THE OBJECTIVE OF THIS PROPOSAL IS TO FILL IN AN URGENT GAP IN OUR UNDERSTANDING OF EARTH S ATMOSPHERE-ICE-OCEAN INTERACTIONS BY PRODUCING OBSERVATIONALLY CONSTRAINED MODELS OF ALL STAGES OF THE LIFE CYCLE OF SNOW OVER ICE SHEETS. TO DEVELOP THESE MODELS WE WILL LEVERAGE SEVERAL UNIQUE SATELLITE MEASUREMENTS COMPLEMENTARY EXISTING AIRBORNE AND IN SITU MEASUREMENTS GLOBAL ATMOSPHERIC MODELING MASS AND ENERGY BALANCE MODELING AND PROCESS MODELING. THE LIFE CYCLE OF SNOW IS ONE OF THE MOST CRITICAL UNKNOWNS OF THE POLAR CLIMATE SYSTEM. OVER THE POLAR REGIONS PRECIPITATION IS PREDOMINANTLY IN ITS SOLID PHASE HOWEVER THE ACTUAL MAGNITUDE OF SNOWFALL THAT FALLS ACCUMULATES REDISTRIBUTES METAMORPHOSIZES MELTS AND SUBSEQUENTLY RUNS OFF REMAINS POORLY QUANTIFIED LARGELY DUE TO THE LACK OF COHERENT OBSERVATIONS. EVALUATING THE IMPACT OF THIS EVOLUTION ON THE EARTH SYSTEM HAS SUBSTANTIAL IMPLICATIONS FOR THE MASS BALANCE OF ICE SHEETS AND THEIR CONTRIBUTION TO SEA-LEVEL RISE FRESHWATER INPUT INTO THE OCEAN AND SEA ICE GROWTH AND WATER MASS FORMATION AND SUBSEQUENT IMPACT ON GLOBAL OCEAN CIRCULATION. THUS OUR PRIMARY RESEARCH HYPOTHESIS IS: COMBINING DIFFERENT SATELLITE PRODUCTS CAN WE MEASURE HOW MUCH SNOW FALLS OVER EARTH S ICE SHEETS ACCUMULATES AND ULTIMATELY RETURNS TO THE OCEAN? IF SO HOW DOES IT VARY IN TIME AND SPACE? WHAT IS THE RELATIVE IMPACT OF EACH COMPONENT OF THE SNOW LIFE CYCLE ON EARTH S CLIMATE SYSTEM? NASA S INTEGRATED MULTI-SATELLITE RETRIEVALS FOR GLOBAL PRECIPITATION MEASUREMENT (IMERG) HAS MADE SIGNIFICANT PROGRESS TOWARDS GENERATING TWO DECADES OF GLOBAL SATELLITE PRECIPITATION HOWEVER RETRIEVALS DO NOT EXIST OVER ICE SHEETS DUE TO: (1) LIMITATIONS IN THE GLOBAL PRECIPITATION MEASUREMENT (GPM) MISSION S SATELLITE COVERAGE AT HIGH LATITUDES AND (2) COMPLICATIONS IN INTERPRETING PASSIVE MICROWAVE PRECIPITATION RETRIEVALS OVER ICE-COVERED TERRAIN. WE WILL INSTEAD LEVERAGE MULTIPLE INDEPENDENT SATELLITE MEASUREMENTS OF SEVERAL STAGES OF THE LIFE CYCLE OF SNOW TO IMPROVE OUR UNDERSTANDING OF FALLING SNOW AND FALLEN SNOW AS WELL AS ITS SUBSEQUENT EVOLUTION OVER THE GREENLAND AND ANTARCTIC ICE SHEETS. SPECIFICALLY WE EMPLOY OBSERVATIONS OF SNOWFALL AND RAINFALL RATES (CLOUDSAT) SURFACE HEIGHT CHANGE (ICESAT ICESAT-2 GNSS REFLECTOMETRY AWS) SNOW THICKNESS (OPERATION ICEBRIDGE SNOW RADAR) BLOWING SNOW FLUXES (ICESAT ICESAT-2) ALBEDO AND TIMING OF BARE ICE EXPOSURE (MODIS) MASS CHANGE (GRACE GRACE-FO) AND MICROWAVE BRIGHTNESS TEMPERATURES (SSM/I SSM/IS AMSR-E AMSR2). INDIVIDUALLY THESE OBSERVATIONS PROVIDE MEDIOCRE CONSTRAINTS ON SNOW EVOLUTION SINCE THEY TARGET A SPECIFIC STAGE OF THE LIFE CYCLE IN CONJUNCTION THEY PROVIDE INDEPENDENT CONTROLS ON THE ENTIRE SYSTEM. GLOBAL ATMOSPHERIC MODELING SURFACE ENERGY AND MASS BALANCE MODELING AND RADIATIVE TRANSFER MODELING PROVIDE THE PLATFORM THROUGH WHICH WE WILL EVALUATE OUR PRESENT UNDERSTANDING OF SNOW EVOLUTION OVER THE ICE SHEETS BY ENSURING THAT OUR MODELS FALL WITHIN THE OBSERVATIONAL CONSTRAINTS. THUS WE WILL PRODUCE THE MOST ROBUST OPERATIONAL ESTIMATES OF SNOWFALL RATES AND THEIR EVOLUTION OVER THE ICE SHEETS VIA INTERDISCIPLINARY STUDIES OF THE ATMOSPHERE CRYOSPHERE AND OCEAN USING AN ARRAY OF REMOTE SENSING AND MODELING TECHNIQUES. THE WORK IS AIMED AT SUBELEMENT 4: THE LIFE CYCLE OF SNOW AND FULFILLS THE IDS REQUIREMENTS FOR APPROACHES THAT INTEGRATE THE TRADITIONAL DISCIPLINES OF THE EARTH SCIENCES AS WELL AS INNOVATIVE AND COMPLEMENTARY USE OF MODELS AND DATA AND EXPLOITING THE VAST WEALTH OF DATA FROM NASA SATELLITE AND AIRBORNE SENSORS. FURTHERMORE THE PROPOSED RESEARCH HAS A TEAM THAT IS COMPRISED OF SCIENTISTS WITH UNIQUE EXPERTISE AT VARIOUS STAGES IN THEIR CAREER (GRADUATE STUDENTS POST DOCS EARLY-CAREER SCIENTISTS) AND WILL CONTINUE THE SUCCESS OF THE IDS PROGRAM IN TRAINING THE NEXT GENERATION OF INTERDISCIPLINARY SCIENTISTS AS DETAILED IN THE SOLICITATION.