Hindcasting the January 2009 Arctic Sudden Stratospheric Warming and Its Influence on the Arctic Oscillation with Unified Parameterization of Orographic Drag in NOGAPS. Part I: Extended-Range Stand-Alone Forecast

This study is Part II of the effort to improve the forecasting of sudden stratospheric warming (SSW) events by using a version of the Navy Operational Global Atmospheric Prediction System (NOGAPS) that covers the full stratosphere. In Part I, extended-range (3 week) hindcast experiments (without data assimilation) for the January 2009 Arctic major SSW were performed using NOGAPS with a unified orographic drag parameterization that consists of the schemes employed by Webster et al., as well as Kim and Arakawa and Kim and Doyle. Part I demonstrated that the model with upgraded middle-atmospheric orographic drag physics better forecasts the magnitude and evolution of the SSW and better simulates the trend of the Arctic Oscillation (AO) index. In this study (Part II), a series of 5-day hindcast experiments is performed with cycling data assimilation using the Naval Research Laboratory Atmospheric Variational Data Assimilation System-Accelerated Representer (NAVDAS-AR), a four-dimensional variational data assimilation (4DVAR) system. Further efforts are made to improve the hindcasting of SSW by improving the satellite radiance bias correction process that strongly affects the data assimilation. The innovation (observation minus background) limit is optimally determined to reduce the rejection of useful radiance data. It is found that when the innovation limit is properly set, both the analysis and forecast of the SSW event can be improved, and that the orographic drag helps improve the SSW forecast.

Using NCEP/NCAR reanalysis data, we investigate the statistical characteristics and the long-term variations of major sudden stratospheric warming (SSW) events in the Northern Hemisphere. We find that the strength and duration of major SSW events has increased from 1958 to 2019 and that this is due to the strengthening of the winter planetary wave activity. We find that the frequency of displacement and split SSW events differs between early, middle and late winter. Early and middle winter are dominated by displacement and split SSW events, respectively, but the frequency of the two types of event is almost equal in late winter. This is due to the differences in the relative strength of wavenumber-1 and wavenumnber-2 planetary wave activity in the three winter periods. As a result of the increase in upward planetary wave activity and the decrease in westerly winds around the polar vortex in middle winter, a shift in the timing of SSW events toward middle winter is detected. In addition, we revealed the influence of the downward propagation of different types of SSW event on the surface temperature anomaly. There were surface cold centers in Russia and northern China after the middle split SSW events; by contrast, there were more cold events in North America after the middle split SSW events.

Abstract. Using European Centre for Medium-Range Weather Forecasts fifth-generation (ERA5) and second Modern-Era Retrospective analysis for Research and Applications (MERRA-2) reanalysis and surface meteorological observation data, this study explores the possible impact of sudden stratospheric warming (SSW) events on air quality in the Beijing–Tianjin–Hebei (BTH) region. Major SSW events are divided into polar vortex displacement SSW and polar vortex split SSW. As the duration of split SSW events is longer and the stratospheric signal pulses propagate further downward than displacement SSWs, subseasonal variability of the atmospheric particulates in the BTH is larger during split SSWs. The air particulate concentration is light before the SSW onset due to the enhanced perturbation in the troposphere associated with strengthened planetary waves. The air particulate concentration around the SSW onset dates begins to rise due to weakening of the tropospheric disturbance as the enhanced planetary waves enter the stratosphere. In the decaying period of the SSW, the air particulate concentration decreases as the stratospheric negative northern annular mode (NAM) signal propagates downward. Specifically, in the pre-SSW period of displacement (split) SSW events, a wavenumber-1-like (wavenumber-2-like) anomaly pattern is strengthened. The East Asian winter monsoon intensifies as the East Asian trough is deepened, especially before the split SSW event onset, leading to a cleaning period. Around the SSW onset period as the tropospheric perturbation diminishes and the East Asian winter monsoon weakens, a surge of air particulate concentration is observed. After the SSW onset, due to the downward propagation of the stratospheric negative NAM signal, cold anomalies form in northeastern East Asia, especially for split SSWs, corresponding to a cleaning period in the BHT region. The local meteorological conditions during the SSWs are also discussed.

A quasi 2‐day wave (QTDW) during the austral summer period usually coincides with a sudden stratospheric warming (SSW) event in the winter hemisphere, while the SSW influences on QTDWs are not totally understood. In this work, the hourly Navy Operational Global Atmospheric Prediction System‐Advanced Level Physics High Altitude reanalysis data sets during January/February 2006 are utilized to study the contribution of a major SSW on the anomalous QTDW activities during the same period. Our new findings are generalized as follows: (1) The summer easterly is enhanced during a SSW event due to the interhemispheric coupling, which is clearly indicated by the anomalous cross‐equator circulation from the winter to summer mesosphere. (2) The enhanced summer easterly could sustain critical layers for QTDWs with larger phase speeds (e.g., smaller zonal wave number or shortwave period) and strengthen the summer easterly barotropic/baroclinic instabilities, which are essential for the QTDW amplification through wave‐mean flow interactions. This is why a strong westward QTDW with zonal wave number 2 is identified besides the conventionally dominant wave mode of wave number 3, and their periods are only ~42–45 hr during January 2006. (3) The strong winter planetary waves during SSW periods facilitate the occurrence of the nonlinear interaction between QTDWs and stationary planetary waves, which is strongly suggested by the abnormal temporal variations of wave number 2 and wave number 3 QTDWs. We conclude that the anomalous QTDW behaviors in summer mesosphere during January 2006 are associated with the major SSW event in the winter stratosphere.

 We investigate the statistical characteristics and the long-term variations of major sudden stratospheric warming (SSW) events in the Northern Hemisphere. We find that the strength and duration of major SSW events have increased from 1958 to 2019 and that this is due to the strengthening of the winter planetary wave activity. We find that the frequency of displacement and split SSW events differs between early, middle, and late winter. Early and middle winter are dominated by displacement and split SSW events, respectively, but the frequency of the two types of events is almost equal in late winter. This is due to the differences in the relative strength of wavenumber-1 and wavenumber-2 planetary wave activity in the three winter periods. As a result of the increase in upward planetary wave activity and the decrease in westerly winds around the polar vortex in middle winter, a shift in the timing of SSW events toward middle winter is detected. In addition, we revealed the influence of the downward propagation of different types of SSW events on the surface temperature anomaly. There were surface cold centers in Russia and northern China after the middle split SSW events; by contrast, there were more cold events in North America after the middle split SSW events. 

In June 1990, the assimilation of synthetic tropical cyclone observations into the Navy Operational Global Atmospheric Prediction System (NOGAPS) was initiated at Fleet Numerical Oceanography Center (FNOC). These observations are derived directly from the information contained in the tropical cyclone warnings issued by the Joint Typhoon Warning Center (JTWC) and the National Hurricane Center. This paper describes these synthetic observations, the evolution of their use at FNOC, and the details of their assimilation into NOGAPS. The results of a comprehensive evaluation of the 1991 NOGAPS tropical cyclone forecast performance in the western North Pacific are presented. NOGAPS analysis and forecast position errors were determined for all tropical circulations of tropical storm strength or greater. It was found that, after the assimilation of synthetic observations, the NOGAPS spectral forecast model consistently maintained the tropical circulations as evidenced by detection percentages of 96%, 90% ...

The paper evaluates the meteorological quality and operational utility of the Navy Operational Global Atmospheric Prediction System (NOGAPS) in forecasting tropical cyclones. It is shown that the model can provide useful predictions of motion and formation on a real-time basis in the western North Pacific. The meterological characteristics of the NOGAPS tropical cyclone predictions are evaluated by examining the formation of low-level cyclone systems in the tropics and vortex structure in the NOGAPS analysis and verifying 72-h forecasts. The adjusted NOGAPS track forecasts showed equitable skill to the baseline aid and the dynamical model. NOGAPS successfully predicted unusual equatorward turns for several straight-running cyclones.

Effects of the Northern Hemisphere sudden stratospheric warmings on the Sporadic-E layers in the Brazilian sector

The major sudden stratospheric warming (SSW) events strongly influence the mean structure of the entire atmosphere, from the troposphere to the thermosphere. These events disrupt the compositional and thermal structure of the mesosphere and lower thermosphere (MLT), causing spatiotemporal variations in the concentration of trace species of this region. Currently, the role of dynamical changes during SSW events on radiative cooling in the MLT region is not well understood. An investigation of the SSW‐induced changes in carbon dioxide () radiative cooling in the MLT region is presented by examining the changes in the dynamics and transport of key species, such as and atomic oxygen (O). A composite analysis has been performed to understand these changes during the major SSW events that occurred between 2005 and 2020. The variation of trace species is found to be associated with the change in vertical residual circulation. The results also show that radiative cooling decreases during the mesospheric cooling that occurs during the stratospheric warming over the polar region. During the recovery stage of the SSW event, the radiative cooling enhances in the mesosphere. These variations in radiative cooling are mainly caused by temperature perturbations and oxygen transport in the MLT region. The contribution of temperature change and transport have also been investigated in detail.

Dynamical response of low-latitude middle atmosphere to major sudden stratospheric warming events

Abstract. The impact of major sudden stratospheric warming (SSW) events and early final stratospheric warming (FSW) events on ozone variations in the middle atmosphere in the Arctic is investigated by performing microwave radiometer measurements above Ny-Ålesund, Svalbard (79° N, 12° E), with GROMOS-C (GRound-based Ozone MOnitoring System for Campaigns). The retrieved daily ozone profiles during SSW and FSW events in the stratosphere and lower mesosphere at 20–70 km from microwave observations are cross-compared to MERRA-2 (Modern-Era Retrospective Analysis for Research and Applications, version 2) and MLS (Microwave Limb Sounder). The vertically resolved structures of polar ozone anomalies relative to the climatologies derived from GROMOS-C, MERRA-2, and MLS shed light on the consistent pattern in the evolution of ozone anomalies during both types of events. For SSW events, ozone anomalies are positive at all altitudes within 30 d after onset, followed by negative anomalies descending in the middle stratosphere. However, positive anomalies in the middle and lower stratosphere and negative anomalies in the upper stratosphere at onset are followed by negative anomalies in the middle stratosphere and positive anomalies in the upper stratosphere during FSW events. Here, we compare results by leveraging the ozone continuity equation with meteorological fields from MERRA-2 and directly using MERRA-2 ozone tendency products to quantify the impact of dynamical and chemical processes on ozone anomalies during SSW and FSW events. We document the underlying dynamical and chemical mechanisms that are responsible for the observed ozone anomalies in the entire life cycle of SSW and FSW events. Polar ozone anomalies in the lower and middle stratosphere undergo a rapid and long-lasting increase of more than 1 ppmv close to SSW onset, which is attributed to the dynamical processes of the horizontal eddy effect and vertical advection. The pattern of ozone anomalies for FSW events is associated with the combined effects of dynamical and chemical terms, which reflect the photochemical processes counteracted partially by positive horizontal eddy transport, in particular in the middle stratosphere. In addition, we find that the variability in polar total column ozone (TCO) is associated with horizontal eddy transport and vertical advection of ozone in the lower stratosphere. This study enhances our understanding of the mechanisms that control changes in polar ozone during the life cycle of SSW and FSW events, providing a new aspect of quantitative analysis of dynamical and chemical fields.

Response of equatorial and low latitude mesosphere lower thermospheric dynamics to the northern hemispheric sudden stratospheric warming events

In this paper, sub-ionospheric Very Low Frequency/Low Frequency (VLF/LF) radio signals over the North Atlantic zone and North Pacific zone have been analysed during the major Sudden Stratospheric Warming (SSW) event of January, 2009. Signals from the transmitters like, NAA (24.0 kHz) and NKR (37.5 kHz) received at Germany and signals from the transmitters like, NLK (24.8 kHz) and NPM (21.4 kHz) received at Japan are considered for possible lower ionospheric response to the SSW event. Very clear and significant increase/decrease of average nighttime and daytime VLF/LF amplitudes from the quiet signals have been found associated with the SSW event.

Mid-latitude ionospheric disturbances during the major Sudden Stratospheric Warming event of 2018 observed by sub-ionospheric VLF/LF signals

This paper presents effects of the major sudden stratospheric warming (SSW) event of 2009 on the subionospheric very low frequency/low frequency (VLF/LF) radio signals propagating in the Earth‐ionosphere waveguide. Signal amplitudes from four transmitters received by VLF/LF radio networks of Germany and Japan corresponding to the major SSW event are investigated for possible anomalies and atmospheric influence on the high‐ to middle‐latitude ionosphere. Significant anomalous increase or decrease of nighttime and daytime amplitudes of VLF/LF signals by ∼3–5 dB during the SSW event have been found for all propagation paths associated with stratospheric temperature rise at 10 hPa level. Increase or decrease in VLF/LF amplitudes during daytime and nighttime is actually due to the modification of the lower ionospheric boundary conditions in terms of electron density and electron‐neutral collision frequency profiles and associated modal interference effects between the different propagating waveguide modes during the SSW period. TIMED/SABER mission data are also used to investigate the upper mesospheric conditions over the VLF/LF propagation path during the same time period. We observe a decrease in neutral temperature and an increase in pressure at the height of 75–80 km around the peak time of the event. VLF/LF anomalies are correlated and in phase with the stratospheric temperature and mesospheric pressure variation, while minimum of mesospheric cooling shows a 2–3 day delay with maximum VLF/LF anomalies. Simulations of VLF/LF diurnal variation are performed using the well‐known Long Wave Propagating Capability (LWPC) code within the Earth‐ionosphere waveguide to explain the VLF/LF anomalies qualitatively.