The Sensitivity of the Cloud Imaging and Particle Size Experiment to Gravity Wave Density Perturbations near the Stratopause
Abstract:Nadir images of Rayleigh scattered UV sunlight (265 nm) from the Cloud Imaging and Particle Size (CIPS) instrument on the Aeronomy of Ice in the Mesosphere (AIM) satellite contain many periodic structures. These structures are believed to be the result of Gravity Waves (GW). In this work, we investigate the theoretical impact of GWs on the albedo imagery. We show that GWs are indeed capable of producing the observed structures. The sensitivity to GW perturbations peaks at an altitude of ~50 km with a FWHM of ~15 km with images taken at mid to polar latitudes. For vertical wavelengths greater than 15 km, this corresponds to GWs with momentum fluxes greater than ~0.1-0.01 mPa. Vertical wavelengths less than ~10 km require amplitudes larger than static stability considerations allow, so these GWs are not observable. Observable horizontal wavelengths range from approximately 20 to 300 km. These wavelength ranges correspond to a sensitivity to GWs with an intrinsic period shorter than ~2.5 hrs.
Existing satellite GW studies in this altitude region use limb scanners or microwave nadir imagers, which are sensitive to much longer horizontal wavelengths. This tends to limit the sensitivity to longer period waves. Therefore, the addition of CIPS imagery to the existing coverage of GW measurements is complimentary - significantly expanding the spectral coverage of GWs near the stratopause. Short period GWs observed by CIPS carry significantly larger momentum flux than the climatological averages seen for longer period GWs, so important differences may exist. GWs are critically important to middle atmosphere dynamics, and their representation in global climate models is not well constrained by observations.