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chemical reactions leading ultimately to ozone
loss and radiative cooling.
The goals of this project are to evaluate the
GEOS-5 model, in particular relating to model
behavior in the Arctic and Antarctic
stratospheres in light of the key role of PSCs in
ozone loss. Model transport of long-lived
tracers is a focus, as well as the calculation and
observation of PSCs, the climatology of the
polar winter stratosphere being an important
factor in ozone loss. Comparisons with
Microwave Limb Sounder (MLS) data are used
to gauge the accuracy of GEOS-5 transport
within the polar stratosphere. In addition, the
GEOS-5 assimilation system is used in
conjunction with Atmospheric Infrared Sounder
(AIRS) data to identify PSC locations.
Accomplishments during the Reporting Period
Several GEOS-5 simulations spanning seven
years were conducted to examine transport
characteristics of the model. The GEOS-5
model is run under “online CTM” conditions,
which allows analyzed wind, temperature, and
water vapor measurements to drive the model
dynamics. Simulations were compared to the
GEOS-4 model run under similar conditions, as
well as to MLS data.
The mixing ratios of N2O and NOy in the
Northern polar stratosphere calculated by the
GEOS-5 CTM were compared to observations
from the MLS instrument. Because of the
relative inertness of these compounds, this
comparison informs the overall evaluation of
transport characteristics of the GEOS-5 model.
The two tracers have somewhat differing
properties in the winter stratosphere; while both
are chemically inert, HNO3 is incorporated into
PSCs at low temperatures. Therefore, while
N2O can be used predominately to evaluate
transport to and through the polar stratosphere,
HNO3 can be used in conjunction with that
information to ascertain the accuracy of the
GEOS-5 PSC calculations.
The GEOS-4 model was found to have a
significant high bias in the calculated N2O
mixing ratio compared to MLS measurements,
likely indicative of an erroneously high degree
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CODE 610.1
of mixing in the GEOS-4 model; in particular,
high N2O values associated with the tropical
stratosphere are too readily transported to the
Arctic stratosphere. In contrast, no such bias is
readily observed in the GEOS-5 results,
indicating that mixing is greatly reduced in
GEOS-5 compared to GEOS-4. Similar
comparisons with MLS HNO3 measurements
indicate that the current PSC formation module
used in both GEOS-4 and GEOS-5 causes an
erroneously high degree of calculated PSCs.
In addition to examining the transport
diagnostics, the PI expanded a technique to map
PSCs using observed-minus-forecast (O-F)
residuals from assimilated Atmospheric Infrared
Sounder (AIRS) brightness temperature data in
certain channels. Brightness temperatures were
computed from six-hour GEOS-5 forecasts for
several hundred AIRS channels using a radiation
transfer module. The differences between
collocated AIRS observations and these
computed values are the O-F residuals in the
assimilation system, and contain qualitative
information about PSCs due to the clear-sky
condition required by the radiation transfer
module. Three separate AIRS channels were
used in conjunction to identify PSCs; the O-F
threshold indicative of a PSC for each channel
was identified using CALIPSO measurements.
The three-channel technique is a key
improvement to the one-channel detection
method used previously, and largely eliminates
interference arising from high-altitude cirrus
clouds. The advantages of the technique include
the direct measurement of a PSC, as opposed to
the passive technique of identifying a PSC based
solely on depressions in H2O or HNO3 fields, as
well as excellent spatial coverage from the AIRS
instrument allowing for the tracking of
individual clouds from formation to dispersion.
Several high-resolution GEOS-5 model runs are
also underway, with the purpose of studying a
recently observed stratospheric sudden warming.
Objectives for the Coming Year
Results from the PSC detection technique using
AIRS O-F residuals, including the detection of
infrequent Northern hemisphere ice clouds, will
be submitted for publication to a scientific