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

摘要

Measuring water vapor in the upper troposphere and lower stratosphere isdifficult due to the low mixing ratios found there, typically only a fewparts per million. Here we examine near-infrared spectra acquired with theSolar Spectral Flux Radiometer (SSFR) during the first science phase of theNASA Airborne Tropical TRopopause EXperiment (ATTREX). From the 1400 and1900 nm absorption bands we infer water vapor amounts in the tropicaltropopause layer and adjacent regions between altitudes of 14 and 18 km. Wecompare these measurements to solar transmittance spectra produced with theMODerate resolution atmospheric TRANsmission (MODTRAN) radiative transfermodel, using in situ water vapor, temperature, and pressure profiles acquiredconcurrently with the SSFR spectra. Measured and modeled transmittance valuesagree within 0.002, with some larger differences in the 1900 nm band (up to0.004). Integrated water vapor amounts along the absorption path lengths of 3to 6 km varied from 1.26 × 10 to4.59 × 10 g cm. A 0.002 difference in absorptance at1367 nm results in a 3.35 × 10 g cm change ofintegrated water vapor amounts; 0.004 absorptance change at 1870 nm resultsin 5.50 × 10 g cm of water vapor. These are 27%(1367 nm) and 44% (1870 nm) differences at the lowest measured valueof water vapor (1.26 × 10 g cm) and 7%(1367 nm) and 12% (1870 nm) differences at the highest measured valueof water vapor (4.59 × 10 g cm). A potential methodfor extending this type of measurement from aircraft flight altitude to thetop of the atmosphere is discussed.