ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-17-6113-2017Enhanced stratospheric water vapor over the summertime continental United
States and the role of overshooting convectionHermanRobert L.robert.l.herman@jpl.nasa.govhttps://orcid.org/0000-0001-7063-6424RayEric A.RosenlofKaren H.https://orcid.org/0000-0002-0903-8270BedkaKristopher M.SchwartzMichael J.https://orcid.org/0000-0001-6169-5094ReadWilliam G.TroyRobert F.ChinKeithChristensenLance E.FuDejianhttps://orcid.org/0000-0001-5205-0059StachnikRobert A.BuiT. PaulDean-DayJonathan M.Jet Propulsion Laboratory, California Institute of Technology,
Pasadena, California, USANational Oceanic and Atmospheric Administration (NOAA) Earth System
Research Laboratory (ESRL) Chemical Sciences Division, Boulder, Colorado,
USANASA Langley Research Center, Hampton, Virginia, USANASA Ames Research Center, Moffett Field, California, USABay Area Environmental Research Institute, Sonoma, California, USARobert L. Herman (robert.l.herman@jpl.nasa.gov)17May20171796113612429November201619December20169April201710April2017This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://acp.copernicus.org/articles/17/6113/2017/acp-17-6113-2017.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/17/6113/2017/acp-17-6113-2017.pdf
The NASA ER-2 aircraft sampled the lower stratosphere over North
America during the field mission for the NASA Studies of Emissions and Atmospheric Composition,
Clouds and Climate Coupling by Regional Surveys (SEAC4RS).
This study reports observations of convectively influenced air parcels with
enhanced water vapor in the overworld stratosphere over the summertime
continental United States and investigates three case studies in detail.
Water vapor mixing ratios greater than 10 ppmv, which is much higher than the
background 4 to 6 ppmv of the overworld stratosphere, were measured by the
JPL Laser Hygrometer (JLH Mark2) at altitudes between 16.0 and 17.5 km
(potential temperatures of approximately 380 to 410 K). Overshooting cloud
tops (OTs) are identified from a SEAC4RS OT detection product based on
satellite infrared window channel brightness temperature gradients. Through
trajectory analysis, we make the connection between these in situ
water measurements and OT. Back trajectory analysis ties enhanced water to OT
1 to 7 days prior to the intercept by the aircraft. The trajectory
paths are dominated by the North American monsoon (NAM) anticyclonic
circulation. This connection suggests that ice is convectively transported to
the overworld stratosphere in OT events and subsequently sublimated; such
events may irreversibly enhance stratospheric water vapor in the summer over
Mexico and the United States. A regional context is provided by water
observations from the Aura Microwave Limb Sounder (MLS).
Introduction
Water plays a predominant role in the radiative balance of the Earth's
atmosphere, both in the gas phase as the Earth's primary greenhouse gas and
in condensed phases in cloud and aerosol. Despite its low abundance, upper
tropospheric and lower stratospheric (UTLS) water vapor is critically
important in controlling outgoing long-wave radiation; quantifying UTLS
water vapor and its controlling processes is critical for climate
characterization and prediction. Climate models are sensitive to changes in
stratospheric water (Shindell, 2001) and clouds (Boucher et al., 2013).
Increases in UTLS water are associated with warming at the surface on the
decadal scale (Solomon et al., 2010). As the dominant source of hydroxyl
radicals, UTLS water also plays an important role in the control of the UTLS ozone
(Shindell, 2001; Kirk-Davidoff et al., 1999).
The overworld stratosphere, the altitude region with potential temperature
θ greater than 380 K (Holton et al., 1995), is extremely dry, with
typical mixing ratios of 3–6 parts per million by volume (ppmv). The
importance of low temperatures at the tropical tropopause acting as a “cold
trap” to prevent tropospheric water from entering the stratosphere has been
recognized since Brewer (1949). Tropospheric air slowly ascends through the
tropical tropopause layer (TTL) as part of the hemispheric-scale
Brewer–Dobson circulation. In the TTL, air passes through extremely cold
regions where water vapor condenses in situ to form cirrus ice, and then the
cirrus slowly falls due to sedimentation (e.g., Jensen et al., 2005, 2013).
Additional condensation and sedimentation are thought to be associated with
convection and large-scale waves (e.g., Voemel et al., 2002). The amount of
water that enters the stratosphere is largely a function of the coldest
temperature that a parcel trajectory encounters. This typically occurs in the
tropics, and the coldest temperature is typically near the tropical
tropopause. The saturation mixing ratio at the cold point tropopause thereby
sets the entry value of water vapor.
In contrast to water entry into the overworld stratosphere, water transport
from the troposphere to the lowermost stratosphere (350 K < θ < 380 K over summer CONUS) may occur through several different
pathways. Poleward of the subtropical jet, water may be transported into the
lowermost stratosphere through isentropic troposphere–stratosphere exchange
(Holton et al., 1995) or through convective overshoot of the local
tropopause (Dessler et al., 2007; Hanisco et al., 2007; Liu at el., 2008; Liu and Liu, 2016). Isentropic
transport from the tropics is the dominant pathway for water into the
lowermost stratosphere, with evidence from the seasonal cycle of lower
stratospheric water (e.g., Flury et al., 2013). How important the sublimation of
ice from convective overshoot is for hydrating the stratosphere is a topic
of ongoing debate (e.g., Randel et al., 2015; Wang, 2003). Case studies have
reported extreme events in which ice is transported to the overworld
stratosphere and subsequently sublimates, but the amount of ice that is
irreversibly injected into the stratosphere is poorly known. Airborne
measurements have demonstrated that convective injection occurs both in the
tropics (Webster and Heymsfield, 2003; Corti et al., 2008; Sayres et al.,
2010; Sargent et al., 2014) and at mid-latitudes (Hanisco et al., 2007;
Anderson et al., 2012). Ice injected directly into the stratosphere is
unaffected by the cold trap in the vicinity of the tropopause (Ravishankara,
2012).
The subject of this paper is the role of convective overshooting tops in
enhancing stratospheric water. Paraphrasing Bedka et al. (2010), a
convective overshooting top (OT) is a protrusion above a cumulonimbus anvil
caused by strong updrafts above the equilibrium level. Early observations of OT
include photographs of OT in the stratosphere from a U-2 aircraft (Roach,
1967). Recent observations of elevated water mixing ratios in the summer
overworld stratosphere by aircraft (Anderson et al., 2012) and the Aura
Microwave Limb Sounder (MLS; Schwartz et al., 2013) suggest that ice
injection into the overworld stratosphere by OT, while rare, occurs in three
predominant regions during the summer season. These three regions are the
Asian monsoon region, the South American continent and the focus of this
study, the North American monsoon (NAM) region (Schwartz et al., 2013).
The NASA ER-2 aircraft sampled the summer stratospheric NAM region during
the field mission for the NASA Studies of Emissions and Atmospheric Composition, Clouds and
Climate Coupling by Regional Surveys (SEAC4RS) (Toon et
al., 2016). One of the primary goals of this multi-aircraft mission was to
address the following question: do deep convective cloud systems locally inject water
vapor and other chemicals into the overworld stratosphere over the
continental United States (CONUS)? It is challenging for space- and
ground-based techniques to detect enhanced water vapor injected into the
stratosphere by OTs. Satellite measurements are limited by their horizontal
and vertical resolution in detecting fine-scale three-dimensional variations
in water vapor, while ground-based measurements are confined to sampling at
fixed locations. In contrast, airborne in situ stratospheric measurements of water
have an advantage because the aircraft can be routed to a specific location,
altitude, date and time. Modelers can predict whether air parcels are likely
to have convective influence, and aircraft flight paths are planned to
intercept those air parcels. The purpose of this paper is to report three
new case studies of enhanced water vapor in the overworld stratosphere
during the NASA SEAC4RS field mission and to connect these
observations to deep convective OT over the North American continent.
A summary of the enhanced water vapor measurements in the overworld
stratosphere during SEAC4RS*. The dates are NASA ER-2 aircraft flight dates
in day-month-year format, and the JLH Mark2 maximum water vapor mixing ratios
(ppmv) are shown for potential temperatures greater than 400 K (left) and in
the range of 380–400 K (right).
* SEAC4RS is the Studies of Emissions and Atmospheric Composition, Clouds
and Climate Coupling by Regional Surveys.
The JLH Mark2 stratospheric water vapor profiles from 23 aircraft
flights during SEAC4RS. This altitude range includes the overworld
stratosphere (potential temperature greater than 380 K) and lowermost
stratosphere (tropopause to 380 K). The majority of the observations have mixing
ratios lower than 10 ppmv in the lowermost stratosphere and lower than 6 ppmv
in the overworld stratosphere. Enhanced water measurements are the extreme
outliers with high water mixing ratios and a threshold value of the mean plus
2 standard deviations.
ObservationsAircraft
The airborne in situ water vapor measurements reported here are from the Jet
Propulsion Laboratory Laser Hygrometer Mark2 (JLH Mark2), a tunable laser
spectrometer with an open-path cell external to the aircraft fuselage (May,
1998). Water vapor is reported at 1 Hz (10 % accuracy), although the time
response of the open-path cell is much faster than this because the
instrument samples the free-stream airflow. This instrument has a
redesigned optomechanical structure for greater optical stability and was
first flown in this configuration on the NASA ER-2 high-altitude aircraft
during the SEAC4RS field mission. Pressure and temperature, provided by
the Meteorological Measurement System (MMS; Scott et al., 1990), are used
in the data processing to calculate water vapor mixing ratios from spectra,
as described in May (1998).
During SEAC4RS, nine aircraft flights targeted air parcels with recent
convective influence (see Table 3 of Toon et al., 2016). Figure 1 shows the
combined vertical profiles of JLH Mark2 water vapor from all 23 SEAC4RS
flights. Outliers with high water vapor mixing ratios are the focus of this
study. Enhanced water vapor was measured on 11 flights (Table 1). Here
we define “enhanced water vapor” as mixing ratios greater than 2 standard
deviations above the mean in situ measurement. For the overworld
stratosphere in all 23 SEAC4RS flights, the mean H2O is 6.7 ± 1.5 ppmv at 380–400 K and 5.0 ± 0.8 ppmv at 400–420 K (Fig. 2). Thus, the
threshold for enhanced water vapor is 9.7 ppmv at 380–400 K and 6.6 ppmv at
400–420 K. The majority of the measurements have background water mixing ratios
characteristic of the overworld stratosphere, 4 to 6 ppmv. For the overworld
stratosphere (potential temperature greater than 380 K), Fig. 1 shows
enhanced water vapor at potential temperatures up to approximately 410 K
(17.5 km of altitude). We define the “enhanced water region” as the layer of
the overworld stratosphere where these events have been observed at 380–410 K
of potential temperature corresponding to 16–17.5 km of altitude. Enhanced water
vapor measured in situ by both the JLH Mark2 instrument (Fig. 1) and the Harvard
Water Vapor instrument (Smith et al., 2015) on the NASA ER-2 aircraft
indicated that the aircraft intercepted convectively influenced air. Other
tracers measured on the aircraft did not change significantly in these
plumes. For the SEAC4RS flights, the agreement between these two water
vapor instruments is within ±10 % for stratospheric water. This is
consistent with the AquaVIT laboratory intercomparison (Fahey et al., 2014)
and other aircraft field missions (e.g., Rollins et al., 2014). The largest
enhancements were observed on three flights that are described in detail in
Sect. 4.
The distribution of JLH Mark2 water vapor in the overworld
stratosphere for all flights in the SEAC4RS mission (summer 2013)
plotted as a fraction of the observations in each potential temperature range.
The first trace (black circles and line) is at potential temperatures of 380 to
400 K, corresponding to approximately 16.8 to 17.4 km of altitude (99 to 90 hPa). The second trace (red triangles and line) is at potential temperatures of 400
to 420 K, corresponding to approximately 17.4 to 18.0 km of altitude (90 to
80 hPa).
Aura MLS
Aura MLS measures ∼ 3500 profiles each day of water vapor and
other atmospheric species (Livesey et al., 2016). While the aircraft samples
in situ water in a thin trajectory through the atmosphere, Aura MLS provides a
larger-scale context. Expanding on the analysis of Schwartz et al. (2013),
Aura MLS observations of stratospheric water vapor are presented here for
the SEAC4RS time period in summer 2013. Aura MLS H2O has 0.4 ppmv
of
precision at 100 hPa for individual profile measurements with a spatial
representativeness of 200 km along the line of sight (Schwartz et al., 2013).
The results shown here use MLS version 4.2 data, but are not significantly
different from the previous version 3.3. MLS observations over CONUS are at
∼ 14:10 local time (ascending orbit) and ∼ 01:20
local time (descending orbit) with successive swaths separated by
∼ 1650 km. The vertical resolution of the water vapor product is
∼ 3 km in the lower stratosphere (Livesey et al., 2016).
Aura MLS shows a seasonal maximum in water vapor over CONUS in July and
August. The histogram of Aura MLS water vapor in Fig. 3 indicates that the
July–August 2013 CONUS lower stratosphere was drier than the previous
nine-summer MLS record (2004 to 2012). Nevertheless, enhanced lower
stratospheric water vapor was observed by MLS in 2013 as a rare but detectable
event. From the MLS histogram, the frequency of 100 hPa H2O
> 8 ppmv was 0.9 % of the observations in July–August 2013 in
the blue shaded box. Figure 4 shows that, out of all MLS 100 hPa water vapor
retrievals over the 2-month period from July to August 2013, water greater than
8 ppmv was measured only nine times over North America (in the blue shaded
box), three times near the west coast of Mexico and once over the Caribbean
Sea.
The distribution of Aura MLS v4.2 100 hPa H2O over CONUS (blue
shaded box in insert), corresponding to approximately 17 km of altitude. The
two histograms for July–August 2013 (blue asterisks and trace) and the
previous nine-summer MLS record, for July–August 2004 through 2012 (red circles
and trace), indicate that 2013 was drier than average. The threshold for
MLS-detected “enhanced water vapor” (thick black vertical line) is set at 8 ppmv, the same as in Schwartz et al. (2013), to exclude the larger population of
measurements at 6 to 8 ppmv of water vapor that may have other sources.
Analysis
Here we briefly describe the analytical technique used to determine whether
back trajectories from the aircraft location intersect OT as identified by a
satellite OT data product.
Detection of overshooting tops
In order to link the stratospheric water vapor encountered by the aircraft
to the storm systems from which they may have originated, it is necessary to
have a comprehensive continental-scale catalog of deep convection.
Geostationary Operational Environmental Satellite (GOES) infrared imagery is
used to assemble a catalog of OTs throughout the US and offshore waters.
This catalog was acquired from the NASA LaRC Airborne Science Data for
Atmospheric Composition data archive
(http://www-air.larc.nasa.gov/cgi-bin/ArcView/seac4rs). Because OTs are
correlated with storm intensity, the OT product was primarily developed to
benefit the aviation community for more accurate turbulence prediction as
well as the general public for earlier severe storm warnings. However, the
product is also ideally suited for identifying storm systems that can
moisten the stratosphere.
The 2-month mean map of Aura MLS v4.2 100 hPa H2O (color
scale), corresponding to approximately 17 km of altitude with superimposed
MERRA horizontal winds (arrows) for July–August 2013 during the SEAC4RS
time period. The MLS observations of 100 hPa H2O greater than 8 ppmv in
this 2-month period are shown by the white circles.
Infrared brightness temperatures are used to detect cloud top temperature
anomalies within thunderstorm anvils. OT candidates are colder than the mean
surrounding anvil, with the temperature difference indicative of both the
strength of the convective updraft and the depth of penetration. For a
description of the method, the reader is directed to Bedka et al. (2010).
The horizontal spatial resolution of the OT product is dependent on the
underlying satellite imagery resolution, i.e., the size of the GOES IR
pixel, which is 7 km or less over the CONUS. Additional validation of OTs
requires comparison with the Global Forecast System (GFS) Numerical Weather
Prediction (NWP) model tropopause temperature. The maximum OT cloud height
was derived based on knowledge of the (1) OT–anvil temperature difference,
(2) the anvil cloud height based on a match of the anvil mean temperature near
the OT and the GFS NWP temperature profile and (3) a temperature lapse
rate within the UTLS region based on a GOES-derived OT–anvil temperature
difference and NASA CloudSat OT–anvil height difference for a sample of
direct CloudSat OT overpasses (Griffin et al., 2016). Griffin et al. (2016)
find that 75 % of OT height retrievals are within 0.5 km of CloudSat OT
height, so we conservatively estimate the accuracy of the OT altitude to 0.5 km. For SEAC4RS, every available GOES-East and GOES-West scan
(typically 15 min resolution) was processed for the full duration of the
mission, even for the non-flight days, yielding a detailed and comprehensive
picture of the location, timing and depth of penetration of the convective
storms over the entire CONUS. The output files include the OT coordinates,
time, overshooting intensity in degrees K, which is related to the
temperature difference between the OT and the anvil, and an estimate of
maximum cloud height for OT pixels in meters.
The ability of GOES-East and GOES-West to observe an OT depends on its
lifetime. OTs are transient events with lifetimes typically less than 30 min, but they can exceed 1 hour in well-organized storms such as mesoscale
convective systems and supercell storms (Bedka et al., 2015; Solomon et al.,
2016 and the references therein). Animations such as the following show the
variability in OTs sampled by GOES at 1 min of resolution:
It is clear that some OTs are quite persistent and are both prominent and
detectable in IR imagery, but the majority of OTs in these particular
animations are short lived (< 10 min). Within these OTs, strong
convective updrafts can transport ice to 16–18 km of altitude where turbulent
processes, such as gravity wave breaking, mix tropospheric and stratospheric
air (e.g., Mullendore et al., 2009, 2005; Wang, 2003; Homeyer et al., 2017),
enabling the detrainment of ice and stratospheric hydration.
Bedka et al. (2010) showed that the OT detection algorithm has a false
positive rate of 4.2 to 38.8 %, depending on the size of the
overshooting and algorithm settings. As noted above, OTs are transient and
can evolve quite rapidly. The storm top characteristics and evolution we see
in the GOES data featured in this paper only capture a subset of the storm
lifetimes, even if we were to have a 100 % OT detection rate, due to the
15 min resolution of the GOES imager. In addition, the relatively coarse GOES
spatial resolution (up to 7 km over the northern latitudes of the US) can cause
the Bedka et al. (2010) method to miss some small diameter and/or weak OT
regions. We would be able to better map tracks of storm updrafts using data
at 1 min of frequency like that shown by Bedka et al. (2015), but these data
are not available over the broad geographic domains required for our analysis.
Given uncertainties in back trajectories, GOES under-sampling and the fact that many
OTs can be located in close proximity to one another, we are not able to
make a direct connection between an individual OT and a stratospheric water
vapor plume observed 1 day or more later. Rather, our analysis identifies a
cluster of storms that are the best candidates for generating ice that
sublimates into the enhanced water vapor plumes sampled by the ER-2.
Back trajectory modeling
Back trajectories were run from each flight profile for which enhanced water
vapor was measured to determine whether the sampled air was convectively
influenced. The trajectories were run with the FLEXPART model (Stohl et al.,
2005) using NCEP Climate Forecast System version 2 (CFSv2) meteorology (Saha
et al., 2014); the trajectory time step interval was 1 h.
Trajectories were initialized every second along the flight track profiles
and run backward for 7 days. A sampled air parcel was determined to be
convectively influenced if the back trajectory from that parcel intercepted
an OT region. The tolerances for a trajectory to be considered to have
intercepted an OT cloud were ±0.25∘ latitude and longitude,
±3 h and ±0.5 km in altitude. These tolerances were chosen
primarily due to the resolution of the NCEP meteorology used to run the
trajectories (1∘× 1∘) and based on personal communication with
Leonard Pfister.
Map and profiles of aircraft and satellite water vapor on 8 August
2013 over California (number 1; shown in dark blue) and Texas (number 2; shown
in green). (a) Map of the ER-2 aircraft flight track (solid colored trace) and
nearly coincident Aura MLS geolocations (asterisks and lines). (b) ER-2
aircraft altitude profiles (solid colored trace) color coded by dives and
MLS times (horizontal lines). (c) Vertical profiles of in situ water vapor
measurements from JLH Mark2 (dots) and MLS retrievals of water vapor
(circles and lines). Some measurements exceed the threshold for enhanced
water vapor of 8 ppmv for Aura MLS (after Schwartz et al., 2013) and the
campaign-wide mean plus 2 standard deviations for JLH Mark2, 9.7 ppmv at 380–400 K and
6.6 ppmv at 400–420 K.
Case studies
In this section, we highlight three NASA ER-2 flights during which elevated
stratospheric water was observed by JLH Mark2. These dates are 8, 16 and 27
August 2013. Similar results are seen from other hygrometers on the NASA
ER-2 aircraft (Smith et al., 2015). For each of these ER-2 flights,
the back trajectories are presented along with the intersection of
coincident OT. The cases are described below.
An analysis of the 8 August 2013 NASA ER-2 aircraft flight.
(a) Vertical profiles of JLH Mark2 in situ H2O. Back trajectories were initialized
from all aircraft water measurements at 16 to 17.5 km of altitude. (b) Example
back trajectories (thin blue traces) and coincident overshooting convection
(red). Along the NASA ER-2 flight track (orange line), enhanced water vapor
was measured (thick blue lines). This figure identifies where trajectories
and OT are coincident (red squares) within the tolerances prescribed in Sect. 3.2. The green markers are overshooting convective tops within ±3 h
of the red squares to indicate the main regions of convective overshooting
during the 7 days prior to the ER-2 flight and which of those regions
appeared to contribute most to the water vapor enhancement measured on the
flight. (c) Altitude plot of example back trajectories showing coincident
overshooting (red squares). The green markers are overshooting convective
tops within ±3 h of the red squares. The high resolution of the
convective overshooting data meant that there could be multiple coincident
convective overshooting cells for a single location on a back trajectory.
(d) Days between OT and intercept by the aircraft on 8 August 2013.
First case: 8 August 2013
Figure 5 shows the details of the 8 August 2013 ER-2 aircraft flight. This
flight was the transit flight from Palmdale, California (34.6∘ N,
118.1∘ W), to Ellington Field in Houston, Texas (29.6∘ N, 95.2∘ W). In addition to sending the NASA ER-2 aircraft to the
destination base, the scientific goal of this flight was to create five profiles for the North
American monsoon region plus the aircraft ascent and
final descent. This flight shows a dramatic transition (from west to east) of
background stratospheric water to enhanced water. In the lowermost
stratosphere (350 K < θ < 380 K), water can be
highly variable, but at 90 hPa it is generally unusual to observe water
vapor greater than 6 ppmv. As shown in Fig. 5c, there is a gradient in
water vapor from west to east: 4.0 to 4.4 ppmv at 90 hPa (17 km) over the
west coast of CONUS (black and blue points) and greater than 10 ppmv at 90 hPa over Texas (green points). Simultaneous Aura MLS retrievals also
demonstrate a west-to-east water vapor gradient on this day (lines and
filled circles in Fig. 5c). Both JLH Mark2 and Aura MLS water vapor exceed
the thresholds for enhanced water vapor.
An analysis of the 8 August 2013 case is shown in Fig. 6. For clarity, only
some example trajectories (a subset of our analysis) are shown. These are
displayed as thin blue traces in panels (b) and (c). The intersections of
the example trajectories with coincident OT are shown as red squares in
panels (b) and (c). All overshooting convective tops within ±3 h of
the red squares are shown by the green symbols in panels (b) and (c). Back
trajectories from the flight track follow the anticyclonic NAM circulation
over Western Mexico, the Great Plains and the Mississippi Valley (Fig. 6b). Every
one of the example back trajectories intersects OT, as shown by the red symbols
in Fig. 6b. For this flight, coincidences with overshooting convection are
dominated by overshooting clouds over the Mississippi Valley and the Great
Plains. All overshooting convections within the tolerances prescribed (see
Sect. 3.2) for the back trajectories are shown by the green symbols in
Figs. 6b and 5c. Figure 6c demonstrates the range of altitudes reached by
the coincident overshooting convection and how many convective overshooting
cells were coincident. The high resolution of the convective overshooting
data meant that there could be multiple coincident convective overshooting
cells for a single location on a back trajectory. It is significant that
some of the green overshooting cells are higher altitude than the red
coincident points, suggesting that overshooting air parcels descended
slightly before mixing with the surrounding air. Figure 6d indicates that the
source of the enhanced water was dominated by overshooting clouds within
7 days prior to intercept by the aircraft.
An analysis of the 16 August 2013 NASA ER-2 flight. (a) Vertical
profiles of JLH Mark2 in situ H2O; similar to Fig. 6a. (b) Back trajectories
from the aircraft path; similar to Fig. 6b. (c) Altitude plot of back
trajectories showing coincident overshooting (red) and all overshooting
within ±3 h (green); similar to Fig. 6c. (d) Days between OT and
intercept by the aircraft; similar to Fig. 6d.
An analysis of the 27 August 2013 NASA ER-2 flight. (a) Vertical
profiles of JLH Mark2 in situ H2O; similar to Fig. 6a. (b) Back trajectories
from the aircraft path; similar to Fig. 6b. (c) Altitude plot of back
trajectories showing coincident overshooting (red) and all overshooting
within ±3 h (green); similar to Fig. 6c. (d) Days between OT and
intercept by the aircraft; similar to Fig. 6d.
The fraction of back trajectories that intersected OTs during the 7 previous days for the three SEAC4RS flights on 8 August (blue), 16
August (green) and 27 August 2013 (red) shown in Figs. 6, 7 and 8,
respectively.
Second case: 16 August 2013
The NASA ER-2 flight on 16 August 2013 was designed to survey the North
American monsoon in a triangular flight path from Houston, Texas to the
Imperial Valley in Southern California, to southeastern Colorado and back to
Texas. The NASA ER-2 aircraft performed six dives, encountering enhanced
stratospheric water at 16 to 17 km of altitude (Fig. 7a). As shown in Fig. 7b, back trajectories intersect overshooting tops over the South Central
US (Texas, Oklahoma and Arkansas) and also over the Sierra Madre Occidental
mountain range on the west coast of Mexico. This case is an example of the
classic North American monsoon circulation with a moisture source over the
Sierra Madre Occidental (Adams and Comrie, 1997), in which air parcels are
transported from OT in Mexico around the anticyclone to the CONUS (Fig. 7b). The altitude range of the convective overshoot is typically 16 to 17 km, as shown in Fig. 7c. The time between OT and intercept by the
aircraft ranges from 2 to 7 days (Fig. 7d).
Third case: 27 August 2013
The 27 August 2013 flight performed six dives to sample the North American
monsoon. Stratospheric water was enhanced to 15 to 20 ppmv at altitudes
ranging from 16.0 to 17.5 km (Fig. 8a). The ER-2 aircraft intercepted
highly enhanced stratospheric water from a mesoscale convective complex over
the Upper Midwest, which had overshooting tops over northern Minnesota and
northern Wisconsin (Toon et al., 2016), as shown in Fig. 8b. Figure 8c
shows an abundance of OT above 17 km (green). Generally speaking, the OTs
appear at higher altitudes in the northern CONUS and southern Canada than in the
central CONUS. Figure 8d shows that the air masses were sampled in situ by the ER-2
aircraft over Illinois and Indiana 1 to 2 days after the intense storm.
As is a common theme for all these experiment days, a portion of the
back trajectories also trace back to overshooting tops over the Sierra Madre
Occidental 1 week prior.
Conclusions
In this paper, we have examined in situ measurements of stratospheric water taken by JLH
Mark2 on the ER-2 aircraft during the SEAC4RS field mission. With JLH
Mark2 data, enhanced H2O above background mixing ratios was frequently
encountered in the overworld stratosphere between 16 and 17.5 km of altitude.
Back trajectories initialized at 1 s time stamps along the aircraft
flight track at 16 to 17.5 km connect the sampled air parcels to convective
OT within 7 days prior to the flight. The trajectory modeling indicates
that the identified OTs are associated with larger storm systems over the
South Central US (Fig. 6), deep convection over the Sierra Madre Occidental
(Fig. 7) and deep convection over the Upper Midwest US and southern
central Canada (Fig. 8). For all the back trajectories in the three case
studies, the fraction that connects to OT within the previous 7 days
ranges from 30 to 70 % (Fig. 9). The three aircraft flight dates
analyzed in Fig. 9 have a higher fraction of enhanced water than the other
flights. These three flights deliberately targeted air masses influenced by
convection. For the CONUS in general, the fraction of air parcels at 370–420 K influenced by OT is much smaller.
The concentrations of enhanced water and the connection to OT suggests a
mechanism for moistening the CONUS lower stratosphere: ice is irreversibly
injected into the overworld stratosphere by the most intense convective
tops. The temperatures of the CONUS lower stratosphere are sufficiently warm
to sublimate the ice, producing water vapor mixing ratios elevated to 10 ppmv
or more above background levels. The summertime CONUS has a high frequency
of thunderstorms with sufficient energy to transport ice to the upper
troposphere (Koshak et al., 2015 and the references therein). On rare occasion,
these storms have sufficient energy to loft ice through the tropopause and
into the stratosphere. Further evidence of ice is provided by water
isotopologues. Evaporation and condensation are fractionating processes for
isotopologues, especially semiheavy water HDO relative to H2O (e.g., Craig, 1961;
Dansgaard, 1964). Condensation preferentially concentrates the heavier HDO
isotopologue, so lofted ice is relatively enriched in HDO / H2O compared
to the gas phase (e.g., Webster and Heymsfield, 2003 and the references therein).
Ice sublimation is supported by the enriched HDO / H2O isotopic signature
observed by the ACE satellite over summertime North America (Randel et al.,
2010). Cross-tropopause transport is a consequence of turbulent mixing at
cloud top, possibly enhanced by the existence of breaking gravity waves
often occurring near overshooting cloud tops (Wang, 2003). This study
addresses a primary goal of the SEAC4RS field mission (Toon et al.,
2016) and affirmatively answers the following scientific question: do deep convective
cloud systems locally inject water vapor and other chemicals into the
overworld stratosphere over the CONUS? This water is almost certainly
injected in the ice phase and subsequently sublimated in the relatively warm
stratosphere over CONUS, leading to irreversible hydration. From this study,
we conclude that the depth of injection was typically 16 to 17.5 km of altitude
for these particular summertime events.
Satellite retrievals of water vapor from Aura MLS provide a larger-scale
context. The fraction of Aura MLS observations at 100 hPa (approximately 17 km of altitude) with H2O greater than the 8 ppmv threshold is 0.9 % for
July–August 2013. In comparison, Schwartz et al. (2013) report that, for
the 9-year record of 2004–2012, July and August had 1.4 and 3.2 % of
the observations exceed 8 ppmv, respectively. This reinforces the conclusion of
Randel et al. (2015) that OTs play a minor role in the mid-latitude
stratospheric water budget. At the 100 hPa level in the lower
stratosphere, the year 2013 was slightly drier than the average of 2004–2012
summers (Fig. 3). Despite the relatively dry conditions of summer 2013,
there was sufficient enhanced water to be clearly observed in the Aura MLS
retrievals (Figs. 3, 4 and 5). Limb measurements from Aura MLS come from a
∼ 200 km path through the atmosphere with ∼ 3 km
of vertical resolution in the lower stratosphere (Livesey et al., 2016). The
aircraft profiles of water vapor are very similar on ascent and descent (Fig. 5c), which allows us to estimate the horizontal length of
these features as greater than 180 km and a vertical thickness of
∼ 0.5 km. This size is sufficiently large that the MLS
retrieval is sensitive to enhanced water, as shown in Fig. 5c.
In situ measurements probe air parcels on a small-scale that can be connected to
OTs
that inject ice and, to a lesser extent, trace gases into the stratosphere
(e.g., Ray et al., 2004; Hanisco et al., 2007; Jost et al., 2004). In
contrast, modeling studies tend to focus on large-scale processes. Dessler
et al. (2002) and Corti et al. (2008) concluded that OTs are a significant
source of water vapor in the mid-latitude lower stratosphere. In contrast,
Randel et al. (2015) used Aura MLS observations to conclude that circulation
plays a larger role than OT in controlling mid-latitude stratospheric water
vapor in the NAM monsoon region. Our study shows clear evidence of
observable perturbations to stratospheric water vapor on ER-2 aircraft
flights that targeted convectively influenced air during SEAC4RS. In
future work, we plan a more detailed back trajectory analysis of air parcels
over summertime North America to better understand the transport of ice and
water in the lower stratosphere.
Data discussed in this manuscript are publically available. The NASA
aircraft data are available through the following digital object identifier
(DOI): SEAC4RS 10.5067/Aircraft/SEAC4RS/Aerosol-TraceGas-Cloud.
Robert Herman prepared the manuscript with contributions from all coauthors
and was responsible for all aspects of the JLH Mark2 as principal
investigator. Eric Ray and Karen Rosenlof provided trajectory calculations
and interpretation. Kristopher Bedka provided an overshooting top data
product and interpretation. Robert Troy, Robert Stachnik and Keith Chin
operated the JLH Mark2 instrument in the field and downloaded data. Robert Stachnik, Dejian Fu, Lance Christensen and Keith Chin also developed
software components for the JLH Mark2 instrument. Michael Schwartz and
William Read provided Aura MLS data and statistical analysis. T. Paul Bui
and Jonathan Dean-Day measured pressure and temperature with the MMS
instrument and provided data.
The authors declare that they have no conflict of interest.
Acknowledgements
We thank Jose Landeros and Dave Natzic for providing technical support in
the laboratory and the field and the aircraft crew and flight planners for
making these measurements possible. The JLH Mark2 team is supported by the
NASA Upper Atmosphere Research Program and Radiation Sciences Program. Part
of this research was performed at JPL, California Institute of Technology,
under a contract with NASA.
Edited by: R. Müller
Reviewed by: two anonymous referees
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