THE OBJECTIVE OF THE PROPOSED WORK IS TO TEST THE HYPOTHESIS THAT DENSE POLARIMETRIC GNSS RADIO OCCULTATION (RO) OBSERVATIONS CAN BE USED TO IMPROVE MICROPHYSICAL MODELING IN CASES WHERE IT IS CRITICAL FOR FORECASTING HEAVY PRECIPITATION. ATMOSPHERIC RIVERS ARE IMPORTANT TO THE WATER CYCLE IN THE WESTERN US BECAUSE THEY CAUSE FLOODING AND BECAUSE THEY PROVIDE THE SNOWPACK FOR THE REGION'S WATER RESOURCES. QUANTIFYING THE ONSHORE MOISTURE TRANSPORT IS THE MAIN FACTOR IN DETERMINING THE AMOUNT OF PRECIPITATION THAT FALLS AND DENSE RADIO OCCULTATION OBSERVATIONS ARE AN ADVANTAGEOUS SOURCE OF OBSERVATIONS FOR THIS PURPOSE BECAUSE OF THEIR HIGH VERTICAL RESOLUTION OVER THE OCEAN. HOWEVER THE CHOICE OF MICROPHYSICS PARAMETERIZATION IN THE FORECAST MODELS ALSO HAS A STRONG EFFECT ON THE QUANTITY AND TYPE OF FORECAST PRECIPITATION. POLARIMETRIC GNSS RADIO OCCULTATION OBSERVATIONS OF THE DIFFERENTIAL DELAY BETWEEN HORIZONTALLY VERSUS VERTICALLY POLARIZED SIGNALS (H/VPOL) HAVE THE POTENTIAL TO PROVIDE INFORMATION ON THE VERTICAL DISTRIBUTION OF HYDROMETEORS. THESE OBSERVATIONS OF PRECIPITATION MADE AT THE SAME SCALE AS THE MOISTURE OBSERVATIONS WILL IMPROVE FORECASTS BY PROVIDING VERIFICATION OF THE MICROPHYSICAL PARAMETERIZATION PERFORMANCE AT THE REGIONAL SCALE.WE COLLECTED POLARIMETRIC GPS MEASUREMENTS AS WELL AS GLONASS OCCULTATION OBSERVATIONS DURING A FLIGHT OF OPPORTUNITY DURING A HEAVY PRECIPITATION ATMOSPHERIC RIVER EVENT IN FEBRUARY 2015. THESE AIRBORNE CAMPAIGNS ARE INTERESTING FOR DATA ASSIMILATION STUDIES BECAUSE OF THE HIGH OBSERVATION DENSITY AND THE GLONASS OBSERVATIONS INCREASE THE DENSITY OF OBSERVATIONS STILL FURTHER. WE WILL TEST THE HYPOTHESIS THAT POLARIMETRIC RO OBSERVATIONS CAN BE USED TO DISTINGUISH BETWEEN MICROPHYSICAL PARAMETERIZATIONS AND CARRY OUT A SENSITIVITY STUDY WITH SIMULATED OBSERVATIONS USING THE WEATHER AND RESEARCH FORECASTING MODEL TO UNDERSTAND THE POTENTIAL FORECAST IMPACT. WE WILL ALSO ASSESS THE POTENTIAL IMPROVEMENT IN SNR AND ACCURACY THAT CAN BE ACHIEVED USING A COSMIC-2 TRIG-TYPE OPEN LOOP RECORDING SYSTEM WITH A PHASED ARRAY ANTENNA BEYOND THE CAPABILITIES OF THE STATE OF THE ART AIRBORNE SYSTEM USED IN THE PREVIOUS CAMPAIGN AS A PRELUDE TO ESTABLISHING THE AIRBORNE SYSTEM AS A TESTBED FOR ADDING GNSS H/VPOL RO CAPABILITY TO FUTURE NASA EARTH OBSERVATION MISSIONS BEYOND COSMIC-2.THE APPROACH BUILDS ON THE RECENT WORK ON GNSS POLARIMETRIC DELAYS (CARDELLACH ET AL. 2010) BY DEVELOPING SIMULATION ALGORITHMS FOR POLARIMETRIC DELAYS FOR AIRBORNE RO AND TESTING THEIR SENSITIVITY TO PRECIPITATION FOR AN ATMOSPHERIC RIVER EVENT FOR DIFFERENT MICROPHYSICAL PARAMETERIZATIONS GIVEN THE TECHNICAL CAPABILITIES OF THE EXISTING GISMOS AIRBORNE SYSTEM AND THE TRIG RADIOOCCULTATION SYSTEM. WE WILL ANALYZE H/VPOL AND GLONASS DATASETS COLLECTED DURING FLIGHTS OF OPPORTUNITY DURING THE CALWATER2015 ATMOSPHERIC RIVERS FIELD CAMPAIGN. WE WILL ASSESS THE POTENTIAL IMPACT OF GLONASS OBSERVATIONS FOR THESE EVENTS USING AN OBSERVATION SYSTEM SIMULATION EXPERIMENT (OSSE).THE RESULTS WILL PROVIDE A REALISTIC ASSESSMENT OF THE PERSPECTIVES FOR FUTURE H/VPOL OBSERVATION MISSIONS AND INCREASED SIGNAL CAPABILITIES PROVIDED BY THE TRIG RECEIVER AT A SAMPLING DENSITY THAT WE HOPE TO ACHIEVE WITH CONSTELLATIONS WITH COSMIC-2 AND BEYOND.THE PROPOSED WORK IS HIGHLY RELEVANT TO NASA EARTH SURFACE AND INTERIOR PROGRAM AREAS EMPHASIZING NATURAL HAZARDS FROM FLOODING AND THE WATER CYCLE BECAUSE OF THE HIGH IMPACT HEAVY PRECIPITATION IN ATMOSPHERIC RIVER EVENTS. THESE EVENTS ARE IMPORTANT TO THE SNOWPACK THAT PROVIDES WATER STORAGE FOR THE DROUGHT-STRICKEN WESTERN US. THE PROPOSED WORK DIRECTLY ADDRESSES NASA'S PROGRAMMATIC ANNOUNCEMENT SEEKING INNOVATIVE APPROACHES TO THE DEVELOPMENT OF GNSS REMOTE SENSING TECHNIQUES AND ALGORITHMS AND ADDRESSES NASA'S FOCUS ON BROADER UTILIZATION OF EXISTING GNSS SIGNALS IN PARTICULAR GLONASS IN PREPARATION FOR EXPLOITING THE FUTURE CAPABILITIES OF THE TRIG RECEIVER.