Observation of a Summer Tropopause Fold by Ozonesonde at Changchun, China: Comparison with Reanalysis and Model Simulation

A schematic diagram of global-scale stratosphere-troposphere exchange (STE) was presented by (Holton et al., 1995) and, in the same work, they also described the dynamical mechanism of Brewer-Dobson circulation and the mass flux of the STE that it drives. In extratropical regions, STE is controlled by the synoptic scale weather systems and mesoscale weather systems (e.g., cut-off lows and tropopause folds). Tropopause folds have been identified as a likely major source of STE (Holton et al., 1995; Stohl et al., 2003) outside the tropics. Statistical results show that 50%-70% of STE events are associated with tropopause fold occurrence in the subtropics (Sprenger and Wernli, 2003).

Tropopause folds occur in areas with large vertical shear and strong meridional thermal gradients, and there are two formation mechanisms——one involving the jet stream baroclinic instability zone, and the other being upper-tropospheric frontogenesis. The phenomenon of ozone intrusion from the stratosphere to the troposphere via tropopause folds was first identified by (Reed, 1955). Later, (Danielsen, 1968) proved that stratospheric air in a tropopause fold that occurred over North America was mixed with tropospheric air. (Shapiro, 1980) suggested that turbulent mixing processes are important for STE in the frontal zones of jets associated with tropopause folds. The tropopause fold is the most important and useful process for STE in the midlatitudes, transporting ozone-rich stratospheric air in extratropical regions southward and downward into the free troposphere and boundary layer (Baray et al., 2000; Lelieveld and Dentener, 2000). This ozone-rich air is exchanged with the troposphere through turbulent diffusion and diabatic processes (Lamarque and Hess, 1994), as well as convective erosion and mixing processes during the breakup of stratospheric filaments (Gouget et al., 2000). Whilst some attention has been paid in recent years to tropopause folds associated with cut-off low systems in northeastern Asia (Yang and Lü, 2003; Liu et al., 2013; Chen et al., 2014; Li et al., 2015), very few studies of tropopause folds associated with jets have been conducted over Northeast China, especially in summer.

Ozonesonde data, radar data, aircraft data, and satellite data have all been applied to study STE. Tropopause folds, as the major mechanism of STE, have been successfully sampled by ozonesondes as layers of air with high ozone mixing ratios and low relative humidity in the troposphere (Sørensen and Nielsen, 2001; Baray et al., 2012). Very high frequency (VHF) wind-profile radars have also been used to detect tropopause folds. Some of these can offer long-term series of observations, and so their data can be used to study the climatological features of tropopause folds. (Ravetta et al., 1999) used an ozone lidar, wind field radar and temperature lidar to detect tropopause folds over France. Furthermore, VHF radars have also been used to study folds over Canada by (Hocking et al., 2007) and over the Tibetan Plateau, China, by (Zhang et al., 2010). In addition, aircraft observations have been used to diagnose the finer scale chemical and microphysical structures, and transport and mixing processes, of folds in the extratropical area over North America (Fischer et al., 2000; Beuermann et al., 2002; Pan et al., 2004, 2007).

Alongside observational work, primitive equation models were initially used to simulate tropopause folds. However, because of their coarse resolutions, these models were unable to simulate these folds in detail. The mesoscale structure of tropopause folds also could not be detected. Later, following developments in high-resolution modeling, much research based on mesoscale models was carried out (Bush and Peltier, 1994; Lamarque and Hess, 1994). Thereafter, the highest resolution models became able to simulate the finer scale structures of STE. Key examples include (Lamarque and Hess, 1994), who used a mesoscale model (MM4) to simulate the evolution of a tropopause fold. (Elbern et al., 1998) used the MM5 model to study a tropopause fold and, compared with the European Center for Medium-Range Weather Forecasts (ECMWF) data, found that the model result was good. One-way coupling of a rebuilt version of the Regional Acid Deposition Model (RADM) and MM5 model was applied by (Yang and Lü, 2004) to simulate the STE process due to a tropopause fold. Following these successful modeling studies, we used the Weather Research and Forecasting model with Chemistry (WRF-Chem) model, which allows for very consistent simulations in different spatial resolutions, to simulate the tropopause fold case in the present study.

Although much attention has been paid to tropopause folds that have occurred over North American and European regions, the structure of tropopause folds over northeastern China has been substantially less well studied. In the present reported work, we investigated a tropopause fold that took place in the midlatitudes in association with the East Asian trough, as simulated by a mesoscale chemistry model and Lagrangian trajectory model. In section 2, the observation data and reanalysis data used in the study are briefly introduced, as well as the mesoscale model and trajectory model. Then, in section 3, the ozonesonde observations and meteorological background of the case are described. Section 4 discusses the results of the numerical simulation and, finally, a summary and conclusions are provided in section 5.

Ozone profile data were obtained from ozonesondes that were launched at Changchun Observatory [(43.9^°N, 125.2^°E), 237 m MSL], Northeast China, at 0600 UTC 2-30, June 2013. (Zhang et al., 2014a) evaluated the quality of the ozone data and concluded that they are of a high level of reliability. Changchun Observatory is located in the west of the urban area of the city, 6.5 km from the center. There are no factories or densely populated areas within 2 km. The observation experiments were designed to investigate the ozone structure of the upper troposphere and lower stratosphere in Northeast Asia. The ozonesonde was developed at the Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences (referred to as the "IAP ozonesonde"), and was based on a previous single-cell Global Positioning System Ozonesonde sensor (GPSO3) (Wang et al., 2003; Xuan et al., 2004). The IAP ozonesonde is a kind of double-cell ozonesonde and a type of electrochemical concentration cell. A detailed description of the IAP ozonesonde is provided by (Zhang et al., 2014b). In the same work, they also showed that the average difference of ozone partial pressure between the IAP and ECC ozonesondes is 0.3 mPa in the lower troposphere, close to 0 in the middle troposphere, and less than 1 mPa in the upper troposphere and lower stratosphere. Balloons carrying radiosondes and ozonesondes measure temperature, humidity, wind direction, wind speed, and ozone partial pressure profiles.

Total column ozone data were obtained from the ozone monitoring instrument (OMI), which is a nadir-viewing near-ultraviolet/visible charge-coupled spectrometer device aboard National Aeronautics and Space Administration (NASA) Earth Observing System's Aura satellite. In the present study, we used the OMI Level 3e TO3 0.25^°× 0.25^° data. The algorithms originated from the Total Ozone Mapping Spectrometer (TOMS) V8 method. The TOMS V8 algorithm retrieves vertical column ozone data essentially using 317.5 and 331.2 nm wavelengths (Balis et al., 2007). The OMI's additional hyperspectral measurements provide better estimates and corrections of the factors that induce uncertainty in ozone retrieval (e.g., cloud and aerosol, sea-glint effects, profile shape sensitivity, SO₂, and other trace gas contamination).

ECMWF Interim Reanalysis (ERA-Interim) (Dee et al., 2011) data with a horizontal resolution of 0.75^°× 0.75^° and a temporal resolution of 6 h (0000, 0600, 1200, 1800 UTC) and 37 vertical pressure levels spanning 1000-1 hPa were used to examine the synoptic situation of the tropopause fold event, and as a dynamical field to drive the trajectory model. Note that the vertical resolution of the data is typically between 0.5-1.0 km in the upper troposphere and lower stratosphere (UTLS). Grid data were also used to estimate the height of the thermal tropopause based on the standard (World Meteorological Organization, 1957) lapse-rate criterion. The tropopause is defined as "the lowest level at which the lapse rate decreases to 2^°C km^-1 or less, provided that the average lapse rate between this level and all higher levels within 2 km does not exceed 2^°C km^-1. If above the first tropopause the average lapse rate between any level and all higher levels within 1 km exceeds 3^°C km^-1, then a second tropopause is defined by the same criterion". The WMO criterion of the second tropopause was modified to 2 K km^-1 (rather than 3 K km^-1), following the empirical finding that this produces results that are consistent with radiosonde data (Randel et al., 2007).

For further analysis of the structure of the tropopause fold observed by the ozonesonde, the WRF-Chem model, version 3.4.1 (Grell et al., 2005), was used in this study. Documentation providing details of the model can be down- loaded from http://www2.mmm.ucar.edu/wrf/users/pub-doc.html. Version 2 of the Regional Acid Deposition Model (RADM2) (Stockwell et al., 1990) gas-phase chemical mechanism was used, but the anthropogenic emissions and biogenic emissions were not employed in our numerical experiment. Other parameterizations used in this simulation included the microphysics scheme of (Lin et al., 1983), the new cumulus parameterization scheme of (Grell and Dévényi, 2002), the Yonsei University planetary boundary layer scheme, the Goddard shortwave radiative transfer scheme, the Rapid Radiative Transfer Model longwave radiation scheme, and the unified Noah land-surface model.

The simulations presented in this study were performed with the non-hydrostatic model option. Thirty-seven vertical levels were configured, extending from the Earth's surface to 50 hPa, with a relatively high vertical resolution (400 m) within the UTLS region. The domain had a horizontal resolution of 27 km with 140× 130 points. The initial and lateral boundary meteorological conditions for the model were from the National Centers for Environmental Prediction global analyses data. Final Global Data Assimilation System data, which comprise four-times-daily datasets with a horizontal resolution of 1^° and 26 levels in the vertical direction, spanning from 1000 hPa to 10 hPa (see http://rda.ucar.edu/datasets/ds083.2/data/), were also used. The initial and boundary conditions for the chemistry were from the output of the Model for Ozone and Related Chemical Tracers, version 4 (Emmons et al., 2010).

The TRAJ3D trajectory model was used to investigate the motion of air parcels and the kinematic structure of the air flow by calculating the parcels' three-dimensional trajectories. The parcel trajectory was the solution to the equation \begin{equation} \label{eq1} \dfrac{d{x}}{dt}={v}({x},t), \end{equation} where v(x,t) is the velocity as a function of position and time. If it is known, the new position of the air parcel can be determined by solving Eq. (2) with the air parcel initial position x₀ at time t₀. Equation (2) can be rewritten as x(t)=x(x₀,t). The velocity field v was from the ERA-Interim data. Wind fields were computed by linear interpolation in space and time. A detailed introduction to the model was provided by (Bowman, 1993) and (Bowman and Carrie, 2002).

The initial locations for the air parcels in a cross section along 43.5^°N were restricted to the area between 112^°E and 129^°E and between the lapse rate tropopause and 1 PVU surface. The horizontal and vertical spacing between individual trajectories was taken as 0.75^° and 25 hPa, respectively. The 3-day backward trajectories for air parcels were initialized at 0600 UTC 12 June 2013 (the simulation period covered 0600 UTC 9 June through 0600 UTC 12 June).

Figure 1.  Observed vertical profiles of ozone mixing ratio (ppbv, blue), temperature (^°C, red), and relative humidity (%, black) at 0600 UTC on (a) 11 June, (b) 12 June, and (c) 13 June. Ozone profiles observed during 2-30 June are shown in gray. The dashed red line marks the lapse-rate tropopause.

Figure 2.  Three typical atmospheric profiles of static stability, or Brunt-Väisälä frequency N, at 0600 UTC on (a) 11 June, (b) 12 June, and (c) 13 June. The dashed line marks the lapse-rate tropopause.

High-resolution profiles of temperature and relative humidity during 11-13 June and 29-day profiles of ozone mixing ratio from the soundings are shown in Fig. 1. The 3-day profiles suggest that the tropopause height, relative humidity, and ozone mixing ratio changed remarkably. The height of the first thermal tropopause dropped from 16.5 km to 15.2 km, and then to 10 km. The ozone concentration in the middle and lower troposphere at 0600 UTC 11 June (Fig. 1a) was lower than on other days (Figs. 1b and c). On the contrary, the relative humidity was higher. The reason may be that free tropospheric ozone can be depressed by a warm conveyor belt that flushes the troposphere with a low ozone concentration of clean air at lower latitude. One day later, an anomalous dry layer with low humidity (less than 1%) and high ozone concentration (peaks reaching 180 ppbv) was observed at 6 km in the troposphere (Fig. 1b). The width of the ozone peaked between the first thermal tropopause (near 10 km) and 16 km at 0600 UTC 13 June (Fig. 1c).

The Brunt-Väisälä frequency, or static stability of the atmosphere, for three days (during 11-13 June) is shown in Fig. 2. The static stability N is the solution to the equation $$ N^2=\dfrac{{\rm g}}{\theta}\dfrac{\partial\theta}{\partial z}=\dfrac{{\rm g}}{T}\left(\dfrac{\partial T}{\partial z}+\Gamma_{\rm d}\right), $$ where g is the gravitational constant, θ is potential temperature, T is temperature, z is altitude, and Γ _d=9.8^°C km^-1 is the dry adiabatic lapse rate. As can be seen, there was a broad transition in static stability (Fig. 2a) from the troposphere to the stratosphere, and a sharp transition (Fig. 2c). However, the static stability was unstable from the surface to the lower stratosphere (Fig. 2b). The most unstable N (5-8 km) coincided with peak ozone, according to the measurements.

Figure 3.  Synoptic conditions during 11-13 June: (a-c) the lapse-rate tropopause height (hPa, color scale) and 300 hPa geopotential height (contours every 4 dagpm); (d-f) the 300 hPa horizontal wind field (m s^-1, color scale). The stars mark the location of ozonesondes.

Figure 4.  The FY-2C satellite image of cloud coverage (left) and total column ozone (DU, color scale) as measured by the OMI with potential vorticity (contours, PVU, 1 PVU = 10^-6 m² K kg^-1 s^-1) on the 330 K isentropic surface at 0600 UTC 12 June 2013. The star marks the location of the ozonesonde.

Figure 3 displays the daily evolution of the geopotential height, pressure height of the lapse rate tropopause, and wind speed at the 300 hPa level, together with the ozonesonde launched position (pentagon). On 9 June, the upper-level trough that was centered over Lake Baikal showed a southeastward extension (figure not shown). During the following days, the trough moved slowly. On 11 June, the trough was located near the border between Mongolia and Northeast China, then deepened, and a closed isoline in the trough was found. The thermal tropopause, as defined by the WMO definition, was indistinct near the cyclonic side of the jet stream, and the height of the thermal tropopause on the cyclonic side of the jet was lower than on its anticyclonic side (Fig. 3a). The core of the jet was located in the Gulf of Bohai (Fig. 3d). One day later, the low pressure system situated on western side of Northeast China was shallow, but had a strong jet streak on its eastern side (Fig. 3b). This southwesterly upper-level jet streak extended from East China into Northeast China with wind speed of more than 60 m s^-1, based on the 300 hPa wind field (Fig. 3e). On 13 June, the contours behind the trough became straight. The trough weakened and moved across Changchun Station. Meanwhile, the height gradient of the thermal tropopause decreased (Fig. 3c). The jet moved eastward, followed by a decreasing of the curvature of the jet as the trough weakened (Fig. 3f).

Figure 5.  Temperature (^°C, color scale) and geopotential height (gpm, contours) at the (a, b) 300 hPa level and (c, d) 500 hPa level from the (a, c) ERA-Interim data and (b, d) WRF-Chem model at 0600 UTC 12 June 2013. The stars mark the location of the ozonesonde.

Figure 6.  Zonal-vertical cross sections (pressure-longitude) of (a, c, e) water vapor mixing ratio and (b, d, f) ozone mixing ratio centered at the latitude of Changchun (43.5^°N) at (a, b) 1800 UTC 12 June, (c, d) 0600 UTC 13 June, and (e, f) 1800 UTC 13 June. The meridional wind (m s^-1, black contours), 310 K, 340 K, and 370 K isentropes (dashed black lines), and PV (1, 2, 4, 6, and 8 PVU, brown line) are shown. The primary tropopause (white dots) and locations of the balloons (dashed white lines) are also shown.

The left panel of Fig. 4 illustrates the clouds in a true color image from the FY-2C (Feng-Yun) data at 0600 UTC 12 June 2013. The upper-level cold front clouds crossed Northeast China and North China. The boundary of the clouds near the cold area was smooth and the side near the warm side was obscure. The cloud belt was parallel to the core of the jet and was situated on the anticyclonic side of the jet axis. The head of clouds was cyclonic. Behind the cold front, the weather was clear. However, there was thick cloud ahead of the cold front. Total column ozone (TCO) measured by the OMI, together with the potential vorticity on the 330 K isentropic surface, are displayed in the right panel of Fig. 4. The results show a high degree of correlation between the axe-shaped area of TCO increase over Northeast China on 12 June and the high PV on the isentropic surface, indicating there was an intrusion of stratospheric air into the troposphere. Meanwhile, the distribution of the TCO increase was also found to correspond with the clear-sky regions in the FY-2C cloud image. The intrusion is further discussed in section 4.3.

Figure 5 displays the geopotential height and temperature field on 300 hPa and 500 hPa surface outputs from the WRF model and ERA-Interim reanalysis at 0600 UTC 12 June. The intensity and position of the East Asian trough from the model agreed well with the ERA-Interim data, except the closed contour inside the trough on the 500 hPa surface (Figs. 5c and d). The dense isotherm at the eastern flank of the trough was also simulated. The fact that the simulation results agreed well with the reanalysis results indicates that the WRF-Chem model is capable of performing precise analyses of tropopause folds. Therefore, to further analyze the structure of the tropopause fold observed by the ozonesonde in this study, the vertical cross sections of ozone volume mixing ratios across the station, as determined from the model results, are presented and discussed in section 4.2.

Figure 7.  Time series of ozone mixing ratio (ppbv, color scale), temperature (^°C, contours), and primary tropopause from the WRF-Chem model over Changchun Station during 10-13 June 2013 (a). Dashed white lines mark the locations of the balloons. Comparisons of WRF-Chem ozone profiles interpolated to the observation station (blue) with ozonesonde profiles (red) at 0600 UTC on (b) 11 June, (c) 12 June, and (d) 13 June.

The structure and evolution of the fold in the vertical cross sections along the latitude 43.5^°N can be seen in Fig. 6. Vertical cross sections of water vapor mixing ratio (Figs. 6a, c, e, ERA-Interim) and ozone mixing ratio (Figs. 6b, d, f, WRF-Chem) are shown. The dynamical tropopause is represented by a 2 PVU isoline and the position of the ozonesonde observation is marked with a white vertical dashed line. It is clear that the core of the jet stream along the cross section from the reanalysis agreed well with the output from the model. The core of jet the stream lay near the 250 hPa height. The fold was positioned on the west flank of a large jet stream, which was more visible in the WRF model data than in the ERA-Interim data. The water vapor mixing ratio with low values (less than 180 ppbv) intruded down to 500 hPa from the cyclonic side of the subtropical jet over Changchun, suggesting dry intrusion of stratospheric air through the tropopause fold (Fig. 6a). This phenomenon was also proven by the fact that, in the model simulation, the ozone tongue coinciding with the high PV air intruded into the middle troposphere beneath the jet stream (upper-level frontogenesis zone) (Fig. 6b). The WRF-Chem model, with its high resolution, revealed the fold structure in detail, supporting the interpretation of the structure seen by the ozonesonde (Fig. 1b). During the next 12 hours, as the jet moved eastward, the ozone volume mixing ratio in the tropopause fold zone decreased (Fig. 6d). However, a high ozone concentration center departing from the stratosphere can be seen in this vertical cross section. This feature suggests that the tropopause fold was bringing stratospheric ozone-rich air into the middle troposphere over Changchun. According to the ERA-Interim data, the fold passed over the observation station at 0600 UTC 13 June 2013 (Fig. 6e). In contrast, according to the model, the location of the fold was farther to the west (Fig. 6f).

Figure 8.  Initial location of the air parcels initialized along a cross section trough the jet at 0600 UTC 12 June. The meridional wind (m s^-1, black contours), the isentropic contours (K, dashed blue lines), and PV (1, 2, 4, 6, and 8 PVU, brown line) are shown.

Figure 9.  Three-day backward trajectories for the points in Fig. 8.

For a more detailed structure of the tropopause fold, the time series of the ozone mixing ratios from 10-13 June at Changchun Station are shown in Fig. 7. The results show that there was a high ozone concentration center in the middle troposphere between 12 and 13 June (Fig. 7a). The intrusion subsided to at least 700 hPa (∼3 km). The stratospheric air observed in the troposphere retained the chemical signature for longer than the dynamic signature. This may be due to the slow chemical transformation of ozone in the free troposphere above the surface layer and, owing to relatively weak turbulent mixing with tropospheric air, the intruded air mass can then be identified, e.g., by ozonesonde (Sørensen and Nielsen, 2001). This stratospheric intrusion was authenticated by the mesoscale WRF-Chem model. A comparison of the observed and modeled vertical profiles of ozone mixing ratios is provided in Figs. 7b-d. At 0600 UTC 11 June, the ozone profiles and the intrusion layer from the model were nearly the same as those observed (Fig. 7b). With time, the model result was less accurate, especially at 0600 UTC 13 June (Fig. 7d). At the time of the final observation, the high ozone layer situated in the lower stratosphere was not simulated. This may have been the result of the steep temperature gradient in the upper-level front.

Tropopause folds beneath the subtropical jet over the eastern Mediterranean and central Europe were reported by (Tyrlis et al., 2013) and (Trickl et al., 2011), respectively. (Li et al., 2015) also pointed out that the jet stream associated with the cut-off low system brought about increased ozone mixing ratios over Northeast China in early summer. This further supports our findings that stratospheric influenced air can penetrate into the middle troposphere because of dynamics associated with the subtropical jet over Northeast China in early summer.

TRAJ3D was used to estimate the origins of the ozone-rich air parcels observed by ozonesonde at Changchun Station at 0600 UTC 12 June. The calculations of the TRAJ3D model, based on (Bowman, 1993), were driven by ERA-Interim wind fields. Trajectories were calculated kinematically at pressure levels using the wind field from the 6-hourly ERA-Interim reanalysis data. Figure 8 shows the initial location of air parcels in a vertical cross section through the observation station. The potential temperature, PV, and tropopause height are also shown.

Parcels were classified and marked with different colors according to their upstream regions of origin, which can be seen clearly in Fig. 9. The results show that most air parcels (marked blue) came from central Siberia, with low water vapor and high ozone mixing ratios. Furthermore, the high ozone region (blue) in Fig. 8 is consistent with the result of the model shown in Fig. 6b. In contrast, air parcels (marked green) with low ozone volume mixing ratios originated from the westerly wind belt in the mid-latitudes. The air parcels (marked red) with low ozone concentration came from the eastern coastal region of China. This differs to the findings of (Weigel et al., 2012), who studied a stratospheric intrusion associated with the upper-level jet over the Mediterranean.

Their study showed that air parcels with high ozone concentration (>200 ppbv) originated from the westerly wind belt along the subtropical jet. Furthermore, air parcels with low ozone concentration came from the Asian monsoon anticyclone via easterly wind transport.

A remarkable tropopause fold that occurred over Northeast China was observed by the balloon-borne IAP ozonesonde in early summer 2013. The vertical profiles of temperature, relative humidity, and ozone mixing ratio were measured at 0600 UTC 11-13 June 2013. The observations were compared with the behavior of the temperature lapse-rate tropopause as seen in ERA-Interim meteorological reanalysis data and high-resolution WRF-Chem model results.

The ERA-Interim reanalysis data and OMI ozone data show that the tropopause fold was associated with the southwesterly jet stream on the eastern flank of the trough, producing an intrusion of stratospheric air. The mesoscale WRF-Chem model was used to reproduce the evolution of stratospheric constituents, e.g., ozone. The results clearly indicated an increase in the downward transport of ozone-enriched stratospheric air, which caused the total column ozone increase seen in the OMI data. In conclusion, the high-resolution WRF-Chem model is capable of describing events such as deep tropopause folds and, together with relatively simple observational tools, e.g., ozonesondes, a detailed picture of such events can be obtained.

We also used the TRAJ3D model, driven by ERA-Interim data, to trace the origins of the intruded air parcels. Air parcels corresponding to observed ozone peaks at Changchun Station passed through the tropopause fold, thus bringing down ozone-rich air from the lower stratospheric at high latitudes (central Siberia), southward and downward into the middle troposphere over Changchun. It seems that the trajectory model is a useful tool for further studies on the origin of measured ozone peaks in the free tropopause, tracing them back, for example, to intrusions through tropopause folds.