A New Instrument For In-Situ Measurement of Total Ice Water Content - Observations of Sub-visible Cirrus at the Tropical Tropopause

The recently developed Harvard Total Water Instrument measures the ice water content of cirrus, in conjunction with the Harvard water vapor instrument. The instrument samples liquid and/or solid water particles without perturbing the ambient particle density, and uses the well established lyman-α photo-fragment fluorescence technique to make accurate and precise measurements of the total water content of the ambient air. Data obtained aboard the WB-57 aircraft on flights out of Costa Rica during the summer of 2001 show both agreement with the Harvard water vapor instrument in dry air, and sufficient sensitivity to detect sub-visible cirrus near the tropical tropopause. Preliminary analysis shows evidence of high supersaturations in the presence of these cirrus. Intercomparisons with an airborne tunable diode laser hygrometer during the CRYSTAL-FACE mission show that contamination of our water vapor measurement due to evaporation in our sampling duct is at most 5%, and cannot explain the observed supersaturations. We will discuss the implications of high supersaturation in the context of cloud microphysics, and the processes controlling water vapor in the upper troposphere and lower stratosphere.