OH-equivalent temperatures derived from ACE-FTS and SABER temperature profiles – a comparison with OH*(3-1) temperatures from Maynooth (53.2° N, 6.4° W)

摘要

OH-equivalent temperatures were derived from all of the temperature profilesretrieved in 2004 and 2005 by the ACE-FTS instrument in a 5 degree band oflatitude centred on a ground-based observing station at Maynooth. A globallyaveraged OH volume emission rate (VER) profile obtained from WINDII data wasemployed as a weighting function to compute the equivalent temperatures. Theannual cycle of temperature thus produced was compared with the annual cycleof temperatures recorded at the ground-based station more than a decadeearlier from the OH*(3-1) Meinel band. Both data sets showed excellentagreement in the absolute value of the temperature minimum (~162 K)and in its time of occurrence in the annual cycle at summer solstice. Awayfrom mid-summer, however, the temperatures diverged and reach a maximumdisagreement of more than 20 K in mid-winter. Comparison of the Maynoothground-based data with the corresponding results from two nearby stations inthe same time-period indicated that the Maynooth data are consistent withother ground stations. The temperature difference between the satellite andground-based datasets in winter was reduced to 14–15 K by lowering the peakaltitude of the weighting function to 84 km. An unrealistically low peakaltitude would be required, however, to bring temperatures derived from thesatellite into agreement with the ground-based data.OH equivalent temperatures derived from the SABER instrument using the sameweighting function produced results that agreed well with ACE-FTS. When theOH 1.6 μm VER profile measured by SABER was used as the weightingfunction, the OH equivalent temperatures increased in winter as expected butthe summer temperatures were reduced resulting in an approximately constantoffset of 8.6±0.8 K between ground and satellite values with theground values higher. Variability in both the altitude and width of the OHlayer within a discernable seasonal variation were responsible for thechanges introduced. The higher temperatures in winter were due to primarilyto the lower altitude of the OH layer, while the colder summer temperatureswere due to a thinner summer OH layer. We are not aware of previous reportsof the effect of the layer width on ground-based temperatures.Comparison of OH-equivalent temperatures derived from ACE-FTS and SABERtemperature profiles with OH*(3-1) temperatures from Wuppertal at 51.3° Nwhich were measured during the same period showed a similar pattern to theMaynooth data from a decade earlier, but the warm offset of the groundvalues was lower at 4.5±0.5 K. This discrepancy between temperaturesderived from ground-based instruments recording hydroxyl spectra andsatellite borne instruments has been observed by other observers. Furtherwork will be required by both the satellite and ground-based communities toidentify the exact cause of this difference.