Interhemispheric differences in polar mesospheric clouds observed by the HALOE instrument

Abstract

HALOE data have been used to study Polar Mesospheric Clouds (PMCs) and to investigate differences in the characteristics of the clouds between the northern and southern hemispheres. HALOE has been observing the high latitude summer regions since the fall of 1991, which has provided observations of 14 northern hemisphere and southern hemisphere PMC seasons. HALOE infrared extinction profiles are used to extract PMC extinction and the altitude of the PMC for each cloud observation. We present evidence of higher PMC occurrence frequencies, lower cloud altitudes and larger PMC extinction in the NH compared to the SH. When the PMC data are combined for all years between the latitudes of 55° and 70°, we find that the northern hemisphere clouds are brighter by 29±5% and the peak altitude occurs at 0.9±0.1 km lower than the southern hemisphere clouds. In addition, we find that PMCs occur twice as frequently in the northern hemisphere. Seasonal distributions of PMC extinction and occurrence frequency for the combined multi-year datasets reveal an offset between the two hemispheres with the SH season starting about 10 days earlier than for the NH. Interhemispheric PMC altitude differences are supported by a lower altitude region of saturation in the NH and by a lower H2O peak altitude in the NH. HALOE mean temperature profiles for PMC events alone reveal a colder NH up to ∼82 km, but NH and SH profile error bars overlap above this altitude. Analysis of all temperature profiles (PMC and non-PMC) measured during the high-altitude polar summer reveal a colder NH at all altitudes suggesting this as one possible cause for the observed interhemispheric differences in PMC properties. © 2006 Elsevier Ltd. All rights reserved.