Rayleigh lidar observations of mesospheric inversion layers over Gadanki (13.5^oN, 79.2^oE) and their relationship with gravity wave activity

Indian Journal of Radio & Space Physics Vol 43, February 2014, pp 83-90 Rayleigh lidar observations of mesospheric inversion layers over Gadanki (13.5°N, 79.2°E) and their relationship with gravity wave activity K Ramesh1, S Sridharan2,$,*, S Vijaya Bhaskara Rao1, K Raghunath2 & Y Bhavani Kumar2 1 Department of Physics, Sri Venkateswara University, Tirupati 517 502, India 2 National Atmospheric Research Laboratory, Gadanki 517 112, India $ E-mail: susridharan@narl.gov.in Received 20 April 2013; revised 4 July 2013; accepted 23 August 2013 Fifteen nights of Rayleigh lidar temperature profiles over Gadanki (13.5ºN, 79.2ºE) for the period January - February 1999 show the presence of mesospheric inversion layers (MILs) on ten nights indicating their frequent occurrences over low latitudes. The role of gravity waves in causing the MILs has been investigated using the temperature profiles derived from the half an hour integrated photon counts. The temperature deviations from the nightly mean temperature are inferred as due to gravity waves and potential energy per unit mass due to these waves; and their spectra are examined separating MIL nights and non-MIL nights. The spectra of potential energy per unit mass (Ep) computed from the temperature fluctuations attain broad band convective instability limit at mesospheric heights on MIL nights, whereas the spectra just tend to attain the limit on non-MIL nights at longer wavelengths. These results indicate that enhanced gravity wave growth and breaking due to convective instability may be the potential mechanism for the occurrence of these MILs. Keywords: Mesospheric inversion layer, Convective instability, Gravity wave, Temperature profile, Brunt-Väisälä frequency, Potential energy density, Energy content PACS Nos: 92.60.hc; 92.60.hh 1 Introduction An inversion of the vertical temperature gradient from negative to positive observed in the mesosphere over several kilometers in thickness is called mesospheric inversion layer (MIL)1. The MILs have been found to occur more frequently in winter months at mid latitudes2 and in equinox months3 at low latitudes. Though, the occurrence characteristics of MILs have been reported from several sites, it is still unknown about their causative mechanisms. A few mechanisms, namely gravity wave breaking, gravity wave-tidal interactions, chemical heating and planetary wave critical level interaction have been suggested for their occurrence1. Of these, the planetary wave critical level interaction has been suggested mainly for the lower MIL which occurs in the altitude region ~65-80 km. Mlynczak & Solomon4 suggested that chemical heating arising from the quenching of excited species such as OH, which is maximum at ~90 km during night time, could form MILs. For the upper mesospheric inversion layers, the gravity wave tidal interaction has been suggested as the potential mechanism between 85 and 100 km, where tidal amplitudes are much larger. Gravity wave breaking has also been considered as a potential mechanism for the occurrence of MILs because when MILs occur, they are accompanied by the regions of convective or shear instabilities. Convective instability occurs when the gradient of the total potential temperature or Brunt-Väisälä frequency square (N2) becomes negative and the dynamic or shear instability occurs when the Richardson number Ri=N2/(du/dz)2 is in between 0 and 0.25. Here, N2, is Brunt-Väisälä frequency square; du/dz, is the wind shear. Gravity wave can become unstable at the height, where the zonal winds begin to decelerate as a part of the development of a critical layer arising from the matching of the wave phase speed (c) with the background zonal wind ( u ), which implies u = c or u − c = 0 . The decrease in N caused by the higher temperatures in MIL region reduces the stability, causing the waves to break and the turbulent heating arising from the breaking waves provides a feedback mechanism which can be maintained as a MIL5. In this paper, the Rayleigh lidar temperature observations carried out over Gadanki (13.5°N, 79.2°E) during January-February 1999 are used to find the causative mechanisms for the occurrence of a 84 INDIAN J RADIO & SPACE PHYS, FEBRUARY 2014 few mesospheric inversion layers and their night-tonight variabilities. Gravity waves inferred from the temperature perturbations and their spectra are compared for MIL and non-MIL nights and the results are discussed. 2 Observations and Data analysis The Rayleigh lidar system at National Atmospheric Research Laboratory (NARL), Gadanki (13.5°N, 79.2°E) has been operated since March 1998 during cloud-free nights according to scientists’ requirements. It employs the second harmonic generation of Nd:YAG solid state pulsed laser at 532 nm with pulse repetition frequency of 20Hz and has the energy about 550 mJ per pulse with pulse width of 7 ns. The Rayleigh lidar receiver employs a Newtonian type telescope with a diameter of 750 mm and the back scattered photons are detected by two photo multiplier tubes of high (90%) and low (10%) gains referred as R and U channels and counted sequentially into successive 300 m range bins using Multi Channel Scalar (MCS) averager for the height range of ~30-85 km. The strong backscatter from altitudes below 12 km is blocked by electronic gating, which is synchronized to trigger the laser. The four minute (250 sec) averaged photon count profiles corresponding to 5000 laser shots are averaged for 30 minutes and the temperature and standard errors are determined using the method given by Hauchecorne & Chanin6. The Rayleigh lidar technique involves range resolved detection of molecular backscattered laser radiation from altitudes above 30 km where the atmosphere is free from aerosols so that the signal strength is proportional to molecular number density. Using the number density taken from an appropriate model (CIRA-86) for the height of 50 km where the signal-to-noise ratio is fairly high, the constant of proportionality is evaluated and thereby, the density profile (z) is derived. Taking the pressure (P) at the top of the height range (90 km) from the atmospheric model (CIRA-86), the pressure profile is computed using the measured density profile, assuming the atmosphere to be in hydrostatic equilibrium. Using the ideal gas equation, the vertical temperature profile T(z) is calculated using the following expression: T ( zi ) = ρ ( zi ) g ( zi ) ∆z M g ( zi ) ∆z , where X= P( zi + ∆z / 2) R Log (1 + X ) … (1) The uncertainty in the temperature is calculated by: δT ( zi ) T ( zi ) = δLog (1 + X ) Log (1 + X ) = δX … (2) (1 + X ) Log (1 + X ) where, M, is mean molecular weight of air; g, the acceleration due to gravity; z, the thickness of the atmospheric layer; R, the universal gas constant; , the atmospheric density; and P, the pressure. 3 Results 3.1 Mesospheric thermal structure during January-February 1999 In this paper, the Rayleigh lidar temperature observations carried out during January-February 1999 are presented. As the system was operated for several cloud free nights continuously under equatorial wave campaign, it has provided a valuable and continuous data set for about 24 nights with relatively less noise. Figure 1 shows the nightly mean temperatures during 27 January-19 February 1999 over Gadanki. It can be observed from the figure that the mesospheric temperatures are relatively larger during a few nights in the altitude region of ~75-85 km. As the uncertainties in temperature measurements are larger due to poor signal (less number of photon counts) at this height region, out of these 24 nights of observations, only 15 (27-31 January 1999 and 2-3, 5-6, 9-13, 19 February 1999) nights are selected for the present study. The fundamental characteristic of an inversion layer is its amplitude. In the present work, inversion nights are identified based on the following criteria. The inversion amplitude can be defined as the difference between the temperatures at top (~81 km) and bottom levels (~74 km) of the inversion layer6. Figure 2(a) shows the inversion amplitude and the uncertainty values in the inversion region for 27 January-19 February 1999 and the inversion events Fig. 1 — Contour plot of Rayleigh lidar nightly mean temperatures (~5 h) during 27 January - 19 February 1999 (24 nights) over Gadanki (13.5°N, 79.2°E) RAMESH et al.: MILs OVER GADANKI AND THEIR RELATIONSHIP WITH GRAVITY WAVE are identified as the observed amplitude to be greater than 20 K, which is reasonably larger than the maximum uncertainty in temperature (~15K). Thus, from this figure, the inversion nights are considered to be 27-29, 31 January 1999, 2-3, 5, 11, 13, 19 February 1999, whereas the non-inversion nights are 30 January, 6, 9, 10, 12 February 1999. The same can be inferred from Fig. 2(b), which represents the difference between the inversion amplitude and uncertainties and the differences are negative (amplitude < uncertainty) on non-inversion nights and positive (amplitude > uncertainty) during inversion nights. Since the data quality is poor due to low signal-to-noise ratio above ~70 km and operational duration (<2.5 h) of Rayleigh lidar observations is less, the remaining nights (30 January, 1, 4, 6-10, 12, 14-18 February 1999) are not considered, even though the inversion amplitudes are larger or smaller than the uncertainties. Figure 3(a and b) shows the lidar vertical temperature profiles for inversion and noninversion events, respectively and they are compared with MSIS-90 model profiles and each profile is shifted by 30 K. It is observed clearly from Fig. 3(a) that on inversion nights, the nightly mean lidar temperatures are found to be larger (~10-40 K) than the MSIS-90 model temperatures in the inversion region, whereas during non-inversion events, the lidar temperatures are almost matching with the MSIS-90 model temperatures at ~80 km as shown in Fig. 3(b). It can also be observed from Fig. 3(a and b) that the maximum temperature is above 220 K for inversion nights, whereas it is less than 220 K for non-inversion nights. The thickness of the inversion is equal to the difference between the altitudes of the top and bottom levels of the inversion layer and it is found that the inversion layer thickness is varying from ~ 2.5 km to Fig. 2 — (a) Mesospheric inversion amplitudes and the corresponding uncertainties [2 T(z)]; and (b) difference between inversion amplitude and uncertainties for all the inversion and non-inversion nights 85 8 km during the inversion nights. It is also observed that for inversion nights, the environmental lapse rate ( = -dT/dz) is close to the dry adiabatic lapse rate ( d=10 K km-1) at the top level of the inversion but during non-inversion events, the lapse rate is much smaller than the dry adiabatic lapse rate (not shown). 3.2 Brunt-Väisälä frequency The Brunt-Väisälä frequency square is an important parameter to determine the stability of the atmosphere. The Brunt-Väisälä frequency squared values are calculated using the following relation: N 2 ( z) = g ( z ) ∂T ( z ) g ( z ) + T ( z ) ∂z Cp … (3) where, g(z), is the acceleration due to gravity; T(z), the nightly mean temperature profile; ∂T / ∂z , temperature gradient; and Cp, the specific heat at constant pressure (1004 J kg-1 K-1). The atmosphere is convectively stable if N2>0 and unstable if N2 0. Fig. 3 — Altitude profiles of nightly mean temperatures (blue thick lines) for: (a) mesospheric inversion nights during 27-31 January and 2, 3, 5, 11, 13, 19 February 1999; and (b) non-mesospheric inversion nights during 30 January and 6, 9, 10, 12 February 1999 [corresponding MSIS-90 model temperature profiles are shown by black thin lines; each profile is shifted by 30 K] 86 INDIAN J RADIO & SPACE PHYS, FEBRUARY 2014 Figure 4 shows the Brunt-Väisälä frequency squares between 30 and 84 km for all the inversion and noninversion events. The first 10 plots (first two rows) are corresponding to the inversion nights and the next five plots (third row) are for non-inversion nights. It is observed from these figures that for the inversion nights, the N2 values are positive from 30 km to 80 km, and above that they became zero or negative at the top of the inversion layer. The N2 values vary from ~5×10-4 (rad s-1)2 at 30 km to ~8.5 - 10×10-4 (rad s-1)2 at 80 km. On the other hand, for non-inversion nights, the N2 values are positive from 30 km to almost 80-84 km and the values vary from ~5×10-4 (rad s-1)2 at 30 km to ~6.0 - 9.3×10-4 (rad s-1)2 at ~74-79 km. Thus, it is clear that the atmosphere is stable for the height regions up to which the N2 values are positive and the gravity waves propagate with increasing amplitude in response to the decreasing density and when N2 becomes negative, the atmosphere becomes unstable; and the gravity waves reaching these heights becomes convectively unstable and breaks. The turbulence generated due to gravity wave breaking causes the temperature enhancement at that particular height region, which is maintained as an inversion layer. 3.3 Vertical wave number spectra of gravity wave potential energy per unit mass The average potential energy per unit mass can be calculated from the following equation: 1 g Ep = 2 N ( z) 2 2 T ′( z ) T ( z) o … (4) where, g, is the acceleration due to gravity; N(z), the Brunt-Väisälä frequency; T (z)/To(z), fractional temperature perturbations7. The power spectral density (PSD) calculated from the Fast Fourier Transform (FFT) analysis employed on relative temperature perturbations is scaled as [g/N(z)][T (z)/To(z)], which gives the vertical wave number potential energy spectral density. Assuming that the wave induced temperature gradients are constrained by convective instability and the adiabatic lapse rates, the vertical wave number (m) and the potential energy spectral densities are limited to approximately N2/2m3 for a single mode and N2/10m3 for a broadly distributed spectrum of waves Fig. 4 — Altitude profiles of Brunt-Väisälä frequency squares for inversion (top and middle rows) and non-inversion (bottom row) nights [vertical lines indicate the limit of N2=<0] RAMESH et al.: MILs OVER GADANKI AND THEIR RELATIONSHIP WITH GRAVITY WAVE interacting only due in superposition. Figure 5 shows the spectra of relative temperature perturbations as a function of vertical wave numbers for three different height regions 30-48.9 km, 48.9-67.8 km and 64.8-83.7 km for the inversion nights. This figure also shows the broad band convective instability limit for these height regions calculated from (1/2 )(N2/10m3). As expected, these spectra reflect that vertical growth in the spectral magnitude is observed mainly at longer wavelengths. At higher altitudes (64.8-83.7 km), the spectrum at longer wavelengths (or shorter wave number), such as ~10 km, not only attains the convective instability limit but exceeds this limit during most of the MIL nights. At lower altitudes (30.0-48.9 km and 48.9-67.8 km), the spectral magnitudes are near or below the convective instability limit indicating that it is yet to attain the convective instability limit. For example, on 11 February 1999 MIL event, the wave of wavelength ~10 km having spectral power of 5.91×105 m3 s-1 is not attaining the convective instability limit in the height region of 30.0–48.9 km but it tends to attain in the height region of 48.9–67.8 km with spectral power of 7.29×105 m3 s-1 and in the height region of 87 64.8–83.7 km (superposition height region), the spectral power of the same wave is 3.69×106 m3 s-1. This indicates the gravity waves are propagating upwards with increasing amplitude and attaining the convective instability limit at a particular height during the inversion events. Whenever the gravity wave attains the convective instability limit, the wave breaks and deposits the energy and momentum carried by it, which causes an increase in the temperature at that particular height region. Figure 6 shows the power spectral densities of temperature perturbations as a function of vertical wave numbers during nonMIL events. It is clearly observed from this figure that the gravity waves at higher altitudes (64.8-83.7 km) do not attain or just tend to attain the convective instability limit at longer wavelengths (~10 km) during most of the non-inversion nights. It indicates that these gravity waves have not attained enough amplitude to get saturated. As a consequence, the inversion layers are not present on these noninversion nights. Figure 7(a and b) shows the averaged spectra of temperature perturbations for all the inversion nights and non-inversion nights, respectively. It is clear from Fig. 5 — Nightly averaged vertical wave number potential energy spectra for inversion nights [broadband convective instability limit (1/2 )(N2/10m3) lines also shown for corresponding heights] 88 INDIAN J RADIO & SPACE PHYS, FEBRUARY 2014 this figure that at lower altitudes, the gravity wave is not attaining the convective instability limit on both inversion and non-inversion events but at higher altitudes (64.8-83.7 km), the gravity wave of longer wavelength (~10 km) is having the spectral power much above the convective instability limit with maximum value of 2.89×106 m3 s-1 on inversion events and the wave just tend to attain the convective Fig. 6 — Nightly averaged vertical wave number potential energy spectra for non-inversion nights [broadband convective instability limit (1/2 )(N2/10m3) lines also shown for corresponding heights] Fig. 7 — Total mean vertical wave number potential energy spectra for: (a) inversion and (b) non-inversion nights [broadband convective instability limit (1/2 )(N2/10m3) lines also shown for corresponding heights] RAMESH et al.: MILs OVER GADANKI AND THEIR RELATIONSHIP WITH GRAVITY WAVE instability limit having the averaged spectral power of 1.02×106 m3 s-1 on non-inversion events. 3.4 Energy content spectra The average energy content spectra calculated by multiplying the power spectral density (PSD) and the vertical wave number at the three different altitude regions 30.0-48.9 km, 48.9-67.8 km and 64.8-83.7 km for the MIL and non-MIL events are shown in Fig. 8(a and b), respectively. Here, the PSD is taken in linear scale and the vertical wave number in log scale. The area under any portion of this curve is proportional to the potential energy density contained in the corresponding wave number band. It is clear from these figures that the gravity wave of vertical wavelength of ~10 km (wave number 1×10-4 m-1) has the lower energies of ~60-70 (ms-1)2 at the lower height regions (30-67.8 km) but as it propagates upwards, its amplitude increases and the wave has higher energy of ~300 (ms-1)2 on inversion events and ~106 (ms-1)2 on non-inversion events in the height region of 64.8–83.7 km. Thus, the energy content of the gravity wave is larger in the inversion region (above 80 km) during MIL events when compared to that of non-MIL events. But there is not much difference in energy contents observed on both MIL and non-MIL events in the lower height region. Thus, the enhancement in the energy content at higher altitudes during the MIL events is due to the gravity wave breaking which did not occur during the case of non-inversion events due to their smaller energy at higher altitudes. 4 Discussion and Summary In this study, a few mesospheric inversion layers are identified in the Rayleigh lidar temperature profiles over Gadanki (13.5ºN, 79.2ºE) during Fig. 8 — Total mean energy content spectra for: (a) inversion and (b) non-inversion nights 89 January-February 1999 based on the criteria that the inversion amplitude is greater than 20 K, which is above the uncertainty limit in the temperature measurement. The comparison of lidar and MSIS-90 model temperature profiles show that in the height region of 60-85 km, the lidar temperatures are much larger than that of model values during inversion events and almost matching with the model values on non-inversion nights in the inversion region. Moreover, the MILs found for ten nights, during January-February 1999, indicates the frequent occurrence of MILs over low latitude region Gadanki and the amplitude of the inversions varies as ~20-48 K with the thickness of ~2.5-8 km during these MIL events. The temperature lapse rates reaching the dry adiabatic lapse rate (10 K km-1) indicates the existence of the turbulent layers at the inversion heights. Though several mechanisms have been proposed for the formation of MILs, the present work investigates the role of gravity waves in causing the MILs using Rayleigh lidar temperature observations over Gadanki. The stability of the atmosphere, which is essential for the vertical propagation of the gravity waves, was estimated from the temperature lapse rates and the Brunt-Väisälä frequencies for MIL and non-MIL events. These parameters are further used to obtain gravity wave potential energy and to verify the condition for convective instability. Cutler & Collins8 discussed the mesospheric inversion layers above 60 km with the amplitude of 18 K using Rayleigh lidar observations over a high latitude station Poker Flat, Alaska (65ºN, 147ºW). They showed the temperature gradients at the top of the inversion layer approaching the dry adiabatic lapse rate which represents the turbulent layers at that altitude region. The gravity wave generated in the troposphere propagate vertically upwards with increasing amplitude as the atmospheric density decreases in proportion to e(z/2H) would at some height induce convective or shear (dynamic) instability. The amplitude growth saturates above this height (breaking level) by irreversible extraction of the energy from the wave to produce the turbulence. Atmospheric gravity waves exist only when N2 is positive and if N2 becomes negative, the layer becomes convectively unstable and the gravity wave amplitudes cannot grow beyond that level. It is observed that the N2 values are negative above 80 km indicating the convective instability, which leads to saturation and breaking of gravity waves during MIL events. 90 INDIAN J RADIO & SPACE PHYS, FEBRUARY 2014 The temperature deviations from the nightly mean temperature are inferred as due to gravity waves and the potential energy per unit mass due to these waves and their spectra are examined for MIL and non-MIL events separately. The spectra of potential energy per unit mass (Ep) calculated from the temperature fluctuations attained the broadband convective instability limit at the mesospheric heights during MIL events, whereas the spectra just tend to attain the limit for non-MIL events at longer wavelengths. These results denote the saturation of wave amplitudes and the wave breaking due to convective instability are the potential causative mechanisms for the occurrence of these MILs. Some main features of the power spectral density (PSD) have been described here. The PSD of available potential energy increases for all wave numbers by roughly one order of magnitude from the stratosphere to mesosphere over the height regions of 30.0-48.9 km, 48.9-67.8 km and 64.8-83.7 km. But the observation of the large MIL amplitudes at high altitudes between ~74 and 81 km is consistent with longer wavelength gravity waves (~10 km), which are more stable for a given amplitude than shorter wavelength waves propagating to higher altitudes. It is now established by many observational studies that the amplitude of monochromatic gravity waves increases with vertical wavelength8. The variation of PSD with vertical wave number is studied for different wave numbers for both MIL events and nonMIL events. From the averaged spectral density, it is clearly shown that on MIL events, the gravity wave of wavelength ~10 km propagates from the lower atmosphere and attain the convective instability limit with larger power spectral density amplitude above ~80 km when compared to lower height regions and causing the wave breaking and thereby, producing the mesospheric temperature inversions. But, the wave of same wavelength (~10 km) during non-MIL nights just tends to attain the convective instability limit and it has less PSD when compared to the MIL events. The averaged energy content, calculated for all the MIL and non-MIL events, shows that the energy content on MIL nights is much larger than that on non-MIL nights in the height region of ~64.8-83.7 km. Thus, it is concluded that the enhanced gravity wave amplitude and the wave breaking due to convective instability could be the potential mechanism for the occurrence of the MILs during January-February 1999 over Gadanki. Acknowledgement The authors would like to acknowledge NASA for providing the MSIS-E-90 model data through the website http://omniweb.gsfc.nasa.gov/vitmo/msis_vitmo.html. References 1 Meriwether J W & Gardner C S, A review of the mesospheric inversion layer phenomenon, J Geophys Res (USA), 105 (2000) pp 12405-12416. 2 Leblanc T & Hauchecorne A, Recent observations of mesospheric temperature inversions, J Geophys Res (USA), 102 (1997) pp 19471-19482. 3 Siva Kumar V, Rao P B & Krishnaiah M, Lidar measurements of stratosphere-mesosphere thermal structure at low latitude: Comparison with satellite data and models, J Geophys Res (USA), 108 (2003) 4342, doi: 10.1029/2002JD003029. 4 Mlynczak M G & Solomon S, Middle atmosphere heating by exothermic chemical reactions involving odd-hydrogen species, Geophys Res Lett (USA), 18 (1991) pp 37-40. 5 Hauchecorne A, Chanin, M L & Wilson R, Mesospheric temperature inversion and gravity wave dynamics, Geophys Res Lett (USA), 14 (1987) pp 935-939. 6 Hauchecorne A & Chanin M L, Density and temperature profiles obtained by lidar between 35 and 70 km, Geophys Res Lett (USA), 7 (1980) pp 565-568. 7 Whiteway J A & Carswell A I, Rayleigh Lidar observations of thermal structure and gravity wave activity in the high arctic during a stratospheric warming, J Atmos Sci (USA), 51 (1994) pp 3122-3136. 8 Cutler J L, Collins L R, Mizutani K & Itabe T, Rayleigh lidar observations of mesospheric inversion layers at Poker Flat, Alaska (65°N, 147°W), Geophys Res Lett (USA), 28 (2001) pp 1467-1470.