RAYLEIGH LIDAR OBSERVATION OF TROPICAL MESOSPHERIC INVERSION LAYER: A COMPARISON BETWEEN DYNAMICS AND CHEMISTRY

The Rayleigh lidar at National Atmospheric Research Laboratory, Gadanki (13.5°N, 79.2°E), India operates at 532 nm green laser with ~600 mJ/pulse since 2007. The vertical temperature profiles are derived above ~30 km by assuming the atmosphere is in hydrostatic equilibrium and obeys ideal gas law. A large mesospheric inversion layer (MIL) is observed at ~77.4-84.6 km on the night of 22 March 2007 over Gadanki. Although dynamics and chemistry play vital role, both the mechanisms are compared for the occurrence of the MIL in the present study.


INTRODUCTION
LIDAR (LIght Detection And Ranging) is the most efficient technique to capture the nocturnal mesospheric inversion layers with high spatial and temporal resolution. The term Mesospheric Inversion Layer (MIL) refers to the inversion of temperature gradient from negative to positive superposed upon the characteristically decreasing mesospheric thermal structure. However the causative mechanisms of MILs are quite complex, a few suggested mechanisms for their occurrence are gravity wave breaking, gravity wave-tidal interactions, chemical heating, and planetary wave critical level interaction [1,2]. The study of MILs is essential to understand the energetics in the mesosphere and lower thermosphere (MLT) region. Most of the waves which are generated in the lower atmosphere can propagate to the MLT region with increasing amplitude in response to decreasing density. When the waves break, the energy and momentum carried by the wave will be deposited in the background and thereby modulate the thermal and dynamical structures. In addition, the chemical heating by the dominant exothermic reactions among H, O, O 2 , O 3 , OH, HO 2 enhances the background temperature in the MLT region [3,4].
In the present study, a large MIL observed on 22 March 2007 over a tropical region, Gadanki (13.5°N, 79.2°E) has been selected to investigate the dominant causative mechanism using mainly the Rayleigh lidar temperature profiles for the duration of ~6 hours.

NARL LIDAR SYSTEM
The Mie and Rayleigh lidar system installed at National Atmospheric Research Laboratory (NARL), Gadanki (13.5°N, 79.2°E) employs a Qswitched solid state (Nd:YAG) pulsed laser (PL9050) from Continuum, USA. The system operates at 532 nm green laser with the pulse repetition frequency (PRF) of 50 Hz and the energy of ~600 mJ per pulse. The laser beam diameter is 9 mm and it will be expanded into 10 times in order to reduce the beam divergence from ~0.45 mRad (before expansion) to less than 0.1 mRad (after expansion) so that the beam remains within the FOV of the receiver system at all ranges of interest. The Mie (Schmidt-Cassegrain telescope) and Rayleigh receivers (Newtonian telescope) collect the atmospheric back scattered photons which are allowed to fall on photo multiplier tubes (PMTs) of different gains. The PMT output signal will be collected and processed through the MCS-plus (Multi Channel Scalar) PC based photon count system which is working with four data acquisition channels, two from Rayleigh receiver (R, U channels) and two from Mie receiver (P, S channels) for simultaneous photon counting with 12500 laser shots averaged for the time resolution of 250 sec with range resolution (dwell time=2µsec) of 300 m. For example, the typical MCS photon count profiles for P-channel and R-channel are as shown in Figure 1. More details on NARL lidar system can be obtained from Ramesh and Sridharan, [2012] [5].

DATA ANALYSIS
The lidar temperature profiles are determined from the method given by Hauchecorne and Chanin, [1980] [9].
In the height range above 30 km where the aerosol contribution is negligible (scattering ratio is greater than one calculated from Mie lidar), the range corrected signal and atmospheric transmission is proportional to the molecular number density. Using the number density taken from an appropriate model (MSIS-90 or CIRA-86) for the height of 50km where the signal-tonoise ratio is fairly high, the constant of proportionality is evaluated and there by the density profile r(z) is derived. Taking the pressure (P) at the top of the height range (90km) from the model, the pressure profile is computed using the measured density profile, assuming the atmosphere to be in hydrostatic equilibrium. Using the ideal gas relation, the temperature at i th layer T(z i ) is given by and the statistical standard error in the temperature is given by  As shown in Figure 2c, the associated chemical heating rates due to the reactions O+OH→H+O 2 , H+O 2 +M→HO 2 +M, H+O 3 →OH+O 2 , O+HO 2 → OH+O 2 , O+O+M→O 2 +M, O+O 2 +M→O 3 +M and their total increases drastically above ~77 km with the dominant reaction between H and O 3 (~3 K/day at ~85 km). However the total heating rate varies as 0.1-5.6 K/day at ~77-85 km which is much smaller than that shown (~30 K/day) by Ramesh et al., [2013] [4]. Figure 3 shows the half-hour integrated lidar temperatures and it can observed from this figure that the inversion layer descends between ~80 and 86 km at the rate of (~1.2 km/hr) diurnal tidal phase speed. Also the maximum inversion temperature decreases from ~286 K (~21:00 hrs) to ~223 K (~02:30 hrs) between ~80 and 85 km during the observation period   Further Figure 5 shows the vertical profile of lidar nightly mean Brunt-Väisälä frequency square (N 2 ) calculated from the equation (3).

RESULTS
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.
From this figure it can be observed that the condition for convective instability (N 2 <0) is satisfied at ~85 km which indicates the role of gravity wave breaking for the occurrence of the inversion layer.

SUMMARY AND CONCLUSIONS
In the present study, the dominant causative mechanism for the occurrence of a large MIL has been investigated mainly using the Rayleigh lidar observations for 22 March 2007 over Gadanki. A small scale gravity wave (T~38 min, λ z~6 .4) which is propagated from the lower atmosphere attains maximum amplitude at ~ 85 km and breaks at this height region (N 2 <0). Due to the wave breaking, the heat and momentum flux carried by the wave is deposited in the background and cause the inversion layer. The inversion layer descends at the rate of diurnal phase speed which gives the evidence for the gravity wave tidal interaction during this MIL event. However the smaller values of O 3 vmr and chemical heating rates suggest their negligible role for the occurrence of this MIL.