Upper-troposphere and lower-stratosphere water vapor retrievals from the 1400 and 1900 nm water vapor bands

Abstract. Measuring water vapor in the upper troposphere and lower stratosphere is difficult due to the low mixing ratios found there, typically only a few parts per million. Here we examine near-infrared spectra acquired with the Solar Spectral Flux Radiometer (SSFR) during the first science phase of the NASA Airborne Tropical TRopopause EXperiment (ATTREX). From the 1400 and 1900 nm absorption bands we infer water vapor amounts in the tropical tropopause layer and adjacent regions between altitudes of 14 and 18 km. We compare these measurements to solar transmittance spectra produced with the MODerate resolution atmospheric TRANsmission (MODTRAN) radiative transfer model, using in situ water vapor, temperature, and pressure profiles acquired concurrently with the SSFR spectra. Measured and modeled transmittance values agree within 0.002, with some larger differences in the 1900 nm band (up to 0.004). Integrated water vapor amounts along the absorption path lengths of 3 to 6 km varied from 1.26 × 10⁻⁴ to 4.59 × 10⁻⁴ g cm⁻². A 0.002 difference in absorptance at 1367 nm results in a 3.35 × 10⁻⁵ g cm⁻² change of integrated water vapor amounts; 0.004 absorptance change at 1870 nm results in 5.50 × 10⁻⁵ g cm⁻² of water vapor. These are 27% (1367 nm) and 44% (1870 nm) differences at the lowest measured value of water vapor (1.26 × 10⁻⁴ g cm⁻²) and 7% (1367 nm) and 12% (1870 nm) differences at the highest measured value of water vapor (4.59 × 10⁻⁴ g cm⁻²). A potential method for extending this type of measurement from aircraft flight altitude to the top of the atmosphere is discussed.