Technical Note: On HALOE stratospheric water vapor variations and trends at Boulder, Colorado
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.
Status: final response (author comments only)
RC1: 'Comment on egusphere-2023-431', Anonymous Referee #1, 19 Apr 2023
- AC1: 'Reply on RC1', Ellis Remsberg, 27 Apr 2023
RC2: 'Comment on egusphere-2023-431', Anonymous Referee #2, 24 May 2023
- AC2: 'Reply on RC2', Ellis Remsberg, 28 May 2023
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The technical note written by Ellis Remsberg makes a valuable and insightful contribution to the current research on the variability and trends of stratospheric water vapor, particularly in relation to the comparability of satellite-based (HALOE) and in-situ-based frost point hygrometer (FPH) observations above Boulder. The FPH method is assumed to be the most accurate. The paper presents several new and interesting findings, including:
(i) Identification of problems with the HALOE sampling at 40N from 2001 to 2005, particularly during late winter and spring.
(ii) Demonstration of good agreement between the trends derived from HALOE at 30 hPa and FPH data, by considering a limited time span from 1993 to 2002.
(iii) Giving few hints that both FPH observations and HALOE closest data points can be strongly influenced by moist polar air isentropically transported as remnants of the NH polar vortex during late winter and spring.
Overall, the technical note is an important addition to the current research on stratospheric water vapor variability and trends, and provides new insights on the comparability of satellite-based and in-situ-based observations. Here, few detailed comments:
(*) To improve readability, it may be helpful to present Figures 1-3 on the left panel and Figures 4-6 on the right panel of a single figure, allowing for easier comparison between the HALOE and FPH data at each step of the data handling process.
(*) The correction procedure discussed by Scherer et al. (2008) for FPH data is not applied in this study. As the trends for HALOE and NOAA appear different in their Figure 6b compared to the current study, it should be noted that the current study utilizes slightly modified HALOE data by excluding data from 2001 and 2005 and FPH data without the correction discussed by Scherer et al. (2008). Further discussion on the potential impact of this correction on the results would be valuable.
(*) While the comparison at 30 hPa is important, it would be informative to also compare the trends at 70-100 hPa, which have been widely discussed for trend analysis in the past (Solomon 2010, Hegglin 2014, Konopka 2022). These studies have shown strong positive trends from FPH observations for the period 1993-2001, whereas other data used in these studies (UARS HALOE shifted to MLS, merged satellite, SWOOSH) have exhibited weaker or even negative trends. Examining these pressure levels may provide additional insight into the ongoing discussion of the trends.
(*) My most concern is related to the comparison between your Figs 3 and 6, i.e. between HALOE and FPH based trends for the period 1993-2002 at 30 hPa. Yes, even if the trends are very similar, the variability derived from the MLR (thin solid lines) is very different (i.e. thin line in Fig.3 compared with the thin line in Fig. 6). Why?. I guess the correlation between these two thin solid lines is very weak. So why we should trust the trends derived from these two so different lines?
(*) In the second part of your paper, you discuss isentropic polar intrusions into the mid latitudes as potential reasons for additional variability both in FPH data and collocated HALOE observations. Following your analysis, this effect should be expected mainly in the late spring and winter. This is certainly an interesting point not well discussed in the past. However, as discussed in Konopka et al., 2022 (see their Fig 4d and 4e), the winter/spring effect on the trends after 2000 was mainly diagnosed in the tropics and not in the Boulder region. Thus, "intrusion effects" on HALOE/FPH observations around 40N and the largest trends after 2000 during winter/spring seasons are probably two different issues.
"auto-regressive effects" - too technical here, please explain it
please explain the abbreviations "SS", "SR"
"have a significant (CI)" ?
"....from the LIMS experiment" please explain the abbreviation