Recent observations of mesospheric temperature inversions over a tropical station (13.5°N,79.2°E)

Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 323 – 334

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Recent observations of mesospheric temperature inversions over a tropical station (13:5◦N; 79:2◦E) M. Venkat Ratnama , J.B. Neea;∗ , W.N. Chena , V. Siva Kumarb , P.B. Raob a Department

b National

of Physics, National Central University, Chung-Li 32054, Taiwan MST Radar Facility, Gadanki, Post Box No. 123, Tirupati 517502, India Received 29 November 2001; accepted 3 December 2002

Abstract Present study mainly deals with recent observations of mesospheric temperature inversions (MTI) over Gadanki (13:5◦ N; 79:2◦ E), a tropical station in India using for about 40 months of Nd:YAG lidar data. Long-term measurements of halogen occultation experiment, high resolution Doppler imager on board upper atmospheric research satellite and solar mesospheric explorer have been used to compare the characteristics (amplitude, height and percentage of occurrences) of the inversions observed with ground-based lidar measurements for the >rst time. In general, the height and percentage occurrence of the inversions are matching well between ground and satellite based measurements most of the time and there exists a large discrepancy in the amplitudes of MTI measured by these instruments as expected. The height occurrence of these inversions is at ∼ 76 km and for about 60% of the time, strong inversions can be seen at this tropical latitude. The height occurrence of the inversions shows annual oscillation with peak during summer and minimum in winter. The percentage occurrences of these inversions are showing semi-annual oscillation with peak during equinoxes and minimum during solstice. The possible causative mechanism for the frequent occurrence of these inversions over this tropical latitude are explained in light of current understanding of the gravity wave activity and also with the chemical heating/cooling taking place at these heights. c 2003 Elsevier Science Ltd. All rights reserved.  Keywords: Temperature inversions; Gravity waves; Chemical heating; Turbulence

1. Introduction Mesospheric temperature inversion (positive gradient of temperature), which is frequently observed in low and mid-latitudes, is one of the important parameter to the current research. Though it was >rst observed by Schmidlin (1976) almost three decades back, the follow up implications are very slow due to various limitations of the currently available instruments. Recently, Meriwether and Gardner (2000) well documented these limitations and explained the possible mechanisms for the occurrences of these mesospheric temperature inversions (MTI). This phenomenon was reported by many investigators using

∗

Corresponding author. Fax: +886-3-425-1175. E-mail address: [email protected] (J.B. Nee).

various instruments which includes falling spheres by Schmidlin (1976) and Lubken et al. (1994), ground based lidar over mid-latitudes by Hauchecorne et al. (1987), Whiteway et al. (1995), Meriwether et al. (1994), Leblanc et al. (1998, 1999a, b), over tropical latitudes by Siva Kumar et al. (2001) and Ratnam et al. (2002), Sodium lidar by She et al. (1990), Bills and Gardner (1993), Satellite measurements by Clancy and Rusch (1989) (solar mesospheric explorer (SME)) and Leblanc and Hauchecorne (1997) (upper atmospheric research satellite (UARS)). Most of these observations are con>ned to mid and high latitudes and measurements in the tropical latitudes are very sparse. There is a critical need to get more observations in the tropical latitudes for better understanding the possible mechanism for the occurrence of these MTI. In general, MTI has been seen at two distinct levels (near 75 and 95 km) separated by approximately 20 km

c 2003 Elsevier Science Ltd. All rights reserved. 1364-6826/03/$ - see front matter
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M. Venkat Ratnam et al. / Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 323 – 334

(Dao et al., 1995). Still it is not yet understood clearly whether inversions at these two heights are occurring through the same mechanism. With the invention of Na lidar (She et al., 1993), it is possible to get 24 h observations between 80 and 105 km and revealed that the upper inversion layer was mainly due to tidal activity and chemical heating (Dao et al., 1995; States and Gardner, 2000a, b). Meriwether et al. (1998) suggested that these inversions are caused by ampli>cation mechanism based upon the interactions of gravity wave with tidal structure. Whether the same phenomenon will apply to the lower inversion is still not yet clear due to lack of 24 h temperature observations. Very recently Duck et al. (2001) had given >rst 24 h observations of MTI using a Nd:YAG lidar located at Millstone Hill/MIT Haystack observatory (42:6◦ N; 71:5◦ W) and observed no evidence of a residual inversion layer in the 24 h mean temperature pro>le. Until now, the observed inversion near 75 km is likely linked with gravity wave breaking at mid latitudes (Hauchecorne et al., 1987). The gravity wave breaking through wave dissipation will provide heating rate of about 10 K=day (Fritts and van Zandt, 1993). However, Lubken (1997) showed that the turbulent heating rates are very small (1–2 K=day) at these heights. Evidences also showed that these inversions were associated with near adiabatic lapse rate, indicates a well-mixed turbulent layer (Whiteway et al., 1995; Liu et al., 2000). These inversions will develop often in conjugation with the long period wave through the mesopause (Bills and Gardner, 1993; Berger and von Zahn, 1999). Gravity wave breaking can heat up the environment but not strong enough to produce the observed heating rates (40 K) (Duck et al., 2001). The other possible mechanism for the occurrences of these inversions with large amplitudes could be due to chemical heating through exothermic reactions as reported by Meriwether and Mlynczak (1995). Mlynczak and Soloman (1993) had given various possible exothermic reactions responsible for heating in the middle atmosphere during the daytime. Here it is believed that chemical processes involving Ox and HOx species might be responsible to the observed inversions (Berger and von Zahn, 1999). This paper deals with the statistics of MTI observed by NMRF-CRL lidar located at a tropical station, Gadanki (13:5◦ N; 79:2◦ E) and their comparison with satellite-based measurements (halogen occulation experiment (HALOE) and high resolution Doppler image (HRDI)) for the >rst time. Brief description of all the instruments used for the present study is given in Section 2. Database and method of analysis are discussed brieKy in Section 3. Observed results on statistics of MTI (amplitude, height and percentage occurrence) observed with lidar, HALOE and HRDI are presented in Section 4. Monthly and annual mean variations of these inversions observed with all the instruments are given in Sections 5 and 6, respectively. Section 7 deals with the typical examples of MTI observed across the globe by HALOE and HRDI measurements. Discussion on the

observed results and possible causative mechanisms are discussed in detail in Section 8. Finally, the overall summary and conclusions drawn from the present study are given in Section 9. 2. Instrument description 2.1. NMRF-CRL Nd:YAG lidar The lidar system comprises of a laser transmitter, receiving telescopes, data acquisition and processing sub-systems. The lidar employs the second harmonic of Nd:YAG pulsed laser at 532 nm with an energy of about 550 mJ at a pulse repetition rate of 20 Hz and a pulse width of 7 ns. A Kat mirror oriented at 45◦ to the beam axis directs the transmitted beam, having a divergence of 0:1 m rad, vertically. The Rayleigh receiver employs a Newtonian telescope with primary mirror having an eLective diameter of 75 cm. A narrow band interference >lter with full-width at half-maximum (FWHM) of 1:07 nm is used to reject much of the background light. The signal is then splitted into two channels in the ratio of 9:1, the high gain channel (R) covers 50 – 80 km where the signal is weak and the low gain channel (U) covers 30 –50 km where the signal is relatively stronger. The signals are directed to photomultiplier tubes (PMTs), which operates in photon count mode for both channels. The outputs of the PMTs are ampli>ed with threshold adjustable comparator and sharper circuit. The method of analysis for the determination of temperature pro>le from the Rayleigh channel follows closely that given by Hauchecorne and Chanin (1980) and Chanin and Hauchecorne (1984). More details of this instrument and method of analysis can be had from Siva Kumar et al. (2001) and Ratnam et al. (2002). 2.2. Halogen occultation experiment HALOE is one of four instruments on board UARS, which is used to measure temperature. UARS was launched on September 12, 1991 and started making observations from October 1991. This instrument uses solar occultation by the limb of the earth’s atmosphere to measure vertical pro>les of transmission in eight infrared bands. Transmission pro>les measured in the 2.80-
m CO2 band play an essential part in processing the HALOE temperature measurements. As this instrument mainly uses solar occultation for measuring atmospheric parameters, the observations are con>ned to sunrise and sunset conditions only giving total 15 pro>les at each event. The latitudes of the sunrise and sunset change slowly with the drift of UARS orbit. More details of this instrument, its geographical coverage, calibration and preliminary validation of various species measured by this instrument are given by Russell et al. (1993). Hervig et al. (1996) has given detailed validation of temperature measurements of this instrument.

M. Venkat Ratnam et al. / Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 323 – 334

2.3. High resolution Doppler imager HRDI is another instrument on board UARS, which measures the brightness in the O2 atmospheric ‘A’ band by observing the earth limb with the line-of-sight tangent heights between 50 and 115 km. The pro>les of brightness measurements from two consecutive limb scans are inverted together to provide both temperature and band volume emission rate pro>les. It consists of a triple etalon Fabry–Perot interferometer and a two-axis gimbaled telescope. The UARS orbit is inclined to 57◦ with respect to the equator and it can view the atmosphere up to 73◦ from the equator in one hemisphere and 41◦ in the other. Hays et al. (1993), Grassl et al. (1995) and Skinner et al. (1996) gave complete description of HRDI. Ortland et al. (1998) has given detailed description of the temperature retrieval.

325

son of amplitudes, height and percentage occurrence of MTI have been made between all these instruments. For all the instruments (lidar, HALOE, HRDI) an algorithm has been developed to detect the temperature inversions and their characteristics following Leblanc and Hauchecorne (1997). In brief, the amplitude of an inversion is de>ned as temperature diLerence between the top and bottom of the inversion layer and the thickness is de>ned as altitude diLerence between top and bottom. In a particular data set, the number of inversion pro>les observed to the total number of pro>les gives the percentage occurrence of MTI. In developing this algorithm, temperature inversions greater than the error in diLerent instruments (HALOE and HRDI ∼12 K; lidar ∼10 K at 75 km) at a particular height has been considered as signi>cant. More details regarding de>nition of inversions and their characteristics can be had from Leblanc and Hauchecorne (1997).

3. Database and analysis 4. Results 4.1. NMRF-CRL lidar observations In the present study, characteristics of inversion layer, i.e., amplitude, height and percentage occurrence have been compared between lidar, HALOE and HRDI even though it is not desirable to compare the amplitudes between ground based instrument like lidar and the satellite measurements like HALOE and HRDI. Main reason for not comparing between these data is that the satellite measurements will give smoothed values showing less amplitude and also altitude of occurrence sometimes up and down. This study will enable to understand clear picture of MTI throughout the globe although amplitudes may diLer. Fig. 1 shows the comparison of mesospheric temperature inversions as observed by HALOE, HRDI and NMRF-CRL

90

180

200

220

240

260

280

13.5 N

80 70

Height (km)

The lidar at Gadanki (National MST Radar Facility— NMRF) was installed by Communication Research laboratory (CRL), Japan under Indo-Japanese collaboration (hereafter it will be referred as NMRF-CRL lidar). This lidar was used to measure temperature between 30 and 85 km from March 1998 to the present with 300-m vertical resolution and 5 min time resolution. Observations were recorded throughout the night starting from 2200 h to morning 0500 h (local time) only on clear nights (cloud free sky). This gives on average of 5 h integration of temperature measurements and ∼ 3 days a week. Error in estimating temperature while integrating for about 5 h at 85 km is ∼ 20 K including all noises which drops to 8–10 K at 75 km. Details of uncertainty in estimating the temperature using this lidar is given by Parameswaran et al. (2000). After getting the timely averaged pro>les, three point running mean (1 km) have been applied further to remove short period Kuctuations from the observed data. Database also includes observations from HALOE instrument (version 19), and also HRDI instrument of level 3AT version 11 data, which are available through GSFC/DAAC. The vertical resolution of these instruments are ∼3 km below 85 km but the data was interpolated to 1 km for comparing the data obtained from other instruments. The error in observed temperature is about 20 K at 90 km but reduces to 10 –12 K at 75 km height region, the region of present interest. Data from October 1991 to August 2001 are used for the present study. Unlike HALOE, HRDI measures brightness in the O2 atmospheric ‘A’ band by observing the earth limb with line-of-sight tangent heights between 50 and 115 km. In general, the observations using this instrument are con>ned between 0800 and 1600 h. Data starting from January 1992 to December 1998 are used for the present study. In constructing the time series, the missing data have been linearly interpolated. The observed results were also compared with the monthly averaged values of SME data. Compari-

60

HALOE MSIS-2000 NMRF-CRL Lidar HRDI

50 40 30

18 March 1998 180

200 220 240 Temperature (K)

260

280

Fig. 1. Intercomparison of MTI observed by NMRF-CRL lidar, and near coincidences of HALOE, HRDI on UARS on 18 March 1998. MSIS-2000 model pro>le for the same day is also plotted for comparison.

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M. Venkat Ratnam et al. / Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 323 – 334 NMRF-CRL Lidar

HALOE/UARS

HRDI/UARS

Height of Occurrence (km)

85

(a)

(d)

(g)

80 75

70

65 35

15

15

(e)

(b)

(h)

Amplitude (K)

30 10

10

25 5

20

5

15 0

0

Percentage of Occurrence

100 80

(c)

(f)

(i)

60 40 Instrument observation Least Squares fit of AO Least Squares fit of SAO

20 0 J

F M A M J

J

A S O N D

J

F M A M J

J

A S O N

D

J

F M A M J

J

A S O N D

Month

Fig. 2. Monthly mean variations observed in the characteristics (height of occurrence, amplitudes and percentage of occurrences) of MTI over Gadanki by NMRF-CRL lidar (a) – (c), HALOE (d) – (f) and HRDI (g) – (i). Least-square >t of SAO and AO is also shown in the >gure.

lidar on 18 March 1998. Here for comparison, near coincident pro>les have been selected. The vertical pro>le of mass spectrometer incoherent radar (MSIS)-2000 model (Hedin, 1991) for the same day with 1 km height resolution is also plotted in this >gure. From the >gure it is clear that there is a good agreement between HALOE, HRDI and lidar data even though amplitudes and peaks of inversions are diLerent. Interestingly, the height of bottom level of inversion is matching well between these instruments. The amplitudes seen by lidar in this example is ∼ 20 K and sometimes this amplitude reaches ∼ 40 K, whereas amplitudes seen by HALOE and HRDI are only about 8 K. The MSIS-2000 model values were showing good comparison between 30 and 90 km except at the inversion. Earlier it was extensively compared between the CIRA-86 model and the NMRF-CRL lidar data and found that these two are matching well at mesospheric heights except at the zones of inversion (Nee et al., 2002). At the zones of inversions, MSIS-2000 model values are showing ∼20 K warmer (or lidar values are showing 20 K cooler). From statistical study using lidar data we could see that for about 40% of the time, at the zones of inversions, there is an increase in temperature (above normal) and for about 60% of the time there is a decrease in temperature (cooling). From CIRA-86 model and MSIS-2000 model comparison, it is found that there is not much difference in the observed temperature values at this tropical latitude. One thing to be noted here is that the MSIS-2000 model values are based upon several satellite and rocket measurements across the globe, which have poor spatial and time resolutions whereas lidar values are of high spatial and time resolution. Leblanc et al. (1998) also observed that the

model values (CIRA-86) are warmer for about 10 K at these heights. Here only one typical example, which is showing good agreement, is shown and it is to be noted that sometimes there is no comparison between these diLerent instruments and reason for this will be explained later in this study. 4.2. Statistics of MTI observed by the NMRF-CRL lidar As explained earlier, an algorithm has been developed to see the characteristics of MTI (amplitude, height and percentage of occurrences) using NMRF-CRL lidar located at Gadanki. On each individual day the algorithm has been subjected and characteristics of MTI is regained, and then averaged over a month to get monthly variation of these parameters. Fig. 2(a) – (c) show the height, amplitude and percentage occurrences of MTI, respectively, observed over Gadanki using lidar data collected for about 40 months. To understand the nature of the variation better, a least-square >t of the semiannual oscillation (SAO) and annual oscillation (AO) has been made to the observed data and is plotted in the same >gure. From the Fig. 2(a) it is clear that the height occurrences of these MTI is not constant throughout the year and is showing annual oscillation (RMS deviation of AO is 0.46 and SAO is 1.11) with peak in summer and minimum in winter. On the average the height of occurrence is ∼76 km. The amplitude of inversion shown in Fig. 2(b) is also not constant throughout the year and shows peak during the month of May and minimum during winter months. On the average the amplitude of these MTI is ∼ 20 K (after deducting the error of 10 K). The percentage of occurrences (Fig. 2(c)) shows clear semi-annual oscillation (RMS

M. Venkat Ratnam et al. / Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 323 – 334

deviation of SAO is 8.56 and AO is 12.48) with peak during equinoxes and minimum during solstice. On the average for about 60% of time MTI are seen at this latitude. These results are in contrast to mid-latitude observations where the occurrences of these MTI shows annual oscillation with peak during winter and minimum during summer (Hauchecorne et al., 1987; Whiteway et al., 1995; Leblanc et al., 1998). Using mid-latitude lidar data, Hauchecorne et al. (1987) had made a statistical study and inferred that temperature inversion occurs in the height region of 55 –72 km in winter, while in summer season, the inversion occurs in 70 –83 km region. Whiteway et al. (1995) at Toronto (44◦ N; 80◦ W) found similar kind of mesospheric inversion around 70 km. Leblanc et al. (1998) reported the climatology of these inversions using long-term lidar measurements at middle (44◦ N; 1:0◦ W; 40:6◦ N; 105:1◦ W; 34:4◦ N; 117:7◦ W) and low latitudes (19:5◦ N; 155:6◦ W). They found annual oscillation in occurrence of these inversions at mid-latitudes and semi-annual oscillation at low latitudes. 4.3. Statistics of MTI observed by HALOE and HRDI Similar to the above, characteristics of MTI observed with HALOE and HRDI are shown in the same Fig. 2(d) – (i). HALOE observations starting from October 1991 to August 2001, almost covering 10 years and HRDI observations from January 1992 to December 1998 covering 7 years of the data are used in this >gure. For selecting the data from these satellite observations we have considered ±2◦ (from 13:5◦ N) latitude and ±60◦ (79:2◦ E) longitude separation. First on each individual day at this latitude, characteristics of MTI have been obtained and then averaged for monthly. Later all the similar months have been averaged and shown in >gure. From the >gure it is clear that in HALOE observations also the height of occurrences are not constant and most of the time during summer months it shows at higher heights and during winter at lower heights. In general the average height of occurrences is at ∼ 76 km. The amplitude of MTI is found to be ∼ 8 K (after deducting 12 K error) and is showing peak in the month of June. The percentage of occurrences is showing a clear semi-annual oscillation with the peak formed 1 month later to equinoxes. In general after comparing lidar and HALOE data it is clear that the height and percentage of occurrences are closely matching but not the amplitudes. It is well known that the amplitudes will not match well between ground based and satellite measurements, as ground based measurements are point observations >xed for one latitude and time, whereas the satellite measurements are zonal averaged (smoothed). Moreover the time of observations was also diLerent. But as the height and percentage of occurrences are closely matching, if one could get the amplitudes along various latitudes (land) then it will be possible to get good model which will >t throughout the globe. Hence ground based instruments are very important in this regard. Leblanc and Hauchecorne (1997) reported similar results using lidar (located in the

327

south of France), HALOE and improved stratospheric and mesospheric sounder (ISAMS). They found clear annual oscillation in mid-latitudes with maximum during winter using lidar and also with HALOE and ISAMS. They also observed the semi-annual cycle in the tropical latitudes with maximum 1 month after the equinoxes from the satellite observations. The percentage of occurrences also follows annual and semi-annual cycles in mid and tropical latitudes, respectively. Similarly, the same algorithm has been applied for the HRDI data also and characteristics of MTI are regained as shown in the same Fig. 2(g) – (i). From this >gure it is clear that the height of occurrences in HRDI measurements are also showing clear annual oscillation with peak during summer months and minimum during winter. In general the height of occurrences in HRDI is at ∼79 km; 3 km higher than the HALOE and lidar. The amplitudes of MTI are showing more or less similar to the HALOE observations and in general, the amplitudes are of the order of 8 K (after deducting error). The percentage of occurrences shows a clear semi-annual oscillation with peak appearing 1 month later of equinoxes. The over all statistics of the MTI observed over this latitudes using various instruments and their best >t (SAO and AO) are given in Table 1. 5. Monthly mean variability of MTI amplitudes Monthly mean amplitudes of MTI observed by the HALOE starting from October 1991 to August 2001, HRDI from October 1992 to December 1998 and lidar from March 1998 to June 2001 are shown in Fig. 3. The SAO >t is showing better with a RMS deviation of 1.93 (shown in the >gure) than the AO >t (not shown here) with a RMS deviation of 2.17. From the >gure it is clear that the MTI amplitudes are showing a semi-annual oscillation with peak during equinoxes and minimum during solstices. Most of the time it occurs 1 month later from equinoxes. The amplitude in general observed by HRDI and HALOE are of the order of 8 K and started an increasing trend from the year 1998 which may be caused due to enhanced solar activity. The amplitudes observed with lidar are shown in right side of the >gure. The trend observed by lidar also shows same results but with larger amplitude than the satellite observations. The monthly mean amplitudes observed by lidar reaches as high as 40 K. Mostly peak of the amplitudes are seen during equinoxes. However, amplitudes observed by Leblanc and Hauchecorne (1997) using HALOE data (annual mean) over this latitude is of the order of 4 K using the data until the year 1995. The reason for this discrepancy may be related to the solar activity in which amplitudes start increasing little bit from the year 1998. So on the average it is showing higher amplitudes than reported using HALOE data. The monthly mean temperature observed at 76 km by all the instruments are shown in Fig. 4. The least-square >t

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M. Venkat Ratnam et al. / Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 323 – 334

Table 1 Overall characteristics of MTI observed by NMRF-CRL lidar, HALOE and HRDI observations and their RMS deviation of least-square >t for SAO and AO Characteristics of MTI

NMRF-CRL lidar

HALOE

HRDI

Height (km) Percentage (%) Amplitude (K) Oscillation (height) Oscillation (percentage)

76 60 20 Annual Semiannual (peak in equinoxes)

76 60 8 Annual Semiannual (1 month later)

79 60 8 Weak annual Semiannual (1 month later)

RMS deviation AO SAO

AO

SAO

AO

SAO

0.46 4.49 13.9

1.43 1.66 12.5

2.27 2.22 9.94

0.83 1.67 8.02

1.00 2.46 6.90

Height Amplitude Percentage

1.11 7.31 8.56

Highlighted values are for best >ts.

1992

Amplitude (K)

20

1993

1994 1995

1996

1997 1998

1999

2000

2001

45

NMRF-CRL Lidar HRDI/UARS HALOE/UARS Least Squares fit of SAO

15

40 35 30

10 25 20

5

15

13.5 N

0

10 0

12

24

36

48

60

72

84

96

108

120

Months (Oct'91-Sep'2001) Fig. 3. Monthly mean amplitudes of MTI observed by HALOE, HRDI and NMRF-CRL lidar showing inter annual variability. Amplitudes observed by lidar are given on right side of the >gure. Least-square >t for SAO is also shown in the >gure.

1992 1993 1994 1995 1996 1997 1998 1999 2000 2001

Temperature (K)

230 220

230

76 Km

NMRF-CRL Lidar HALOE/UARS HRDI/UARS Least Squares fit of SAO

220

210

210

200

200 190

190 180

13.5 N 0

12

24

36

48

60

72

84

96

108

180 120

Months (Oct'91-Sep'2001) Fig. 4. Monthly mean temperatures at 76 km height region observed with HALOE, HRDI and NMRF-CRL lidar measurements. Temperature observed by lidar is given on right side of the >gure. Least-square >t for SAO is also shown in the >gure.

M. Venkat Ratnam et al. / Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 323 – 334

Annual mean variability of MTI observed by SME, HALOE, HRDI and NMRF-CRL lidar is shown in Fig. 5. It is to be noted that the contour interval is not constant for all the instruments. The contour interval of SME is 5 K, HALOE and HRDI is 6 K whereas for NMRF-CRL lidar it is 7 K. From SME observations, it is clear that MTI occurs in between 76 and 80 km height region during equinoxes with phase shifting downwards. From HALOE observations it can be seen that the MTI occurs at the height of 76 km during 1 month later of equinoxes. From HRDI observations, it is noted that the MTI occurs slightly above the HALOE observations. The NMRF-CRL lidar observations reveal that MTI occurs during equinoxes around 76 km height region. The noisy contours in NMRF-CRL observations might be due to the gravity wave activity, which was not >ltered out completely even in the long time averages. The amplitude of the observed MTI is very low in case of all satellite observations when compared to ground based observations.

Height (km)

235 230 225 220 215 210 205 200 195 190

85 80 75 70

HALOE/UARS

Height (km)

90

229 223 217 211 205 199 193 187 181 175

85 80 75 70

(b) 65

HRDI/UARS

90

(c)

215

85

209

80

203

75

197

70

191

65

185

NMRF-CRL Lidar, Gadanki, India

80

227 220

75

213 206

70

199 192

65

185

J

F M A M J

(d)

7. Typical observations of MTI across the globe by HALOE and HRDI Fig. 6(a) and (b) shows typical examples of MTI observed with HALOE across the globe during the month of May and November, respectively. These are the composite >gures obtained by averaging the observed data for about 10 years (1992–2001) during the similar months. From Fig. 6(a) it is clear that during the month of May, amplitudes and percentage of occurrences of MTI are maximum in tropical latitudes and goes on decreasing as one move towards the poles. During the month of November (Fig. 6(b)), the amplitudes and percentage of occurrences are more in mid-latitudes than the tropical latitudes. Similar results has been seen with HRDI also across the globe during the month of May and December and are shown in Fig. 7(a) and (b), respectively. Again these are the composite >gures obtained by averaging the observed data for about 7 years (1992–

T(K)

(a) 65

Height (km)

6. Annual mean variation of MTI

Solar Mesospheric Explorer 90

Height (km)

for the semi-annual oscillation is also shown in the same >gure. The observed RMS deviation for SAO is 4.25, which is slightly better than the RMS deviation of AO (5.5). Monthly mean temperature also shows semi-annual variation with the peak occurring 1 month later of equinoxes. In general the minimum and maximum temperatures seen by satellite measurements are 180 and 210 K, respectively. The lidar observation shows high temperatures than satellite measurements. The monthly mean minimum and maximum temperatures seen by lidar at 76 km is 190 and 220 K, respectively, which is 10 K higher than the satellite measurements. The reason for this is well known and is already explained above.

329

J

A S O N D

Month

Fig. 5. Annual mean time-height contours showing the variation of MTI in (a) SME (b) HALOE, (c) HRDI and (d) NMRF-CRL lidar observations.

1998) during the similar months. These examples show the capability of the satellite observations in getting the characteristics of MTI across the globe. Since the other characteristics of MTI like height and percentage of occurrences are matching well with the ground based instrument then by putting the amplitudes of the MTI observed at various latitudes, it is possible to build suitable model to understand the physical processes of the MTI across the globe. 8. Discussion Possible explanation for the occurrences of these MTI is still not yet well de>ned as reviewed by Meriwether

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M. Venkat Ratnam et al. / Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 323 – 334

Fig. 6. Composite map showing the mean amplitudes of MTI observed by HALOE during (a) May and (b) November. Contour interval between diLerent shades is 2 K.

and Gardner (2000). The main limitation is the lack of ground-based observations of the MTI during daytime. Most of the lidar observations are con>ned to nighttime only and hence there is diPculty in estimating the tidal inKuences, if any, due to lack of 24-h observations. Recently, for the >rst time Duck et al. (2001) was able to give diurnal variations of MTI using a Rayleigh lidar over Millstone using limited data set. The 24-h measured mean temperature showed no evidence of residual inversion layer and concluded that gravity wave activity alone cannot give the observed MTI amplitudes. Whether the same phenomenon will occur at the tropical altitudes is not yet known. This kind of observations should be done throughout the globe for better understanding the MTI. Satellite observations can give information on these MTI even during the daytime (HRDI, HALOE only during sunrise and sunset events) but with poor spatial resolution. The other important limitation to study the causative mechanism of MTI is lack of background wind observations simultaneously. Though coherent radar (Indian MST

radar operating at 53 MHz) is located at Gadanki, it cannot detect the backscattered echoes during night times, as it needs electron density Kuctuations to backscatter the echoes, which is a daytime phenomenon. Onsite wind measurements are necessary simultaneously to study this phenomenon in more depth, which can be possible only with the lidar with Doppler capability. It is now generally accepted that dynamics and structure of the mesosphere are strongly inKuenced by the propagation of gravity waves into this region. Gravity waves originating in the troposphere can propagate into higher altitudes with increase in amplitude. A vertically propagating wave begins to break at the level where there is a sudden change in the temperature lapse rate (Hauchecorne et al., 1987). At zones of gravity waves breaking, a large amount of energy and momentum is lost by the waves. Ratnam et al. (2002) reported this phenomenon using nighttime lidar and daytime radar observations over this site. This wave deposits its momentum, decelerates the local wind and its breaking produces

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Fig. 7. Composite map showing the mean amplitudes of MTI observed by HRDI during (a) May and (b) December. Contour interval between diLerent shades is 2K.

turbulence inside and above the inversion layer. Moreover, the Richardson number (Ri = N 2 =(windshear)2 , where N is Brunt Visalia frequency) calculated using temperature from lidar and wind shear from MST radar is found to be less than 0.25 at the inversion indicating that this region is highly turbulent (dynamic or shear instabilities). The eddy diLusivity (measure of turbulence), K, calculated (K = 0:1 2 =N , where is corrected spectral width taken from MST radar observations) using corrected spectral width (Rao et al., 2001), during the equinoxes is found to be very high at these heights. Hence breaking of gravity waves will contribute some heating/cooling (even though small) at these heights. No doubt, breaking of gravity waves do heat/cool environment but not strong enough to produce the observed high heating rates (40 K). The gravity wave breaking, through wave dissipation will provide a heating rate of about 10 K=day ( Fritts and van Zandt, 1993). However, Lubken (1997) reported that

the turbulent heating rates are very small (1–2 K=day) for winter and high during summer (l0 –20 K=day) around summer mesopause (90 km). Gardner and Yang (1998) reported that the breaking of gravity waves have a far greater cooling inKuence by transporting heating downwards at mesopause heights which is also coinciding with our observations even at 75 km. From statistical study using lidar data we could see that for about 40% of the time, at the zones of inversions, there will be an increase in temperature (above normal) and for about 60% of the time there is a decrease in temperature (cooling) (for example see Fig. 1). Even though the dynamics associated with the gravity wave breaking is considered as important source, Meriwether and Mlynczak (1995) drawn attention on the possibility of additional source involving exothermic chemical reactions contributing signi>cantly to the formation of MTI. Using the 2D model, it is observed that the region of

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largest semi-annual amplitudes in the water vapour at these heights. It is also noticed that the ozone mixing ratios are also increasing during the equinoxes (Fig. 8c). This may be one of the reasons for increasing in temperature at these heights. But still, analysis should be done in depth by taking into considerations of all possible chemical mechanisms and we are working on role of chemistry of various minor species responsible for these MTI using HALOE observations. Berger and von Zahn (1999) also concluded that the gravity waves with enhanced chemical species might produce temperature inversion at 70 –80 km. In fact in their calculation about thermal structure of mesosphere, temperature inversion is a normal phenomenon at 70 –80 km. Heat budget is balanced between radiative cooling due to CO2 and chemical heating due to ozone and odd hydrogen throughout the mesosphere and mostly at mesopause. 9. Summary and conclusion

Fig. 8. Time height contours showing monthly variations of (a) temperature (b) water vapour and (c) ozone observed by HALOE using the data from October 1991 to August 2001.

80 –95 km may be heated up as much as 3–10 K=day, which is quite high similar to that of due to dissipation of gravity waves calculated by Fritts and van Zandt (1993). Since the reactions involve ozone and atomic hydrogen, it is thought that the chemical heating will be more during summer when ozone levels are high than in winter at mid latitudes. Fig. 8 shows contours of monthly variations of temperature, water vapour and ozone observed with HALOE during October 1991 to August 2001. In the topical latitudes (13◦ N), it is noticed that the water vapour (H2 O) mixing ratios are increasing just below the MTI heights during the equinoxes and is showing semi-annual oscillation (Fig. 8b). At higher levels in the mesosphere, the water vapour decreases with height, because it is photolysed by Lyman-

radiation, leading to the production of odd hydrogen. Using HALOE observations, Jackson et al. (1998) observed

Characteristics of MTI over a tropical station are presented by making use of 40 months of NMRF-CRL lidar data, which is located at Gadanki, India. The MTI at this latitude occurs around 76 km height regions and shows annual oscillation with peak during summer and minimum during winter. On the average the magnitude of MTI is about 20 K and exceeds 40 K on individual days. For about 60% of the time MTI occurs at this latitude and shows a clear semi-annual oscillation with peak during equinoxes and minimum during solstices. The observed characteristics of MTI are compared with the HALOE and HRDI on board UARS measurements. The height and percentage of occurrences of MTI observed by HALOE are matching well with that of ground based lidar but HRDI observations shows the occurrences at a little higher heights. Large discrepancy is seen in the amplitudes measured by the ground and satellite measurements as expected. In general the occurrences of MTI are more frequent in tropical latitudes during months of equinoxes and in mid-latitudes during winter months. The causative mechanism of these MTI at these latitudes is discussed in the light of gravity wave breaking and also with chemical heating/cooling. Using combined measurements of MST radar (daytime) and lidar (nighttime) revealed breaking of gravity waves at the zones of MTI over this site (Ratnam et al., 2002). This wave deposits its momentum, decelerates the local wind and produces turbulence inside and above the inversion layer. Using MST radar observations, estimated eddy diLusivity (using corrected spectral widths due to beam and shear broadening), K, is found to be more during equinoxes similar to the occurrences of MTI. These gravity waves breaking are not capable in explaining the observed amplitudes of MTI (40 K). It is thought that chemical heating can play a major role in providing the heat/cool source not only at mesopause but also throughout the mesosphere. In future studies much attention should be focused on various

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