Validation of Aura Microwave Limb Sounder water vapor by balloon‐borne Cryogenic Frost point Hygrometer measurements

Abstract

[1] Here we present extensive observations of stratospheric and upper tropospheric water vapor using the balloon-borne Cryogenic Frost point Hygrometer (CFH) in support of the Aura Microwave Limb Sounder (MLS) satellite instrument. Coincident measurements were used for the validation of MLS version 1.5 and for a limited validation of MLS version 2.2 water vapor. The sensitivity of MLS is on average 30% lower than that of CFH, which is fully compensated by a constant offset at stratospheric levels but only partially compensated at tropospheric levels, leading to an upper tropospheric dry bias. The sensitivity of MLS observations may be adjusted using the correlation parameters provided here. For version 1.5 stratospheric observations at pressures of 68 hPa and smaller MLS retrievals and CFH in situ observations agree on average to within 2.3% ± 11.8%. At 100 hPa the agreement is to within 6.4% ± 22% and at upper tropospheric pressures to within 23% ± 37%. In the tropical stratosphere during the boreal winter the agreement is not as good. The ''tape recorder'' amplitude in MLS observations depends on the vertical profile of water vapor mixing ratio and shows a significant interannual variation. The agreement between stratospheric observations by MLS version 2.2 and CFH is comparable to the agreement using MLS version 1.5. The variability in the difference between observations by MLS version 2.2 and CFH at tropospheric levels is significantly reduced, but a tropospheric dry bias and a reduced sensitivity remain in this version. In the validation data set a dry bias at 177.8 hPa of À24.1% ± 16.0% is statistically significant.

NASA/Aura/Microwave Limb Sounder water vapor validation at Mauna Loa Observatory by Raman lidar

“…The . At 215 hPa (11.9 km) the MLS measurements show a dry bias of 22.3% ± 22% and are consistent with the dry bias of 23% ± 37% measured by the cryogenic frost point hygrometer (CFH) at many latitudes [Vömel et al, 2007b]. At 316 hPa there is also a dry bias of 19.9% ± 46% which is consistent with the CFH's 4% ± 62% measured at many latitudes, but the large spread in the result limits the usefulness for validating MLS.…”

“…At the 147 hPa altitude the MLS average mixing ratio was 7.3 ppmv and at this altitude and low level of water vapor, the lidar signal-to-noise ratio is low. The large dry bias measured (44.8%) is larger than the CFH result of about 14% [Vömel et al, 2007b] that was measured at many latitudes. Although there is no reason to suspect the lidar data at this pressure, a small background on the water vapor channel would introduce higher water vapor levels in the lidar data, which could explain some of the difference.…”

“…This was also found to be true for cryogenic frost point hygrometer measurements by Vömel et al [2007b]. But the variability (standard deviation) of the relative differences was affected by how the overpasses were further screened.…”

Remote sensing ice supersaturation inside and near cirrus clouds: a case study in the subtropics

“…This radiosonde does not require calibration and can be considered an absolute reference for water vapor. Its uncertainty in frost point measurement is better than 0.5 K, leading in the Tropics to a mixing ratio uncertainty of ∼4% in the lower troposphere to ∼10% in the tropopause (Vömel et al, 2007). As Times (UTC) (hhmm) the instrument configuration was not modified during the case study, we consider the calibration coefficient stable.…”

Variability of water vapour in the Arctic stratosphere

“…However, such measurements have been made at Sodankylä (67.4 • N, 26.6 • E), northern Finland, since early 2000 (Vömel et al, 2007a, c). The Sodankylä site is representative of high-latitude conditions in northern Europe, and the upper air soundings in winter and spring sample air both inside and outside the polar stratospheric vortex.…”