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Atmospheric effects of the total solar eclipse of 4 December 2002 simulated with a high-altitude global model

Abstract : The atmosphere s response to the total solar eclipse of 4 December 2002 is studied using a prototype high-altitude global numerical weather prediction model (NOGAPS-ALPHA). Local reductions in solar ultraviolet (UV) radiation during the eclipse are estimated using astronomical calculations of umbral and penumbral surface trajectories and observed solar limb darkening at ~ 200-300 nm. In NOGAPS-ALPHA these UV eclipse shadows yield stratospheric radiative cooling rate footprints peaking near 27 K day 1, a value 2 3 times larger than assumed in previous modeling. Difference fields between NOGAPS-ALPHA runs with and without this eclipse forcing reveal vertically deep middle atmospheric responses, with three-dimensional horizontal structures very similar to the large-scale bow-wave response first proposed by Chimonas (1970). Such structure appears clearly only at later times when total eclipses have abated and gravity waves generated in the stratosphere have had time to propagate vertically. Bow-wave amplitudes and direct thermal cooling responses are both small (]1 K for temperature and ]2 3 m s 1 for horizontal winds), contradicting some rocketsonde measurements that suggest much larger responses near 50 60 km altitude. We also find clear evidence of a bow-wave-like response in the model s surface pressure fields, with an amplitude 0.1 0.5 hPa, while surface air temperatures in NOGAPS-ALPHA show 4 K cooling over Africa during the eclipse. Both findings are consistent with surface atmospheric data acquired during previous eclipse passages.