Thunderstorms Enhance Tropospheric Ozone by Wrapping and Shedding Stratospheric

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Geophysical Research Letters

RESEARCH LETTER Thunderstorms enhance tropospheric ozone 10.1002/2014GL061921 by wrapping and shedding stratospheric air Key Points: Laura L. Pan1, Cameron R. Homeyer1,2, Shawn Honomichl1, Brian A. Ridley1, Morris Weisman1, • Tropopause-reaching MCSs entrain 1 3 3 3 3 ozone-rich stratospheric air Mary C. Barth , Johnathan W. Hair , Marta A. Fenn , Carolyn Butler , Glenn S. Diskin , 3 4 4,5 4,5 into troposphere James H. Crawford , Thomas B. Ryerson , Ilana Pollack , Jeff Peischl , • Airborne lidar measurement is key to and Heidi Huntrieser6 revealing this transport mechanism • A missing transport pathway for 1National Center for Atmospheric Research, Boulder, Colorado, USA, 2Now at School of Meteorology, University of ozone budget in major global models Oklahoma, Norman, Oklahoma, USA, 3NASA Langley Research Center, Hampton, Virginia, USA, 4Chemical Sciences Division Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado, USA, 5Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, Colorado, USA, Correspondence to: 6Institute of Atmospheric Physics, German Aerospace Center, Oberpfaffenhofen, Germany L. L. Pan, [email protected] Abstract A significant source of ozone in the troposphere is transport from the stratosphere. The Citation: stratospheric contribution has been estimated mainly using global models that attribute the transport process Pan, L. L., et al. (2014), Thunderstorms largely to the global-scale Brewer-Dobson circulation and synoptic-scale dynamics associated with upper enhance tropospheric ozone by wrap- tropospheric jet streams. We report observations from research aircraft that reveal additional transport of ping and shedding stratospheric air, Geophys. Res. Lett., 41, 7785–7790, ozone-rich stratospheric air downward into the upper troposphere by a leading-line-trailing-stratiform mesoscale doi:10.1002/2014GL061921. convective system with convection overshooting the tropopause altitude. The fine-scale transport demonstrated by these observations poses a significant challenge to global models that currently do not resolve storm-scale Received 17 SEP 2014 dynamics. Thus, the upper tropospheric ozone budget simulated by global chemistry-climate models where Accepted 5 NOV 2014 Accepted article online 7 NOV 2014 large-scale dynamics and photochemical production from lightning-produced NO are the controlling factors Published online 22 NOV 2014 may require modification.

1. Introduction Tropospheric ozone plays an important role in atmospheric chemistry and in climate forcing. Chemically, ozone controls the atmospheric oxidizing capacity via its photochemical link to OH [Brasseur et al., 1999]. It can be a major pollutant at the surface with adverse effects on human health [National Research Council, 1991]. It is a significant greenhouse gas, especially in the upper troposphere [Lacis et al., 1990; Forster and Shine, 1997], but its contribution to the radiative forcing of climate change continues to have significant uncertainty [Intergovernmental Panel on Climate Change, 2013]. The burden of tropospheric ozone is controlled by photochemical production and loss, deposition at the Earth’s surface, and net transport from the stratosphere. Changes in anthropogenic and natural emissions of ozone precursors, in circulation concomitant with climate change, and in stratospheric input all contribute to trends in tropospheric ozone. Observational and modeling studies have largely attributed transport of stratospheric ozone to the troposphere by large-scale circulation and synoptic-scale dynamical processes [Holton et al., 1995; Sprenger and Wernli, 2003; Hsu et al., 2005], such as the intrusion of stratospheric air near upper level fronts associated with the jet stream [Danielsen, 1968; Shapiro, 1980; Pan et al., 2010; Lin et al., 2012; Langford et al., 2012]. Aircraft observations made in 1985 and 1989 suggested that perturbation of the tropopause by a deep convective thunderstorm led to net ozone transport to the troposphere [Dickerson et al., 1987; Poulida et al., 1996; Stenchikov et al., 1996]. Upper tropospheric ozone enhancement from the stratosphere was also observed near thunderstorm anvils in aircraft studies described by Dye et al. [2000] and Hitchman et al. [2004], but details of the transport process remain obscure, largely because the limited sampling of in situ measurements could not provide complete information of chemical versus dynamical structures of the air mass at the cloud scale. Here we report the first unambiguous observation of thunderstorm-driven stratosphere-to-troposphere transport of ozone made during the Deep Convective Clouds and Chemistry (DC3) experiment conducted over the central U.S. in the summer of 2012. The DC3 experiment was based in Salina Kansas from mid-May to late June of 2012.

Figure 1. (top) The vertical distribution of upper tropospheric ozone mixing ratio measured by DIAL along approximately 30 min (~400 km in distance) of the DC-8 flight leading to the location shown in Figure 2. Note that the cross section is shown in reversed time scale to ease the connection with the east to west flight track shown in Figure 2. The black color indicates the region when the DIAL ozone channel produced no data due to cloud attenuation. In situ ozone is shown by the color of the flight track. (bottom) The depolarization ratio from the DIAL 355 nm wavelength channel provides a signature of cloud/storm edge and outflow pattern in the neighborhood of the storm. The outflow structure shown in light blue is most consistent with low number density of ice particles, which complements the stretching and shedding shown in the ozone structure (Figure 1 (top)).

A principal objective was to investigate the impact of deep convection on atmospheric trace gases, including ozone. Three research aircraft participated in the campaign: the National Science Foundation (NSF) and National Center for Atmospheric Research Gulfstream V (GV), the National Aeronautics and Space Administration (NASA) DC-8, and the German Aerospace Center (DLR) Falcon, each equipped with an extensive payload of meteorological and chemical sensors. The NASA Langley Differential Absorption Lidar (DIAL) was included on the DC-8 to obtain proﬁle measurements of ozone and aerosols above and below the ﬂight track [Hair et al., 2008]. It is the measurements from DIAL within and near thunderstorms that revealed novel evidence for the deep convective pathway of ozone transport from the stratosphere into the troposphere.

2. Ozone Observations Near Deep Convective Thunderstorms The transport event was observed on the ﬂight of 30 May 2012 when the DC-8 aircraft passed over a large mesoscale convective system (MCS) on its return to Salina, Kansas. At the time of the overpass, the storm system had evolved into a classic leading-line trailing-stratiform (LLTS) MCS [Houze et al., 1989]. The convective overshooting behavior of this MCS has been analyzed in detail using high-resolution radar observations. The radar analysis shows that the convective elements of the storm overshot the tropopause up to 3 km. This analysis and the convective transport of tropospheric air into the stratosphere by this storm including water vapor injection into the lower stratosphere are detailed in Homeyer et al. [2014b]. Figure 1 shows that around 02 UTC of 31 May, the DC-8 was ﬂying at 12.5 km altitude, above the tropopause located

Figure 2. The position of the MCS and the DC-8 aircraft at 21:25 CDT 30 May 2012 (02:25 UTC 31 May). The MCS is shown using the GOES Infrared satellite cloud field (gray shading) and the Next Generation Weather Radar Weather Surveillance Radar, 1988 Doppler radar reflectivity (color shading). The DC-8 flight track is colored by the in situ ozone mixing ratio. The aircraft was at lower altitudes during most of the flight and sampling ozone in the tropospheric value range (100 ppb or lower, indicated by blue-green colors). A triangle symbol is used to mark the location of the aircraft at 01:53 UTC, corresponding to the earliest time shown in Figures 1 and 3. Thin red lines indicate the state borders.

near 12 km, on a track nearly perpendicular to the leading convective line. DIAL retrieved a two-dimensional proﬁle “curtain” of ozone along this track revealing the ﬂow structure that has the shape of a “ram’s horn” in front of the leading edge of the storm. The edge of the storm cloud is determined from the 355 nm wavelength channel of DIAL (lower panel of Figure 1). The location and structure of the MCS and position of

Figure 3. Time series of DC-8 in situ measurements for the ﬂight time period shown in Figure 1. Note that the time axis is reversed to be consistent with Figure 1. The in situ O3 (red) and CO (blue) show anticorrelated turbulent structure during the time period of 02:08–02:15 UTC, consistent with the cloud edge indicated by the 355 nm channel (Figure 1 (bottom)). Also shown are the DC-8 ﬂight altitude (solid black line) and the large-scale analysis of the tropopause (green dashed line).

Figure 4. Vertical cross section of the WRF-simulated 30 May storm. The wind vectors (black arrows) show storm-relative wind, produced by subtracting the mean storm motion (14 m/s). Transport of stratospheric air is shown by the distribution of a passive stratospheric tracer (blue color ﬁll), initiated as 100% at each grid point above the tropopause (black dots). The observed wrapping of stratospheric air from the edge of the storm is reproduced by the nonzero fraction of this tracer below the tropopause and around the cloud edge (gray line). The potential temperature ﬁeld (thin black lines, 2 K interval) is also included to show the dynamics of the convection. The cross section shows a span of 1084 km and represents the average of a 40 km wide segment of the simulated storm (perpendicular to the leading convective line).

the DC-8 aircraft at the time of the encounter are shown in Figure 2. Figure 3 shows that during this time period, the in situ ozone sensor measured mixing ratios between 150 and 400 ppbv. This high ozone and its anticorrelation with in situ CO shown in the time series confirm that the aircraft was in the stratosphere. The ozone-rich stratospheric air, with mixing ratios of 150 ppbv or more, is shown to be wrapping around the edge of the storm cloud and to reach altitudes as low as ~8 km or about 4 km below the local tropopause. The figure also shows that thin filaments of the enhanced ozone extended about a hundred kilometers from the cloud edge. The ozone enhancements resulting from the wrapping and the filaments are irreversible, since they are produced by diabatic processes and are associated with turbulent storm motion. Tropospheric mixing ratios of ozone external to the storm systems at this altitude were much lower at 60 to 100 ppbv.

3. Model Simulation and Analyses To further examine the transport process for the LLTS MCS, a numerical simulation of the MCS of 30 May was performed using the Weather Research and Forecasting (WRF) model [Skamarock et al., 2008]. The simulation used a nested domain with 3 km horizontal resolution and 200 m vertical resolution in the tropopause region and was initialized using European Centre for Medium-Range Weather Forecasts reanalyses, following that described by Homeyer et al. [2014a]. A passive stratospheric tracer was incorporated in the simulation to diagnose the flow around the storm. This simulation reproduced the key features of the observations (Figure 4). Although the model does not reproduce the “leading-trailing” asymmetric structure of the LLTS storm system well, it does reproduce the ram’s horn wrapping of stratospheric air around both the leading and trailing edges of the storm as shown by the distribution of the model tracer. The dynamical field of potential temperature further reveals the perturbation of the tropopause produced by the storm. Perturbation to the tropopause is shown both in the region of the updraft center and the subsiding edge of the storm. The model results indicate that the deep convective storms cause significant perturbations of the tropopause and are capable of strongly perturbing this otherwise stable transport barrier.

Figure 5. Schematic of the wrapping and shedding induced by the LLTS MCS (adapted from Houze et al. [1989]). Additional elements in the schematic are the tropopause (dotted line), perturbed and distorted by the MCS ﬂow, and the impact of the storm dynamics on air mass transport.

The key elements of the convective pathway for exchange are illustrated in a modiﬁed schematic of a LLTS system that was originally given by Houze et al. [1989]. The schematic (Figure 5) highlights the effect of storm ﬂow on storm environment and the perturbation of the tropopause. It also emphasizes the importance of the storm systems not only to weather but also to atmospheric composition.

4. Discussions The observation on the 30 May flight is unique in capturing the ram’s horn-shaped ozone enhancement around cloud edge, since the process was not targeted in the experiment and the aircraft was primarily utilized for in situ sampling. Nevertheless, in situ observations on all three aircraft sampled frequent ozone enhancement near the edges of active tropopause-penetrating storms. An analysis of all DC-8 data collected while crossing the cloud edges of tropopause-reaching storms of various types revealed that more than half of the identified cases (17 out of 26 flight segments) sampled significant ozone enhancements of stratospheric origin, based on anticorrelated CO variability. This type of enhancement was also observed by in situ measurements on the Falcon and the GV aircraft, thus providing good evidence to support frequent occurrence of this transport pathway in thunderstorms that reach or exceed the tropopause altitude. These new observations and modeling results have several implications. First, they challenge global chemistry climate models with a “missing pathway” of stratospheric ozone input to the troposphere relevant to quantifying tropospheric ozone for climate assessments. This may be a significant pathway because hundreds of these deep convective storms occur over the continental U.S. during summer [Bedka et al., 2010]. Although the observations made near the 30 May storm were not extensive enough to allow an estimate of the ozone flux from the stratosphere, a previous modeling study using parameterized convection has estimated that convective-scale fluxes increase the ozone transport from the stratosphere by up to 19% of the total annual Northern Hemisphere ozone flux and seasonally as much as 49% (e.g., northern midlatitudes in June) [Tang et al., 2011]. This leads to the second aspect that the new results complicate the understanding

of the impact of lightning-generated nitrogen oxides (LNOx, where NOx =NO+NO2)[Dye et al., 2000; Schumann and Huntrieser, 2007] on upper tropospheric ozone. Recurring upper tropospheric ozone enhancement over North America during summer has been a topic of focused studies. The photochemical

production of ozone from LNOx has been considered the main cause of this enhancement [Cooper et al.,

2006, 2007]. Since the type of deep convective storms that produce the wrapping and shedding are also

electrically active and responsible for generating LNOx, a better understanding of this novel transport pathway is essential for quantifying the role of LNOx in ozone production. The signiﬁcance of these observations also needs to be considered in the context of global change. Since storm behavior may change in an evolving climate, the recognition and the research toward incorporation of this process into the global chemistry-climate models is important for being able to predict the climate forcing associated with the coupling of weather and atmospheric composition.

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