Distributions of ozone in the region of the subtropical jet: An analysis of in situ aircraft measurements

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109,, doi: /2003jd004143, 2004 Distributions of ozone in the region of the subtropical jet: An analysis of in situ aircraft measurements Eric A. Ray, 1,2 Karen H. Rosenlof, 1 Erik Richard, 1,2 David Parrish, 1 and Roger Jakoubek 1 Received 8 September 2003; revised 15 December 2003; accepted 3 March 2004; published 23 April [1] In situ measurements of ozone and meteorological fields from the NASA WB-57F and National Oceanic and Atmospheric Administration (NOAA) Gulfstream IV airborne platforms in the region of the subtropical jet are investigated. The high resolution and precision of the aircraft measurements allow the ozone distribution to be examined on a wide range of spatial scales. Probability distribution functions (pdfs) of ozone, temperature, and wind speed are calculated in a coordinate system centered on the maximum jet wind speeds. There are significant differences in the pdfs near the jet maximum in winter versus spring seasons. The largest gradients in the ozone pdfs are seen at the location of the jet maximum in winter, whereas during spring the largest gradients are most often poleward of the jet by up to several thousand kilometers. These seasonal differences do not appear to be directly related to the strength of the jet on the basis of the limited geographical sampling of the airborne platforms. INDEX TERMS: 3362 Meteorology and Atmospheric Dynamics: Stratosphere/troposphere interactions; 0340 Atmospheric Composition and Structure: Middle atmosphere composition and chemistry; 0341 Atmospheric Composition and Structure: Middle atmosphere constituent transport and chemistry (3334); 0368 Atmospheric Composition and Structure: Troposphere constituent transport and chemistry; KEYWORDS: ozone, subtropical jet, stratosphere-troposphere exchange Citation: Ray, E. A., K. H. Rosenlof, E. Richard, D. Parrish, and R. Jakoubek (2004), Distributions of ozone in the region of the subtropical jet: An analysis of in situ aircraft measurements, J. Geophys. Res., 109,, doi: /2003jd Introduction [2] Exchange of air between the stratosphere and troposphere in the region of the subtropical jet has important implications for the chemical, radiative and dynamical balance of this part of the atmosphere. The subtropical jet typically divides the tropical upper troposphere and the extratropical lower stratosphere at altitudes from 10 to 16 km and within the latitude range from 20 to 40. The region of the jet is often referred to as the tropopause break since the tropopause height changes from near 16 km equatorward of the jet to km poleward of the jet. Isentropes of potential temperature cross the jet, allowing the possibility of quasi-horizontal, adiabatic exchange of air between the stratosphere and troposphere. However, this exchange of air is often inhibited near the location of the maximum jet wind speeds where large horizontal gradients of potential vorticity exist on the cyclonic shear side of the jet. Wave activity near the jet, typically from Rossby waves, has been shown to result in stirring and quasi-horizontal exchange of air [e.g., Vaughan and Timmis, 1998; Postel and Hitchman, 1999; Reid et al., 2000; Horinouchi et al., 2000; Scott and Cammas, 2002]. The exchange of air as a result of wave activity is often in the form of filaments, 1 NOAA Aeronomy Laboratory, Boulder, Colorado, USA. 2 Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA. Copyright 2004 by the American Geophysical Union /04/2003JD which can be stretched into very fine horizontal scales [e.g., Appenzeller et al., 1996a]. These fine scales are not resolved by general circulation models (GCMs), which can impact the model estimates of mass flux between the stratosphere and troposphere [e.g., Kentarchos et al., 2000]. It is also likely that chemistry in the unmixed filaments is important in order to understand the chemistry of the upper troposphere and lower stratosphere (UT/LS) region [e.g., Esler et al., 2001]. [3] In situ measurements from airborne platforms provide the only high-resolution, horizontal sampling of the UT/LS. Characteristics of the variability of a trace gas with a local lifetime longer than several months, such as ozone, can reveal features of the transport and dynamics in a region. High-resolution aircraft measurements provide information on long-lived tracer variability over a range of spatial scales from less than a kilometer to hundreds of kilometers. Measurements from airborne platforms have often been used in case studies to analyze model output or compare to satellite data [e.g., Folkins and Appenzeller, 1996; Beuermann et al., 2002]. Total ozone has also been used to investigate transport regimes in the lower stratosphere [e.g., Hudson et al., 2003]. However, variability of total ozone includes variability in the tropopause height and a large depth of the stratosphere, which makes total ozone difficult to compare to in situ aircraft measurements in the UT/LS. This study will use probability distribution functions (pdfs) to summarize data from a number of flights, but also minimize averaging of the data. Probability distributions have been shown to be useful in summarizing a 1of12

2 Figure 1. Distributions of flight locations in (a) geographical space and (b) potential temperature space. The contours in Figure 1a are 1e2, 1e3 and 1e4, which represent the number of 1 Hz measurements at each location. The bins used in Figure 1a were 4 latitude 4 longitude, and those in Figure 1b were 4K. large amount of measurements and revealing predominant features as well as outliers in the data [Sparling, 2000; Hu and Pierrehumbert, 2001]. [4] The goal of this research was to examine ozone measurements in the vicinity of the subtropical jet from two airborne platforms, the NASA WB-57F and the National Oceanic and Atmospheric Administration (NOAA) Gulfstream-IV (G-IV), in order to gain insight into the horizontal variability in this region. This variability can be used to help interpret the spatial extent, location and longlived tracer characteristics of the tropopause at the subtropical jet. Thus one of the purposes of this study is to compute a seasonal climatology of ozone and its variability near the subtropical jet. The measured ozone variability can also be used to compare to tracer variability in model output, in particular those models with Lagrangian transport schemes and mixing parameterizations, such as CLaMS [McKenna et al., 2002]. Additional description of the distribution of ozone in the UT/LS is also important in and of itself because of the lack of satellite measurements in this region. More accurate representations of ozone gradients near the tropopause in chemical transport models can have an impact on the chemical and radiative balance of tropospheric climate simulations [McLinden et al., 2000]. In particular, the ozone gradients near the subtropical jet are interesting since they have implications for stirring and mixing of air across the tropopause. The measurements from these aircraft are unique in both the predominant altitude range of km, the large number of winter flights and the number of flights over the Pacific Ocean (Figure 1a). By contrast, most of the MOZAIC airborne program data has been taken at altitudes below 12 km and over North America, the Atlantic Ocean and Europe [e.g., Cammas et al., 1998; Morgenstern and Carver, 2001]. 2. Data [5] In this study we used in situ ozone, temperature, pressure and wind speed measurements from two different airborne platforms and several different missions. NASA WB-57F measurements come from the WAM, ACCENT I and II, and CRYSTAL-FACE missions, which took place from 1998 to 2002, while NOAA G-IV measurements come from the Winter Storms missions [Szunyogh et al., 2002]. All of the individual flight dates are listed in Appendix A, and a map of the distribution of flight locations is shown in Figure 1a. The contours in Figure 1a represent the number of measurements at each location and begin at 100, with each contour increasing by an order of magnitude. The flights were concentrated primarily over North America and the northeast Pacific Ocean. Note that all of the winter flights were obtained over the Pacific Ocean by the G-IV. All of the measurements were taken at 1 Hz or higher resolution and were analyzed here at 1 Hz. Typical plane speeds were 200 m/s and thus the spatial resolution of the measurements is roughly 200 m. The onesigma precision at standard temperature and pressure of the WB-57F ozone is approximately 0.6 ppbv and the G-IV ozone is approximately 1 ppbv [Proffitt and McLaughlin, 1983]. [6] National Centers for Environmental Prediction (NCEP) Aviation (1 1 resolution) global analysis output is used to place the measurements in a meteorological frame of reference relative to the subtropical jet core. Measured wind speed relative to the NCEP jet maximum is used as the horizontal coordinate, as opposed to potential vorticity or equivalent latitude, to minimize the use of assimilated model output in analyzing the measurements. Thus the only quantities used from the NCEP analysis are the location and strength of the maximum subtropical jet wind speed at a range of potential temperatures from 320 to 380 K. 3. Analysis 3.1. Ozone Pdfs [7] In order to gain insight into the spatial structure of ozone with relation to the subtropical jet, we computed twodimensional pdfs of in situ ozone measurements versus measured wind speed difference from the jet maximum wind speed. The wind speed difference is used as a dynamical horizontal coordinate rather than a model derived dynamical quantity, such as potential vorticity. We chose wind speed difference as a horizontal coordinate for two reasons. One reason is that we are focusing on ozone variability relative to the position of the subtropical jet; hence the jet maximum wind speed is a natural center of a coordinate system. Potential vorticity has commonly been used as a dynamical definition of the tropopause in the 2of12

3 extratropics, but there is no single value appropriate for all locations and times, which we found to be true in particular near the subtropical jet. The second reason is the much coarser resolution of the model compared to the aircraft measurements. A model-based coordinate would smear out the small-scale features in the aircraft measurements. To calculate the wind speed differences, NCEP maximum subtropical jet wind speeds at each model pressure level and longitude were interpolated to the measured isentropic levels and longitudes and the aircraft in situ wind speeds were subtracted from the jet maximum wind speeds. By definition, this wind speed difference is positive, but to separate locations north and south of the jet maximum we assign negative values to the wind speed differences at measurement locations south of the jet maximum. If the WB-57F measured wind speeds are larger than the NCEP jet maximum, then the wind speed difference is set to zero. A comparison between in situ wind speeds and NCEP wind speeds interpolated to the flight tracks revealed a mean difference of 5 m/s, with the in situ wind speeds having the higher mean. This relatively small difference implies that the NCEP maximum jet wind speeds are consistent enough with the in situ wind speeds to be used in the manner described above. The difference between the in situ and NCEP winds is a systematic error in the wind speed difference analysis and thus should not affect the gradient features. [8] Figure 2 shows an example of the vertical distributions of wind speed and potential temperature at 170 W longitude during the G-IV flight of 16 February This flight crossed the region of maximum jet wind speeds from north to south as indicated by the flight profile overlaid on Figure 2a. Maximum jet wind speeds were in excess of 80 m/s on this day over the central Pacific. The 2 PVU contour is shown poleward of the latitude of the jet maximum to give an indication of the tropopause height in this region. Equatorward of the jet maximum the 380 K potential temperature surface, shown as a bold dashed line, is often used as an indicator of the tropopause height. From north to south across the subtropical jet the potential temperature contours are seen to bulge upward above 350 K and downward below 350 K. Shown in Figure 2b are ozone measurements taken on this particular flight as a function of latitude. Near the location of the jet maximum at 40 N there is a gradient of ozone from roughly 50 ppb at 38 N to roughly 200 ppb at 42 N. Figure 2c shows the ozone measurements from this flight as a function of wind speed difference from the jet maximum. This clearly shows that the ozone gradient occurs from roughly 10 m/s south of the jet maximum to m/s north of the jet maximum. Note that much larger gradients in ozone are seen further north of the jet maximum, in the lowermost stratosphere; these will be discussed later. [9] Ozone pdfs from the winter and spring seasons are shown in Figure 3. We focus on the winter and spring seasons because of the lack of flight paths that crossed the subtropical jet during the summer and fall seasons. The pdfs are also divided into 20 K potential temperature intervals from 320 to 380 K since there is a substantial vertical gradient of ozone in the lower stratosphere. The pdfs are normalized at each wind speed difference value so that the probabilities represent the chance that a particular Figure 2. (a) Pressure-latitude cross section of NCEP wind speed and potential temperature from 17 February 2002, 0Z, at longitude 170 E. The 320, 350 and 380 K theta levels are thickened to indicate the levels that are highlighted in the following plots. The 2 PVU contour is also shown to roughly indicate the tropopause location. The pressure profile from the G-IV flight from this day is also shown. (b) Ozone measurements from the 16 February 2002 G-IV flight as a function of latitude. (c) Ozone measurements from the same flight plotted as a function of wind speed difference from the jet maximum wind speed. Positive values indicate locations poleward of the jet and negative values equatorward of the jet. See text for details. ozone mixing ratio was measured at a particular wind speed difference location. The mean ozone mixing ratio at each wind speed difference is shown by the solid black line. 3of12

4 Figure 3. Probability distributions of ozone as a function of wind speed difference from the jet maximum wind speed. Positive wind speed differences indicate locations poleward of the jet maximum, and negative wind speed differences are equatorward of the jet maximum. An approximate horizontal distance of the measurement location from the jet maximum location is included as an additional x axis in Figure 3a. The wind speed difference bin size is 2 m/s, and the ozone bin size is 10 ppb. The pdfs are normalized at each wind speed difference such that the probability shown is the percent occurrence of an ozone mixing ratio at each particular wind speed difference. The mean ozone mixing ratio as a function of wind speed difference is shown by the solid line in each plot. The pdfs were calculated separately for the winter (Figures 3a, 3c, and 3e) and spring (Figures 3b, 3d, and 3f) seasons and over three different potential temperature ranges, K (Figures 3a and 3b), K (Figures 3c and 3d), and K (Figures 3e and 3f). The contours in percent are 0.01, 0.1, 1, 3, 10, and 30%. 4of12

5 [10] There are several features of note in the seasonal ozone pdfs. One is the contrast between the winter and spring within 10 m/s of the jet maximum. In the 0 10 m/s wind speed difference range, which is on the cyclonic shear side of the jet within roughly 500 km of the maximum wind speed, the winter ozone mixing ratios are significantly larger than in the spring. The difference is most noticeable in the K range but is seen at the lower levels as well. The mean ozone in the K range just poleward of the jet core is ppbv in the spring compared to ppbv in the winter. The pdf contours show an even larger difference between the seasons. There is a significant probability of mixing ratios up to 800 ppb in the winter in contrast to 500 ppb in the spring. This feature is interesting since ozonesonde and MOZAIC aircraft measurements show a peak in mean ozone in the spring in the N latitude range [Logan, 1999; Stohl et al., 2001]. These other ozone measurements seem to contrast with the seasonal ozone differences shown here but there are significant differences in the measurement locations and characteristics, which make direct comparisons difficult. As mentioned previously, the MOZAIC flights were mostly below the level of the WB-57F and G-IV flights so the MOZAIC data are best compared with the K results. There is a smaller difference between the seasons at these lower levels, as shown in Figures 3e and 3f, but the ozone mixing ratios are still larger in the winter. The different geographical sampling between the two aircraft data sets may also contribute to the different ozone features. The ozonesondes represent measurements from fixed locations, which will vary between north and south of the jet during the course of a month. Thus monthly mean ozonesonde measurements will likely average a large range of locations relative to the jet core. [11] The relatively low ozone mixing ratios just poleward of the jet core during spring seem to imply an increased amount of mixing of air across the jet from the troposphere into the stratosphere compared to during winter. Studies of extratropical troposphere to stratosphere (TST) exchange using global analysis data sets generally do not provide significant detail on winter versus spring differences. A study of one year of ECMWF data does show an increase in TST mass exchange during spring compared to winter in the N latitude range over the Pacific Ocean [Wernli and Bourqui, 2002]. This increase in TST mass exchange in spring does not appear to be associated with an increase in synoptic wave activity since there was actually larger transient wave activity in the troposphere during winter in the year studied. A seasonal climatology of tropopause fold events by Elbern et al. [1998] reveals a slight increase in the number of folds in the subtropical eastern Pacific during spring compared to winter. Tropopause folds mostly contribute to stratosphere to troposphere exchange so they are not likely to significantly impact the tracer mixing ratios poleward of the jet. Thus the cause of the seasonal differences in ozone mixing ratios does not appear to be explained by previous studies of extratropical TST. [12] A second feature of the seasonal ozone pdfs is the higher probability of large ozone mixing ratios in the spring above 340 K at wind speed differences of m/s. These are locations north of the jet, often 1000 or more kilometers from the center of the jet, in the midlatitude lowermost stratosphere. The seasonal difference is not present in the K range, possibly because of sampling differences between the potential temperature ranges. The average ozone is higher in the winter up to 25 m/s north of the jet maximum, while between 25 and 50 m/s the spring average is nearly the same, but more variable compared to the winter. The characteristic of the pdf at these locations is much different between the seasons; in winter the most probable values are near the mean and there is a smooth decrease to higher and lower values, whereas in the spring the most probable values are typically lower than the mean and the distribution of the pdf is bimodal. Relatively high ozone has been shown to accumulate in the high-latitude lowermost stratosphere because of descent of stratospheric air in the winter and spring [Pan et al., 1997; Logan, 1999; Prados et al., 2003]. In the spring, this pool of high-ozone, high-pv air is often separated from the subtropical jet by a thousand kilometers or more. An example of the contrasting seasonal meteorology and synoptic features of the subtropical jet are discussed in the next section. [13] To examine the vertical distribution of ozone in more detail than shown above, we computed pdfs in 5 K potential temperature bins and for three different horizontal locations relative to the jet maximum (Figure 4). On the tropospheric side of the jet, at wind speed differences from 60 to 10 m/s (Figures 4a and 4b), the average ozone mixing ratios below 360 K are mostly between 30 and 60 ppbv. In the winter, the average at 340 K is 100 ppbv because of a significant number of ozone mixing ratios greater than 150 ppb at this level. These relatively high mixing ratios are due to intrusions or filaments of stratospheric air which were transported to the equatorward side of the jet, typically in an exit region where the jet is zonally distorted. Aside from the 340 K level, there is generally a greater chance of ozone mixing ratios larger than 100 ppbv in the spring compared to the winter. Part of the higher spring time mixing ratios equatorward of the jet may be due to exchange of air with higher mixing ratios poleward of the jet. However, as mentioned above, during spring there is often a large region poleward of the jet with ozone mixing ratios only slightly elevated above tropospheric values. Thus the higher spring mixing ratios on the tropospheric side of the jet are consistent with more frequent exchange of air across the jet in spring compared to winter. Ozone mixing ratios greater than 100 ppbv below 360 K are rarely if ever measured in the deep tropics [e.g., Highwood and Hoskins, 1998; Folkins et al., 1999; Fujiwara et al., 2000]. This suggests some latitudinal gradient in ozone mixing ratios exists in the tropical mid to upper troposphere from the deep tropics to the subtropical jet. The size of the gradient is likely small on average but highly variable on the basis of the pdfs shown here. [14] Another interesting feature is the shape of the vertical profile of tropospheric ozone in both seasons. The average profile in spring has an S shape with low mixing ratios below 310 K, slightly higher mixing ratios between 310 and 340 K and a region of low mixing ratios centered at 350 K. Although the average profile in winter is affected by the high mixing ratios at 340 K, the most likely values of the pdf show a similar vertical structure as in the spring. In the winter there were no measurements taken above 365 K but in the spring there were measurements taken up to 380 K 5of12

6 Figure 4. Vertical profiles of ozone probability distributions as a function of potential temperature. The pdfs were calculated for separate wind speed difference regions and seasons. Wdiff refers to the wind speed difference from the jet maximum as used in Figure 3. Wdiff less than 0 (Figures 4a and 4b) include locations equatorward of the jet maximum, in the troposphere, and Wdiff greater than 0 (Figures 4c, 4d, 4e, and 4f) include locations poleward of the jet maximum, in the stratosphere. Note the different scales of the ozone axis for Figures 4a and 4b compared to Figures 4c, 4d, 4e, and 4f. and a sharp increase in ozone is seen above 360 K. The region from 350 to 400 K is often referred to as the tropical tropopause layer (TTL) [e.g., Highwood and Hoskins, 1998; Folkins et al., 1999], since it has distinct chemical and thermodynamic characteristics from the tropical troposphere below 350 K. Most of the study of the TTL has focused on the deep tropics and thus the latitudinal features of the TTL into the subtropics are not well documented. The s shape of the mean tropospheric ozone profiles for both winter and spring is similar to the shape of ozonesonde profiles taken in the deep tropics [e.g., Folkins et al., 1999] even though very few of the aircraft measurements used in this study were taken at latitudes equatorward of 20. The low ozone mixing ratios in the upper troposphere are thought to be due to preferential convective outflow in the K level. The convection brings up low ozone mixing ratios from near the surface and deposits them in this layer in the upper troposphere creating a minimum in the ozone profile. The presence of a similar shape vertical profile in the subtropics implies that either the same convective outflow region is present in the subtropics as in the tropics, or that there is efficient horizontal transport of low ozone mixing ratios from the tropics to the subtropics. Considering that there is far less convective activity in the subtropics compared to the tropics, it is likely that horizontal transport contributes more to the shape of the subtropical ozone profile than local convection. [15] The ozone profiles on the poleward side of the jet, in the lowermost stratosphere, generally show the expected increase with higher potential temperatures from 300 to 380 K. The pdfs for wind speed differences of 0 20 m/s (Figures 4c and 4d) highlight the seasonal differences just poleward of the jet maximum wind speeds. In this region, the mean ozone is slightly larger in winter compared to spring but the largest difference between the seasons is seen in the pdf. The probability of ozone values larger than 250 ppbv is typically an order of magnitude larger in winter than spring at all levels. This difference is most noticeable above 360 K as was seen in Figure 3. For wind speed differences of m/s (Figures 4e and 4f), the average profiles are similar, but the distribution of the pdf is substantially different between the seasons. The spring pdf is bimodal above 360 K, with the most probable ozone mixing ratios either 800 ppbv or 400 ppbv. By contrast, in the winter the pdf is evenly distributed around the mean with most probable values of ppbv above 360 K. [16] Alternative horizontal coordinates with a dynamical significance, most commonly PV and equivalent latitude, have been used in previous studies to characterize ozone in the upper troposphere and lower stratosphere [e.g., Beekmann et al., 1994; Morgenstern and Carver, 2001]. We have interpolated NCEP PV and equivalent latitude to the flight tracks to compare to the pdfs of ozone versus measured wind speed difference. Only the ozone versus PV pdf in the K range is shown here in Figure 5. In the winter (Figure 5a), the mean ozone has a small slope up to 4 PVU, above which the slope increases by roughly a factor of 5. However, there is a range of 400 ppbv or more 6of12

7 Figure 5. Probability distributions of ozone as a function of NCEP potential vorticity for the (a) winter and (b) spring seasons in the potential temperature ranges. The NCEP PV was interpolated in time and space onto the aircraft flight tracks. The ozone bin size is the same as in Figure 3, and the PV bin size is 0.5 PVU. in the ozone pdf for any PV value greater than 4 PVU. This suggests that although there is a linear shape to the mean ozone versus PV, the correlation is small and thus PV is not an effective predictor of ozone in the lower stratosphere. The lack of a sharp increase in ozone at any particular PV value also makes it difficult to choose a PV value to represent the tropopause in this region. In the spring (Figure 5b), the slope of the mean ozone versus PV is similar to that in winter up to 4 PVU, but between 4 and 6 PVU the slope is less than in winter and above 6 PVU the spring slope is much larger. Again, as in the winter there is a large spread in the pdf at most PV values, particularly for PV greater than 6 PVU. Pdfs of ozone versus equivalent latitude (not shown) have a similar behavior to that of PV, which is expected since the equivalent latititude is derived from the PV distribution. In both seasons the slope of mean ozone increases for equivalent latitudes greater than 30, and there is a large range of ozone mixing ratios for the high equivalent latitudes. Figure 6. (a d) Probability distributions of measured wind speed computed and shown in a similar manner as was done for ozone. The wind speed difference bin size is the same as in Figure 3, and the wind speed bin size is 2.5 m/s. 7of12

8 [17] The use of PV or equivalent latitude as a dynamical horizontal coordinate is certainly useful for many applications, particularly for model or satellite data sets that have low horizontal resolution. Poleward of the subtropical jet the wind speed difference coordinate certainly does not predict ozone any better than PV. Wind speed is not necessarily expected to be a predictor of ozone since they have different sources and sinks in the UT/LS. Potential vorticity may be considered as a surrogate long-lived tracer and thus an effective predictor of ozone in this region. However, the sources and sinks of PV are significantly different than those of ozone and so even though they have similar lifetimes there are locations where the different sources and sinks will effect the ozone and PV distributions in different ways. So even if the model resolution were not an issue, the PV would not be a perfect predictor of ozone. However, in the region near the center of the subtropical jet the wind speed relative to the jet maximum seems to more consistently reveal the region of large ozone gradients on the basis of the pdfs shown here. This region near the jet maximum is typically where the tropopause is located and is very dynamically active. Although wind speed is not a long-lived tracer, in the region near the center of the jet wind speed identifies where the dynamical activity, or lack of activity, has occurred. This dynamical activity, such as Rossby wave breaking, establishes the tracer gradients and so perhaps is more fundamentally linked to ozone in this region compared to PV. Thus, for the purpose of identifying tracer gradients across the tropopause and providing a nearly entirely in situ measurement analysis, the wind speed difference coordinate is preferable to PV Meteorology of the Subtropical Jet [18] The pdfs of the measured wind speed were computed to determine whether the strength of the jet was responsible for any of the observed features in the seasonal ozone variability and also as an interesting summary of in situ measurements of the structure of the subtropical jet. The pdfs were divided into two potential temperature ranges, K and K, since there were no additional features revealed when the pdfs were further divided. The wind speed pdfs as a function of wind speed difference from the jet maximum (Figure 6) have an expected shape with the highest wind speeds near the region of the jet maximum. The mean jet wind speeds are quite similar between the two seasons, with slightly larger peak wind speeds in the spring when the ozone gradients are smallest across the jet. The similarity of the jet strength between the two seasons does not necessarily exclude some dependence of ozone gradients on jet strength. However, a sub sampling of the ozone pdfs as a function of wind speed difference for different maximum jet wind speeds revealed very little correlation between jet strength and ozone gradients across the jet. This suggests that any differences in tracer characteristics between the two seasons are likely not due to the strength of the jet, but rather due to other features such as the zonal asymmetry of the jet. A couple of interesting features of the pdfs are the asymmetry on the poleward side of the jet in the K range, with the highest probabilities shifted toward lower wind speeds, and the bimodal feature on the equatorward side of the jet during spring. Figure 7. NCEP horizontal wind speed interpolated onto the 360 K surface and averaged over the flight dates during (a) winter , (b) spring , and (c) spring These are not complete seasonal averages but rather averages over the flight dates used in the analysis. The spring years were separated since all of the flights in took place over the continental United States whereas nearly all of the flights in took place over the Pacific Ocean. [19] To examine the general character of the subtropical jet during the period of aircraft measurements, we computed seasonal averages of NCEP horizontal wind speed on the 360 K surface (Figure 7). The averages are not true seasonal averages since only the flight dates plus one day on either side of each flight were included in the averages. The spring dates were divided into the years sampled by the WB-57F ( ) and those sampled by the G-IV ( ) to distinguish the different geographical regions sampled by 8of12

9 Figure 8. NCEP Aviation wind speed and potential vorticity for the flights of (a and b) 2 February and (c and d) 6 March The NCEP data have been interpolated onto potential temperature surfaces that correspond roughly to the flight level for each day. Contour intervals are 10 m/s for wind speed and 1 PVU for potential temperature. the two planes during spring. The winter and spring (Figures 7a and 7b) jet characteristics are quite similar over the Pacific where the G-IV sampled during these seasons. The jet was on average zonally symmetric in the western Pacific during both seasons with a region of decelerating wind speeds in the jet exit region northwest of Hawaii. There is a slight curve northward in the jet exit region during spring that is not seen in winter. The spring average jet (Figure 7c) is significantly different over the Pacific compared to the winter but the WB-57F only sampled over North America where the average jet is mostly similar during the two seasons. [20] The seasonal averages shown in Figure 7 suggest that the zonal character of the jet was not significantly different between the two seasons. However, the averages can conceal some of the distinct synoptic situations that occurred in the specific flight times and regions. An example of a meteorological contrast between winter and spring is shown by the wind speed and PV maps in Figure 8. On 3 February 2002 (Figures 8a and 8b) the jet was zonally oriented across most of the Pacific Ocean. On the 360 K isentropic surface the PV values were mostly uniform and ranged from 6 to 9 PVU across the entire Pacific north of the jet maximum. On 7 March (Figures 8c and 8d) the jet was zonally distorted and the regions of highest PV were separated from the jet maximum by up to thousands of kilometers. The PV on 7 March was distributed into two distinct regions north of the jet maximum. Within of latitude of the jet maximum the PV ranged from 3 to 6 PVU and the measured ozone was ppbv. This is much lower PV and ozone mixing ratios than normally observed poleward of the jet in winter. Regions of PV greater than 8 PVU occurred in filaments that protruded from higher latitudes and were associated with regions of very weak wind speeds. These filaments of high PV were associated with the highest measured ozone values, upwards of 1000 ppmv. [21] As discussed above, the wind speed pdfs are very similar between the two seasons, however the ozone pdfs are quite different poleward of the jet maximum. Since ozone is roughly positively correlated with PV, the PV field might be expected to show similar seasonal differences as were seen in the ozone. Near the jet maximum, the two days shown in Figure 8 have similar PV values. However, the biggest difference in the PV fields is poleward of the jet where the wind speeds are very light, less than 20 m/s, as mentioned above. The tongues of high PV are more prevalent in the spring and coincide with regions of high ozone mixing ratios. An interesting feature of the highozone, high-pv regions is that they do not correspond to regions of significant horizontal wind speed gradients. The wind speed pdfs are similar in that they do not show an 9of12

10 Figure 9. Normalized one-dimensional pdfs of ozone and measured wind speed in the K potential temperature range. The normalization was performed by removing the mean and dividing by the root mean square of the mean-removed data. The pdfs were computed separately for winter (solid lines) and spring (dashed lines) as well as for wind speed differences from (a and b) 50 to 10 m/s, (c and d) 0 to 20 m/s, and (e and f) 20 to 50 m/s. asymmetric behavior in the spring as is seen in the ozone. Thus very subtle changes in wind speed and temperature in the lowermost stratosphere are in some regions associated with ozone gradients larger than those seen across the jet Normalized Pdfs [22] The characteristics of the pdf of a passive tracer in a turbulent flow have been the subject of a number of theoretical and experimental studies in recent years [e.g., Warhaft, 2000; Pierrehumbert, 2000; Hu and Pierrehumbert, 2001]. The tracer pdfs subject to turbulent flow have often been found to have non-gaussian distributions. The mixing and stirring processes lead to the presence of exponential tails to the pdf which signify a much higher probability of extreme events than would be predicted by a simple Gaussian shape. The relatively high probability of extreme events is important for a variety of different applications. In the atmosphere, the presence of extremely high or low mixing ratios of a reactive chemical species or radiatively important species can impact the chemical and/or radiative balance of the region in a nonlinear manner. [23] High-resolution aircraft measurements are uniquely suited to investigate the normalized pdf of a tracer in the atmosphere. Satellite measurements cannot resolve the small-scale features that make up the tails of the pdfs. Balloon-based or sonde measurements do not have the spatial coverage to obtain significant pdf statistics. Figure 9 shows normalized pdfs of measured ozone and wind speed for three different locations relative to the jet. The normalization is performed by taking the difference from the mean and dividing by the root mean square of this difference. The mean is taken only over the specified wind speed difference range for each plot. The top row of plots shows ozone pdfs and the bottom row measured wind speed pdfs. From left to right the pdfs represent different locations relative to the jet with the tropospheric side of the jet on the left and stratospheric side on the right. In general, all of the pdfs for both seasons have a similar shape with a broad maximum and long positive tails. Between 0 and 2 standard deviations the slope of all of the pdfs are much less than exponential, and several of the pdfs have similar features out to 4 standard deviations. The spring ozone pdfs in the troposphere (Figure 9a) and well poleward of the jet (Figure 9e) have a double peak structure that is not seen in other locations or seasons. The double peak in the troposphere is due to the increase in ozone above 360 K as seen in Figure 4. Whereas the double peak in the stratosphere is due to the regions of high PV and ozone encountered in the spring as discussed in section 3.1. [24] Small features in the normalized pdfs shown in Figure 9 are likely not significant because of the limited sampling. However, the general features discussed above, such as the broad tails and seasonal differences, are important characteristics of the meteorological and trace gas distributions in the UT/LS. Pdfs based on measurements, such as shown here, can be used to compare to model output in order to evaluate the dynamics and mixing parameterizations in a model. The measured pdfs can provide a 10 of 12

11 stringent test for model performance, particularly at the smallest scales, where model resolution plays a role. However, the broad features of the pdfs near boundary regions such as the subtropical jet are also of importance. 4. Summary and Discussion [25] We have used high-resolution aircraft measurements to examine the characteristics of ozone and wind speed in the vicinity of the subtropical jet during the winter and spring seasons. The UT/LS near the subtropical jet is an important and highly undersampled region of the atmosphere. Aircraft measurements of a long-lived trace gas such as ozone can reveal much about the transport and mixing in this region. We have attempted to summarize the aircraft measurements by calculating pdfs of ozone and the measured meteorological variables. The pdfs were placed in a wind speed coordinate system that is centered on the subtropical jet in order to show gradients and contrast features seen on the cyclonic and anticyclonic shear sides of the jet. [26] The pdfs of ozone reveal a high probability of a significant gradient across the region of maximum jet wind speeds during winter. This is consistent with the wellknown presence of a large PV gradient on the cyclonic shear side of the jet, which may act to inhibit quasiisentropic transport. The size of the ozone gradient across the jet is not as well known because of the lack of measurements in this region and thus the pdfs shown here add information about the gradient of a long-lived tracer across the jet. The ozone pdfs during spring show a much smaller gradient across the region of maximum jet wind speeds compared to winter. Instead, the largest gradients in ozone during spring are most likely to be located 1000 or more kilometers poleward of the maximum jet wind speed region. These gradients are associated with tongues of air with polar latitude characteristics, high PV and high ozone. The edges of these tongues are not necessarily associated with a significant wind speed or temperature feature. Thus, during spring there often exists a large region of the lowermost stratosphere between 350 and 380 K with ozone mixing ratios elevated by only 100 ppbv or less over the upper tropospheric ozone mixing ratios. [27] These features in the ozone pdfs contribute to the discussion of a couple of basic questions, which have been investigated by previous studies without definitive answers. The first question is as follows: Is the jet a less effective barrier to isentropic transport during spring compared to winter? The pdfs of measured wind speed show very little difference in the strength of the jet between winter and spring in the regions sampled (Figure 6). The seasonal average wind speeds in the Pacific and North America are also mostly similar in the zonal character and jet exit features. Although this is not a comprehensive seasonal sample of subtropical jet wind speed, it is suggestive that jet wind speed does not play the predominant role in the size of ozone gradients. A comparison of ozone versus PV (Figure 5) also reveals a seasonal difference in the ozone on the cyclonic side of the jet, in the 4 6 PVU range, where ozone is higher in the winter compared to spring. Postel and Hitchman [1999] have shown that Rossby wave activity near the N.H. subtropical tropopause is at a minimum in March when most of the spring measurements used in this study were obtained. The wave activity is only slightly higher in January, February and April. So exchange of air across the jet by this mechanism should be roughly comparable during late winter and early spring. Another common diagnostic of isentropic transport used to estimate the strength of the subtropical jet barrier is effective diffusivity [e.g., Scott et al., 2003; Haynes and Shuckburgh, 2000]. On the basis of this technique the isentropic barrier is located at the jet maximum wind speed region and does not vary much in strength from the winter through early spring. From this one might imply that the ozone gradient across the jet should actually increase from winter to spring since the descent of air in the high latitudes brings higher ozone into the lowermost stratosphere throughout this period. The ozone pdfs show that the highest ozone during spring is located well poleward of the jet, separated by a barrier that is not readily apparent from effective diffusivity calculations. [28] Thus the question posed about the seasonal variability of the jet as a barrier does not appear to have a simple answer since most diagnostics of isentropic transport do not fully explain the ozone distributions. One factor that was only briefly mentioned is the variability of the jet upstream from the sampled region, such as over the Tibetan plateau. The Asian monsoon is thought to have a large impact on the composition of the lowermost stratosphere but there are very few measurements in this region. The onset of the Asian monsoon is typically in June, which is later in the year than the months analyzed here. Northward movement of the jet over the Asian continent may have some impact on the composition of the lowermost stratosphere in the spring but it is likely much smaller than during the monsoon season. [29] A second question is as follows: What effect do the seasonal ozone gradients have on estimates of ozone flux across the subtropical and midlatitude tropopause? Because of a lack of ozone measurements in the subtropical UT/LS, most estimates of cross-tropopause ozone flux use estimates of air mass flux across the tropopause combined with proxies for ozone. The typical proxies used are PV-ozone correlations or satellite measurements of total ozone. Both of these methods have limitations in the lowermost stratosphere, which is partly why estimates of annual downward cross-tropopause ozone flux vary by a factor of 3 [Olsen et al., 2002; Appenzeller et al., 1996b]. The downward crosstropopause air mass flux is known to peak during spring so errors in estimated ozone mixing ratios in the lowermost stratosphere during spring will have the most impact on annual ozone flux calculations. The distribution of ozone in the spring lowermost stratosphere shown by the pdfs in Figure 3 suggest that cross-tropopause ozone flux would be small because of the small gradient across the jet. However, without more ozone measurements in the subtropical UT/LS it is difficult to know if the limited geographical sample shown here applies to the rest of the globe. [30] The high precision, accuracy and spatial resolution of the aircraft measurements shown here provide a unique view of long-lived tracer variability and, subsequently, transport and mixing in the UT/LS. Further insight into the mixing and stirring of air in a region can be obtained by analysis of the spatial gradients of long-lived tracers [e.g., Cho et al., 2001]. Future work will involve the use of 11 of 12

12 scaling analysis on the ozone measurements. This type of analysis can be useful as a comparison with global general circulation model output of tracer variability in a region. The UT/LS near the subtropical jet is a difficult region for models to simulate accurately because of the complicated transport and mixing which occurs there. Comparison of measurement diagnostics such as the pdfs shown here and the scaling analysis to model output will provide a stringent and unique test of model performance. Appendix A: List of All of the Flights Used in the Analysis [31] Winter flights were all from the G-IV aircraft and took place on the following dates: In 2001, 31 January and 2, 4, and 17 February; in 2002, 20, 22, 26, and 30 January and 1, 2, 7, 8, 12, 16, 19, 26, and 27 February; in 2003, 15, 16, 18, 29, and 31 January and 3, 8, 13, 17, 19, 21, 22, 23, 24, and 25 February. 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Weather Rev., 130, Vaughan, G., and C. Timmis (1998), Transport of near-tropopause air into the lower midlatitude stratosphere, Q. J. R. Meteorol. Soc., 124, Warhaft, Z. (2000), Passive scalars in turbulent flows, Annu. Rev. Fluid Mech., 32, Wernli, H., and M. Bourqui (2002), A Lagrangian 1-year climatology of (deep) cross-tropopause exchange in the extratropical Northern Hemisphere, J. Geophys. Res., 107(D2), 4021, doi: /2001jd R. Jakoubek, D. Parrish, E. A. Ray, E. Richard, and K. H. Rosenlof, NOAA Aeronomy Laboratory, MS R/AL6, 325 Broadway, Boulder, CO 80305, USA. (jakoubek@al.noaa.gov; dparrish@al.noaa.gov; eray@al. noaa.gov; erik.c.richard@noaa.gov; kosenlof@al.noaa.gov) 12 of 12