A semi-empirical model for mesospheric and stratospheric NO<sub><i>y</i></sub> produced by energetic particle precipitation

Abstract

Abstract. The MIPAS Fourier transform spectrometer on board Envisat has measured global distributions of the six principal reactive nitrogen (NO y ) compounds (HNO 3 , NO 2 , NO, N 2 O 5 , ClONO 2 , and HNO 4 ) during 2002-2012. These observations were used previously to detect regular polar winter descent of reactive nitrogen produced by energetic particle precipitation (EPP) down to the lower stratosphere, often called the EPP indirect effect. It has further been shown that the observed fraction of NO y produced by EPP (EPP-NO y ) has a nearly linear relationship with the geomagnetic A p index when taking into account the time lag introduced by transport. Here we exploit these results in a semiempirical model for computation of EPP-modulated NO y densities and wintertime downward fluxes through stratospheric and mesospheric pressure levels. Since the A p dependence of EPP-NO y is distorted during episodes of strong descent in Arctic winters associated with elevated stratopause events, a specific parameterization has been developed for these episodes. This model accurately reproduces the observations from MIPAS and is also consistent with estimates from other satellite instruments. Since stratospheric EPP-NO y depositions lead to changes in stratospheric ozone with possible implications for climate, the model presented here can be utilized in climate simulations without the need to incorporate many thermospheric and upper mesospheric processes. By employing historical geomagnetic indices, the model also allows for reconstruction of the EPP indirect effect since 1850. We found secular variations of solar cycleaveraged stratospheric EPP-NO y depositions on the order of 1 GM. In particular, we model a reduction of the EPP-NO y deposition rate during the last 3 decades, related to the coincident decline of geomagnetic activity that corresponds to 1.8 % of the NO y production rate by N 2 O oxidation. As the decline of the geomagnetic activity level is expected to continue in the coming decades, this is likely to affect the long-term NO y trend by counteracting the expected increase caused by growing N 2 O emissions.

Influence of Enhanced Planetary Wave Activity on the Polar Vortex Enhancement Related to Energetic Electron Precipitation

“…These lags for Ap are thus employed in all analyses. Similar time lags were already previously found optimal (Maliniemi et al, , ; Salminen et al, ) and agree with the time it takes for majority of NO x /NO y created by energetic particle precipitation to descend from the mesosphere to the stratosphere (Funke et al, , ). Note that these lags also correspond well to the effective lags, which Funke et al () found optimal for correlation between EPP produced NO y in the stratosphere/lower mesosphere and the Ap index.…”

Solar forcing for CMIP6 (v3.2)

“…In other cases, a simple parameterization in dependence of the seasonally averaged Ap index (Baumgaertner et al, 2009) was employed (e.g., Baumgaertner et al, 2011;Rozanov et al, 2012), enabling extended simulations over multidecadal time periods. We recommend the use of the UBC model described in Funke et al (2016) which is designed for the latter application and represents an improved parameterization due to its more detailed representation of geomagnetic modulations, latitudinal distribution, and seasonal evolution. This semi-empirical model for computing time-dependent global zonal mean NO y concentrations (in units of cm −3 ) or EPP-NO y molecular fluxes (in units of cm −2 s −1 ) at pressure levels within 1-0.01 hPa is available at http://solarisheppa.geomar.de/solarisheppa/cmip6.…”

NO<sub><i>y</i></sub> production, ozone loss and changes in net radiative heating due to energetic particle precipitation in 2002–2010

“…Geomagnetic forcing of NO y in the mesosphere is considered by applying an upper boundary condition (UBC) of NO y , parameterized by the geomagnetic Ap index (newly developed submodel UBCNOX). This applies an online version of the upper boundary condition for the amount of NO x described in Funke et al (2016) and Matthes et al (2017). All three parts of the parameterization (background, energetic particles and elevated stratopause events (ESEs)) are calculated directly on the model grid.…”

“…Additionally, all models carried out one scenario including full particle forcing as available to the respective model: 3dCTM including protons and electrons from AIMOS model v1.6 and photoionization (v1.6 phioniz); KASIMA including protons and electrons from AIMOS model v1.6 (v1.6); and EMAC including protons from the database and NO y upper boundary conditions as a constraint for the EPP indirect effect (UBC, Funke et al, 2016).…”

“…All three parts of the parameterization (background, energetic particles and elevated stratopause events (ESEs)) are calculated directly on the model grid. ESEs are detected online by the criteria suggested in Funke et al (2016) (Matthes et al, 2017) are used. NO is prescribed in the four highest levels of the model (pressure at midpoint 0.01-0.09 hPa) instead of NO y , and NO 2 is set to 0 in those levels to balance NO x (see Matthes et al, 2017).…”