Technique to produce daily estimates of the migrating diurnal tide using TIMED/SABER and EOS Aura/MLS Vu Nguyen n , S.E. Palo Department of Aerospace Engineering, University of Colorado, Boulder, USA article info Article history: Received 11 April 2013 Received in revised form 8 July 2013 Accepted 15 July 2013 Available online 31 July 2013 Keywords: Atmospheric tide Migrating Day-to-day variability GSWM model abstract A technique to explicitly compute the day-to-day variability of the migrating diurnal tide (DW1) between 20 km and 80 km on a global scale is presented and analyzed. Our method employs temperature data from two satellite instruments: the MLS (Microwave Limb Sounder) instrument on the EOS (Earth Observing System) Aura spacecraft and the SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) instrument on the TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics) spacecraft. By taking advantage of the four daily solar local time measurements from the two instruments, a least squares fit representing the DW1 is constructed. Consequently, the daily zonal mean, DW1 amplitude and phase are all estimated on a daily basis. Before the implementation of our technique, a comparative analysis between the instrument data sets is conducted. The analysis reveals temperature biases of up to 10 K, which are removed to improve our estimates. To evaluate performance, our method is applied to a model atmosphere constructed from tidal fields obtained from the Global Scale Wave Model (GSWM). Performance results indicate that the DW1 is most effectively extracted from the background atmosphere and other tidal components when each latitude circle is well sampled and the local time sampling is evenly spaced. A comparison of our results to the GSWM and past observations support the conclusion that our method produces daily estimates of the DW1 that can be utilized for scientifically useful investigations of short term tidal variability. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction Atmospheric tides are observed as global scale oscillations of density, temperature and wind with periods that are harmonics of a solar day. Forced primarily by solar absorption in the lower atmosphere, but also by latent heat release and non-linear inter- actions between global-scale waves, tidal components are either confined to their source region or propagate vertically. Propagat- ing tidal amplitudes reach maximum values in the mesosphere- lower thermosphere (MLT) as a result of energy conservation before dissipation forces serve to dampen the tides. In addition to vertical propagation, tides also propagate in the horizontal direction. Tides are commonly categorized by wave number, which describes the number of oscillation maxima along a latitude circle. They are also classified as either migrating or non-migrating; migrating tides propagate sun-synchronously and are fixed in local time. Of the tidal components, the migrating diurnal tide, commonly denoted as DW1 (propagating to the west with a 24 h period and zonal wave number one) is the most dominant below 100 km. The large DW1 amplitude values present in the MLT significantly alter the dynamics of the region through mass, momentum and energy transport. Because the MLT serves as a transition from the well-mixed lower atmosphere to the molecular diffuse upper atmosphere and a coupling region between neutral and ionized gas, the DW1 plays an important role in understanding the atmosphere as a complete system. The DW1 and other atmospheric oscillations have traditionally been observed through ground-based and satellite-borne instru- ments. Ground-based systems such as radar and lidar possess the capability of monitoring a particular region of the atmosphere for continuous intervals. As a result, ground-based observation tech- niques can spectrally analyze continuous time series to determine the periods of local oscillations. The lack of spatial coverage from a single ground-based system however prevents scientists from distinguishing between global and local oscillations. Since the DW1 is a global scale phenomenon, data obtained from a ground- based system cannot be utilized to separate the DW1 from non- migrating diurnal tides. Multiple data sets along a latitude circle are required to effectively extract the DW1 from the background atmosphere. In contrast, satellite-borne instruments observe the earth at multiple locations, which yields the potential to extract the global Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jastp Journal of Atmospheric and Solar-Terrestrial Physics 1364-6826/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jastp.2013.07.008 n Correspondence to: ECOT-634 Aerospace Engineering Sciences Department, University of Colorado 429 UCB, Boulder, CO 80309-0429, USA. Tel.: +1 719 291 6572. E-mail address: vu.a.nguyen1@gmail.com (V. Nguyen). Journal of Atmospheric and Solar-Terrestrial Physics 105-106 (2013) 39–53