On the dependence of the OH<sup>*</sup> Meinel emission altitude on vibrational level: SCIAMACHY observations and model simulations

Abstract

Measurements of the OH Meinel emissions in the terrestrial nightglow are one of the standard ground-based techniques to retrieve upper mesospheric temperatures. It is often assumed that the emission peak altitudes are not strongly dependent on the vibrational level, although this assumption is not based on convincing experimental evidence. In this study we use Envisat/SCIAMACHY (Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY) observations in the near-IR spectral range to retrieve vertical volume emission rate profiles of the OH(3-1), OH(6-2) and OH(8-3) Meinel bands in order to investigate, whether systematic differences in emission peak altitudes can be observed between the different OH Meinel bands. The results indicate that the emission peak altitudes are different for the different vibrational levels, with bands originating from higher vibrational levels having higher emission peak altitudes. It is shown that this finding is consistent with the majority of the previously published results. The SCIAMACHY observations yield differences in emission peak altitudes of up to about 4 km between the OH(3-1) and the OH(8-3) band. <br><br> The observations are complemented by model simulations of the fractional population of the different vibrational levels and of the vibrational level dependence of the emission peak altitude. The model simulations well reproduce the observed vibrational level dependence of the emission peak altitude – both qualitatively and quantitatively – if quenching by atomic oxygen as well as multi-quantum collisional relaxation by O₂ is considered. If a linear relationship between emission peak altitude and vibrational level is assumed, then a peak altitude difference of roughly 0.5 km per vibrational level is inferred from both the SCIAMACHY observations and the model simulations

Variability of OH(3–1) emission altitude from 2003 to 2011: Long-term stability and universality of the emission rate–altitude relationship

“…Based on the MSIS-90E temperature profile for July 1, 2005 at 0°latitude and 0°longitude, a downward shift of the entire OH emission profile by 0.5 km leads to an increase in the observed temperature -weighted by the emission rate profile -by about 0.65 K, as discussed in more detail by von Savigny et al (2012). This implies that for ground-based OH temperature observations, the semi-annual variation in retrieved temperature associated with the sampling altitude variation reported here has an amplitude on the order of 1.0 K. This may correspond to about 25% of the actual semi-annual variation of mesopause temperatures at low latitudes (Zhao et al, 2007).…”

“…The retrieval of vertical OH(3-1) volume emission rate profiles from SCIAMACHY limb observations on the Earth's nightside was recently described in detail by von Savigny et al (2012), and only the most important features are repeated here. The retrieval of vertical OH(3-1) volume emission rate profiles from the observed limb emission rate profiles is performed with a constrained leastsquares technique with statistical weighting.…”

“…However, in a recent study von Savigny et al (2012) have shown that OH Meinel emissions originating from different vibrational levels peak at slightly different altitudes, with emissions from higher vibrational levels having greater emission peak altitudes. Measurements with SCIAMACHY/Envisat and model simulations implied that emission peak altitudes for emissions from adjacent vibrational levels differ by roughly 0.5 km.…”

“…2), which corresponds to the spectral range typically employed for retrieving OH(3-1) rotational temperatures (e.g., Bittner et al, 2002;von Savigny et al, 2004). In contrast to von Savigny et al (2012) -who performed the inversion to vertical volume emission rate profiles on zonally and monthly averaged limb emission measurements -the inversion is applied to individual limb measurements in this study. The OH(3-1) volume emission rate profiles are retrieved on a 3.3 km altitude grid, which is identical to the tangent height grid of the The main focus of the following analyzes is on the mean or centroid emission altitude z determined by weighting altitude z with the retrieved vertical OH(3-1) volume emission rate profile:…”

On the latitudinal distribution of mesospheric temperatures during sudden stratospheric warming events

“…Interestingly, the enhancement in the mesospheric temperatures as seen from NIRIS data described here was found to be occurring during a major SSW event (7–27 January 2013; Figure h) which occurs at polar latitudes. In this context it is relevant to note that OH band emissions from higher vibrational levels peak at higher altitudes [ von Savigny et al , ]. Using Scanning Imaging Absorption spectrometer for Atmospheric Cartography, on Envisat data, it was shown that the OH (6‐2) emission rate profile is typically vertically shifted upward by 1–2 km relative to the OH (3‐1) band, and the OH (8‐3) band is generally found to peak about 1 km higher than the OH (6‐2) band [ von Savigny et al , ].…”