Impact of planetary waves and solar proton events on long-term variations of noctilucent clouds II
Final Report Abstract
The Earth's middle atmosphere is affected by variability covering a large range of spatial and temporal scales and which is be driven from above and below. Within this project we employed noctilucent clouds (NLC), also known as polar mesospheric clouds (PMC), as a sensitive tracer for variability in the polar summer mesopause region of the terrestrial atmosphere. NLCs consist of tiny H2O ice particles with radii typically on the order of a few tens of nanometers. They occur at an altitude of about 83 km during a 3-month period around summer solstice at polar latitudes in the summer hemisphere. It is well established that NLCs react very sensitively to small variations in ambient conditions, particularly to temperature changes. They are discussed as sensitive indicators of climate change in the middle atmosphere. As part of this project we studied two processes that affect the occurrence and distribution of NLC, i.e. (a) planetary waves occurring in the summer mesosphere and (b) the effect of solar variability on NLC. In terms of the effect of planetary waves at the polar summer mesopause we were able to extract clear signatures of the quasi 2-day wave and the quasi 5-day wave in satellite observations of NLCs made with SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) on Envisat and the SBUV (Solar Backscatter Ultraviolet) instruments as well as in temperature observations with the MLS (Microwave Limb Sounder) instrument on Aura. We were also able to show that enhanced planetary wave activity is generally associated with a reduction in NLC occurrence frequency. This behavior is expected on theoretical grounds, because the westward propagating planetary waves oppose the action of gravity wave breaking and lead to a reduced residual circulation. This in turn leads to reduced upwelling above the summer pole and subsequently to a reduced adiabatic cooling at the polar summer mesopause. In terms of solar effects on the polar summer mesopause region and NLCs the project addressed two different aspects. First, the effect of solar proton events (SPEs) on the thermal conditions near the polar summer mesopause and consequently on NLCs was investigated. An analysis of the SBUV(/2) satellite time series to identify examples of NLC depletion caused by SPEs showed that NLCs are probably frequently affected during strong SPEs. As part of this study a physical mechanism explaining a dynamically induced warming at the polar summer mesopause during and after SPEs was investigated using model simulations with the Kühlungsborn Mechanistic General Circulation Model (KMCM). A second aspect related to the effect of SPEs on NLCs is the SPE-induced ion-chemical conversion of H2O to HOx leading to a possible sublimation of NLCs. However, this effect was found to be of minor importance compared to the dynamically induced temperature effect. Second, the project investigated the recently discovered 27-day solar cycle signature in NLCs, which was identified in SCIAMACHY as well as SBUV satellite observations of NLCs using cross correlation analysis and the superposed epoch method. NLC occurrence rate and albedo were found to be anti-correlated with solar proxies with a time-lag of 1 day at most. The sensitivities of the NLC albedo anomalies to Lyman-alpha forcing in terms of the 27-day and the 11-year solar cycle were found to agree within their uncertainties. This finding suggests that the same underlying physico-chemical mechanisms drive the 27-day as well as the 11-year solar cycle signature in NLCs. The exact mechanism is still unknown, however.
Publications
- Satellite observations of the quasi 5-day wave in noctilucent clouds and mesopause temperatures. Geophys. Res. Lett., 34, 2007
C. von Savigny, C. Robert, H. Bovensmann, J. P. Burrows, and M. Schwartz
- Daytime ozone and temperature variations in the mesosphere: A comparison between SABER observations and HAMMONIA model. Atmos. Chem. Phys., 10:8331–8339, 2010
S. Dikty, H. Schmidt, M. Weber, C. von Savigny, and M. G. Mlynczak
- Dynamical heating of the polar summer mesopause induced by solar proton events. J. Geophys. Res., 115, 2010
E. Becker and C. von Savigny
- First evidence of a 27 day solar signature in noctilucent cloud occurrence frequency. J. Geophys. Res., 115, 2010
C. E. Robert, C. von Savigny, N. Rahpoe, H. Bovensmann, J. P. Burrows, M. T. DeLand, and M. J. Schwartz
- Modulations of the 27-day solar cycle signal in stratospheric ozone from SCIAMACHY (2002-2008). J. Geophys. Res., 115:D00I15, 2010
S. Dikty, M. Weber, C. von Savigny, T. Sonkaew, A. Rozanov, and J. P. Burrows
- Impact of solar proton events on noctilucent clouds. J. Atmos. Solar-Terr. Phys., 2011
N. Rahpoe, C. von Savigny, C. E. Robert, M. T. DeLand, and J. P. Burrows
- Impacts of the January 2005 solar particle event on noctilucent clouds and water at the polar summer mesopause. Atmos. Chem. Phys. Discuss., 12:1151–1190, 2012
H. Winkler, C. von Savigny, J. P. Burrows, J. M. Wissing, M. J. Schwartz, A. Lambert, and M. Garcıa-Comas
(See online at https://doi.org/10.5194/acp-12-5633-2012) - Impact of short term solar variability on the polar summer mesopause and noctilucent clouds. In F.-J. Lübken, editor, Climate And Weather of the Sun-Earth System (CAWSES): Highlights from a priority program, chapter 20, pages 365–382. Springer Atmospheric Sciences, Dordrecht, The Netherlands, 2013
C. von Savigny, C. Robert, N. Rahpoe, H. Winkler, E. Becker, H. Bovensmann, J. P. Burrows, and M. T. DeLand
- Investigation of solar irradiance variations and their impact on middle atmospheric ozone. In F.-J. Lübken, editor, Climate And Weather of the Sun-Earth System (CAWSES): Highlights from a priority program, chapter 3, pages 39–54. Springer Atmospheric Sciences, Dordrecht, The Netherlands, 2013
M. Weber, J. Pagaran, S. Dikty, C. von Savigny, J. P. Burrows, M. DeLand, L. E. Floyd, J. W. Harder, M. G. Mlynczak, and H. Schmidt
