Long-term studies of mesosphere and lower-thermosphere summer length definitions based on mean zonal wind features observed for more than one solar cycle at middle and high latitudes in the Northern Hemisphere

Abstract

Abstract. Specular meteor radars (SMRs) and partial reflection radars (PRRs) have been observing mesospheric winds for more than a solar cycle over Germany (∼ 54∘ N) and northern Norway (∼ 69∘ N). This work investigates the mesospheric mean zonal wind and the zonal mean geostrophic zonal wind from the Microwave Limb Sounder (MLS) over these two regions between 2004 and 2020. Our study focuses on the summer when strong planetary waves are absent and the stratospheric and tropospheric conditions are relatively stable. We establish two definitions of the summer length according to the zonal wind reversals: (1) the mesosphere and lower-thermosphere summer length (MLT-SL) using SMR and PRR winds and (2) the mesosphere summer length (M-SL) using the PRR and MLS. Under both definitions, the summer begins around April and ends around middle September. The largest year-to-year variability is found in the summer beginning in both definitions, particularly at high latitudes, possibly due to the influence of the polar vortex. At high latitudes, the year 2004 has a longer summer length compared to the mean value for MLT-SL as well as 2012 for both definitions. The M-SL exhibits an increasing trend over the years, while MLT-SL does not have a well-defined trend. We explore a possible influence of solar activity as well as large-scale atmospheric influences (e.g., quasi-biennial oscillation (QBO), El Niño–Southern Oscillation (ENSO), major sudden stratospheric warming events). We complement our work with an extended time series of 31 years at middle latitudes using only PRR winds. In this case, the summer length shows a breakpoint, suggesting a non-uniform trend, and periods similar to those known for ENSO and QBO.

Validation of Multistatic Meteor Radar Analysis Using Modeled Mesospheric Dynamics: An Assessment of the Reliability of Gradients and Vertical Velocities

“…These products are used to study tides, planetary waves, and gravity wave activity in the MLT region (e.g., Conte et al., 2017; Murphy et al., 2006; Sandford et al., 2006). SMR horizontal wind measurements are also used for long‐term studies of the MLT region (e.g., Jaen et al., 2021; Wilhelm et al., 2019). Previous comparative studies between meteor radars and other ground‐based instruments have also shown agreement.…”

Progress in investigating long-term trends in the mesosphere, thermosphere and ionosphere

“…Based on radar observations at Andoya (69.5 o N, 16.7 o E) over 1994-2020, Latteck et al (2021) obtained after eliminating effects of solar and geomagnetic activity a polar mesospheric summer echo trend of 3.2%/decade, which might be related to the observed negative trend of mesospheric temperatures in polar latitudes. Mesospheric wind measurements by specular meteor radars and partial reflection radars over northern Germany (~54 o N) and northern Norway (~69 o N) over 2004-2020 using two definitions of summer length provided a positive trend of summer length for one definition and no trend for the other definition; 31 year midlatitude partial reflection radar data indicate break point and non-uniform trend of summer length(Jaen et al, 2022).…”

Ground-based noontime D-region electron density climatology over northern Norway

“…During this period, a very pronounced and consistent feature occurs in the mesosphere, namely the zonal wind reversal (see e.g. Keuer et al, 2007;Hoffmann et al, 2010;Jaen et al, 2022). This rises the question of whether the changes in the S2-tidal amplitude and zonal wind reversal have any impact on the D-region electron density, and if so how?…”

“…Discussion started: 25 April 2023 c Author(s) 2023. CC BY 4.0 License.at middle latitudes, which in the past was mostly used for the study of dynamics (see e.g Hoffmann et al, 2010;Jaen et al, 2022)…”