Attribution of Chemistry-Climate Model Initiative (CCMI) ozone radiative flux bias from satellites
<p>The top-of-atmosphere (TOA) outgoing longwave flux over the 9.6 <span class="inline-formula">µm</span> ozone band is a fundamental quantity for understanding chemistry–climate coupling. However, observed TOA fluxes are hard to estimate as they exhibit consid...
Main Authors: | , , , , , , , , , , , , , , , , , |
---|---|
Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2020-01-01
|
Series: | Atmospheric Chemistry and Physics |
Online Access: | https://www.atmos-chem-phys.net/20/281/2020/acp-20-281-2020.pdf |
Summary: | <p>The top-of-atmosphere (TOA) outgoing longwave flux over the 9.6 <span class="inline-formula">µm</span>
ozone band is a fundamental quantity for understanding chemistry–climate
coupling. However, observed TOA fluxes are hard to estimate as they exhibit
considerable variability in space and time that depend on the distributions
of clouds, ozone (<span class="inline-formula">O<sub>3</sub></span>), water vapor (<span class="inline-formula">H<sub>2</sub>O</span>), air temperature
(<span class="inline-formula"><i>T</i><sub>a</sub></span>), and surface temperature (<span class="inline-formula"><i>T</i><sub>s</sub></span>). Benchmarking present-day
fluxes and quantifying the relative influence of their drivers is the first
step for estimating climate feedbacks from ozone radiative forcing and
predicting radiative forcing evolution.</p>
<p>To that end, we constructed observational instantaneous radiative kernels
(IRKs) under clear-sky conditions, representing the sensitivities of the TOA
flux in the 9.6 <span class="inline-formula">µm</span> ozone band to the vertical distribution of
geophysical variables, including <span class="inline-formula">O<sub>3</sub></span>, <span class="inline-formula">H<sub>2</sub>O</span>, <span class="inline-formula"><i>T</i><sub>a</sub></span>, and <span class="inline-formula"><i>T</i><sub>s</sub></span>
based upon the Aura Tropospheric Emission Spectrometer (TES) measurements.
Applying these kernels to present-day simulations from the Chemistry-Climate
Model Initiative (CCMI) project as compared to a 2006 reanalysis
assimilating satellite observations, we show that the models have large
differences in TOA flux, attributable to different geophysical variables. In
particular, model simulations continue to diverge from observations in the
tropics, as reported in previous studies of the Atmospheric Chemistry
Climate Model Intercomparison Project (ACCMIP) simulations. The principal
culprits are tropical middle and upper tropospheric ozone followed by tropical
lower tropospheric <span class="inline-formula">H<sub>2</sub>O</span>. Five models out of the eight studied here have
TOA flux biases exceeding 100 mW m<span class="inline-formula"><sup>−2</sup></span> attributable to tropospheric ozone
bias. Another set of five models have flux biases over 50 mW m<span class="inline-formula"><sup>−2</sup></span> due
to <span class="inline-formula">H<sub>2</sub>O</span>. On the other hand, <span class="inline-formula"><i>T</i><sub>a</sub></span> radiative bias is negligible in all
models (no more than 30 mW m<span class="inline-formula"><sup>−2</sup></span>). We found that the atmospheric component (AM3) of the Geophysical Fluid Dynamics<span id="page282"/> Laboratory (GFDL) general circulation model and Canadian Middle Atmosphere Model (CMAM) have the
lowest TOA flux biases globally but are a result of cancellation of opposite
biases due to different processes. Overall, the multi-model ensemble mean
bias is <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M17" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">133</mn><mo>±</mo><mn mathvariant="normal">98</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="52pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="5389b518f84f2067694b56b2b3c81d83"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-20-281-2020-ie00001.svg" width="52pt" height="10pt" src="acp-20-281-2020-ie00001.png"/></svg:svg></span></span> mW m<span class="inline-formula"><sup>−2</sup></span>, indicating that they are too
atmospherically opaque due to trapping too much radiation in the atmosphere
by overestimated tropical tropospheric <span class="inline-formula">O<sub>3</sub></span> and <span class="inline-formula">H<sub>2</sub>O</span>. Having too much
<span class="inline-formula">O<sub>3</sub></span> and <span class="inline-formula">H<sub>2</sub>O</span> in the troposphere would have different impacts on the
sensitivity of TOA flux to <span class="inline-formula">O<sub>3</sub></span> and these competing effects add more
uncertainties on the ozone radiative forcing. We find that the inter-model
TOA outgoing longwave radiation (OLR) difference is well anti-correlated
with their ozone band flux bias. This suggests that there is significant
radiative compensation in the calculation of model outgoing longwave
radiation.</p> |
---|---|
ISSN: | 1680-7316 1680-7324 |