A search for mountain waves in MLS stratospheric limb radiances from the winter Northern Hemisphere: Data analysis and global mountain wave modeling

Despite evidence from ground-based data that flow over mountains is a dominant source of gravity waves (GWs) for the Northern Hemisphere winter middle atmosphere, GW-related signals in global limb radiances from the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS) have shown little direct evidence of mountain waves. We address this issue by combining a renewed analysis of MLS limb-track and limb-scan radiances with global mountain wave modeling using the Naval Research Laboratory Mountain Wave Forecast Model (MWFM). MLS radiance variances show characteristics consistent with mountain waves, such as enhanced variance over specific mountain ranges and annual variations that peak strongly in winter. However, direct comparisons of MLS variance maps with MWFM-simulated mountain wave climatologies reveal limited agreement. We further develop a detailed "MLS GW visibility function'' that accurately specifies the three-dimensional in-orbit sensitivity of the MLS limb-track radiance measurement to a spectrum of GWs with different wavelengths and horizontal propagation directions. On postprocessing MWFM-generated mountain wave fields through these MLS visibility filters, we generate MWFM variance maps that agree substantially better with MLS radiance variances. This combined data analysis and MLS-filtered MWFM modeling leads us to conclude that many MLS variance enhancements can be associated with mountain waves forced by flow over specific mountainous terrain. These include mountain ranges in Europe (e.g., Scandinavia; Alps; Scotland; Ural, Putoran, Altai, Hangay and Sayan Mountains; Yablonovyy, Stanovoy, Khingan, Verkhoyansk and Central Ranges), North America (e.g., Brooks Range, MacKenzie Mountains, Colorado Rockies), southeastern Greenland, and Iceland. Our results show that given careful consideration of the in-orbit sensitivity of the instrument to GWs, middle atmospheric limb radiances measured from UARS MLS, as well as from the new MLS instrument on the Earth Observing System (EOS) satellite, can provide important global information on mountain waves in the extratropical Northern Hemisphere stratosphere and mesosphere.