The climatology, propagation and excitation of ultra-fast Kelvin waves as observed by meteor radar, Aura MLS, TRMM and in the Kyushu-GCM

Abstract

Wind measurements from a meteor radar on Ascension Island (8° S, 14° W) and simultaneous temperature measurements from the Aura MLS instrument are used to characterise ultra-fast Kelvin waves (UFKW) of zonal wavenumber 1 (E1) in the mesosphere and lower thermosphere (MLT) in the years 2005–2010. These observations are compared with some predictions of the Kyushu-general circulation model. Good agreement is found between observations of the UFKW in the winds and temperatures, and also with the properties of the waves in the Kyushu-GCM. UFKW are found at periods between 2.5–4.5 days with amplitudes of up to 40 m s<sup>−1</sup> in the zonal winds and 6 K in the temperatures. The average vertical wavelength is found to be 44 km. Amplitudes vary with latitude in a Gaussian manner with the profiles centred over the equator. Dissipation of the waves results in monthly-mean eastward accelerations of 0.2–0.9 m s<sup>−1</sup> day<sup>−1</sup> at heights around 95 km, with 5-day mean peak values of 4 m s<sup>−1</sup> day<sup>−1</sup>. Largest wave amplitudes and variances are observed over Indonesia and central Africa and may be a result of very strong moist convective heating over those regions. Rainfall data from TRMM are used as a proxy for latent-heat release in an investigation of the excitation of these waves. No strong correlation is found between the occurrence of large-amplitude mesospheric UFKW events and either the magnitude of the equatorial rainfall or the amplitudes of E1 signatures in the rainfall time series, indicating that either other sources or the propagation environment are more important in determining the amplitude of UFKW in the MLT. A strong semiannual variation in wave amplitudes is observed. Intraseasonal oscillations (ISOs) with periods 25–60 days are evident in the zonal background winds, zonal-mean temperature, UFKW amplitudes, UFKW accelerations and the rainfall rate. This suggests that UFKW play a role in carrying the signature of tropospheric ISOs to the MLT region

Quasi Two‐, Three‐, and Six‐Day Planetary‐Scale Wave Oscillations in the Upper Atmosphere Observed by TIMED/SABER Over ~17 Years During 2002–2018

“…This allows us to determine both the period and zonal wave number of a fast travelling planetary wave using only a few days of data. Observations show that the two‐, three‐, and six‐day waves have a lifetime lasting over days to weeks (e.g., Davis et al, ; Pancheva et al, ; Tunbridge et al, ). An 11‐day running window stepped by one day at a time is thus sufficient to resolve the features of interest.…”

“…Many observations have revealed the presence of fast travelling planetary waves with short periods (≤6 days) in the equatorial mesosphere and lower thermosphere (MLT; e.g., Garcia et al, ; Pancheva et al, , , ; Takahashi et al, ; Tunbridge et al, ; Davis et al, ; England et al, ; Gu et al, ; Liu et al, ). These include the westward propagating Rossby‐gravity waves (with periods approximately two days), and eastward propagating ultrafast Kelvin (with periods approximately three days), and fast Kelvin waves (with periods approximately six days).…”

“…Nonzero meridional winds also arise from the effects of mean winds and dissipation (Forbes et al, ). The amplitudes of Kelvin waves have been observed to vary with latitude in a Gaussian form with the peak centered over the equator (e.g., Davis et al, ).…”

“…Global sampling by satellite allows for the identification of the spatial structure and propagation of the wave. Recently, long‐term satellite observations over many years have become available, allowing for the study of global wave activities during extended times (e.g., Davis et al, ; Pancheva et al, ; Tunbridge et al, ). However, most studies are for one single wave period and there is no statistical study of different wave periods over a wide range, and the wave climatological patterns and behaviors have not yet been comprehensively studied using a decade‐long satellite data set.…”

Signatures of the 3‐day wave in the low‐latitude and midlatitude ionosphere during the January 2010 URSI World Day campaign

“…A detailed study of the identification of this wave is reported by England et al [2012]. The month of January 2010 coincides with the onset of the SSW, but this wave occurs before the SSW and the 3-day wave often occurs in January [e.g., Vincent, 1993;Yoshida et al, 1999;Davis et al, 2011]. It is thus believed that this wave is not related to the SSW.…”