MISSION
STATUS ARCHIVE:
SCIENCE STATUS
04-25-07 to 02.07.08
Current scientific objectives and
findings
Six baseline mission objectives, crafted in the form of questions,
were formulated in order to address the overall AIM mission
goal including: (1) What is the global morphology of PMC particle
size, occurrence frequency and dependence upon H2O and temperature?;
(2) Does gravity wave activity enhance PMC formation by perturbing
the required temperature for condensation and nucleation?;
(3) How does dynamical variability control the length of the
cold summer mesopause season, its latitudinal extent and possible
inter-hemispheric asymmetry?; (4) What are the relative roles
of gas phase chemistry, surface chemistry, dynamics and condensation
/sublimation in determining the abundance and variability
of water vapor in the polar mesosphere?; (5) Is PMC formation
controlled solely by changes in the frost point or do extraterrestrial
forcings such as cosmic dust influx or ionization sources
play a role?; (6) What is needed to establish a physical basis
for the study of mesospheric climate change and its relationship
to global change? This last objective will be accomplished
by using results from the first five to validate mechanisms
using the NCAR Whole Atmosphere Community climate Model (WACCM)
PMC model and other models. The method of “hindcasting”
will be used with AIM and past satellite data to validate
the models and then forecasting will be done to assess future
changes.
All three AIM instruments are working well and returning excellent
scientific data. Specific contributions of these instruments
and scientific accomplishments thus far are summarized in
the next two sections.
Specific contributions of instruments
SOFIE
The Solar Occultation For Ice Experiment (SOFIE) instrument
began science observations on 14 May 2007. SOFIE performs
solar occultation measurements in 16 spectral bands that are
used to retrieve vertical profiles of temperature, O3, H2O,
CO2, CH4, NO, and PMC extinction at 10 wavelengths. Each day
SOFIE provides 15 sunset measurements at latitudes ranging
from 65° - 85°S (depending on time of year) and 15
sunrise measurements at latitudes from 65° - 85°N.
SOFIE images the sun with a 2D detector array to obtain pointing
knowledge of the FOV position relative to the sun center to
within ±0.1 arc second through image edge analysis.
As a result, SOFIE obtains an unprecedented fidelity of observation
altitude and angle. This allows the inference of refraction
angle profiles, from solar extent measurements, that are used
to retrieve temperature from ~55km down into the upper troposphere,
independent of transmission models. SOFIE is the first occultation
sensor to successfully use this technique to such high altitudes.
A high quality first version of all products, except for CO2
and NO, is now being produced routinely. The latter two products
are being delayed because of minor algorithm and instrument
issues, typical of early periods of satellite missions. These
data will be released soon. The first public release of the
current data occurred in February 2008. Figure 2.2.1-1 shows
the H2O and O3 evolution over the course of the 2007 PMC NH
season illustrating the high data quality. Profiles are shown
at 5-day increments. Note the rapid H2O changes in the profiles
in the early part of the season (green to yellow) . The spectral
bandpass for the water channel was positioned at 2.5 µm,
where the ice refractive index is at a minimum, providing
H2O retrievals that are completely insensitive to PMCs. This
allows simultaneous and totally independent water and ice
measurements, an immeasurable benefit to AIM science.
SOFIE has 8 channels, each consisting of two broadband radiometer
measurements, and their differences measured at high gain.
The difference signals are effectively high precision extinction
measurements of the primary gas for the relevant channel.
Signal-to-noise observed in-orbit has been steady, ranging
from 2.7x105 to 5.0x106, depending on channel, all meeting
or exceeding science requirements.
SOFIE depends on spacecraft attitude control, which has been
excellent, for pointing and maintaining the FOV pointed towards
the center of the sun. Both pointing precision and accuracy
are typically ± 3-5 arc seconds, much better than required.
The experiment is being operated at 100% duty cycle in a 100%
autonomous mode. Data collection success is better than 99%.
All systems have been stable with no degradation detected
or predicted.
CIPS
The Cloud Imaging and Particle Size (CIPS) experiment on AIM
is a wide angle (120° along track by 80° across track)
imager consisting of four identical cameras arranged in a
cross pattern. The CIPS instrument is fully operational on
orbit. All four cameras are performing flawlessly. CIPS provides
images of PMCs with a spatial resolution of 1 x 2 km in the
nadir and about 5 km at the edges of the forward and aft cameras.
The spatial resolution of CIPS is unprecedented, resulting
in a many fold increase over previous PMC measurements especially
with respect to its very large wide angle field of view. Although
limited narrow strips of PMC images were acquired by the MSX
UVISI instrument [Carbury et al., 2003], CIPS provides full
spatial coverage at high resolution over the entire polar
cap. Furthermore, CIPS is unique in its ability to image clouds
at a large range of scattering angles, which is crucial for
ice particle size determination.
The four CIPS camera images are merged and binned to form
a single display we call a scene with an averaged spatial
resolution of 5 x 5 km. A scene is depicted in Figure 2.2.2-1.
The scene spatial coverage is 2000 km along the satellite
track and 1000 km across track. The red and yellow areas depict
strong PMCs in the field of view. As the satellite moves in
orbit, any given cloud is viewed seven times at a large range
of scattering angles.
The orbit-to-orbit changes of the PMC features are revealing
new information regarding their horizontal motions, and ice
particle lifetimes. Measurement of mean particle size is a
critical quantity for understanding the microphysics of PMCs.
The unique ability of CIPS to measure the same PMC region
in seven consecutive images (at seven different scattering
angles) represents an observing technique for measuring particle
sizes not possible from any other ground- or space-based observations.
For the first time, CIPS data now make quantitative comparisons
possible with global models, because of the availability of
near-simultaneous data for temperature, water vapor, meteoric
dust, and other gases that are the key forcing parameters
for mesospheric ice formation.
CDE
The Cosmic Dust Experiment (CDE) on the AIM satellite is an
instrument designed to monitor the variability of the cosmic
dust influx into Earth’s mesosphere in order to address
its role in the formation of PMCs. CDE determines the magnitude
and characterizes the temporal and spatial variability of
the cosmic dust influx, allowing for direct correlation studies
with PMC frequency and brightness.
CDE has a sensor area of approximately 0.1 m2, and can detect
particles greater than approximately 1 ?m in radius, by recording
impact generated signals on thin plastic-film detectors. The
detectors are made of permanently polarized Polyvinylidene
Fluoride films (PVDF), a mechanically and thermally stable,
radiation resistant material. Twelve detectors face the zenith
direction, allowing them to record cosmic dust impacts, while
two detectors are completely covered and located on the underside
of the instrument deck in order to measure the background
noise.
CDE was successfully commissioned on May 23, 2006 and has
thereafter been continuously recording events. Since commissioning,
the instrument has experienced a higher than anticipated level
of noise across all detectors that is uncorrelated with spacecraft
activities. The extreme temperature variations seen by CDE
are a possible noise source that was not anticipated, and
is currently being investigated in laboratory testing. Concurrently,
ground algorithms are being refined to identify and remove
noise events from the science data. These successful algorithms
are based on pattern recognition techniques that exploit the
spatial and temporal properties of the observed noise patterns.
The success of the approach is verified by comparing the cumulative
dust flux measured by CDE to earlier observations (Fig. 2.2.3-1).
The reliability of these newly developed algorithms indicate
that CDE will be capable of resolving the spatial (Fig. 2.2.3-2)
and temporal variability of the cosmic dust influx. An extended
period of observations will greatly improve our counting statistics
allowing for the observation of even the possible year-to-year
variability of the dust influx.
Scientific accomplishments since May
2007 Observatory Commissioning
PMC Maps, Seasonal Morphology,
Structures
Images from the CIPS instrument have been used to generate
the first maps of daily cloud brightness at high spatial resolution
for the northern and southern 2007 seasons, to determine mesospheric
ice particle properties and ice mass, and to unveil new cloud
features. An example of a daily cloud map in the heart of
the cloud season is shown in Figure 2.3.1-1. The pixel size
is 5 x 5 km, making this the best spatially resolved cloud
image ever taken from space. Similar maps are generated each
day of the cloud season and show the time evolution of cloud
presence in great detail. The hole over the pole is due to
lack of data.
In addition to the clear gravity wave features in the CIPS
scenes, cloud features such as 'ice rings' and spatially small
but bright clouds have prompted a fresh look into the dynamics
controlling the summer mesosphere. The ice ring features (see
Figure 2.2.2-1) can be caused by gravity waves, but may also
be due to turbulent mixing from vertically displaced air.
Ice rings appear in circular or oval shapes with diameters
that vary from ten to several hundred kilometers or greater.
They have been photographed from the ground, but never explained.
Another indication of dynamical control of the summer mesosphere
is seen in the relatively frequent appearance of spatially
small (~20 - 30 km diameter) but bright clouds. The cloud
brightness may approach four times the Rayleigh background
at small scattering angles. These bright clouds have not been
reported previously and may be a manifestation of an intense
small-scale upwelling. These represent still another unsuspected
aspect of the complicated dynamics that controls transport
and temperature in the heart of the cloud season.
PMC Environment
SOFIE is producing unprecedented measurements of key parameters
from the lower stratosphere up to 95 km and higher, depending
on product. Temperature, H2O, CH4, and O3 are included in
the January 2008 release, with CO2 and NO to be added in the
next release. The time versus height cross sections of SOFIE
H2O in Figure 2.2.1-1 illustrates the development of water
vapor versus altitude in the PMC environment during the 2007
northern cloud season. Water vapor, an essential gas for PMC
formation, is measured at a vertical resolution of 1.6 km
and precision of less than 21 ppbv. The seasonal evolution
of water vapor is characterized by a steady increase in mixing
ratio, which is caused by vertical transport due to upwelling,
and also by sublimation of PMC particles below ~83 km. The
abrupt decrease in H2O above PMC altitudes is consistent with
a loss of water vapor due to ice growth, and also with destruction
of H2O by increasing photolysis at higher altitudes. An example
of the quality of the underlying profiles is demonstrated
in Figure 2.3.2-1, which shows a typical set of retrieved
water vapor profiles, in this case for August 12, 2007. The
various profiles indicate where PMCs have formed as evident
by reduced water vapor near 85 km, and where PMCs have sublimated
resulting in enhanced water vapor just above 80 km. The mesospheric
ozone minimum is observed near 79 km altitude, with a peak
observed near 90 km (Figure 2.2.2-1). O3 enhancement is observed
at altitudes from roughly 80 to 95 km during times that ice
is present. For example, at 92 km the mixing ratio increases
from 0.8 ppmv on 22 May (30 days before solstice) to ~1.1
ppmv by 11 July, returning to ~0.8 ppmv by 15 August. Siskind
et al. [2007] discuss the increase in O3 above PMCs and determined
that the change was consistent with dehydration above the
ice layer resulting in lowered HOx. These observations of
highly transient features, combined with the other SOFIE products
are giving a first-time detailed look at the physics of PMCs.
PMC Microphysical Properties
The multi-wavelength SOFIE observations are shedding new light
on PMCs with unprecedented sensitivity and signal-to-noise.
While previous instruments suggested that ice is present about
half the time near 70?N, SOFIE now shows that ice is almost
always present at these latitudes during the PMC season. This
is demonstrated in Figure 2.3.3-1a, where a time series of
SOFIE ice occurrence frequency is compared to SME and HALOE
results. Note that downgrading SOFIE sensitivity to be consistent
with HALOE and SME yields good agreement between SOFIE and
the SME and HALOE frequencies, confirming that the lower ice
frequencies recorded by the older satellites were a result
of reduced sensitivity. SOFIE has also demonstrated that mesospheric
ice exists as a continuous layer from ~80 km to altitude to
the mesopause (~87 km) and often higher, as shown in Figure
2.3.3-1b. This view is supported by model predictions [Rapp
and Thomas, 2006], but is in contrast with previous measurements
that were not sensitive enough to detect the most tenuous
ice populations.
These new discoveries were possible because SOFIE is 15 -
200 times more sensitive to PMC particles than previous instruments,
including lidars. SOFIE measures PMC extinction at 11 wavelengths
from 0.330 to 5.006 ?m, with 1.5 km vertical resolution and
a precision of better than 10-7 km-1 in extinction. In addition,
SOFIE contains channels located at the peak in the ice absorption
spectrum (~3 ?m wavelength) where PMC extinction is ~100 times
greater than for surrounding NIR and IR wavelengths. The unique
combination of measurement wavelengths allows SOFIE observations
to be used to determine vertical profiles of PMC ice mass
density, particle shape, and Gaussian size distributions.
Time series of SOFIE ice layer altitudes, occurrence frequency,
mass density, particle shape (axial ratio of a spheroid),
effective radius, and water vapor are also shown in Figure
2.3.3-1 for 69?N average latitude. Ice layers with greater
vertical extent are found to have higher mass densities, which
supports current growth /sedimentation theories which suggest
that thicker saturated regions that are thicker in vertical
extent will provide increased growth time for falling particles.
SOFIE offers the first observations of PMC particle shape
versus altitude. SOFIE observations indicate that PMC particles
are non-spherical at all altitudes, with axial ratios of about
2.2 (i.e., oblate spheroids) at the PMC extinction peak (Zmax).
Examining particle shape as a function of altitude reveals
axial ratios in excess of 3 at altitudes above and below the
extinction peak, and a strong anti-correlation to ice mass
density (Figure 2.3.3-2). These new results suggest that changes
should be made to the current treatment of microphysical processes
including particle sedimentation and possibly nucleation and
growth. SOFIE indicates PMC particle effective radii at Zmax
increasing from ~20 nm in the early season to over 40 nm by
early August. A correlated and steady increase is observed
in water vapor measured at Zmax (Figure 2.3.3-1), in conjunction
with decreasing particle concentrations (not shown). A robust
relationship between particle size and water vapor is found
to hold when examining variability over time at one altitude,
and when examining the altitude dependence of H2O and particle
size. These findings point to a fundamental connection between
ice particle characteristics and water vapor. Increased water
vapor near 83 km is expected due to upwelling and also from
ice sublimation [Hervig et al., 2003], and it now appears
that that increased H2O is in turn modifying the ice particles
characteristics. SOFIE results have been compared to concurrent
PMC measurements from the ALOMAR lidar (69?N) during 2007
[Baumgarten et al., 2008], which were used to determine various
PMC properties including Gaussian size distributions. Example
results are shown in Figure 2.3.3-3, where histograms of the
Gaussian size distribution parameters determined at the extinction
peak altitude from SOFIE and the lidar are compared. The agreement
is excellent for particle concentration, median radius, and
distribution width. The notable difference is that SOFIE indicates
the presence of smaller particles at higher concentrations
than the lidar, a difference that is consistent with SOFIE
having greater sensitivity to smaller particles. SOFIE is
also in good agreement with the range of values indicated
by model results and previous lidar measurements, as shown
in Figure 2.3.3-3.
Recent publications
(on-line bibliography)
The AIM data processing and validation has been moving at
a faster pace than anticipated based on past experience with
large satellite data sets. Even though launch occurred less
than 10 months ago, Nine AIM papers were presented at the
international conference on Layered Phenomena in the Mesopause
Region in Fairbanks Alaska in August, 2007 and 15 were presented
at the 2007 Fall AGU meeting. The fact that so many papers
could be presented so soon after launch attests to the high
quality of the AIM results. In addition to these presentations,
14 papers have been submitted for publication in the Journal
of Atmospheric and Terrestrial Physics. A full AIM post-launch
bibliography and submitted versions of the journal articles
are available on the AIM web site at “aim.hamptonu.edu”.
Impact of results as evidenced
by citations, press releases and other activities
The AIM mission was only recently launched, so there are no
citations of papers published. However, the mission has received
extensive media attention world wide both before and after
launch. A press conference was held at the 2007
Fall AGU meeting that was attended by approximately 15
reporters including major news organizations such as the AP,
Reuters and the BBC. The conference went for about an hour
and had to be terminated because of room use. The mission
has been publicized broadly throughout the world. A few of
the many news organizations that have published AIM
stories either online, in print, or both include: Space
News, Chemical & Engineering News, The BBC World News-Online,
Cosmos Magazine in Australia, Forbes-Online, MSNBC-Online,
The Examiner-Online (Philadelphia and Minneapolis), The Baltimore
Examiner, CBSNews.com, Los Angeles Times- Online, Houston
Chronicle- Online, Washington Post- Online, the New York Times,
USA Today – Online, Northwest Florida Daily News, Discovery
Channel TV in Canada, KFMB-TV (CBS affiliate- San Diego, CA),
WFED-AM (Washington D.C.), WTOP-FM (Washington D.C.), WVEC-TV
(Norfolk, Virginia), WHEC-TV (NBC affiliate -Rochester, NY),
KAAL-TV (ABC affiliate – Rochester-MasonCity-Austin,
IA –MN), and KSL-TV (NBC affiliate Salt Lake City, UT).
The AIM mission was a focus at the Fairbanks, Alaska meeting
in August, 2007 on Layered Phenomena in the Mesopause Region.
Also, even though it was a late breaking session for the 2007
Fall AGU meeting, the noctilucent cloud session attracted
over 40 papers of which 15 were addressing AIM.
Productivity and vitality of
the science team: Ongoing Publishable Research
Baseline Science Question 1:
PMC Microphysics:
Achieving this objective as well as all other AIM objectives
requires, first and foremost, that the AIM data be validated.
Substantial progress has been made toward this goal in the
eight months since data were first acquired. For example,
in Figure 2.6.1-1 CIPS and SBUV PMC brightness measurements
are compared. For this analysis, CIPS data were binned to
match the SBUV field of view, and a cloud detection algorithm
based on the SBUV algorithm [DeLand et al., 2003] was applied.
The agreement is excellent, with average differences of only
2%; we thus have confidence that the CIPS data are valid for
scientific investigations [Benze et al., 2008].
Detailed microphysical models suggest that after nucleation,
PMC cloud particles will grow large enough to fall into a
region of warmer temperatures where they sublimate. The resultant
evaporated H2O can then be re-lofted into the region of cold
temperatures where the condensation/growth/decay process cyclically
repeats. One signature of this process would be a layer of
enhanced H2O lying just below the cloud layer, and AIM measurements
of water vapor are being used to investigate the occurrence
of such water vapor enhancements (see section 2.6.4). The
cycling time for this microphysical process will depend on
upwelling rates, which will determine whether particles remain
buoyant long enough to grow. Investigation of the PMC “rings”
described in Section 2.3.1, which are likely related to mesospheric
convection, is providing additional information allowing this
fundamental hypothesis of PMC formation, growth, and decay
to be tested.
Applied to studies such as these, the AIM measurement complement
is poised to address outstanding issues regarding PMC microphysics.
Using statistical correlations between PMCs and H2O and temperature,
we will isolate which of the two, if either, is the key driver
for cloud formation. We will estimate the amount of water
taken up in clouds and compare this with the measured cloud
densities and particle sizes.
Baseline Science Question 2:
Gravity Waves:
The CIPS UV imagery has provided exceptional data on the structure
and variability of PMCs at high, summer-time latitudes. A
key result is that the PMCs appear to be much more patchy
than previously thought, exhibiting extensive “cloud-like”
features. While a number of strong gravity wave events have
been observed within the polar region they do not appear to
be as numerous as wave events observed at lower latitudes
(as determined from prior ground-based NLC and airglow measurements).
Instead, the CIPS imagery shows a mixture of wave activity
and large convective-like features (which may also be related
to the underlying gravity wave activity). This is illustrated
in Figure 2.6.2-1 which shows a processed CIPS image containing
a well-defined wave event (horizontal wavelength 45 km) with
many coherent crests extending over 1000 km in length. This
event is evident in the upper left of the image, with a distinct
boundary. Elsewhere in the scene the PMC are dominated by
large dark circular structures that AIM has shown to be a
common feature of PMCs.
Our initial research has focused on characterizing the properties
of the most prominent wave events (Chandran et al., 2008),
which comprise a variety of medium and small-scale wave events
(horizontal scales typically 50-300 km), often extending over
exceptionally large areas. Planned research includes:
1. In-depth investigation of occurrence frequency and latitudinal
dependence of gravity waves in the northern polar cap region
to determine their summer-time climatology for the first time.
2. Measurement of the occurrence and properties of convective-like
features and investigation of their potential association
with gravity waves and lower atmospheric sources.
3. Novel inter-hemispheric comparison of gravity wave occurrence
and characteristics at polar latitudes using new southern-hemisphere
PMC data.
4. Exploratory investigation of gravity waves at lower latitudes
using their stratospheric ozone signal in the CIPS data.
5. Modeling the effects of the observed gravity wave events
on the nucleation and growth and/or dissipation of PMCs
Baseline Science Question
3: Temperature Variability:
We are beginning to understand the dynamics that controls
the occurrence of clouds in the high latitude summer mesopause
region. We have identified wave modes in the CIPS data that
compare favorably with waves seen in the SABER temperature
fields [Merkel et al., 2007]. We have developed a model (NOGAPS-ALPHA)
which can validate the temperature and H2O fields observed
by SOFIE, simulate the waves observed by CIPS and can hindcast
the locations of the bright clouds seen by CIPS. One important
next step is to quantify and understand the variation in cloud
occurrence, brightness and properties between the northern
and southern hemispheres. This is a key part of Objective
3 as defined in the original AIM proposal. Since the proposal
was written new results have confirmed that interhemispheric
differences exist; the AIM data will allow us to better quantify
those differences and to understand them for the first time.
We are just now collecting data for our first southern summer.
However, even at this early point, important differences have
emerged between the two hemispheres. In general, SH clouds
are dimmer and less frequent. This is summarized in the SOFIE
data shown in Figure 2.6.3-1. The left hand figure is current
data for the NH (top row: cloud altitude, bottom row: occurrence
frequency) while the right hand column shows data from the
2007 SH.
Whereas clouds are practically ubiquitous in the NH data,
with an occurrence frequency greater than 95% at solstice,
in the SH, the occurrence frequency hovers in the 70-90% range.
For the brightest clouds, this difference is greater; there
is up to a 3X greater chance of seeing a bright cloud in the
NH. This is also reflected in the CIPS imagery. We are just
now beginning to compare the albedo differences seen by CIPS
with the frequency and brightness differences seen by SOFIE.
Furthermore, with the detailed composition measurements of
SOFIE, we can put these differences on a physical basis. Preliminary
data suggest that SH clouds are 3 km higher than in the NH
(this is a greater difference than suggested from ground based
measurements) and take up less overall H2O. The region from
80-90 km appears to be several degrees K significantly warmer
in the SH, relative to the NH (see Figure 2.6.3-2). This is
consistent with HALOE (Hervig and Siskind, 2006), but with
the more precise SOFIE data and more consistent polar sampling,
we will be able to better study these differences throughout
the season.
Further, unlike the HALOE data which had to be averaged over
11 years, with each year in the AIM extended mission, we will
be able to build up a true year-by-year climatology of these
differences. This is critical because there is currently great
debate within the community as to the cause of the temperature
difference between the NH and SH. Possibilities such as the
orbital eccentricity [Lubken and Berger, 2007], differential
filtering of gravity waves [Siskind et al., 2003], and teleconnections
to the winter hemisphere [Karlsson et al., 2007] have all
been raised. We will address these questions with three dimensional
models of the atmosphere. The AIM data sampling pattern is
identical in latitude for both hemispheres (though different
in local time) and nearly identical form year to year, ideal
for evaluating trends and hemispheric differences.
Baseline Science Question 4:
Hydrogen Chemistry:
PMC particles are composed of ice that forms through condensation
of water vapor under supersaturated conditions. Thus understanding
the sources and sinks of water vapor is foundational to understanding
the formation and evolution of PMCs. The objective of the
Hydrogen Chemistry investigation is to understand the abundance
and variability of water vapor along with the traces gases
which are related to water vapor through chemical and dynamical
processes. Study of HALOE water vapor measurements in the
polar summer regions led to discovery of a narrow water vapor
layer coincident with the PMC layer (Summers et al., 2001).
This layer has important implications for the formation and
growth of PMC particles and may be an important diagnostic
for water vapor sequestering by ice particles.
Figure 2.6.4-1 shows that initial results for SOFIE water
vapor confirm an enhancement of water vapor within and near
the bottom of the PMC layer as observed in the HALOE data.
This layer develops simultaneously with the PMC layer as shown
by Hervig et al. (2003) for HALOE data. The dramatically improved
S/N of SOFIE over HALOE, along with SOFIE’s better vertical
resolution, provide a much more accurate picture of water
vapor and its connection to PMCs than was possible with HALOE
data. SOFIE measurements of water vapor reveal a smaller and
narrower peak in the water vapor layer near 82 km than that
seen by HALOE (a 12 year climatology of HALOE measurements).
These differences in the layer’s peak value and thickness
persist throughout the PMC season. In monthly averages where
the HALOE peak values typically exceed 10 ppmv and occasionally
reaches 13 ppmv during July and August, the SOFIE water rarely
exceeds 8 ppmv. These differences may be explained by the
required averaging of HALOE measurements that resulted in
denser thinner clouds that vary with altitude being averaged
into thicker less dense clouds. The HALOE 6.6 µm location
of the HALOE water channel also endured spectral interference
from ice and lower S/N, both factors likely producing positive
biases in retrieved water results.
We believe the formation of the enhanced water layer is due
to sedimentation of PMC particles which is then followed by
sublimation near the base of the PMC layer (see Section 2.6.1).
Taken at face value, the lower values of water vapor in the
PMC layer as seen by SOFIE data imply significantly less sequestration
of water than was implied by HALOE. The vertical width of
the water layer in SOFIE data is very narrow, approximately
3-5 km max, whereas the HALOE layer was broader by more than
a factor of two, probably due to the averaging and lower vertical
resolution. Also, HALOE showed an indication of a second,
albeit smaller, layer near 75 km. There is no evidence of
this second layer in the SOFIE data. SOIFE is leading to an
improved understanding of the HALOE data analysis.
Changes in the amount of water vapor within the PMC layer
near 82 km might be driven by changes in residual circulation
(Karlsson et al., 2007), solar UV, temperature (DeLand et
al., 2003), or possibly even diurnal effects. Any of these
could influence the PMC water vapor layer. If the peak water
vapor is connected to solar cycle effects, then future years
may exhibit higher values of water vapor as seen in some years
of the HALOE observations. Also, SOFIE and HALOE sampling
is not identical, so diurnal effects may explain some of the
differences in water vapor seen by the two instruments. Current
validation efforts are underway to understand these differences.
Whether the differences are due to better SOFIE S/N, changes
in mesospheric circulation, or perhaps diurnal effects, the
new SOFIE data will have important implications for understanding
the processes which control the water vapor sequestration
by PMC particles.
Photolysis of mesospheric water vapor initiates the catalytic
odd-hydrogen (HOx) destruction of ozone in the upper mesosphere.
Recently published results from HALOE [Siskind et al., 2007]
suggest that ozone is enhanced above the PMC cloud layer.
This might be due to dehydration and the expected anticorrelation
between odd oxygen (Ox) and odd hydrogen chemistry. However,
in the Siskind et al. [2007] study the effects of PMCs on
temperature were shown to depend upon several poorly understood
model assumptions. The use of HALOE temperatures in that study
was hindered by the fact that the HALOE temperature retrieval
becomes less valid above 80 km where PMC’s form. SOFIE
water vapor and ozone data are of vastly better quality than
the HALOE data and thus have the potential of significantly
improving our understanding of the processes which control
mesospheric ozone and its variation.
Baseline Science Question
5: Nucleation:
Most microphysical studies argue that meteoric smoke is the
primary PMC nucleation site by default. This is because homogeneous
and ion nucleation of PMCs are highly improbable under mesospheric
conditions [e.g. Witt, 1969; Gumbel, 2003]. A set of measurements
relevant to the topic of cloud nucleation is that from the
Cosmic Dust Experiment (CDE) on AIM. CDE determines the magnitude
and characterizes the temporal and spatial variability of
the cosmic dust influx, allowing for direct correlation studies
with PMC frequency and brightness.
As CDE data accumulates we expect to be able to better identify
the spatial and temporal variability of the dust influx (Section
2.2.3). A preliminary comparison of a model based on radar
observations to the CDE measurements of the latitude dependence
of the dust influx is shown in Figure 2.2.3-1. The absolute
number of hits per day is qualitatively similar and we are
studying the differences as more data becomes available.
The resultant distribution of smoke from ablating meteors
is critical in quantitatively understanding PMC microphysics.
However, smoke has only been measured below 20 km [Murphy
et al., 1998] and its vertical distribution is only modeled
above this altitude [e.g. Hunten et al., 1980]. As a result,
smoke concentrations are uncertain to several orders of magnitude,
which lead to uncertainties of at least a factor of two for
modeled cloud brightness and particle radius [Rapp and Thomas,
2006]. Furthermore, recent work has shown that coagulating
smoke to sizes large enough for PMC nucleation (~1 nm) is
slow compared to meridional transport so that smoke concentrations
in the polar summer are extremely low [Megner et al., 2008].
This has only heightened interest in the problem.
We are currently studying the SOFIE data for evidence of smoke
particles in the mesosphere. This was never considered possible
when casting the original AIM science goals but the unprecedented
sensitivity of SOFIE may allow for this detection. The weak
smoke signal can be compromised by the presence of PMCs, so
we are focusing on the SOFIE data from the winter hemisphere.
The challenge in isolating the smoke signal is in distinguishing
its weak extinction from the Rayleigh scattered signal in
the upper mesosphere. A positive smoke detection would be
the first of its kind in the mesosphere and provide new and
valuable constraints to models of PMC formation.
Baseline Science Question 6:
Long Term Change
Our original proposal asked: What is needed to establish a
physical basis for the study of mesospheric climate change
and its relationship to global change? This objective is the
most far-reaching goal of AIM, and thus has the longest time-table,
perhaps requiring data from all four seasons of the nominal
AIM mission. The necessary milestones are: (1) validation
of the AIM instruments, and (2) establishing causal links
between PMC properties and the forcing variables.
Validation: An important milestone in validation of CIPS with
the well-calibrated SBUV experiments (three of which are operating
contemporaneously with AIM) has already been accomplished,
as discussed in Sec. 2.6.1. The continuous time series of
data from eight identical SBUV experiments indicate a clear
increase of PMC brightness and frequency over the past three
decades [DeLand et al., 2007]. In addition, the SOFIE extinction
measurements are being compared to HALOE measurements (Section
2.3.3). Validation of CDE is more challenging since this is
the first spatially and temporarily resolved measurement of
the dust influx. However, the total average influx reported
by CDE shows a good agreement with existing models based on
a variety of sources, including in situ measurements by the
Long Duration Exposure Facility (LDEF), and remote sensing
radar observations (see Figure 2.2.3-1).
Establish causal links: This involves establishing statistical
relationships between PMC properties and the forcing atmospheric
variables, which are believed to be temperature, water vapor
and cloud seed nuclei [Thomas, 1991]. SOFIE and CIPS measure
both PMC and the relevant atmospheric variables in a common
volume. The AIM data set, plus the global TIMED SABER measurements
of water vapor and temperature, provide for the first time,
the data needed to establish these causal links. Another tool
essential to this endeavor is a comprehensive model that includes
coupled chemistry, dynamics and ice microphysics. Dynamical
transport of ice particles between their genesis in the cold
polar regions and their appearance at lower latitudes may
be a critical element in this problem [Berger and von Zahn,
2007]. Two versions of the WACCM GCM, combined with detailed
ice nucleation, growth, bulk motion, and sublimation are now
available for detailed simulations of both the dynamics and
microphysical processes [Marsh et al, 2007; Bardeen et al.,
2007].
Training Young Professionals
The education of future scientists is a priority with the
AIM science team and we actively involve students in every
facet of the mission. Currently there is one post doctorial
scientist, ten graduate students working toward doctoral degrees,
and six undergraduate students. Our students work on every
aspect of the AIM science questions and initiatives including
data analysis, modeling, laboratory data analysis, and algorithm
development. Eight of the graduate students and one undergraduate
student have been the first author on papers presented at
scientific meetings and will be first authors on papers submitted
to peer reviewed scientific journals that have been submitted
for publication. In addition, 24 undergraduate and 5 graduate
students have participated in the AIM satellite control activities
at LASP.
|