Meteor Radar Estimations of Gravity Wave Momentum Fluxes in the Mesosphere -Lower Thermosphere and their source spectra characterisation using Ray tracing modelling

Atmospheric gravity waves play a vital role in coupling the various layers of the atmosphere. Meteor radars are widely used to study gravity wave (GW) variances and their momentum fluxes at the altitudes where meteor counts are sufficient to yield good statistical fits to the data (Mesosphere-Lower Thermosphere).Hocking's matrix inversion method is widely used for obtaining GWMF from meteor radar, but in this method perturbation obtained after the removal of background winds are assumed to be GWs but this perturbation can be contributed by tides and planetary waves. By using Hocking method in Modified Composite Day (MCD) analysis contribution from tide and planetary waves can be removed.The present study evaluates the meteor radar observations of GW momentum fluxes obtained from Thumba (8.5°N, 77°E; 2006-2015),Kototabang (0.2°S, 100.3°E; 2002-2017), and Tirupati (13.63°N, 79.4°E; 2013-2018) using three dimensional wind field simulations, which include specified tidal, planetary and GW fields in MCD analysis.The results showed that the retrieved and specified GW momentum fluxes agree very well over Tirupati followed by Thumba and Kototabang which is depend on the number of meteor detections used in the analysis. Being a sub-grid process, the gravity waves are parameterized in the global atmospheric models.One of the important aspects of the gravity wave parameterisation is to identify the source spectrum in the lower atmosphere. Earlier studies employed GROGRAT (Gravity-wave Regional Or Global Ray Tracer) model to fine tune the source spectrum by comparing model results with the observations. An attempt is made in the present study to identify the best fit source spectra for the gravity waves which are observed at mesospheric altitude using GROGRAT model and meteor radar located at Tirupati. In this regard, monthly mean climatologies of background atmosphere is developed (using MST Radar, MF Radar, HRDI/UARS, TIMED/SABER, Radiosonde, Rocketsonde) in and around ±50 at Tirupati. Considering this as background atmosphere for GROGRAT model and by considering different source spectra (isotropic as well as anisotropic, symmetric as well as antisymmetric) with various combinations of spectral width, horizontal wavelength, spectral amplitude, model runs were carried out at different altitudes and zonal momentum fluxes are obtained. These simulated fluxes are compared with that of the meteor radar observed monthly mean gravity wave momentum flux to get the best fit source spectra at Tirupati. The significance of the present study lies in identifying the best fit source spectra of gravity waves by tuning it with gravity wave momentum flux observations from meteor radar in the mesosphere and lower thermosphere region.