This study investigates the response of the semidiurnal tide (SDT) to the 2013 major sudden stratospheric warming (SSW) event using meteor radar wind observations and mechanistic tidal model ...simulations. In the model, the background atmosphere is constrained to meteorological fields from the Navy Global Environmental Model—High Altitude analysis system. The solar (thermal) and lunar (gravitational) SDT components are forced by incorporating hourly temperature tendency fields from the ERA5 forecast model, and by specifying the M2 and N2 lunar gravitational potentials, respectively. The simulated SDT response is compared against meteor wind observations from the CMOR (43.3°N, 80.8°W), Collm (51.3°N, 13.0°E), and Kiruna (67.5°N, 20.1°E) radars, showing close agreement with the observed amplitude and phase variability. Numerical experiments investigate the individual roles of the solar and lunar SDT components in shaping the net SDT response. Further experiments isolate the impact of changing propagation conditions through the zonal mean background atmosphere, non‐linear wave‐wave interactions, and the SSW‐induced stratospheric ozone redistribution. Results indicate that between 80 and 97 km altitude in the northern hemisphere mid‐to‐high latitudes the net SDT response is driven by the solar SDT component, which itself is shaped by changing propagation conditions through the zonal mean background atmosphere and by non‐linear wave‐wave interactions. In addition, it is demonstrated that as a result of the rapidly varying solar SDT during the SSW the contribution of the lunar SDT to the total measured tidal field can be significantly overestimated.
Key Points
Simulations of the semidiurnal tide (SDT) are compared against meteor wind observations in the mid‐to‐high latitude northern hemisphere during the 2013 sudden stratospheric warming
Individual lunar and solar SDT simulations find that the net tidal response is largely driven by the solar component
The response of the solar SDT is driven by changing zonal mean propagation conditions and by non‐linear interactions with planetary waves
This study uses hourly meteor wind measurements from a longitudinal array of 10 high-latitude SuperDARN high-frequency (HF) radars to isolate the migrating diurnal, semidiurnal, and terdiurnal tides ...at mesosphere–lower-thermosphere (MLT) altitudes. The planetary-scale array of radars covers 180∘ of longitude, with 8 out of 10 radars being in near-continuous operation since the year 2000. Time series spanning 16 years of tidal amplitudes and phases in both zonal and meridional wind are presented, along with their respective annual climatologies. The method to isolate the migrating tides from SuperDARN meteor winds is validated using 2 years of winds from a high-altitude meteorological analysis system. The validation steps demonstrate that, given the geographical spread of the radar stations, the derived tidal modes are most closely representative of the migrating tides at 60∘ N. Some of the main characteristics of the observed migrating tides are that the semidiurnal tide shows sharp phase jumps around the equinoxes and peak amplitudes during early fall and that the terdiurnal tide shows a pronounced secondary amplitude peak around day of year (DOY) 265. In addition, the diurnal tide is found to show a bi-modal circular polarization phase relation between summer and winter.
The present work describes the implementation of the state of the art Cloud-J v7.3 photolysis rate calculation code in the EMEP MSC-W chemistry-transport model. Cloud-J calculates photolysis rates ...and accounts for cloud and aerosol optical properties at model run time, replacing the old system based on tabulated values. The performance of Cloud-J is evaluated against aerial photolysis rate observations made over the Pacific Ocean and against surface observations from three measurement sites in Europe. Numerical experiments are performed to investigate the sensitivity of the calculated photolysis rates to the spatial and temporal model resolution, input meteorology model, simulated ozone column, and cloud effect parameterization. These experiments indicate that the calculated photolysis rates are most sensitive to the choice of input meteorology model and cloud effect parameterization while also showing that surface ozone photolysis rates can vary by up to 20 % due to daily variations in total ozone column. Further analysis investigates the impact of Cloud-J on the oxidizing capacity of the troposphere, aerosol–photolysis interactions, and surface air quality predictions. Results find that the annual mean mass-weighted tropospheric hydroxyl concentration is increased by 26 %, while the photolytic impact of aerosols is mostly limited to large tropical biomass-burning regions. Overall, Cloud-J represents a major improvement over the tabulated system, leading to improved model performance for predicting carbon monoxide and daily maximum ozone surface concentrations.
Simulations of the solar thermal migrating semidiurnal (SW2) tide in the mesosphere‐lower‐thermosphere (MLT) are compared against meteor wind observations from a longitudinal chain of high‐latitude ...Super Dual Auroral Radar Network radars. The simulations span the year 2015 and are performed using a mechanistic primitive equation model. The model employs a whole‐atmosphere tide forcing based on temperature tendency fields from the Specified Dynamics Whole Atmosphere Community Climate Model with Thermosphere and Ionosphere Extension, and a background atmospheric specification based on zonal wind and temperature data from the Navy Global Environmental Model‐High Altitude meteorological analysis system. Results show that the model accurately reproduces the observed seasonal variability of the SW2 tide in both the amplitude and phase. Numerical experiments are performed to investigate how the tidal forcing, dissipation terms, and seasonal variations in the background atmosphere shape the seasonal variations of the simulated SW2 tide. Notable results are that the background atmosphere most strongly impacts the SW2 tide forced in the troposphere, and that the specification of a narrow surface friction profile enhances the net SW2 amplitude in the MLT between April and October. Eddy diffusion is found to damp the simulated tide predominantly around summer solstice and in December.
Key Points
Simulations of the migrating semidiurnal (SW2) tide are validated against observations from a high‐latitude array of Super Dual Auroral Radar Network meteor radars
Numerical experiments investigate the impact of the background atmosphere, tidal dissipation, and tidal forcing on the simulation results
The simulated SW2 is largely shaped by the background atmosphere, while being sensitive to eddy diffusion and surface friction
The energetic particle precipitation (EPP) indirect effect (IE) refers to the downward transport of reactive odd nitrogen (NOx = NO + NO2) produced by EPP (EPP‐NOx) from the polar winter mesosphere ...and lower thermosphere to the stratosphere where it can destroy ozone. Previous studies of the EPP IE examined NOx descent averaged over the polar region, but the work presented here considers longitudinal variations. We report that the January 2009 split Arctic vortex in the stratosphere left an imprint on the distribution of NO near the mesopause, and that the magnitude of EPP‐NOx descent in the upper mesosphere depends strongly on the planetary wave (PW) phase. We focus on an 11‐day case study in late January immediately following the 2009 sudden stratospheric warming during which regional‐scale Lagrangian coherent structures (LCSs) formed atop the strengthening mesospheric vortex. The LCSs emerged over the north Atlantic in the vicinity of the trough of a 10‐day westward traveling planetary wave. Over the next week, the LCSs acted to confine NO‐rich air to polar latitudes, effectively prolonging its lifetime as it descended into the top of the polar vortex. Both a whole atmosphere data assimilation model and satellite observations show that the PW trough remained coincident in space and time with the NO‐rich air as both migrated westward over the Canadian Arctic. Estimates of descent rates indicate five times stronger descent inside the PW trough compared to other longitudes. This case serves to set the stage for future climatological analysis of NO transport via LCSs.
Plain Language Summary
Energetic particles from the sun and the magnetosphere impinge upon Earth's upper atmosphere and create reactive odd nitrogen (NOx) in the mesosphere and lower thermosphere. Descent in the winter polar vortex effectively transports this NOx down to the stratosphere where it can destroy ozone. State‐of‐the‐art models currently underestimate this vertical transport by a factor of 4. Previous studies have examined the NOx descent averaged over the entire polar region, but this study considers longitudinal variations. We examine a case study during late January 2009 and find a closed circulation coincident with the trough of a planetary wave over the north Atlantic at 90 km with shear zones inhibiting horizontal mixing to the north, east, and south. This circulation (1) contains elevated NOx, (2) is associated with five times stronger descent compared to other longitudes, and (3) is the natural upward continuation of the westward tilting polar vortex in the stratosphere and mesosphere. Thus, this meteorological feature near the mesopause provides a transport pathway for air to enter the top of the polar vortex. This is the first work to illustrate the zonally asymmetric nature of NOx descent in the polar winter upper mesosphere and couple it to the vortex below.
Key Points
First demonstration of the impact of the split Arctic vortex on the geographic distribution of nitric oxide at the winter mesopause
First evidence that a Lagrangian coherent structure inhibits horizontal transport of nitric oxide at the polar winter mesopause
Descent of nitric oxide is five times stronger between 80 and 90 km in a westward traveling planetary wave trough compared to the ridge
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