Source of Acquisition NASA Goddard Space Flight Center Chemical Observations of a Polar Vortex Intrusion M. R. Schoeberl, S. R. Kawa, A. R. Douglass, T. J. McGee NASA Goddard Space Flight Center, Greenbelt, Md. E. V. Browell NASA Langley Research Center Hampton, Va. J. Waters, N. Livesey, W. Read, L. Froidevaux, M. L. Santee NASA Jet Propulsion Laboratory, Pasadena, Ca. H. C. Pumphrey School of Geosciences The University of Edinburgh Edinburgh, UK L. R. Lait and L. Twigg SSAI Corp. Greenbelt, Md. Abstract An intrusion of vortex edge air in D the in zrior of the Arctic polar vortex was observed on the January 3 1,2005 flight of the NASA DC-8 aircraft. This intrusion was identified as anomalously high values of ozone by the AROTAL and DIAL lidars. Our analysis shows that this intrusion formed when a blocking feature near Iceland collapsed, allowing edge air to sweep into the vortex interior. Analysis of Aura MLS observations made along the DC-8 flight track also shows the intrusion in both ozone and HNO3. Polar Stratospheric Clouds (PSCs) were observed by the DIAL lidar on the DC-8. The spatial variability of the PSCs can be explained using MLS HNO3 and HzO observations and meteorological analysis temperatures. We also estimate vortex denitrification using the relationship between NzO and HNo3. Reverse domain fill back trajectory calculations are used to focus on the features in the MLS data. The trajectory results improve the agreement between lidar measured ozone and MLS ozone and also improve the agreement between the HNO3 measurements PSC locations. The back trajectory calculations allow us to compute the local denitrification rate and reduction of HCl within the filament. We estimate a denitrification rate of about lO%/day after exposure to below PSC formation temperature. 1. Introduction The Arctic stratospheric polar vortex, unlike the Antarctic, exhibits both inward and outward wave breaking events [Plumb et al., 1994, hereafter P941. Outward wave breaking is the term used to indicate erosion of the stratospheric polar vortex by mid- latitude wave breaking events [Mchtyre and Palmer, 19831 and these events are commonly seen as the vortex erodes during winter [e.g. Schoeberl and Newman, 19961. Inward wave breaking events occur when air moves into the vortex from mid-latitudes and is subsequently entrained and mixed within the vortex. Inside the vortex, the wind shear forms filaments that can be identified by composition that is not characteristic of the polar vortex. In winter, as the polar vortex cools, sulfate aerosols swell and polar stratospheric clouds (PSCs) can form. These swollen sulfate aerosols and PSCs allow for the heterogeneous reaction of CION02 and HC1 to form Clz and HNO3, drawing down both chlorine reservoirs. Clz photolyzes and reacts with ozone to form C10 which begins the catalytic destruction of ozone [Solomon, 19991. There are three predominant types of PSCs in the lower stratosphere: ice which forms at temperatures below -185K, nitric acid trihydrate (NAT) which forms near 195K, and swollen sulfatehitric acid ternary aerosols that form near the same temperature as NAT. It is believed that rapid denitrification of the vortex can occur when ice clouds form [Fahey et al. 19901. This is common in the Antarctic vortex. If temperatures below -195K but above ice formation are sustained, large NAT particles can also form and settle out gravitationally [Fahey et al., 20011, permanently removing HNO3 and halting subsequent NAT formation. Aerosol lidars can be used to distinguish between the liquid aerosols, NAT and ice clouds using information from depolarization of the backscattered lidar beam and the wavelength dependence of the backscattering, which is representative of the size distribution of the aerosols [Browell et al., 1990a; Brooks et al., 20041. The inward breaking events can alter the chemical composition of the vortex by replenishing ozone, nitric acid and the chlorine reservoirs. If inward breaking events occur, the correlations among the trace gases change, making ozone loss and denitrification more difficult to diagnose [Plumb et al., 20001. P94 analyzed the inward breaking events observed during the 1992 AASE I1 aircraft experiment. This experiment took place a few months after the eruption of Mt. Pinatubo. The Pinatubo stratospheric aerosol cloud was not present when the Arctic polar vortex formed and thus the vortex was initially free of volcanic aerosol. During DC-8 flights through the vortex, the DC-8 DIAL lidar measurements showed Pinatubo aerosol intrusions into the vortex Ip941. These intrusions could be traced to an inward breaking event that occurred several days earlier. P94’s analysis also showed that inward breaking events were not uncommon in the NJ3 vortex but were almost never seen in the more stable Southern Hemisphere vortex. P94 discussed the dynamics of the event comparing various transport simulations with the lidar aerosol observations. P94 did not analyze ozone observations or assess the chemical implications of the inward breaking event. Bird et al. [ 19971 analyzed ozone structures observed within the polar vortex using lidar and balloon measurements made near Eureka in the Canadian Arctic. Most of the laminae they observed in ozone were outside the vortex edge and appeared to be generated by the motion of the vortex. They also noted laminae within the vortex. The ozone laminae were correlated with the potential vorticity field and consistent with back- trajectory analysis. Glatthor et al. [2005] used MIPAS data to analyze the chemistry of the unusual Antarctic vortex split of September 2002. They noted a strong correlation between MIPAS N20 and PV. They also noted that the CH4-N20 correlation showed evidence of mixing into the Antarctic vortex during the period prior to the vortex break-up. This evidence appears as increased scatter in the correlation curve. From mid-January to early February 2005, the Polar Aura Validation experiment (PAVE) conducted flights along the Aura sub-satellite track. On Jan. 31,2005, ozone lidars on board a DC-8 flight north of Hudson Bay measured an anomaly in the stratospheric vortex ozone field suggesting the presence of vortex edge air within the polar vortex. This anomaly could be traced to an inward breaking vortex intrusion that occurred on Jan. 29,2005. The Aura Microwave Limb Sounder (MLS) also observed the anomaly. In addition to the ozone filament, the lidars detected regions of Polar Stratospheric Clouds (PSCs) just south of the filament and co-located with the filament. The combined measurements from the lidars, MLS and trajectory analysis provide a detailed chemical picture of the inward breaking event and also explain the spatial distribution of the PSCs. 2.0 The Vortex Intrusion 2.1 Potential vorticity Fig. 1 shows the potential vorticity field at 480K from the Goddard Modeling and Assimilation Office GEOS-4 analysis [Bloom et al.., 2005; Douglass et al., 20031 as well as a 6-day reverse domain fill (RDF) analysis of the PV field [Sutton et al., 1994, Newman and Schoeberl, 19951. RDF is a trajectory technique that provides an assessment of the unmixed transport. An n-day RDF uses n-day back trajectories from a given day. Mixing ratios of the trace gas at the earlier date are then interpolated onto the trajectory end points and copied forward along the trajectory path to describe the unmixed, non-reactive transported field. The analysis field in Fig. la shows the vortex with a weak PV gradient near the east European sector (longitude -45"). The RDF shows relatively low vorticity intruding into the vortex moving counter-clockwise around the pole. The flight track of the DC-8 is shown on the same figure. The northbound segment of the DC-8 flight follows the Aura ascending node orbit track. The flight was tirned so that the Aura overpass occurred near the northern most part of the track. Because of this close coincidence we can assume that the DC-8 and MLS measurements are essentially simultaneous. 2.2 Ozone Figure 2a shows AROTAL ozone observations made during the north-bound segment of the DC-8 flight. This ozone cross section was the same as observed by the DIAL ozone lidar also on the DC-8 (not shown). Because the DC-8 flight was made during the day to match the 1: 30 PM Aura overpass time, the AROTAL instrument was operated primarily as an ozone differential absorption lidar and was not equipped for aerosol detection. Appendix I describes the AROTAL instrument in more detail. The DIAL lidar capabilities are described by Browell et al. [1990a, 1990b, 19931 and Brooks et al. [2004]. The AROTAL measurements are reported in geometric coordinates while MLS L2 data are reported in pressure coordinates. We have converted the AROTAL ozone and DIAL aerosol data (discussed later) to pressure coordinates using the geopotential height from the meteorological analysis. Hereafter we use log-pressure height (7 log,( lOOO/p) km, where p is pressure in hPa) which we refer to as height. The vortex intrusion forms the filament outlined by the 3 ppmv contour line in the AROTAL data and MLS data (Fig. 2a, b). As expected the filament structure is more detailed in the AROTAL observations that are reported every 5 minutes with a vertical resolution of 0.75-3 km. MLS level 2 data are reported about every 1.5" degrees in latitude (this number decreases approaching the pole due to the orbital inclination) and the instrument has 2-3 km vertical resolution. Appendix 11 describes the Aura MLS measurements in more detail, and AII. 1 discusses MLS ozone. Comparing Figures 2a and 2b, the filament observed by AROTAL is clearly evident in the MLS measurements, although the observations do not show the complex vertical and horizontal structure seen by both AROTAL and DIAL because of the lower spatial resolution of the MLS instrument. The filament also appears in MLS HNO3 and N20 measurements shown in Figures 7a and loa, respectively, below. Using RDF calculations we can try and explain some of the structure shown in the lidar data. Figure 2c shows the 6-day RDF from MLS observations. The 3 ppmv contour in RDF can be compared to the same contour in the MLS and AROTAL. Figure 3 shows ozone values at 22 km,t he MLS 4- and 6-day RDFs are able to capture the secondary filament (left arrow in the figure) not seen by MLS on January 3 1 but observed by AROTAL. The 6-day RDF seems to follow the AROTAL ozone measurements with slightly more fidelity than the 4-day RDF especially near the edge of the vortex (58"N). From here on we will be using the 6-day RDF of the various trace gases. Figure 4a shows the MLS ozone mapped to 480K potential temperature surface along with the MLS level 2 points and the DC-8 flight track. The map is created by averaging and linearly interpolating the level 2 data for each MLS level 2 pressure level onto a 2" latitude by 9" longitude grid. Using the GEOS-4 temperature analysis we then interpolate the data to the 480K surface. The 4 PVU contour is also shown (see Fig. 1) as a reference. The map shows that the highest ozone values are near the vortex edge where diabatic descent is the strongest and ozone displaced downward relative to the exterior region as seen in Fig. 2. The filament observed by MLS is near 90"W just north of the region where the outbound and return DC-8 flight tracks cross. The filament is not visible on the map due to the averaging. Using the MLS ozone map from six days earlier, we can construct a high resolution RDF of the ozone field. The RDF very clearly shows the ozone filament seen by MLS and AROTAL in Fig. 2b. The RDF shows that the high ozone filament is air drawn from the vortex edge and is brought around to the DC-8 flight track by the cyclonic circulation. The RDF ozone values are higher outside the vortex than the ozone map because MLS measurements six days earlier are higher. It is clear that inward breaking events that bring high ozone values into the vortex will make ozone loss calculations difficult. 2.3 Dynamics of the filament formation P94 described the dynamics of filament formation for the case they studied. The vortex intrusions they described resulted from a blocking feature over the Northeast Atlantic that generated ridging in the lower stratosphere. In the winter 2004/5. a Northeast Atlantic blocking feature produced inward breaking events on December 29, January 16, January 31 and February 10. The blocking feature became so strong in late February that it caused the vortex to split on February 25 only to immediately rejoin when the block collapsed. By March 14 the vortex split again and never recovered. The dynamics of the filament formation in late January 2005 are very similar to the P94 case. P94 discussed filament formation but did not discuss the role that the variability in the blocking feature plays in the formation of the filament. The sequence of events leading up to the formation of the filament is shown in Figure 5. The sequence begins with the strong blocking feature near zero longitude on January 28th( parts a, b) near Iceland. The map shows that this feature locally distorts the vortex; the vortex edge is displaced toward the pole. Then on January 29&,t he blocking feature collapses (parts c, d). The vortex wind field becomes more zonal and begins to pull the edge material around into the vortex (parts e, f). The edge material then shears out and moves from Iceland to Northern Russia by Jan. 30 and is subsequently stretched out and carried around the pole. By January 3 1 the blocking ridge has reestablished itself. To show that this process can bring edge material to the interior of the vortex, the trajectory of an air parcel is shown on the maps (Fig. 5 a,c,e,g). The parcel (small triangle) begins at the edge of the vortex on the 28tha nd by the 31Sti s well inside the main vortex at the location where the filament was observed. To summarize, it is the sequence of formation and collapse of the blocking ridge that leads to the filament development. The collapse of the blocking feature allows the vortex winds to sweep the edge air into the interior. Over the course of the 2004/2005 winter, temperatures below NAT formation temperature first occur in early December. By very late December, the vortex cools to ice formation temperatures. At 480K, the vortex reaches ice formation temperatures sporadically throughout January and into mid February. The vortex warms up rapidly by