HEPPA-II model-measurement intercomparison project: EPP indirect effects during the dynamically perturbed NH winter 2008–2009

Abstract

<p><strong>Abstract.</strong> We compare simulations from three high-top (with upper lid above 120 km) and five medium-top (with upper lid around 80km) atmospheric models with observations of odd nitrogen (NO<sub>x</sub> = NO + NO<sub>2</sub>), temperature, and carbon monoxide from seven satellite instruments (ACE-FTS on SciSat, GOMOS, MIPAS, and SCIAMACHY on Envisat, MLS on Aura, SABER on TIMED, and SMR on Odin) during the Northern Hemisphere (NH) polar winter 2008/2009. The models included in the comparison are the 3d Chemistry Transport model (3dCTM), the ECHAM5/MESSy Atmospheric Chemistry (EMAC) model, FinROSE, the Hamburg Model of the Neutral and Ionized Atmosphere (HAMMONIA), the Karlsruhe Simulation Model of the Middle Atmosphere (KASIMA), the modeling tools for SOlar Climate Ozone Links studies (SOCOL and CAO-SOCOL), and the Whole Atmosphere Community Climate Model (WACCM4). The comparison focuses on the energetic particle precipitation (EPP) indirect effect, that is, the polar winter descent of NO<sub>x</sub> largely produced by EPP in the mesosphere and lower thermosphere. A particular emphasis is given to the impact of the sudden stratospheric warming (SSW) in January 2009 and the subsequent elevated stratopause (ES) event associated with enhanced descent of mesospheric air. The chemistry climate model simulations have been nudged toward reanalysis data in the troposphere and stratosphere while being unconstrained above. An odd nitrogen upper boundary condition obtained from MIPAS observations has further been applied to medium-top models. Most models provide a good representation of the mesospheric tracer descent in general, and the EPP indirect effect in particular, during the unperturbed (pre-SSW) period of the NH winter 2008/2009. The observed NO<sub>x</sub> descent into the lower mesosphere and stratosphere is generally reproduced within 20 %. Larger discrepancies of a few model simulations could be traced back either to the impact of the models' gravity wave drag scheme on the polar wintertime meridional circulation or to a combination of prescribed NO<sub>x</sub> mixing ratio at the uppermost model layer and low vertical resolution. In March–April, after the ES event, however, modelled mesospheric and stratospheric NOx distributions deviate significantly from the observations. The too fast and early downward propagation of the NO<sub>x</sub> tongue, encountered in most simulations, coincides with a temperature high bias in the lower mesosphere (0.2–0.05 hPa) being likely caused by an overestimation of descent velocities. On the other hand, upper mesospheric temperatures (at 0.05–0.001 hPa) are generally underestimated by the high-top models after the onset of the ES event, being indicative for too slow descent and hence too low NO<sub>x</sub> fluxes. As a consequence, the magnitude of the simulated NO<sub>x</sub> tongue is generally underestimated by these models. Descending NO<sub>x</sub> amounts simulated with medium-top models are on average closer to the observations but show a large spread of up to several hundred percent. This is primarily attributed to the different vertical model domains in which the NO<sub>x</sub> upper boundary condition is applied. In general, the intercomparison demonstrates the ability of state-of-the-art atmospheric models to reproduce the EPP indirect effect in dynamically and geomagnetically quiescent NH winter conditions. The encountered differences between observed and simulated NO<sub>x</sub>, CO, and temperature distributions during the perturbed phase of the 2009 NH winter, however, emphasize the need for model improvements in the dynamical representation of elevated stratopause events in order to allow for a better description of the EPP indirect effect under these particular conditions.</p>

Analysis and Hindcast Experiments of the 2009 Sudden Stratospheric Warming in WACCMX+DART

“…The improved representation of NO descent in WACCMX+DART is related to better representation of the elevated stratopause in this simulation (e.g., Meraner et al, ). Odin Sub‐Millimeter Radiometer observations in Figure 2 of Funke et al () show that NO of 0.1 ppmv descends to almost 0.1 hPa (∼65 km). By comparison, at 0.1 hPa the WACCMX+DART NO is 0.02–0.05 ppmv.…”

“…This is clearly captured in satellite observations that show descent of nitrogen oxides (NO x = NO + NO 2 ) and carbon monoxide (CO) into the stratosphere following the SSW (Manney et al, ; Randall et al, ). The enhanced descent is generally poorly captured by constrained chemistry climate models (e.g., Funke et al, ), which can partly be attributed to inaccurate representation of the dynamics in the upper stratosphere‐mesosphere due to lack of direct constraint at these altitudes (Siskind et al, ). The ability to accurately reproduce enhanced NO x and CO descent is therefore a useful indirect method for assessing the large‐scale dynamics in the mesosphere.…”

On the Upward Extension of the Polar Vortices Into the Mesosphere

“…Global climate models (GCMs) underestimate these effects (Funke et al, ; Hendrickx et al, ; Holt et al, ; Orsolini et al, ; Randall et al, ; Sheese et al, ), and optimum performance occurs when the models are constrained by observations to mesospheric heights (Pedatella et al, ; Sassi et al, ; Siskind et al, ). Indeed, GCMs often simulate a reversal of the winter westerlies above 70–80 km that is not consistent with observations (e.g., Smith, , see their Figure 2).…”

Observations and Modeling of Increased Nitric Oxide in the Antarctic Polar Middle Atmosphere Associated With Geomagnetic Storm‐Driven Energetic Electron Precipitation

“…In contrast, while the chemical lifetime of NO x in the sunlit mesosphere and lower thermosphere is typically ~18 hr, in darkness NO x can persist for months (Shimazaki, ; Solomon et al, ). This long lifetime allows auroral NO in the lower thermosphere, and mesospheric NO produced directly by MEE ionization, to be transported downward inside the polar vortex at high latitudes during winter into the stratosphere where it depletes ozone (e.g., Clilverd et al, ; Funke et al, ; Randall et al, ; Sinnhuber et al, ; Siskind et al, ).…”