SPARC CCMVal PhotoComp-2008
GOALS. Evaluate how models calculate photolysis (and indirectly
heating) rates in the stratosphere and troposphere with the incentive of
locating errors or biases and identifying improved and practical methods. There
are three basic parts to PhotoComp2008:
(1)
Basic test of all J- values for high sun (SZA=15º), w/ & w/o additional scattering
layers (stratiform clouds & stratospheric
volcanic aerosols).
(2)
Test of twilight, sphericity, and 24-hour averages
(SZA = 84º - 96º).
(3)
Test of wavelength integration w/o scattering (SZA = 15º).
There will be one standard
atmosphere, whose primary definition will include air mass, ozone mass, and
temperature in each layer. This
atmosphere is typical of the tropics, ozone column = 260 DU. For efficiency, we will use this same
atmosphere in all sections, even the low-sun, polar cases.
PARTICIPATION. This study is designed to aid development and
testing of the photolysis and short-wave heating codes used in
chemistry-transport models and coupled chemistry-climate models. This project is open: any research group can
participate by running the experiments and reporting the results as specified
below. We also encourage participation
from groups (without CTMs or CCMs) who have participated in other
model-measurement studies (e.g., IPMMI, POLARIS). Many CTM/CCMs will be using “the same”
photolysis scheme (e.g., fast-TUV, fast-J) and think their participation
redundant – this is false. The
implementation of a standard scheme into any CTM/CCM will likely alter
(intended or inadvertent) how the J-values are calculated: thus it is very important when you perform
these tests that the photolysis module that is as close a possible to that
embedded within the CTM/CCM and not the original, standalone version that you
used to derive your inline model.
EXPERIMENTS.
Part 1 is a basic test
of all J-values for high sun (SZA = 15º) over the ocean (albedo = 0.10, Lambertian).
The primary atmosphere (Table 1a)
is specified in terms of pressure layers, mean temperature, and column O3
in each layer. Please do not include absorption
by NO2 or other species in calculating optical depths. For 1b and 1c we recommend that you use the
specified optical properties in Table 1c, interpolating across the 5 specified
wavelengths.
Part 2 tests the
simulation of a spherical atmosphere and twilight conditions that are critical
to the polar regions.
Use the same atmosphere as Part 1 without clouds or aerosols. Assume equinox (solar declination = 0º) and a
latitude of 84ºN. The surface SZA (not
including refraction) varies from 84º (
Part 3 tests the accuracy
of wavelength binning in the critical region 290-400 nm that dominates
tropospheric photolysis. Shut off all
Rayleigh scattering and surface reflection (albedo = 0) giving effectively a simple
Beer’s Law calculation. Repeat the
calculation in Part 1, but report only J-values for J-O3 (i.e., total), J-O3(1D) [O3 => O2 + O(1D)],
and J-NO2 [NO2 => NO + O].
These are the two critical J-values for the troposphere, and they both
have unusual structures in absorption cross section and quantum yields. The organizers will make these calculations
using very high resolution (0.05 nm) cross sections and solar fluxes and for
different options (e.g., JPL-06 vs. IUPAC cross sections) to provide a
benchmark. NOTE that we will only use
results below 20 km (L=
DIAGNOSTICS.
Model Documentation
should include a brief outline of the methods and any references (limit: one
page). Please include brief notes on:
how you treat sphericity and refraction, the
Schumann-Runge bands (J-O2 and J-NO), Rayleigh
scattering, multiple scattering, clouds and aerosols, seasonal changes in
sun-earth distance, solar variability, and any specific parameterizations. Default cross sections are JPL-2006, please
note if you are using alternate.
Report all J-values
and all standard model layers since this is a check on all modeled J-values,
not just the radiative transfer solution.
See Examples or web PDF file for data formatting. We are not specifying the day-of-the-year, so
use solar fluxes for sun-earth distance = 1.0 au and average over the 11-yr
solar cycle if possible. UCI's
high-resolution solar spectrum used in these experiments is the average of two
high and low SUSIM spectra (
DISCUSSION.
Implementation
into a particular model's code will up to the participant. For example,
UCI has two models that they will use in PhotoComp: a fast-JX model within the CTM that uses
layers of uniform composition defined by mass (kg/m2); and a
stand-alone photochemical box model that defines altitude (in cm) as the
vertical grid and uses number densities for air and ozone. For the latter, we have re-mapped the primary
atmosphere (Table 1a) onto a grid-point structure (Table 1b) that has the same
mid-layer properties as the layer mean value and the same columns of O2
and O3.
One question will be: What
is the correct answer? In some cases we may be able to define a
"best" answer based on obvious physics or convergence of some of the
more resolved models, but in others we may not. Thus in all of our
proposed experiments we will begin with a "standard model” result (not
necessarily the best answer) from one of the models and then determine a best
answer, if possible, after analysis of the results.
One approach to defining the
correct answer would be to merge observed radiation fields or photolysis rates
(e.g., IPMMI, POLARIS, see references below), but we feel this may be too
difficult to match the exact observing conditions. One way to include the knowledge
gained by these field studies is to ensure participation from some of the
models (e.g., NCAR-TUV, APL).
We do not recommend reporting
detailed actinic fluxes as a function of wavelength since everyone selects
different ways of integrating over wavelength (e.g., bins) and trying to
reconcile the different wavelength scales is not worthwhile. If major problems show up, then a subgroup of
models can consider how to resolve the differences.
Another major issue with
photolysis and heating rates is the treatment of clouds and cloud fraction.
This is very important, but probably beyond the current PhotoComp.
It would require a special workshop. We do include an option for a
plane-parallel volcanic aerosol layer (aka Pinatubo) and a stratiform cloud.