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Comprehensive Study on Tropical (10°N-15°N) Mesospheric Inversion Layers Using Lidar and Satellite (Timed-Saber) Observations

One of the interesting and poorly understood features of mesosphere and lower thermosphere (MLT) region is the phenomenon of Mesospheric Inversion Layers (MILs). The poor understanding of MILs is due to limited access of their occurrence height region, however the lidars are more efficient tools which provide stratosphere and mesosphere nocturnal temperatures with high temporal and vertical resolutions. The state-of-the-art lidar system comprising Mie, Rayleigh lidars installed at National Atmospheric Research Laboratory (NARL), Gadanki (13.5°N, 79.2°E), India has provided an excellent opportunity to undertake this study. The Nd:YAG laser source with lower power (11W) has been replaced by the one with higher power (30W) in January 2007. As the laser power has been increased, the molecular back scatter signal is also increased and consequently the top height level of the temperature retrieval has been increased to ~90-95 km. In the present study, the role of dominant causative mechanisms for the occurrence of MILs has been discussed using mainly the lidar and satellite (TIMED-SABER) observations over Gadanki region.


INTRODUCTION
Mesospheric Inversion Layer (MIL) is the region of temperature gradient inversion from negative to positive in mesosphere, over several kilometers in thickness [1,2,3]. The study of MILs is essential for the comprehensive understanding of middle atmosphere/MLT energy budget. Alhough the MILs have been reported from several sites using lidar and space borne observations, their morphological characteristics are still unknown. However their causative mechanisms are quite complex, the MILs occur mainly due to Gravity Wave (GW) breaking, Planetary Wave (PW) critical level interaction, chemical heating and GW-Tidal interaction [2,3]. In the absence of damping, all the atmospheric waves viz., GWs, PWs and Tides propagate vertically with exponentially (e z/2H , z=altitude and H=scale height) increasing amplitudes in response to decreasing density up to MLT region. The waves deposit energy and momentum in the background when they break and consequently influence the thermal, dynamical structures and also the chemical composition. In addition to the above dynamical processes, the heating due to several exothermic reactions mainly among H, O, O 2 , O 3 , OH, HO 2 enhances the background temperature which is maintained as a MIL.

The Gadanki Lidar System
The lidar system containing Mie, Rayleigh lidars installed at National Atmospheric Research laboratory (NARL), Department of Space, Gadanki (13.5°N, 79.2°E), India is operational since 1998. The system operates at 532 nm green laser with pulse repetition frequency (PRF) of 20 Hz and the relatively lower energy of ~550 mJ per pulse (PL8020, Continuum, USA). Later in 2007, the lidar system was upgraded with a high power laser of energy ~600 mJ/pulse and PRF of 50 Hz (PL9050, Continuum, USA) which is operated in monostatic biaxial mode. The 10X beam expander is used to expand the beam from its initial diameter of 9 mm to 90 mm so that the beam divergence decreases from ~0.45 mRad to < 0.1 mRad and thus the beam remains within the field of view (FOV) of the receiving telescope at all height ranges. The Schmidt-Cassegrain (ø=35 cm) and Newtonian (ø=75 cm) telescopes serve as receivers to collect the atmospheric back scattered photons due to Mie (aerosols and clouds) and Rayleigh scattering (molecules) respectively. The received photons then allowed to fall on photo multiplier tubes (PMTs) of various gains (Hamamatsu, R3234-01). The PMT output signal will be collected and processed through the MCS-plus (Multi Channel Scalar) PC based photon count system with each two data acquisition channels of the receivers (Mie receiver:P, S channels & Rayleigh receiver : R, U channels). The photons are integrated for the time resolution of ~4 min corresponding to 12500 laser shots with dwell time of 2μsec for range resolution of 300 m. The laser source and the 90 mm expanded laser beam are shown in Figure 1(a-c). The simulated lidar signal from equation (1) is as shown in Figure 2. The photon counts decreases with height in response to the decreasing atmospheric density.

Lidar Temperature Retrieval
The lidar temperature profiles are determined from the method given by Hauchecorne and Chanin,[1980] [4]. Above 30 km where the aerosol (Mie scattering) contribution is negligible and assuming the atmosphere is in hydrostatic equilibrium and obeys ideal gas law, the temperature at i th layer can be given by the following equation.
……………(2) and the statistical standard error in the temperature is given by the following equation.

……………..(3)
The scattering ratio is used to identify the aerosol and molecular layers and it can be defined as ………... (4) In general, the scattering ratio is assumed to be unity above 30 km so that the contribution from aerosol scattering is negligible.

OBSERVATIONS AND RESULTS
The nightly (19:07-23:14 LT) mean Rayleigh lidar temperature (T) over Gadanki along with TIMED-SABER temperature profile for 12.5°N, 79.0°E at 23:58 LT are shown in Figure 3a. The lidar and SABER temperature profiles show large inversion layers above ~75 km with amplitude and thickness of ~93 K, ~85 K and ~8 km, ~5 km respectively. The MSIS-90 (Mass Spectrometer Incoherent Scatter-90) model temperature profile also included in the figure for reference. The profiles of lapse rate, Γ=-dT/dz and the Brunt-vaisala frequency square (N 2 ) calculated from equation (5) are shown in Figure  3b and Figure 3c respectively. The lapse rate and N 2 values represents the presence of turbulent layers and conditions for convective instability. …….. (5) Here g(z), T(z) and c p are acceleration due to gravity, mean temperature and specific heat at constant pressure (1004 J/K/Kg) respectively. It is evident from Figure 3b that the turbulent layers are not present as the lapse rate (Γ ) is not equal to the adiabatic lapse rate (Γ d ~10 K/km). Further it can be observed from Figure 3c that the condition for convective instability (N 2 <0) is not satisfied above ~75-80 km which indicates the negligible role of gravity wave breaking for the occurrence of inversion layer. Further the SABER chemical heating rates among H, O, O 2 , O 3 , OH and HO 2 for 25-03-2011 are shown in Figure 4. The reactions are listed in Table 1. The heating rates increase above ~77 km and the reaction between H and O 3 dominates all the reactions with ~10 K/day. Also the total heating rates are about 20 K/day in the inversion region at ~80 km.

SUMMARY AND DISCUSSION
The increased laser power of the NARL lidar system enabled to achieve the temperature profiles even up to ~90 km with the considerably lower uncertainty values. Sridharan et al. [2010] [8] observed a considerable reduction in the standard error with the new laser (PL9050) at heights ~5 km higher than the same obtained with the low power laser (PL8020). As an example, they obtained the standard error as 20 K at 85 km with low power laser and the same standard error is obtained above 90 km with the high power laser. They concluded that it is best suitable to study upper mesospheric temperature inversions, which takes place at altitudes above 85 km. In this study, it is clearly evident that the dynamics in connection with gravity wave breaking is negligible that can be observed primarily from the vertical profiles of lapse rate and Brunt-vaisala frequency square values. However the total chemical heating due to the exothermic reactionsamong H, O, O 2 , O 3 , OH, HO 2 is dominant for the occurrence of the MIL with the total heating rate of ~20 K/day above ~75-80.