Q16DW in meteor observation
Figure 1 shows the monthly mean zonal (positive eastward) and meridional (positive northward) winds at the height range of 76–100 km observed by the meteor radars at BJ and WH for 7 years from October 2010 to September 2017. At WH, the observational data are missing in two long periods of 4 July–23 November 2011 (143 days), and 1 July–22 August 2016 (53 days). The zonal wind in the MLT at mid-latitudes exhibits an interesting feature of two annual oscillations. One oscillation is centered at about 92 km with the maximum velocity of about 40–50 ms−1 at the two stations in the transition from spring (March–May) to summer (June–August), and the other is centered around 80 km with the maximum value of about 40–50 ms−1 (50–65 ms−1) at BJ (WH) in winter (December–March). Because the two annual oscillations are nearly out of phase, the zonal wind at the height of 80–92 km shows a distinguishable semiannual oscillation. Relative to the zonal wind, the meridional wind is weak with the monthly mean values of about − 15 to 10 ms−1, and the northward wind arises mainly in autumn (September–November) and early winter.
The missing data are minimal at 88 km with a fraction less than 2% (except the two data gaps at WH) due to the Gaussian meteor number distribution centered at this height, thus the winds at 88 km have good representation and high reliability. For investigating the PW activities, we use linear interpolation to fill in the missing data, and then perform a wavelet transform on the zonal and meridional winds at 88 km to obtain the dominant PW modes and the temporal variability of these modes. Morlet wavelet function consisting of a plane wave modulated by a Gaussian envelope is chosen as mother wavelet (Huang et al. 2015; Cheng et al. 2021), and we follow Torrence and Compo (1998) to carry out the wavelet transform with the width of Morlet wavelet function defined as the \(e\)-folding time of wavelet amplitude. Figure 2 presents the wavelet spectra of the zonal and meridional winds at 88 km during the 7 years. One can note that the perturbations with temporal scales of PWs and short-period oscillations are robust in the MLT during winter, similar to the observations from the Super Dual Auroral Radar Network (SuperDARN) radars at latitudes of 51°–66°N (Kleinknecht et al., 2014). As is well known, PW activities are usually associated with SSWs, thus we calculate the daily averaged zonal mean temperature at 10 hPa between 60°N and 90°N and the daily averaged zonal mean zonal wind at 10 hPa over 60°N from 1 October 2010 to 30 September 2017 based on the MERRA2 reanalysis data, which are depicted in Fig. 3. The SSW events are easy to identify from the mean temperature between 60°N and 90°N, and then the major and minor SSWs are distinguished by whether the zonal wind at 60°N reverses or not. We mark the date of the maximum temperature in the major and minor SSWs with the black solid and dashed vertical lines in Figs. 2 and 3, respectively. Stratospheric final warming (SFW) is indicative of the final transition of stratospheric winter circulation from eastward to westward (Andrews et al. 1987). In terms of zonal wind anomalies, SFW is to a certain extent similar to the major SSW; whereas, SFW is still substantially different from the major SSW because the polar winter vortex collapses and the reversed mean zonal wind does not return back until the next transition from summer to autumn (Black and McDaniel 2007). SFW may be a strong warming, and may also consist of several successive weak warming events (Yu et al. 2019). The SFW is marked by the dotted vertical line in Figs. 2 and 3.
It can clearly be seen from Fig. 2 that the PW and oscillation activities in the MLT at mid-latitudes have a close relationship with the SSWs and SFWs. In the 7 years, only a major SSW takes place around 11 January 2013, and correspondingly, an oscillation with a wide period range centered at 22 days is the strongest perturbation in the zonal winds, with the spectral amplitudes of 19.2 ms−1 (17.3 ms−1) at BJ (WH). Generally, the perturbations are considerably weak in the meridional wind relative to in the zonal wind. However, it is interesting that there are PW activities from the autumn of 2013 to the winter of 2013/2014, which seem not to have much relevance to the SSW and SFW in the winter. The PW is strong not only in the zonal wind, but also in the meridional wind, and lasts for a longer duration compared with the other PWs during the SSW and SFW events. In addition, there are no prominent PW activities around the SSW on 9 February 2014 and the SFW on 16 March 2014, which is different from the scenario in the SSW and SFW in the other years.
In order to clearly show the PW evolution with time, the wavelet spectra of the zonal and meridional winds at 88 km from 1 September 2013 to 1 April 2014 are replotted in Fig. 4. In the zonal wind, the PW is active for a long time of nearly 4 months from about October 2013 to January 2014, and has a period range of about 14–20 days, thus we call it Q16DW. The Q16DW intensity decays rapidly from about 21 January 2014, which is obviously earlier than the temperature peak of the SSW on 9 February 2014, and even the onset of the SSW on 4 February 2014. In the meridional wind, the strong Q16DW occurs mainly in December 2013 and January 2014. Overall, the Q16DW onset in both the zonal and meridional winds at WH predates that at BJ. The spectral amplitude has the maximum values of 15.9 ms−1 in the zonal wind and 12.7 ms−1 in the meridional wind at BJ, and the similar magnitudes of 17.3 ms−1 in the zonal wind and 12.2 ms−1 in the meridional wind at WH.
Since the Q16DW strength in the zonal wind is slightly stronger at WH than at BJ, we use the MERRA2 reanalysis data at WH to highlight the Q16DW activity from the troposphere to the stratosphere. The wavelet transform is carried out on the reanalysis zonal wind at WH between 1 September 2013 and 1 April 2014. Figure 5 presents the wavelet spectra at the selected pressure levels. The pressure levels at 375, 56, 10, 2.6, 1.6 and 0.6 hPa correspond approximately to the altitudes at about 7, 20, 32, 40, 45 and 52 km derived from logarithmic pressure height formula under a specified scale height of 7 km, respectively. Figure 5 illustrates that the Q16DW arises in the zonal wind from the troposphere to the stratosphere, and is intense in November and December 2013. Besides, the Q16DW activities can also be seen around the SSW and SFW events. Nevertheless, the later event seems to be another Q16DW event distinguished from the earlier one because their spectrum peaks not only are separated from each other in time, but also show the different evolutions with height. It can be noted from Fig. 4 that in the zonal wind of the MLT, the corresponding Q16DW occurs during the earlier interval, but seems not to obviously arise during the later interval.
Q16DW propagation
With the help of the reanalysis data, we investigate the propagation features of these Q16DWs. By using a band-pass filter with lower and upper cut-off periods of 12.5 and 20 days, the Q16DWs are extracted from the radar observations at 78–98 km and the MERRA2 reanalysis data at 103–10−2 hPa levels (~ 0–80 km) between 1 September 2013 and 1 April 2014. Figure 6 shows the filtered Q16DW in the zonal and meridional winds at BJ and WH. The upper bound at 10–2 hPa level in the reanalysis data is approximately around the altitude of 80 km, thus we can see that the Q16DWs at 78 and 80 km in the radar observations are largely in agreement with those at 10–2 hPa in the reanalysis data, in particular, the consistency of their phases, which shows the reliability of the two datasets. Due possibly to the limitation of observational data used in assimilation to produce the reanalysis data, the Q16DW magnitudes at 10–2 hPa in the reanalysis are sometimes different from those at 78 and 80 km in the observation. For example, in the zonal wind at 78 and 80 km, the Q16DW activities from the radar observations obviously arise at both WH and BJ from 20 October to 10 November 2013, but the corresponding wave activities cannot clearly be seen at 10–2 hPa from the reanalysis data. In contrast, the Q16DW occurs in the reanalysis meridional wind at 10–2 hPa over Wuhan around the SSW event, however, the corresponding wave does not evidently appear in the observed meridional wind at 78 and 80 km. Even so, the small difference should not severely affect the wave analysis from the troposphere to the MLT. In the troposphere, the Q16DW amplitude is weak because of the dense atmosphere. In the stratosphere and MLT, the strong Q16DW activities before the SSW are distinguishable from those around the SSW and SFW events. Before the SSW, the Q16DW displays a downward phase progression from the MLT to the stratosphere, indicating the upward propagation of the Q16DW, while after the SSW, the Q16DW phase is steeper and even progresses upward. For the extraordinary Q16DW of our concern during October 2013–January 2014, the vertical wavelength is estimated to be about 80 km in the MLT over BJ and WH based on the vertical variation of wave phase in the zonal wind at 78–98 km from the radar observations, and to be about 120 km at 10–0.1 hPa levels (~ 32–64 km) in the stratosphere and lower mesosphere from the reanalysis data, respectively.
We use the same filter to extract the Q16DWs along the longitude line of 115°E in the Northern Hemisphere from the reanalysis data. Figure 7 depicts the filtered Q16DWs in the zonal and meridional winds at 0.6, 10, 100 and 375 hPa levels (~ 52, 32, 16 and 7 km) from 1 September 2013 to 1 April 2014. The two Q16DW activities show the different features of phase propagation in the meridional direction. In the stratosphere, the Q16DW around the SSW and SFW exhibits a northward phase propagation, especially in the middle and upper stratosphere at mid- and high-latitudes with a thriving activity prior to the SSW. However, the first Q16DW during October to December 2013 shows a southward phase progression in the troposphere, and a slow phase variation with latitude in the stratosphere at mid-latitudes. Less latitudinal change of wave phase can also be seen in the MLT at the two mid-latitudinal BJ and WH stations from Fig. 6a and e. At high-latitudes, the wave phase displays a trend of southward propagation. The characteristic of phase variation may be connected with the background condition since PW propagation is closely related to the background atmosphere.
The frequency–wavenumber spectrum can be derived from a two-dimensional Fourier transform on the reanalysis data. Based on the wavelet spectrum of the zonal wind at 0.6 hPa level (~ 52 km) in Fig. 5a, we select the zonal wind at this pressure level in the two time durations of 22 November–23 December 2013, and 20 January–20 February 2014 to perform the Fourier spectrum analysis, respectively, and then the spectral frequency is changed into the corresponding period. Figure 8 presents the period–wavenumber spectrum of the zonal wind perturbations obtained by removing the mean value from the reanalysis zonal wind in the two time intervals. The negative zonal wavenumber in Fig. 8 indicates the westward propagation of the wave. Interestingly, the first Q16DW has the predominant wavenumber of − 1 with the maximum spectral amplitude of 8.2 ms−1, whereas the second Q16DW around the SSW has the predominant wavenumber of − 2 with the spectral magnitude of 7.7 ms−1, which is consistent with the continuous predominance of PWs with wavenumber 2 in the stratosphere during the minor SSW occurring in the winter of 2013/14 (Harada and Hirooka, 2017). Besides, in the first time duration, a spectral peak at wavenumber of − 1 and period of 8 days has the magnitude of 6.2 ms−1. There are also other different wavenumber Q16DW components with rather weak intensity, thus the Q16DW components with wavenumbers of − 1 and − 2 are the predominant perturbations in the first and second time intervals, respectively. This indicates that the two westward propagating Q16DWs have the different zonal scales and phase speeds. Therefore, the two Q16DWs can be distinguished from not only the durations of their activities, but also the features of their propagations.