Causes and Consequences of Mesospheric Water Vapor Layers
Address the causes and consequences of the layered water vapor distribution
observed in the low latitude mesosphere (Summers et al., Science, 277, 1967, 1997a;
Summers and Siskind, GRL, 26, 1837, 1999) and its connection with layer(s) of elevated water
vapor observed at high polar latitudes (Summers et al., GRL, 28, 3601, 2001). The former are
arguably a consequence of heterogeneous chemistry, whereas the later may be due to both
water condensation/sublimation physics associated with polar mesospheric clouds
(Stevens et al., GRL, 28, 4449, 2001; Hervig et al., GRL, 28, 971, 2001) and heterogeneous
chemistry. This research will use UARS HALOE water vapor observations (1991-present) to
characterize the mesospheric water vapor layer and its annual variation
1) use HALOE observations (1991-2005) to characterize the mesospheric water vapor
layer and its annual variation (formation and evolution of the water vapor layer).
3) investigate the role of heterogeneous chemistry and surface/gas exchange on
meteoric dust as a source of the water vapor layer, and
Not much here—only articles.
4) explore the possibility that this layer is causally connected to the formation and
evolution of polar mesospheric clouds.
Science Question: (not answered…)
Can heterogeneous chemistry or surface/gas exchange on meteor dust provide the required
source of water to explain the observed mesospheric water vapor layer and its annual
variability, and is this layer causally connected to polar mesospheric cloud formation?
Revised:
What is the variability of water vapor in the mesosphere? How is the observed mesospheric
water vapor layer connected to polar mesospheric cloud formation? What is its annual
variability?
Sources & circulation
The dominant sources and sinks of water vapor in the stratosphere and mesosphere
are generally thought to be reasonably well understood (Allen et al., 1984; Brasseur and
Solomon, 1986; LeTexier et al., 1988). Water vapor enters the tropical stratosphere though
upward advection from the troposphere. The cold tropospheric temperature acts as a valve
to severely limit the amount of water vapor that can actually enter the stratosphere. In
addition, this upward advection carries tropospheric methane into the stratosphere where it
is oxidized to form approximately two water molecules for every methane molecule. These
two sources produce a water vapor profile with a mixing ratio increasing with altitude in
the stratosphere, peaking between 50-60 km altitude at a value of 6-7 ppmv (Summers et
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al., 1997b). Above this altitude the photolysis of water and its reaction with O(1D) leads to a
decreasing water vapor mixing ratio with altitude. This general picture is illustrated in Figure
1 where we show a zonal average V. 19 water vapor distribution taken from the HALOE
(Russell et al., 1993) experiment on the UARS satellite for the time period late March through
April, 2002 (http://haloedata.larc.nasa.gov/home.html). The water vapor abundance at low
latitudes generally peaks between 0.4 and 1.0 mbar (approximately 46 – 55 km).
Water layer
However, it is evident from an examination of HALOE mesospheric water vapor that a
secondary peak is present between 60-70 km altitude (~0.05-0.01 mbar). This second peak
reflects a layer of water vapor that is completely unexpected on the basis of classical gas
phase chemistry (Summers et al., 1997a; 1997b; Summers and Siskind, 1999). The
location of this second layer is exactly where conventional mesospheric chemistry predicts net
water vapor loss, not production (Brasseur and Solomon, 1986). The fact that there is a
peak in water vapor mixing ratio at that location implies a local source (Summers and Siskind,
1999). Confirmation of this layer’s existence comes from independent satellite measurements
of enhanced OH emission from this region of the atmosphere made with the MAHRSI
experiment (Conway et al., 1996; 1999; 2000; Summers et al., 1997). This second layer of
water vapor is a robust feature of the tropical mesosphere, and is present throughout the
year as observed in almost 12 years of HALOE measurements (see also Summers and
Conway, 2000)
… and it appears in the 14-year average.
Although this layer most often appears limited to low latitudes (within 35o of the
equator), it sometimes appears morphologically extended to high latitudes, especially in the
summer hemisphere. This is seen in Figure 1 where the layer appears to dip slightly in altitude
at mid-southern latitudes before it rises to peak near 0.03 mbar (70-75 km) at high southern
latitudes. Figure 2 shows the analogous situation for the northern hemisphere where the low
latitude layer extends to high northern latitudes.
Recent analysis of high latitude HALOE water vapor data show that this mesospheric
water vapor layer extends to Polar Mesosphere Cloud (PMC) latitudes, 65-70oN (Hervig et al.,
2003) as seen in Figure 3, but at a lower altitude than where PMCs form (typically 80-83 km
altitude). Remarkably, this lower altitude enhanced water vapor layer appears to evolve
simultaneously with the enhanced water observed coincident with the PMC layer as shown in
Figure 4(b), and may even “pre-condition” the formation of PMCs by increasing the relative
humidity of the mesosphere and thus decreasing the temperature drop necessary for
condensation to occur (Summers et al., 2001).
The source of the secondary H2O layer in the 60-75 km range is very unlikely to be
condensation/evaporation of ice particles because the atmospheric temperatures there are much
too high for ice particle formation (Reid, 1975, Thomas, 1991). One possibility is conversion
of H2 to H2O by heterogeneous chemistry (Summers and Siskind, 1999). An important clue
here is that the peak H2O mixing ratio is about 8 ppmv, almost exactly the amount of H2O that
would result is all available H2 is converted to H2O. This suggests an unconventional form of
mesospheric chemistry that produces this result.
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Figure 1. Water vapor daily averages from HALOE sunrise measurements during early
Spring, 2002.
Figure 2. Water vapor daily averages from HALOE Sunset measurements for late-spring early
summer, 1999.
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Figure 3. Average profiles based on HALOE measurements from 65°–70°N during 1992–2002
in 10-day time bins. a) Water vapor profiles corresponding to the beginning and end of the
PMC season. b) H2O(ice) compared to the difference between the August 4 and June 5 H2O
profiles (“H2O increase”), from Hervig et al. (2003).
Heterogeneous chemistry
Summers and Siskind (1999) proposed heterogeneous chemistry on the surface of meteoric
dust as a possible means of producing the mesospheric water vapor layer. The reaction
O + H2  H2O
(1)
on meteoric dust is a plausible means of conversion of H2 back into H2O, its source molecule.
Meteoric dust is produced in the atmosphere as a consequence of ablation of incoming cosmic
dust particles, followed by re-condensation (Hunten et al., 1980). Most of these re-condensed
particles are expected to be very small, on the order of 1 nm in radius. Middle atmospheric
winds will exert a controlling influence on the distribution of such small particles. For
example, at low latitudes vertical upwelling will counter their sedimentation, as seen in Figure
5, where the sedimentation time scale is shown for meteoric dust particles of 1, 3, and 10 nm.
The time scales for vertical advection, eddy mixing, and meridional (horizontal) transport here
were obtained from a two dimensional chemical-dynamical model (Summers et al., 1997b) and
averaged over a latitude within 5o of the equator. The coagulation time scale is shown for a
Gaussian dust distribution of dust centered at 70 km altitude with a 6 km half width (see
Summers and Siskind, 1999 for more details). As suggested by the figure, vertical advection
will tend to confine small (1-10nm) dust particles to region between ~60-80km in altitude. The
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ultimate loss of these small particles will be due to both coagulation to form larger particles
that fall faster, and meridional (horizontal) advection to higher latitudes.
Figure 4. Seasonal cross sections of HALOE measurements for 65°–70°N during 1992–2002
averaged in 10-day time bins. Shown are a) temperature, b) water vapor, and c) the water vapor
resulting from PMC evaporation, H2O(ice). d) The average water vapor and H2O(ice)
measured at 83.1 km altitude, with standard deviations indicated by shaded regions. The
increase in water vapor after PMC appearance (“H2O increase”) is also shown. The H2O
increase was determined by subtracting the water vapor amount (4.8 ppmv) for the first time
bin containing PMCs (June 5) from the entire H2O time series (from Hervig et al., 2003).
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Figure 5. Comparison of sedimentation, advective, mixing and coagulation time scales for
meteoritic dust at low latitudes (Summers and Siskind, 1999).
Although the mixing ratio of H2 is expected to be roughly constant in the 60-70 km
altitude region, the abundance of atomic oxygen increases dramatically with altitude over this
range (Brasseur and Solomon, 1986). The enhancement of meteoric dust and the high
abundances of O in the 60-80 km altitude region provide necessary conditions for reaction (1)
to proceed. One also expects the dust layer to evolve similarly to the observed water layer, i.e.,
the dust layer will move to higher altitudes in the summer hemisphere in response to
predominantly upward advection in that hemisphere (driven by solar heating and wave-mean
flow momentum forcing).
Figure 6 shows two-dimensional model calculations of the effect of reaction (1) on the
water vapor abundance, assuming an imposed distribution of meteoric dust. For comparison,
the observed water vapor mixing ratio obtained from a 5 year climatology of HALOE data,
averaged over a 6 week period after equinox, and within 5o of the equator, is also shown. This
H2O data is the same as in Figure 2 of Siskind and Summers (1998) and encompasses the
period studied by Summers et al. (1997a). Here we show two-dimensional model results for
the trace gases H2O and H2 for three different model assumptions. Model case So assumes only
standard gas phase chemistry (DeMore et al., 1997). Model case S1 is the standard model with
the additional of surface chemistry for converting H2 to H2O via equation (1), an assumed
distribution of meteoric dust surface area per unit volume that is constant with altitude in the
mesosphere (cf. Hunten et al., 1980), and a reaction probability of 0.01. Model case S2
assumes a Gaussian distribution of dust peaking at 70 km altitude, with a half width of 6 km,
and a reaction probability of 0.02. Assuming a very modest reaction probability of 0.01-0.02
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leads to the conversion of all H2 into H2O, right at the region where the mesospheric water
vapor layer is observed.
Figure 6. (a) Observed water vapor mixing ratio obtained from a 5 year climatology of
HALOE data, averaged over a 6 week period after equinox, and within 5o of the equator, and
comparisons with two-dimensional model results utilizing gas phase chemistry and putative
heterogeneous chemistry on meteoric dust. (b) Chemical H2O and H2 production (P) and loss
(L) rates for gas phase chemistry and heterogeneous production of H2O (S).
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This entire section goes with the model program…
The two dimensional model calculations shown above suggest that heterogeneous
chemistry on meteoric dust may play an important role in the formation of the mesospheric
water vapor layer, but they are not conclusive. Perhaps the most critical assumption in the
model calculations is that regarding the distribution of meteoric dust. Although the assumed
distribution is plausible based upon a comparison of transport timescales, it has not been
demonstrated quantitatively using an atmospheric model that includes meteoric ablation,
condensation, coagulation, and dust transport. The Hunten et al. (1980) paper was onedimensional and completely ignored the influence of atmospheric winds. The only published
study of the influence of atmospheric winds on meteoric dust that we have been able to find is
that of Fiocco and Grams (1971). However, the meridional transport model used in that study
is vastly out of date with current observations and models of the dynamical structure of the
middle atmosphere. Furthermore, Fiocco and Grams (1971) ignored coagulation as a loss
process for small dust particles, which as can be seen from Figure 5 has a characteristic time
scale comparable to those for vertical and horizontal advection in the region where the
mesospheric water layer is most apparent.
…comparison with zonal averaged HALOE water vapor data.
Importance for understanding the state and variability of the mesosphere:
The photochemical breakup of water vapor is the source of odd-hydrogen species (HOx
= H + OH + HO2) in the middle atmosphere (Brasseur and Solomon, 1986). This
photochemical breakup is a direct consequence of the absorption of solar ultraviolet radiation,
and thus the magnitude of the odd-hydrogen source follows closely the abundance of water
vapor and the variation in solar activity. Odd hydrogen controls both the catalytic loss of
ozone in the mesosphere and the recombination of atomic oxygen to form molecular oxygen.
The condensation of mesospheric water vapor in the summer polar mesosphere is the source of
Polar Mesospheric Clouds (Garcia, 1989), which may be harbingers of climate change as a
result of increasing atmospheric CO2 and humidity (Thomas et al., 1989; Thomas 1991), and
are the focus of the NASA AIM (Aeronomy of Ice in the Mesosphere) SMEX mission to be
launched in September, 2006 (One central goal of AIM is to understand the connection
between mesospheric water vapor and the formation of PMCs). If heterogeneous chemistry
does indeed occur on meteoric dust particles in the polar mesosphere then this may “precondition” the atmosphere for PMC formation by increasing the relative humidity of the
mesosphere. Meteor dust itself may also act as condensation nuclei for PMC particles
(Thomas, 1991). Water vapor is also an infrared active gas and plays an important role in
determining the thermal state of the middle atmosphere. Furthermore, the chemical destruction
of middle atmospheric water vapor fuels the production of H which diffuses into the upper
atmosphere where it eventually escapes from the Earth (Brassuer and Solomon, 1986).
“The loss of hydrogen to space is compensated by the continuous production of H by
water vapor photolysis above 60-70 km altitude.” Brasseur & Solomon, Aeronomy of the
middle atmosphere, p 129.
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Thus understanding the sources and sinks of mesospheric water vapor is at the heart of
understanding the state, variability, and long term evolution of the middle atmosphere and in
particular its response to natural (e.g. solar) and anthropogenic forcing.
III. Approach:
In the proposed work we will [deleted] … the comparison with zonal averaged HALOE
water vapor data.
As a by-product we will ascertain the amount of meteoric dust available to act as
condensation nuclei for PMCs, and determine the degree to which the mesospheric water vapor
layer acts to “pre-condition” the polar mesosphere for the formation of PMCs. Here we
describe the two central models that will be used for this study. The HALOE water vapor data
and some of the basic characteristics of the mesospheric water vapor layers have been
discussed extensively elsewhere (Russell et al., 1993; Summers et al., 1997; Summers et al.,
2001; Hervig et al., 2001; Hervig et al., 2003).
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Figure 7. Results from the CARMA model of PMC formation. Note the evolution of water
vapor and water ice as a function of time and altitude. Contours of ice are in equivalent water
vapor (in ppmv).
IV. Research plan:
Year 1:
During the first year the focus will be on characterizing the zonal average and seasonal
behavior of the mesospheric water vapor layer as seen in the HALOE V.19 H2O data base.
The goal will be to develop a climatology of the water vapor layer to illustrate the “typical”
annual behavior, although some year-to-year differences do appear. The initial approach will
be to repeat the analysis of Hervig et al. (2003), which addressed water vapor profiles at 65o70oN and for time periods just prior to and during the PMC season, but for all latitude ranges
observed by HALOE and at all times during the year (probably weekly on monthly averages).
The final result will be a “movie” of the behavior of the mesospheric water vapor layer against
which to test the model simulations.
dust will represent a sink for all HOx species, along with O3, CO, H2O2, sulfates, etc. in
addition to O and H2
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