanied by temperature perturbation: July 14, 2008.
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occur in order to overcome the static stability. There is in fact experimental evidence for the occurrence of dynamic instability from simultaneous winds and temperature measurements (Sherman and She, 2006). This suggests that the regions in which we see strong positive temperature gradients constitute turbulence-free zones, in which strong gradients in the mixing ratio of a minor constituent such as sodium can develop. The suppression of turbulent transfer in a region of positive temperature gradient explains not only the build-up of strong gradients in sodium mixing ratio, but also the fact that positive temperature gradients are much stronger than negative ones. This follows because negative gradients are limited to the adiabatic rate of around 9 K/km. In other words, increased vertical mixing in regions of negative temperature gradient explains both the reduced gradients in sodium mixing ratio, and also the small magnitude of the temperature gradients themselves. If we are correct in our suggestion that strong positive temperature gradients correspond to turbulence-free zones in which eddy transport is much reduced, and that strong gradients in the mixing ratio of minor constituents form in such regions, then this effect should be taken into account in modelling studies. In particular, the interpretation of observed oscillations in sodium concentration in terms of gravity wave and tidal modulation will be strongly influenced by this effect. Studies of this sort carried out in the past (see, for example, Shelton et al., 1980; Gardner and Shelton, 1985; Batista et al., 1985; Yang et al., 2008a, 2008b) have not taken this effect into account. It should be noted that, even if our interpretation of the observed asymmetry in temperature gradients and their correlation with gradients in sodium mixing ratio is incorrect, their existence still implies that the sodium layer cannot always be treated as a simple passive tracer for atmospheric waves.
5. Conclusions We have made simultaneous measurements of sodium concentration and temperature in the 80–100 km region. We find that strong positive temperature gradients are frequently correlated with similar positive gradients in sodium concentration on the bottomside of the layer, and with negative concentration gradients above the peak. We also find that positive temperature gradients are normally much stronger than negative ones. We suggest that this behaviour is the result of the inhibition of vertical eddy mixing by positive temperature gradients. This inhibition of vertical mixing can explain both the correlation with sodium concentration gradients and the fact that the positive temperature gradients are generally much stronger than the negative ones. This effect, which has important implications for the modelling of wave propagation in the mesopause region, does not appear to have been reported previously.
Acknowledgements We are grateful to Dr. Guotao Yang, who developed the temperature analysis, and acknowledge financial support from the Fundac- a~ o de Apoio a Pesquisa do Estado de Sa~ o Paulo – FAPESP, and the Conselho Nacional de Desenvolvimento Cientı´fico e Te´cnico – CNPq.
References Batista, P.P., Clemesha, B.R., Simonich, D.M., Kirchhoff, V.W.J.H., 1985. Tidal oscillations in the atmospheric sodium layer. J. Geophys. Res. 90, 3881–3888. Chiu, Y.T., Ching, B.K., 1978. The response of atmospheric and lower ionospheric layer structures to gravity waves. Geophys. Res. Lett. 5, 539–542. Clemesha, B.R., Kirchhoff, V.W.J.H., Simonich, D.M., Takahashi, H., 1978. Evidence of an extraterrestrial source for the mesospheric sodium layer. Geophys. Res. Lett. 5, 873–876. ¨ Fricke-Begemann, C., Hoffner, J., 2005. Temperature tides and waves near the mesopause from lidar observations at two latitudes. J. Geophys. Res. 110, doi:10.1029/2005JD005770. ¨ ¨ Fritts, D.C., Williams, B.P., She, C.Y., Vance, J.D., Rapp, M., Lubken, F.-J., Mullemann, A., Schmidlin, F.J., Goldberg, R.A., 2004. Observations of extreme temperature and wind gradients near the summer mesopause during the MaCWAVE/MIDAS rocket campaign. Geophys. Res. Lett. 31, doi:10.1029/2003GL019389. Gardner, C.S., Shelton, J.D., 1985. Density response of neutral atmospheric layers to gravity wave perturbations. J. Geophys. Res. 90, 1745–1754. Gardner, C.S., Tao, X., Papen, G.C., 1995. Observations of strong wind shears and temperature enhancements during several sporadic Na layer events above Haleakala. Geophys. Res. Lett. 22, 2809–2812. Gardner, C.S., Plane, J.M.C., Pan, W., Vondrak, T., Murray, B.J., Chu, X., 2005. Seasonal variations of the Na and Fe layers at the South Pole and their implications for the chemistry and general circulation of the polar mesosphere. J. Geophys. Res. 110, doi:10.1029/2004JD005670. Plane, J.M., Gardner, C.S., Yu, J., She, C.Y., Garcia, R.R., Pumphrey, H.C., 1999. Mesospheric Na layer at 40 N: modeling and observations. J. Geophys. Res. 104, 3773–3788. Quian, J., Gu, Y., Gardner, C.S., 1998. Characteristics of the sporadic Na layers observed during the airborne lidar and observations of Hawaiian airglow/ airborne noctilucent cloud (ALOHA/ANLC-93) campaigns. J. Geophys. Res. 103, 6333–6347. Shelton, J.D., Gardner, C.S., Sechrist, C.F., 1980. Density response of the mesospheric sodium layer to gravity wave perturbations. Geophys. Res. Lett. 7, 1069–1072. Sherman, J.P., She, C.Y., 2006. Seasonal variation of mesopause region wind shears, convective and dynamic instabilities above Fort Collins, CO: a statistical study. J. Atmos. Solar-Terr. Phys. 68, 1061–1074. Sridharan, S., Prasanth, P.V., Kumar, Y.B., Ramkumar, G., Sathishkumar, S., Raghunath, K., 2009. Observations of peculiar sporadic sodium structures and their relation with wind variations. J. Atmos. Terr. Phys. 71, 575–582. Yang, G., Clemesha, B., Batista, P., Simonich, D., 2008a. Lidar study of the characteristics of gravity waves in the Mesopause region at a southern lowlatitude location. J. Atmos. Terr. Phys. 70, 991–1011. Yang, G., Clemesha, B., Batista, P., Simonich, D., 2008b. Improvement in the technique to extract gravity wave parameters from lidar data. J. Geophys. Res., doi:10.1029/2007JD009454. Zhou, Q., Mathews, J.D., Tepley, C.A., 1993. A proposed temperature dependent mechanism for the formation of sporadic sodium layers. J. Atmos. Terr. Phys. 55, 513–521. Zhou, Q., Mathews, J.D., 1995. Generation of sporadic sodium layers via turbulent heating of the atmosphere. J. Atmos. Terr. Phys. 57, 1309–1320.
Contents lists available at ScienceDirect
Journal of Atmospheric and Solar-Terrestrial Physics journal homepage: www.elsevier.com/locate/jastp
Mesopause region temperature structure observed by sodium resonance lidar Barclay Clemesha n, Dale Simonich, Paulo Batista Instituto Nacional de Pesquisas Espaciais, Avenida dos Astronautas, 1758, 12227-010 Sa~ o Jose´ dos Campos, SP, Brazil
a r t i c l e in f o
a b s t r a c t
Article history: Received 31 October 2009 Received in revised form 9 March 2010 Accepted 20 March 2010 Available online 27 March 2010
Mesopause region temperature measurements made with a sodium resonance lidar show two unexpected features: (1) Strong positive temperature gradients are often associated with strong gradients in the sodium concentration and (2) positive temperature gradients are generally much stronger than negative ones. Although the structures we see frequently appear to be associated with gravity waves or tides, the asymmetrical temperature oscillations cannot be explained as the result of simple wave propagation. We suggest that strong positive temperature gradients correspond to regions of high atmospheric stability, where eddy diffusion is inhibited, permitting the build-up of strong gradients in temperature and minor constituent mixing ratio. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Mesopause region Temperature Lidar
1. Introduction It is well-known that gravity waves, tides and planetary waves play a major role in the dynamics of the mesopause region. Generated in the troposphere and lower stratosphere, such waves increase in amplitude as they propagate upwards, depositing energy and momentum in the middle atmosphere. It is generally believed that the cold summer mesopause is the result of wave momentum deposition which provides the major forcing for the meridional circulation. The behaviour of such waves and their interactions with the atmospheric constituents has been the subject of many studies, both experimental and theoretical (Chiu and Ching, 1978; Shelton et al., 1980; Gardner and Shelton, 1985; Batista et al., 1985; Yang et al., 2008b). Lidar measurements of the vertical distribution of meteor metals such as sodium have been used to study tides (Batista et al., 1985; Fricke-Begemann and ¨ Hoffner, 2005) and gravity waves (Shelton et al., 1980; Yang et al., 2008a) in the 80–100 km region. Simultaneous measurements of temperature and sodium concentration (Gardner et al., 2005) have shown, as expected, a high degree of correlation between the time variations of these two parameters, the correlation being positive on the bottomside of the layer and negative on the topside. Although this correlation has been ascribed by some workers to temperature dependent chemistry, mainly involving ions on the topside of the layer and neutrals on the bottom (Gardner et al., 2005), it seems far more likely that it is mainly the result of the
n
Corresponding author. Tel.: + 55 12 3945 6953; fax: + 55 12 3945 6952. E-mail addresses: [email protected] (B. Clemesha), [email protected] (D. Simonich), [email protected] (P. Batista). 1364-6826/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2010.03.017
wave-associated vertical motions in the atmosphere. On the bottomside of the layer, where the mixing ratio of sodium increases with height, upward motion of an air parcel leads to adiabatic cooling and a decrease in sodium concentration relative to the unperturbed concentration at the new height. On the topside of the layer, where the mixing ratio decreases with height, upward motion still produces cooling, but in association with an increase in concentration relative to the background density. This mechanism will inevitably lead to a positive correlation between sodium density and temperature on the bottomside of the layer, and a negative correlation on the top. A detailed analysis by Plane et al. (1999) has shown that only about 20% of the correlated density fluctuations on the bottomside of the layer can be ascribed to chemistry. It is, of course, this relative weakness of short term chemical effects that has made it possible to use sodium as a tracer for gravity wave and tidal studies. The simultaneous temperature and sodium concentration measurements reported in the present paper show the expected correlations, as described above, but also suggest that turbulent vertical mixing has a major effect on the gradients in both temperature and sodium concentration. This latter effect, which has not been reported previously, is the main topic to be explored in this paper.
2. Observations At Sa~ o Jose´ dos Campos we have been making mesopause region temperature measurements with the aid of a sodium lidar since March 2007. Our lidar mixes the outputs from two NdYag oscillators, one operating at 1064 nm and the other at 1319, to generate the 589 nm emission required to probe the sodium layer.
ARTICLE IN PRESS
3. Results Fig. 1 shows typical temperature and Na concentration profiles, averaged over 900 m in height and 15 min in time. The main mesopause is at 101 km, with a temperature of 162 K, but a secondary minimum appears at 92 km with a temperature of 183 K. The appearance of inversion layers of this sort is a very common occurrence in our data; in fact, profiles showing a monotonic temperature decrease from 80 km up to the mesopause are rare. It seems probable that the structure we see in the layer is caused mainly by tides and gravity waves. Fig. 2 shows a typical time history of sodium concentration and atmospheric temperature for a little more than 7 h at a fixed 9
-3
Na Concentration (10 m ) 0
1
2
3
4
105 Temperature
Height (km)
100 95 90 85
Concentration
80 160
180 200 220 Temperature (K)
240
Fig. 1. Temperature and Na concentration profiles, 22:25–22:40 LT on May 13, 2009.
-3
9
12 10 8 6 4 2 0
Temperature (K)
741
Na Concentration at 89 km
270
Na Temperature at 89 km
240 210 180 150 18
19
20
21
22 23 Local Time
24
25
26
Fig. 2. Time histories of the Na concentration and temperature at 89 km for the night of September 14, 2007.
1
Correlation Coefficient
The oscillators are seeded by single frequency CW NdYag lasers in order to achieve a final bandwidth of less than 100 MHz. The 1064 nm seeder is temperature tuned to switch the final output between the D2a peak and the cross-over wavelength. Our normal data taking routine involves taking 250 shots on-line followed by 750 shots at the cross-over wavelength in a continuous sequence which takes 3 min per cycle, including the time required for frequency shifting the 1064 seeder. The lidar receiver uses a 75 cm telescope coupled to two thermoelectrically cooled H7442 Hamamatsu photomultipliers which have a quantum efficiency of nearly 40% at 589 nm. Two pmts are used in order to extend the dynamic range, and a rotating shutter protects the pmts from the strong signal scattered by the troposphere. The photon pulse outputs from the pmts are counted in Ortec multi-channel scalers with a dwell time of 2 ms, corresponding to a range resolution of 300 m. Under good seeing conditions we typically get profiles with one to two thousand counts per channel at the peak of the sodium layer. To reduce measurement noise we typically average over 5 height intervals (1500 m) and 5 time intervals (15 min). The data analysis takes into account the extinction of the lidar beam in the sodium layer and provides us with estimates of the sodium concentration and temperature profiles. We estimate the absolute error in our temperature measurement in regions of high sodium concentration to be 75 K. The absolute sodium concentration is determined by comparing the resonant scattering from the sodium layer with the Rayleigh scattering from around 40 km. We estimate the absolute accuracy of the measured sodium concentration to be about 710%, and the relative error of the time/height-averaged profiles is about 2% at the peak of the layer.
Na Density (10 m )
B. Clemesha et al. / Journal of Atmospheric and Solar-Terrestrial Physics 72 (2010) 740–744
0
-1 80
85
90 Height (km)
95
100
Fig. 3. Correlation between time variations in Na density and temperature as a function of height for the night of September 14, 2007.
height (89 km), below the peak of the layer. A strong positive correlation between temperature and sodium concentration is obvious. In Fig. 3 we show how the correlation varies with height for the same time period, showing a maximum positive correlation of about 0.8 in the region of 89 km, and maximum negative correlation of similar magnitude around 97 km. The correlations shown in Fig. 3 are expected, and are quite similar to those found by Gardner et al. (2005) for data obtained at the South Pole. Since tides and gravity waves are a major source of density and temperature variations in the mesopause region, a close relationship between the temperature profile and the sodium concentration is not unexpected. However, some aspects of the correlated density and temperature variations observed in our data do not appear to simply represent the density and temperature fields of a propagating wave. In Fig. 4 we show a 2-h average temperature profile and the corresponding sodium concentration distribution. The first thing to notice about the temperature profile is that the positive gradient from about 88 to 90 km is about 4 times steeper than the negative gradient above 90 km. Note that the mesopause on this night was above 103 km, outside the range of our measurement. The second point of interest in Fig. 4 is the presence of a strong positive gradient in the sodium concentration at precisely the same height as the positive temperature gradient. Fig. 4 illustrates 2 common
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features of our observations: (1) positive gradients in temperature are generally much stronger than negative ones; (2) strong positive gradients in temperature are frequently associated with strong gradients in sodium density. With respect to the latter feature, in a total of 145 nights with more than 3 h of continuous data, involving more than 1000 h of observations, we observed well-correlated positive temperature and sodium concentration gradients on 106 nights, with the correlation frequently extending over a period of several hours. Below the peak of the sodium layer, typically around 92 km, the correlated concentration gradients are almost always positive, as illustrated by Fig. 4. On the topside of the sodium layer or, more precisely, above the height at which the mixing ratio gradient becomes negative, the correlated concentration gradients are negative, as illustrated in Fig. 5. Note that in Fig. 5 we have plotted sodium mixing ratio (on an arbitrary scale) rather than concentration. Also note that the statistic given above (106 nights in 145) does not include the negatively correlated gradients seen on the topside of the layer. On many occasions the correlated temperature and concentration gradients are seen to propagate downwards, as would be expected for a tidal or gravity wave oscillation. In Fig. 6 we show a sequence of 4 temperature and Na mixing ratio profiles observed over a period of about 2 h on September 14, 2007. It is immediately obvious from this figure that the relationship is maintained from profile to profile. The temperature structure is followed by the Na mixing ratio structure from profile to profile,
with a coincidence in height within less than one km. Particularly convincing is the fact that unequally spaced differences in height between consecutive temperature profiles are exactly reproduced in the Na mixing ratio profiles. The occurrence of sequences such as that shown in Fig. 6, together with the fact that we see strong positive coincident temperature and concentration gradients on 106 out of a total of 145 nights of data, make it quite clear that we are dealing with a consistent phenomenon and not just an occasional coincidence. It should also be noted that, on the majority of the nights when correlated gradients were not seen, the temperature gradients present were small. The existence of sporadic layers of enhanced sodium concentration has been known for many years. Such layers are seen frequently at our location and others, and their characteristics have been reported in a large number of papers (see, for example, Clemesha et al., 1978; Gardner et al., 1995; Quian et al., 1998; Sridharan et al., 2009). At least one mechanism has been suggested for the production of sporadic layers via atmospheric heating (Zhou et al., 1993, Zhou and Mathews, 1995), so it is interesting to see if the temperature within sporadic layers is enhanced. We have examined our data from this point of view with the result that although on some occasions sporadic layers are accompanied by clear temperature enhancements, as shown in Fig. 7, on others there is no effect, as shown in Fig. 8. It is always possible, of course, that the sporadic layer was originally associated with a layer of enhanced temperature which had dissipated before our lidar observed the sporadic sodium layer. Perhaps surprisingly, in view of this inconsistent relationship,
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Fig. 7. Temperature perturbation associated with a sporadic layer: 2027 LT, September 10, 2008.
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there are occasions when a long lived sporadic layer has been tracked by a temperature enhancement for many hours. An example of this is shown in Fig. 9, where enhanced temperatures accompany a downward propagating sporadic layer for more than 6 h. Note that it is a positive gradient in temperature that coincides with the lower edge of the sporadic layer. This is consistent with the relationship between temperature and concentration gradients observed in the main sodium layer. Other workers have also found inconsistent effects with respect to sporadic layers and temperature structure (Gardner et al., 1995), although Qian et al. (1998) did find an average temperature enhancement of 12.9 K in sporadic sodium layers observed in the ALOHA/ANLC-93 campaigns.
4. Discussion At least one other set of measurements has indicated that positive temperature gradients are stronger than negative ones. Fritts et al. (2004), using lidar measurements made at ALOMAR and falling sphere measurements at Andoya, both at around 691N, found temperature gradients up to nearly 100 K/km, with positive gradients generally larger than negative ones. These workers suggested that this asymmetry was caused by the positive mean background temperature gradient above the mesopause. This explanation does not work for our observations because our mesopause is typically at about 100 km, so the mean background temperature gradient is negative, rather than positive. With
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respect to the ALOMAR measurements it is interesting to note that Fritts and his co-workers considered the temperature gradients observed by them to be extreme, and suggested that they resulted from an unusual mesopause circulation and structure. Note, however, that we frequently see positive gradients up to nearly 100 K/km, and at 231S we would not expect to see the same sort of unusual circulation as observed close to the Arctic Circle. At first sight the observations suggest that the correlation between temperature gradients and Na concentration might simply be the result of a propagating tide or gravity wave. If this is the case then modulation of the sodium profile would mainly result from the wave-related vertical wind, in association with vertical gradients in the sodium mixing ratio, and the temperature gradients would be the direct result of the wave field. Supporting this suggestion is the fact that above the peak of the background Na layer, positive temperature gradients correlate with negative Na gradients. As pointed out in the introduction, this phase reversal is to be expected and would result from the reversal in the vertical gradient of Na mixing ratio above the layer peak. Although such wave modulation does, indeed, appear to occur (and has been pointed out in much earlier studies, see, for example, Shelton et al., 1980; Gardner and Shelton, 1985; Batista et al., 1985), it does not fully explain our observations. In particular, we note that only positive temperature gradients are correlated with the gradient in Na concentration. In regions where the temperature gradient is negative, there appears to be no special relationship between the gradients in temperature and Na concentration (although the temperatures and concentrations themselves are correlated). Furthermore, as already pointed out, the observed positive temperature gradients are much stronger than the negative ones. It is our belief that the explanation of the observed behaviour of the temperature structure and the sodium distribution lies in the effects of the temperature gradient on atmospheric stability. Vertical transport in the atmospheric sodium layer is almost entirely due to eddy diffusion, and the magnitude of such diffusion depends on the stability of the atmosphere. In principle, the atmosphere will exhibit static stability provided the lapse rate is less than the adiabatic lapse rate of around 9 K/km. Dynamic stability will obtain for Richardson’s nos. greater than 0.25. In general, dynamic instability will occur in regions of large wind shear. We do not have wind measurements for the lidar site, so we cannot say when conditions for dynamic instability occur, but we can determine the lapse rate, which is invariably less than the adiabatic rate. Since we know that eddy diffusion occurs in the mesopause region it appears that strong dynamic instability must
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Fig. 9. Long-lived sporadic layer accompanied by temperature perturbation: July 14, 2008.
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occur in order to overcome the static stability. There is in fact experimental evidence for the occurrence of dynamic instability from simultaneous winds and temperature measurements (Sherman and She, 2006). This suggests that the regions in which we see strong positive temperature gradients constitute turbulence-free zones, in which strong gradients in the mixing ratio of a minor constituent such as sodium can develop. The suppression of turbulent transfer in a region of positive temperature gradient explains not only the build-up of strong gradients in sodium mixing ratio, but also the fact that positive temperature gradients are much stronger than negative ones. This follows because negative gradients are limited to the adiabatic rate of around 9 K/km. In other words, increased vertical mixing in regions of negative temperature gradient explains both the reduced gradients in sodium mixing ratio, and also the small magnitude of the temperature gradients themselves. If we are correct in our suggestion that strong positive temperature gradients correspond to turbulence-free zones in which eddy transport is much reduced, and that strong gradients in the mixing ratio of minor constituents form in such regions, then this effect should be taken into account in modelling studies. In particular, the interpretation of observed oscillations in sodium concentration in terms of gravity wave and tidal modulation will be strongly influenced by this effect. Studies of this sort carried out in the past (see, for example, Shelton et al., 1980; Gardner and Shelton, 1985; Batista et al., 1985; Yang et al., 2008a, 2008b) have not taken this effect into account. It should be noted that, even if our interpretation of the observed asymmetry in temperature gradients and their correlation with gradients in sodium mixing ratio is incorrect, their existence still implies that the sodium layer cannot always be treated as a simple passive tracer for atmospheric waves.
5. Conclusions We have made simultaneous measurements of sodium concentration and temperature in the 80–100 km region. We find that strong positive temperature gradients are frequently correlated with similar positive gradients in sodium concentration on the bottomside of the layer, and with negative concentration gradients above the peak. We also find that positive temperature gradients are normally much stronger than negative ones. We suggest that this behaviour is the result of the inhibition of vertical eddy mixing by positive temperature gradients. This inhibition of vertical mixing can explain both the correlation with sodium concentration gradients and the fact that the positive temperature gradients are generally much stronger than the negative ones. This effect, which has important implications for the modelling of wave propagation in the mesopause region, does not appear to have been reported previously.
Acknowledgements We are grateful to Dr. Guotao Yang, who developed the temperature analysis, and acknowledge financial support from the Fundac- a~ o de Apoio a Pesquisa do Estado de Sa~ o Paulo – FAPESP, and the Conselho Nacional de Desenvolvimento Cientı´fico e Te´cnico – CNPq.
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