14:00 〜 14:15
[AAS06-14] Sporadic, large gravity wave events over Syowa Station in summer -Comparison between the PANSY radar and the ERA5 reanalysis-
キーワード：大気重力波、再解析、南極域
Gravity waves (GWs) are atmospheric waves whose restoring force is buoyancy. They are generated by mountains, jet streams, etc, and can change a global wind distribution through momentum transport and deposit. They do not only decelerate the mesospheric jet stream, but also affect the horizontal wind in the lower stratosphere, and the global meridional circulation. However, GW observations are not enough to verify their behaviour in the Antarctic, due to the harsh environment. In addition, GWs have a wide range of horizontal wavelength (i.e., from several km to several thousand km) and period (i.e., from Brunt-Väisälä period (approximately 5 minutes) to inertial period (over 12 hours)), which makes it difficult to reproduce GWs in the entire frequency range even in the state-of-the-art atmospheric models in spite of the recent increase of their resolution. In order to implement the effect of subgrid-scale phenomena into the models, which are not explicitly represented, a GW parameterization is introduced. In general, nonorographic GW parameterization assumes nearly constant wave sources and instantaneous upward propagation, but in reality the wave sources are not constant and GWs propagate horizontally as well (Plougonven et al., 2020). Thus it is required to constrain the GW effect in the models based on observations which cover the whole frequency range of GWs and estimate the GW momentum transport in the Antarctic.
Intermittency, a measure of how transient or intermittent a GW event is, has recently received much attention. Even if the total amount of momentum flux is the same, continuous, small amplitude events deposit momentum to higher altitudes, while sporadic, large amplitude events deposit momentum to lower altitudes. As a result, the structure and strength of the driven meridional circulation depend on the GW intermittency (Hertzog et al., 2008). In Antarctica, intermitency has been studied using super pressure balloons (Hertzog et al., 2012) and the Program of the Antarctic Syowa MST/IS radar (PANSY radar) at Syowa Station (Minamihara et al., 2020), suggesting differences in the characteristics of intermittency due to different wave generation mechanisms.
Our purpose of this study is to investigate the characteristics of GWs for sporadic, large amplitude events that have a large impact on the overall momentum transport, and also to investigate how well the reanalysis data reproduces the GWs events in the Antarctic. We used the PANSY radar, for the observation data and the ERA5 reanalysis for the reanalysis data. The PANSY radar, which was installed at Syowa Station (69°S,40°E) in 2011, can observe three dimensional winds in the troposphere and lower stratosphere with high accuracy and high temporal and vertical resolution. It is the only instrument in the Antarctic that enables us to capture GWs in the almost entire frequency range (Sato et. al., 2014). The ERA5 reanalysis is the latest meteorological reanalysis dataset provided by the European Centre for Medium-Range Weather Forecasts. The ERA5 data is distributed at 137 vertical levels from the surface to 0.01 hPa with a horizontal spacing of 0.25 degree every 1 hour.
We use three dimensional winds of the PANSY radar and the ERA5 reanalysis during the time period of January to March 2016. We focus on inertia GWs in austral summer period because they can be extracted from both the PANSY and ERA5 data (i.e., even one hour resolution of ERA5) and their attribution to wave sources are more straightforward (i.e., there are few downward group velocity waves in summer according to the past studies using the PANSY radar (Minamihara et. al., 2018)). The inertia GWs are extracted by applying a bandpass filter with a cutoff period of 4-24 h and a cutoff vertical wavelength of 1.5-8 km. As a result, we found similar wave-like structure in the PANSY radar and the ERA5 analysis.
In order to examine the propagation characteristics of inertia GWs, we use a hodograph analysis. It utilizes the feature that the hodograph (i.e., the altitude change of the horizontal wind vector drawn in the zonal and meridional wind space) becomes an ellipse, in which the amplitude, intrinsic period, vertical wavelength, phase velocity, and group velocity of GWs can be estimated. Although the hodograph analysis generally has an ambiguity of propagation direction by 180°, we exclude it by estimating the ground-based period directly using the data both in time and altitude.
The estimated momentum flux of each GW event is larger than the most-frequently observed GWs by the PANSY radar (Minamihara et al., 2020). We will discuss the wave sources through the ray tracing analysis of the observed GWs.
Intermittency, a measure of how transient or intermittent a GW event is, has recently received much attention. Even if the total amount of momentum flux is the same, continuous, small amplitude events deposit momentum to higher altitudes, while sporadic, large amplitude events deposit momentum to lower altitudes. As a result, the structure and strength of the driven meridional circulation depend on the GW intermittency (Hertzog et al., 2008). In Antarctica, intermitency has been studied using super pressure balloons (Hertzog et al., 2012) and the Program of the Antarctic Syowa MST/IS radar (PANSY radar) at Syowa Station (Minamihara et al., 2020), suggesting differences in the characteristics of intermittency due to different wave generation mechanisms.
Our purpose of this study is to investigate the characteristics of GWs for sporadic, large amplitude events that have a large impact on the overall momentum transport, and also to investigate how well the reanalysis data reproduces the GWs events in the Antarctic. We used the PANSY radar, for the observation data and the ERA5 reanalysis for the reanalysis data. The PANSY radar, which was installed at Syowa Station (69°S,40°E) in 2011, can observe three dimensional winds in the troposphere and lower stratosphere with high accuracy and high temporal and vertical resolution. It is the only instrument in the Antarctic that enables us to capture GWs in the almost entire frequency range (Sato et. al., 2014). The ERA5 reanalysis is the latest meteorological reanalysis dataset provided by the European Centre for Medium-Range Weather Forecasts. The ERA5 data is distributed at 137 vertical levels from the surface to 0.01 hPa with a horizontal spacing of 0.25 degree every 1 hour.
We use three dimensional winds of the PANSY radar and the ERA5 reanalysis during the time period of January to March 2016. We focus on inertia GWs in austral summer period because they can be extracted from both the PANSY and ERA5 data (i.e., even one hour resolution of ERA5) and their attribution to wave sources are more straightforward (i.e., there are few downward group velocity waves in summer according to the past studies using the PANSY radar (Minamihara et. al., 2018)). The inertia GWs are extracted by applying a bandpass filter with a cutoff period of 4-24 h and a cutoff vertical wavelength of 1.5-8 km. As a result, we found similar wave-like structure in the PANSY radar and the ERA5 analysis.
In order to examine the propagation characteristics of inertia GWs, we use a hodograph analysis. It utilizes the feature that the hodograph (i.e., the altitude change of the horizontal wind vector drawn in the zonal and meridional wind space) becomes an ellipse, in which the amplitude, intrinsic period, vertical wavelength, phase velocity, and group velocity of GWs can be estimated. Although the hodograph analysis generally has an ambiguity of propagation direction by 180°, we exclude it by estimating the ground-based period directly using the data both in time and altitude.
The estimated momentum flux of each GW event is larger than the most-frequently observed GWs by the PANSY radar (Minamihara et al., 2020). We will discuss the wave sources through the ray tracing analysis of the observed GWs.