An Intercomparison of Water Vapor Measurements in the TTL and Lower Tropical Stratosphere during CRAVE and TC4: The Importance and Implications of Laboratory Calibrations With water Vapor Mixing Ratios From 0-10 ppmv.

As part of the effort to validate instruments on the Aura satellite, the Costa Rica Aura Validation Experiment (CRAVE) was flown in January and February of 2006. Systematic differences in measured water vapor in the tropopause region and lower stratosphere between in situ instruments on the WB57, the NOAA Colorado frostpoint hygrometer (CFH) and satellite borne instruments such as the Microwave Limb Sounder (MLS) and the Halogen Occultation Experiment (HALOE) are well-documented. Results from CRAVE, presented during a water vapor workshop organized as part of the CRAVE science meeting, provided further confirmation of these differences. The availability for the first time of multiple robust intercomparisons between these instruments led to the conclusion that at low water mixing ratios (less than 10 ppmv), the differences appear to be well-represented by an offset of about 2 ppmv with in situ instruments on the WB57 measuring higher than MLS and CFH. This enduring discrepancy precludes a satisfactory validation of satellite retrievals of stratospheric water vapor profiles. In this talk we will summarize the recent low water calibration runs in our laboratory that provide direct evidence that the Harvard Lyman alpha instrument measures accurately at low water. We will then compare data taken during the recent TC4 campaign in August 2007 with that from CRAVE and previous campaigns. The implications of the results on our understanding of the mechanisms that control the stratospheric water vapor budget will be discussed.