Diurnal propagating tides in the low-latitude middle atmosphere |
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| Institution: | 1. Space Science and Astrobiology Division, NASA/Ames Research Center, Moffett Field, CA 94035, United States of America;2. Laboratoire Atmosphères, Milieux, Observations Spatiales, CNRS, Guyancourt, France;3. Bay Area Environmental Research Institute, NASA/Ames Research Center, Moffett Field, CA 94035, United States of America;4. Space Science Institute, Boulder, CO 80301, United States of America;5. NASA/Ames Research Center, Moffett Field, CA, United States of America;1. Planetary Environmental and Astrobiological Research Laboratory, School of Atmospheric Sciences, Sun Yat-Sen University, Zhuhai, Guangdong, 519082, China;2. CAS Key Laboratory of Geospace Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui, China;3. Department of Earth Science and Engineering, Imperial College, London, UK;4. Aeolis Research, Chandler, AZ, USA;5. CAS Center for Excellence in Comparative Planetology, Hefei, China;6. Space Science Institute, Boulder, CO, 80301, USA |
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| Abstract: | Using an equivalent gravity wave f-plane model it is shown that longitude variations in diurnal insolation absorption by tropospheric H2O can account for longitudinal variations of at least ± 12–15% about zonal mean values in the diurnal wind amplitude at low latitudes (0–20°) between 80 and 100 km, by virtue of the non-migrating propagating tidal modes which are excited. Phase variations of about ± 0.75 h also occur. These percentage variations are conservative estimates, since the background migrating (1,1,1) mode appears to be slightly (20–25%) overestimated in amplitude. In addition, the assumed eddy dissipation values, which appear necessary to model the breaking (1,1,1) mode, are larger than generally considered ‘reasonable’ by photochemical modellers. For a photochemically more reasonable eddy diffusion profile, estimates of longitude differences in diurnal wind amplitude are quite similar to the above values below 87 km, but increase to ± 17–25% near 100 km, with accompanying phase variations of ± 1–2 h about zonal mean values. In addition, it is shown that radiative damping by CO2 parameterized by a scale-dependent Newtonian cooling coefficient accounts for no more than a 20% reduction in the amplitudes of diurnal propagating tides above 80 km. |
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