Technical Note: On HALOE stratospheric water vapor variations and trends at Boulder, Colorado

Abstract

Abstract. This study compares time series of stratospheric water vapor (SWV) data at 30 hPa from 1993 to 2005, based on sets of Halogen Occultation Experiment (HALOE) profiles above the Boulder, CO (40° N, 255° E) region and on local frost-point hygrometer (FPH) measurements. Their differing trends herein agree with previously published findings. The FPH trends are presumed to be accurate within their uncertainties, and there are no known measurement biases affecting the HALOE trends. However, the seasonal sampling from HALOE is deficient at 40° N from 2001 to 2005, especially during late winter and springtime. HALOE time series at 20 hPa clearly show a springtime maximum in SWV at 40° N. The retrievals of HALOE SWV have significant corrections for interfering aerosol extinction following the eruption of Pinatubo, but there is no evidence that those corrections cause incorrect SWV trends after 1992. Accordingly, this study finds that the SWV trends from HALOE and FPH agree within their uncertainties for the more limited time span of 1993 to 2002. Northern hemisphere time series and daily plots of SWV from the Limb Infrared Monitor of the Stratosphere (LIMS) experiment indicate that there is transport of filaments of high SWV from polar to middle latitudes during dynamically active, winter and springtime periods. Although FPH measurements sense SWV variations at all scales, the HALOE time series do not resolve small-scale structure because its time series data are based on an average of four or more occultations within a finite latitude/longitude sector. It is concluded that the variations and trends of HALOE SWV are accurate for 1993 to 2002 at 40° N and in accord with the spatial scales of its measurements and its sampling frequency over time.