Comparison of ECHAM5/MESSy Atmospheric Chemistry (EMAC) simulations of the Arctic winter 2009/2010 and 2010/2011 with Envisat/MIPAS and Aura/MLS observations
We present model simulations with the atmospheric chemistry–climate model
ECHAM5/MESSy Atmospheric Chemistry (EMAC) nudged toward European Centre for
Medium-Range Weather Forecasts (ECMWF) ERA-Interim reanalyses for the Arctic
winters 2009/2010 and 2010/2011. This study is the first to perform an
extensive assessment of the performance of the EMAC model for Arctic winters
as previous studies have only made limited evaluations of EMAC simulations
which also were mainly focused on the Antarctic winter stratosphere. We have
chosen the two extreme Arctic winters 2009/2010 and 2010/2011 to evaluate the
formation of polar stratospheric clouds (PSCs) and the representation of the
chemistry and dynamics of the polar winter stratosphere in EMAC. The EMAC
simulations are compared to observations by the Michelson Interferometer for
Passive Atmospheric Soundings (Envisat/MIPAS) and the observations from the
Aura Microwave Limb Sounder (Aura/MLS). The Arctic winter 2010/2011 was one
of the coldest stratospheric winters on record, leading to the strongest
depletion of ozone measured in the Arctic. The Arctic winter 2009/2010 was,
from the climatological perspective, one of the warmest stratospheric winters
on record. However, it was distinguished by an exceptionally cold
stratosphere (colder than the climatological mean) from mid-December 2009 to
mid-January 2010, leading to prolonged PSC formation and existence.
Significant denitrification, the removal of HNO3 from the
stratosphere by sedimentation of HNO3-containing polar stratospheric
cloud particles, occurred in that winter. In our comparison, we focus on PSC
formation and denitrification. The comparisons between EMAC simulations and
satellite observations show that model and measurements compare well for
these two Arctic winters (differences for HNO3 generally within
±20 %) and thus that EMAC nudged toward ECMWF ERA-Interim
reanalyses is capable of giving a realistic representation of the evolution
of PSCs and associated sequestration of gas-phase HNO3 in the polar
winter stratosphere. However, simulated PSC volume densities are smaller than
the ones derived from Envisat/MIPAS observations by a factor of 3–7. Further,
PSCs in EMAC are not simulated as high up (in altitude) as they are observed.
This underestimation of PSC volume density and vertical extension of the PSCs
results in an underestimation of the vertical redistribution of HNO3
due to denitrification/re-nitrification. The differences found here between
model simulations and observations stipulate further improvements in the EMAC
set-up for simulating PSCs