Recent divergences in stratospheric water vapor measurements by frost point hygrometers and the Aura Microwave Limb Sounder.

Dale F Hurst,Karen H Rosenlof,Holger Vömel,Henry B Selkirk,Samuel J Oltmans,Emrys G Hall,Allen F Jordan,Sean M Davis,William G Read

doi:10.5194/amt-9-4447-2016

Abstract

Balloon-borne frost point hygrometers (FPs) and the Aura Microwave Limb Sounder (MLS) provide high-quality vertical profile measurements of water vapor in the upper troposphere and lower stratosphere (UTLS). A previous comparison of stratospheric water vapor measurements by FPs and MLS over three sites - Boulder, Colorado (40.0° N); Hilo, Hawaii (19.7° N); and Lauder, New Zealand (45.0° S) - from August 2004 through December 2012 not only demonstrated agreement better than 1% between 68 and 26 hPa but also exposed statistically significant biases of 2 to 10% at 83 and 100 hPa (Hurst et al., 2014). A simple linear regression analysis of the FP-MLS differences revealed no significant long-term drifts between the two instruments. Here we extend the drift comparison to mid-2015 and add two FP sites - Lindenberg, Germany (52.2° N), and San José, Costa Rica (10.0° N) - that employ FPs of different manufacture and calibration for their water vapor soundings. The extended comparison period reveals that stratospheric FP and MLS measurements over four of the five sites have diverged at rates of 0.03 to 0.07 ppmv year−1 (0.6 to 1.5% year−1) from ~2010 to mid-2015. These rates are similar in magnitude to the 30-year (1980–2010) average growth rate of stratospheric water vapor (~ 1% year−1) measured by FPs over Boulder (Hurst et al., 2011). By mid-2015, the FP–MLS differences at some sites were large enough to exceed the combined accuracy estimates of the FP and MLS measurements.

Highlights

Water vapor in the Earth’s atmosphere influences the radiation budget by strongly attenuating outgoing long-wave radiation
For Hilo the analysis found no discernable maxima (Fmax) in the time series of F -statistics, probably because the record only began at the end of 2010, after the changepoints that were determined for most other sites
In the remainder of this work we report stratospheric averages of trends and changes in frost point hygrometers (FPs)–Microwave Limb Sounder (MLS) differences

Summary

Introduction

Water vapor in the Earth’s atmosphere influences the radiation budget by strongly attenuating outgoing long-wave radiation. Though the lower troposphere holds the vast majority of atmospheric water vapor, abundance changes in the relatively dry upper troposphere and lower stratosphere (UTLS) can significantly impact global surface temperatures and climate (Forster and Shine, 2002; Solomon et al, 2010). Satellite-based remote sensors have greatly enhanced our ability to monitor UTLS water vapor on a near-global scale. Because of the limited operational lifetimes of satellite sensors, an analysis of trends over decadal or longer scales requires the merging of measurements by different instruments. Efforts to combine UTLS water vapor data sets from different satellites have demonstrated the need to reduce measurement biases between instruments before trend anal-. The necessity of adjusting data sets before they are merged imposes an additional source of uncertainty on any determination of long-term trends

Methods

Results

Conclusion