Spatial heterodyne observations of water (SHOW) from a high-altitude airplane: characterization, performance, and first results

Abstract

Abstract. The Spatial Heterodyne Observations of Water instrument (SHOW) is a limb-sounding satellite prototype that utilizes the Spatial Heterodyne Spectroscopy (SHS) technique, operating in a limb-viewing configuration, to observe limb-scattered sunlight in a vibrational band of water vapour within a spectral window from 1363 to 1366 nm. The goal is to retrieve high vertical and horizontal resolution measurements of water vapour in the upper troposphere and lower stratosphere. The prototype instrument has been configured for observations from NASA's ER-2 high-altitude airborne remote science airplane. Flying at a maximum altitude of ∼21.34 km with a maximum speed of ∼760 km h−1, the ER-2 provides a stable platform to simulate observations from a low-earth orbit satellite. Demonstration flights were performed from the ER-2 during an observation campaign from 15 to 22 July 2017. In this paper, we present the laboratory characterization work and the level 0 to level 1 processing of flight data that were obtained during an engineering flight performed on 18 July 2017. Water vapour profile retrievals are presented and compared to in situ radiosonde measurements made of the same approximate column of air. These measurements are used to validate the SHOW measurement concept and examine the sensitivity of the technique.

Observational evidence of moistening the lowermost stratosphere via isentropic mixing across the subtropical jet

“…The SHOW prototype flew several demonstration flights on NASA's ER-2 aircraft in July, 2017 in order to validate the measurement approach and to demonstrate the along-track sampling capabilities of the instrument. The SHOW measurement technique, retrieval approach and instrument performance was validated during an Engineering flight that was performed on July 17, 2018 (Langille et al, 2019). Comparison with co-located radiosonde measurements were found to be in excellent agreement, with differences of < 1 ppm above 15 km (near the thermal tropopause) and < 2-5 ppm below 15 km, due to both natural variability between the observations and measurement precision.…”

“…These vertically resolved spectral images are inverted using a non-linear optimal estimation approach to obtain the vertical distribution of water vapour. The SHOW measurement technique and retrieval algorithm is discussed in previous publications [Langille et al, 2018;Langille et al, 2019].…”

Convectively Transported Water Vapor Plumes in the Midlatitude Lower Stratosphere

“…We first compute the horizontal area-weighted average GEM water vapor data at each altitude using the horizontal resolutions and the retrieval spacings from the instruments. The horizontal resolutions used in this analysis (along track × cross track) are 200 × 7, 13.5 × 13.5, and 50 × 100 km for MLS, AIRS, and SHOW (designed spatial resolutions are used for SHOW), respectively, and are the same for all altitudes (Langille et al, 2017(Langille et al, , 2018(Langille et al, , 2019Livesey et al, 2017). The retrieval spacings used for MLS, AIRS, and SHOW are 200, 1, and 15 km, respectively.…”

“…For the AIRS vertical averaging kernels, our calculations are based on the joint AIRS-DARDAR retrieval method developed by Feng et al (2021), Feng and Huang (2018), as the standard AIRS retrieval is not sensitive to the water vapor signal from the UTLS (Fetzer et al, 2008;Gettelman et al, 2004;Read et al, 2007). Note that the vertical resolutions in the UTLS region are ∼5.5 km in AIRS (Feng et al, 2021), ∼3 km in the MLS (Livesey et al, 2017), and 0.5-1 km in the SHOW (Langille et al, 2017(Langille et al, , 2018(Langille et al, , 2019. We refer to the water vapor data calculated following the above method as GEM-MLS, GEM-AIRS, and GEM-SHOW.…”

Observational evidence of moistening the lowermost stratosphere via isentropic mixing across the subtropical jet

“…The AURA-MLS satellite instrument obtained measurements along this track at approximately 21:50 UTC -roughly 2 h after the SHOW measurements were performed. Along this track, the MLS instrument sampled the same geophysical feature along a slightly different path with a horizontal resolution of 168-230 km and a vertical resolution of 1.3-3.2 km in the UTLS (316-46 hPa) (Livesey et al, 2018). The MLS measurements have a coarser spatial resolution, and the sampling is not exactly coincident with SHOW.…”