DESPITE ITS LOW CONCENTRATION STRATOSPHERIC WATER VAPOR HAS LARGE IMPACTS ON THE EARTH'S ENERGY BUDGET AND CLIMATE. RECENT STUDIES SUGGEST THAT EVEN SMALL CHANGES IN STRATOSPHERIC HUMIDITY MAY HAVE CLIMATE IMPACTS THAT ARE SIGNIFICANT COMPARED TO THOSE OF DECADAL INCREASES IN GREENHOUSE GASES. FUTURE CHANGES IN STRATOSPHERIC HUMIDITY AND OZONE CONCENTRATION IN RESPONSE TO CHANGING CLIMATE ARE SIGNIFICANT CLIMATE FEEDBACKS. WHILE THE TROPOSPHERIC WATER VAPOR CLIMATE FEEDBACK IS WELL REPRESENTED IN GLOBAL MODELS PREDICTIONS OF FUTURE CHANGES IN STRATOSPHERIC HUMIDITY ARE HIGHLY UNCERTAIN BECAUSE OF GAPS IN OUR UNDERSTANDING OF PHYSICAL PROCESSES OCCURRING IN THE TROPICAL TROPOPAUSE LAYER (TTL -13-18 KM) THE REGION OF THE ATMOSPHERE THAT CONTROLS THE COMPOSITION OF THE STRATOSPHERE. UNCERTAINTIES IN THE TTL CHEMICAL COMPOSITION ALSO LIMIT OUR ABILITY TO PREDICT FUTURE CHANGES IN STRATOSPHERIC OZONE. WE PROPOSE A SERIES OF MEASUREMENT CAMPAIGNS USING THE LONG-RANGE NASA GLOBAL HAWK. (GH) UNMANNED AIRCRAFT SYSTEM (UAS) TO DIRECTLY ADDRESS THESE PROBLEMS.A NUMBER OF TIL PROCESSES AFFECT STRATOSPHERIC HUMIDITY AND OZONE: DEEP CONVECTIVE TRANSPORT OF WATER VAPOR AND OTHER CHEMICAL CONSTITUENTS INTO THE TTL; THE FORMATION OF UBIQUITOUS THIN CIRRUS IN THE TTL (WHICH THEMSELVES HAVE A LARGE EFFECT ON THE RADIATION BUDGET AND CLIMATE); LIMITATION OF STRATOSPHERIC HUMIDITY BY SEDIMENTING ICE CRYSTALS IN TTL THIN CIRRUS; SLOW VERTICAL TRANSPORT THROUGH THE TTL; THE EFFECTS OF TROPICAL ATMOSPHERIC WAVES ON TTL THERMAL STRUCTURE DEHYDRATION AND TRANSPORT; AND THE CHEMISTRY OF SHORT-LIVED TRACE GASES.OBSERVATIONS OF TTL COMPOSITION ARE SPARSE COMPARED TO THOSE IN OTHER CLIMATICALLY IMPORTANT REGIONS PARTLY BECAUSE THE HIGH TTL ALTITUDE LIMITS AIRCRAFT SAMPLING. FURTHER THE UTILITY OF SATELLITE MEASUREMENTS OF TTL COMPOSITION IS REDUCED BY THE CHARACTERISTICALLY STRONG VERTICAL GRADIENTS IN THE TTL. AS A RESULT TTL PROCESSES ARE NOT WELL REPRESENTED IN GLOBAL MODELS. THIS INVESTIGATION WILL PROVIDE DATA NEEDED TO ADDRESS THESE SHORTCOMINGS: HIGH SPATIAL RESOLUTION (ONLY OBTAINABLE BY AIRCRAFT) MEASUREMENTS OVER LARGE GEOGRAPHICAL REGIONS (REQUIRING THE GR RANGE) AND OVER MULTIPLE SEASONS.THE PROPOSED INVESTIGATION FILLS SEVERAL SIGNIFICANT GAPS IN ATMOSPHERIC SCIENCE IDENTIFIED IN THE NASA DECADAL SURVEY INVOLVING CLIMATE CHANGE STRATOSPHERIC OZONE AND STRATOSPHERE-TROPOSPHERE EXCHANGE. SCIENCE QUESTIONS THAT WILL BE ADDRESSED INCLUDE THE FOLLOWING:1. WHAT PROCESSES CONTROL THE TROPICAL TROPOPAUSE TEMPERATURE AND THE HUMIDITY OF AIR ENTERING THE STRATOSPHERE (INCLUDING THEIR SEASONAL CYCLES)?2. WHAT ARE THE DOMINANT PATHWAYS FOR VERTICAL TRANSPORT FROM CONVECTIVE DETRAINMENT ALTITUDES IN THE TTL UP TO THE TROPICAL TROPOPAUSE IN DIFFERENT SEASONS?3. WHAT ARE THE FORMATION PROCESSES MICROPHYSICAL PROPERTIES AND CLIMATE IMPACT OF TTL CIRRUS AND HOW DO THESE CLOUDS REGULATE THE HUMIDITY OF AIR ENTERING THE STRATOSPHERE?4. WHAT ARE THE CHEMICAL AND TRANSPORT PROCESSES THAT DRIVE THE BUDGETS OF PRECURSORS OF OZONE-DESTROYING SUBSTANCES SUCH AS SHORT-LIVED HALOGEN COMPOUNDS?5. HOW WILL TTL CIRRUS STRATOSPHERIC HUMIDITY AND STRATOSPHERIC OZONE RESPOND TO A CHANGING CLIMATE AND WHAT ARE THE RESULTING FEEDBACK EFFECTS?