WATER VAPOR CONCENTRATION AND CLOUD ABUNDANCE IN THE UPPER TROPOSPHERE/LOWER STRATOSPHERE (UTLS) REGION HAVE A STRONG IMPACT ON THE EARTHS RADIATION BUDGET CLIMATE AND STRATOSPHERIC CHEMISTRY; HOWEVER THE PROCESSES CONTROLLING UTLS HUMIDITY AND CLOUD FORMATION ARE NOT WELL UNDERSTOOD AND NOT WELL REPRESENTED IN CLIMATE MODELS. WE PROPOSE USING MEASUREMENTS OF OPTICALLY THIN CIRRUS AND WATER VAPOR FROM SAGE II/III ALONG WITH COMPLEMENTARY MEASUREMENTS OF CLOUDS FROM THE CALIOP LIDAR ON THE CALIPSO SATELLITE CLOUDS FROM OMPS-LP AND WATER VAPOR FROM THE MICROWAVE LIMB SOUNDER (MLS) ON THE AURA SATELLITE TO ADDRESS THE UTLS CLOUD AND HUMIDITY ISSUES. WE PROPOSE TO ADAPT SAGE II CLOUD DETECTION ALGORITHMS FOR SAGE III EXTINCTION DATA IMPLEMENT SAGE III OCCULTATION CLOUD ALGORITHMS AND ADAPT OMPS-LP LIMB RADIANCE ALGORITHMS FOR SAGE III LIMB SCAN DATA. WE PROPOSE TO USE STATE-OF-THE-ART TRANSPORT AND MICROPHYSICAL MODELS DRIVEN BY REANALYSIS FIELDS TO UNDERSTAND THE PROCESSES CONTROLLING CLOUDS AND STRATOSPHERIC WATER VAPOR WITH A FOCUS ON THE ABUNDANCE AND THE ROLE OF OPTICALLY THIN CIRRUS OCCURRING NEAR THE TROPICAL TROPOPAUSE ON THE HUMIDITY OF AIR ENTERING THE STRATOSPHERE. THE SAGE III SOLAR OCCULTATION MEASUREMENTS ARE PARTICULARLY WELL SUITED FOR DETECTION OF OPTICALLY THIN CIRRUS. THE PROPOSED MODELING APPROACHES INCLUDE (1) COMPUTATIONALLY EFFICIENT ENSEMBLES OF FORWARD TRAJECTORIES WITH SIMPLIFIED CLOUD MICROPHYSICS DRIVEN BY MERRA-2 AND ERA- INTERIM METEOROLOGICAL ANALYSES AND (2) ONE-DIMENSIONAL CLOUD SIMULATIONS WITH DETAILED MICROPHYSICS ALONG BACK TRAJECTORIES USING TEMPERATURE FIELDS FROM THE METEOROLOGICAL ANALYSES WITH HIGH-FREQUENCY WAVES SUPERIMPOSED. THE MODEL RESULTS WILL BE EVALUATED AND CONSTRAINED BY COMPARISON WITH MEASUREMENTS OF OPTICALLY THIN CIRRUS BY SAGE II/III AND OMPS-LP SUPPLEMENTED BY CALIOP CLOUD MEASUREMENTS AND AURA MLS WATER VAPOR MEASUREMENTS. FINALLY WE PROPOSE TO USE SAGE III WATER VAPOR MEASUREMENTS ALONG WITH OUR EXISTING SAGE II MLS AND HALOE WATER VAPOR CLIMATOLOGY TO TEST MULTI-DECADAL MODEL SIMULATIONS. THE RESEARCH WILL FOCUS ON THE FOLLOWING SPECIFIC TASKS: DEVELOPMENT OF A SAGE III CLOUD-DETECTION ALGORITHM. THE ALGORITHM WILL BE EVALUATED BY COMPARISON WITH OMPS-LP CALIOP AND HIGH-ALTITUDE AIRCRAFT AEROSOL AND CLOUD MEASUREMENTS. CLOUD PRODUCTS GENERATED WITH OUR ALGORITHM WILL BE PROVIDED FOR COMMUNITY USE. ADAPTATION OF THE OMPS-LP CLOUD ALGORITHMS FOR THE SAGE III LIMB SCAN MEASUREMENTS. USE OF THE SAGE III CLOUD PRODUCT ALONG WITH OMPS-LP CLOUD PRODUCTS TO QUANTIFY THE OCCURRENCE OF HIGH-ALTITUDE OPTICALLY-THIN TROPICAL TROPOPAUSE LAYER (TTL) CIRRUS. COMBINATION OF THE SAGE II HALOE AURA MLS AND SAGE III DATA INTO A UNIFIED MULTI-DECADAL RECORD OF STRATOSPHERIC WATER VAPOR. USE OF FORWARD AND BACKWARD TRAJECTORY MODELS TO ANALYZE THE DEHYDRATION PROCESS IN THE TROPICAL UPPER TROPOSPHERE AND USE OF THE SAGE III CALIOP AND OMPS-LP CLOUD PRODUCTS AS WELL AS SAGE III AND MLS WATER VAPOR PRODUCTS TO CONSTRAIN SIMULATIONS OF OPTICALLY THIN TTL CIRRUS AND WATER VAPOR. THE PROPOSAL TEAM HAS EXTENSIVE EXPERIENCE IN TTL RESEARCH INCLUDING ALGORITHM DEVELOPMENT MODELING AND ANALYSIS OF CLOUD AEROSOL AND TRACE GAS DATASETS. ONE OF US (MRS) HAS BEEN ON THE SAGE III SCIENCE UTILIZATION TEAM FOR THE LAST 5 YEARS. THE PROPOSED MODELING TOOLS ARE MATURE AND IDEALLY SUITED FOR ADDRESSING THE SCIENCE ISSUES DISCUSSED ABOVE.