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

The NASA/Aura/Microwave Limb Sounder (MLS) instrument has been compared to the Mauna Loa Observatory Raman water vapor lidar. Calibration of the lidar used Vaisala RS80-H radiosondes launched from the observatory. The average standard deviation between the sondes and the lidar, in the range 6 km to 11.5 km, is 11.9%. The sondes indicate no overlap correction for the lidar at low altitudes is necessary. A comparison was made between the lidar total column water and a GPS total column water measurement as a check on the calibration, resulting in a correlation slope of 1.026 +/- 0.058 and R-2 = 0.84. The MLS measurements are significantly better in the stratosphere where the lidar has poor sensitivity. The MLS measurement in the troposphere has much lower altitude resolution than the lidar so the validation overlap altitudes are limited. A comparison is made with version 1.5 MLS data for 32 overpasses at the three MLS altitudes in the troposphere. The GPS total column water measurement was used to screen the overpasses by eliminating ones where the water varied by more than 50% during the lidar integration period. At 147 hPa the MLS data show a dry bias of 44.8% +/- 36%. At 215 hPa the MLS measurement also shows a dry bias of 22.3% +/- 22%, and at 316 hPa there is a dry bias of 19.9% +/- 46%. The dry bias seen is consistent with the cryogenic frost point hygrometer (CFH) measurements at many latitudes (23% +/- 37% at 215 hPa and 4% +/- 62% at 316 hPa).