A gravity wave case study for an observation over Antarctica using the cloud imaging and particle size experiment

This work presents a case study using a new data set for space based observations of Gravity Waves (GW) near the stratopause. The data set uses the Cloud Imaging and Particle Size instrument (CIPS) on the Aeronomy of Ice in the Mesosphere (AIM) satellite. The CIPS instrument is a camera array with a wide field of view centered in the nadir. The instrument observes with a band pass centered at the UV wavelength of 265 nm. The GWs are observed via perturbations in the Rayleigh scattered sunlight near the stratopause. Gravity waves are the dominant driver of mesospheric dynamics and they are insufficiently constrained in global climate models. CIPS observes the GWs over much of a hemisphere at a higher altitude than similar imaging observations such as those from the Atmospheric Infrared Sounder instrument (AIRS). The sensitivity to the shorter horizontal wavelengths (~20km) compliments the GW data sets constructed from limb viewing instruments which observe near the stratopause but tend to see the GWs with longer horizontal wavelengths. A GW observed on two consecutive orbits on October 10th 2010 to the east of McMurdo station in Antarctica is presented. The dominant horizontal wavelength of this GW was approximately 150km. From the apparent phase progression between the two orbits, a ground based phase speed of 20 m/s is estimated. The GW was propagating in the upwind direction according to winds from MERRA re-analysis data. From this wind data, the intrinsic frequency of the GW is estimated, and from the dispersion relation, the vertical wavelength (~18km) is derived. Using the three dimensional wave vector, the radiative transfer of the Rayleigh scattered albedo observation is simulated and used to determine the GW amplitude. The resulting momentum flux is estimated to be approximately 10 mPa. Momentum flux is the critical measurement needed to understand the forcing by GWs on the mean flow. This momentum flux is sufficient for a local deceleration of the mean flow of approximately 200 m/s/day.