Shao Dong Zhang

TIMED/SABER observations of lower mesospheric inversion layers at low and middle latitudes

We present the global distribution, seasonal, and interannual variations of the lower mesospheric inversion layers (MILs) using SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) temperature data. We show that both the characteristics and the formation mechanisms of large spatiotemporal-scale lower MILs are latitude dependent. At low latitudes, the monthly zonal mean amplitude of the lower MILs exhibits a semi-annual cycle and reaches a maximum of similar to 40 K in spring and a secondary maximum of similar to 30 K in autumn. On the equator, the semi-annual oscillations in the background and diurnal-migrating-tide temperatures could contribute more than 12 and 25 K, respectively, suggesting they are the key causes of large spatiotemporal-scale lower MILs at low latitudes. At middle latitudes, the monthly zonal mean amplitude of the lower MILs exhibits an annual cycle with its maximum in the range 24-33 K in winter. In addition, their longitudinal distribution and daily variation in winter are closely correlated with the transient structure of a composite wave composed of stationary and westward-propagating quasi-16-day planetary waves with zonal wave number 1. The correlation coefficient between the lower MILs and the composite wave can sometimes reach unity. The composite planetary wave could contribute temperature enhancements of at least 15-20 K to the lower MILs. Thus, we believe that the transient structure of planetary waves is also an important cause of the large spatiotemporal-scale lower MILs in winter at middle latitudes, in addition to previously proposed mechanisms.

A numerical study on amplitude characteristics of the terdiurnal tide excited by nonlinear interaction between the diurnal and semidiurnal tides

A fully nonlinear numerical tidal model has been developed with the aim of presenting amplitude features of the terdiurnal tide excited by nonlinear interaction between the diurnal and semidiurnal tides. Since the superposition of the migrating diurnal and semidiurnal tidal solutions from the GSWM-00 (Global Scale Wave Model 2000) is taken as the initial disturbance for this nonlinear model, the diurnal and semidiurnal tides are allowed to nonlinearly interact with each other. The analyses on the simulations show that the migrating terdiurnal tide can be significantly excited by the nonlinear interaction between the diurnal and semidiurnal tides in the mesosphere and lower thermosphere (MLT) region, especially above 90 km, and have pronounced amplitudes (wind speeds over 15 m s−1 and temperature over 10 K) in the lower thermosphere (90–110 km). In addition, its amplitudes vary strongly with season and maximize during equinoctial months at low and middle latitudes, and its zonal wind component is larger than the meridional wind component. Simultaneously, the diurnal and semidiurnal tides exhibit evident variations, indicating that the wave-wave and wave-mean flow interactions are the important cause of the tidal variability. Our calculations also illustrate the remarkable alteration of the background fields induced by the mean flow/tidal interaction, implying this interaction should be comprehensively considered in describing global atmospheric dynamic and thermal structures in the MLT region.

Low‐frequency oscillations of the gravity wave energy density in the lower atmosphere at low latitudes revealed by U.S. radiosonde data

We adopted 14 year (from 1998 to 2011) radiosonde data from 16 stations at latitudes between 14.33°S to 29.37°N to investigate the low-frequency oscillations of inertial gravity wave energy densities (including kinetic energy density, potential energy density, and total energy density) in the lower atmosphere from 2 km to 30 km. Apparent signatures of 11 year solar cycle and quasi-biennial oscillation (QBO) were found in the gravity wave energy densities in the troposphere and lower stratosphere, respectively. Detailed analyses suggested that the 11 year oscillation and QBO of gravity wave activity might be due to the 11 year oscillation of convection activity and zonal wind QBO modulation, respectively. Besides, the gravity wave energy also showed a 35.2 month oscillation in the lower stratosphere, which might be generated by the interaction between the QBO (with the period of 26.7 months), and the 11 year solar cycle, while more possible causes are the direct modulation from the background zonal wind. Our results showed that the QBO signature in gravity wave total energy density could extend northward to over 20°N in the Northern Hemisphere, while the 11 year solar cycle in gravity wave activity could only extend to lower than 7.5°N. The oscillation with the period of 35.2 months may be a prolonged QBO in both gravity wave activity and background zonal wind.

Simultaneous upward and downward propagating inertia‐gravity waves in the MLT observed at Andes Lidar Observatory

Based on the temperature, and zonal and meridional winds observed with a Na lidar at Andes Lidar Observatory (30.3°S, 70.7°W) on the night of 20-21 July 2015, we report simultaneous upward and downward propagating inertia-gravity waves (IGWs) in the mesosphere and lower stratosphere (MLT). The ground-based periods of the upward and downward IGWs are about 5.4 h and 4.8 h, respectively. The horizontal and vertical wavelengths are about 935 km and 10.9 km for the 5.4-h IGW, and about 1248 km and 22 km for the 4.8-h IGW, respectively. Hodograph analyses indicate that the 5.4-h IGW propagates in the direction of about 23° west of north, while the 4.8-h IGW travels northward with an azimuth of 20°clockwise from north. These wave parameters are in the typical IGW wavelength and period ranges, nevertheless, the downward propagating IGWs in the MLT are rarely reported in previous observations. The ray-tracing analysis suggests that the 5.4-h IGW is likely to originate from the stratospheric jet adjustment over the Antarctic, while the 4.8-h IGW source may be above the MLT because it is unlikely to propagate downward through a reflection in the realistic atmospheric wind field. Although both IGWs do not reach their amplitude thresholds of instability, the Richard number reveals that the dynamical and convective instabilities occur intermittently, which indicates that the instability arising from the multiple-perturbation superposition may have a significant influence on wave saturation and amplitude constraint in the MLT.