Quantifying consistency and biases between aircraft, balloon and remote sensing measurements of UT/LS water vapor during the WB-57 NASA MACPEX mission

Mixing ratios of water vapor in Earth's upper troposphere and lower stratosphere (UT/LS) are low (< 10 ppmv), yet water in this region is a significant driver of climate. Significant discrepancies have repeatedly been observed between multiple high precision measurements of water vapor at these low values in the UT/LS, leading to uncertainty in the absolute value of the direct radiative forcing from stratospheric water vapor. During the NASA Mid-latitude Airborne Cirrus Properties Experiment (MACPEX) mission in March and April of 2011, measurements of water vapor in the UT/LS were made using the NOAA chemical ionization mass spectrometer (CIMS) and Harvard Lyman-α water vapor (HWV) instruments integrated on the NASA WB-57 based out of Ellington Field, TX. This was the first aircraft deployment of the CIMS instrument configured to measure water vapor. The CIMS carried a novel in situ calibration system using two independent water vapor standards that were in excellent agreement throughout the campaign. CIMS was also in excellent agreement with HWV, which operates and is calibrated using fundamentally different principles. The redundant and independent calibration systems of CIMS and HWV afford new confidence in the accuracy of these aircraft measurements. We compare these aircraft measurements of water vapor to those made with the MLS instrument on the AURA satellite, and to frost point balloon borne (NOAA FPH and CFH) measurements coordinated with the WB-57 descents from the LS. A persistent bias of 0.7 ppmv is observed between the frost point and aircraft measurements in the 3 to 10 ppmv range, with frost point being lower. The MLS measurements are less precise, but generally fall between the balloon and aircraft measurements in the LS. These measurements renew interest in comparisons in the tropics where even lower mixing ratios and higher saturations with respect to ice are encountered.