THE OVERARCHING GOAL OF THE PROPOSED WORK IS TO CONTRIBUTE TO THE EVALUATION AND IMPROVEMENT OF GLOBAL PRECIPITATION MEASUREMENT MISSION (GPM) RETRIEVAL ALGORITHMS BY CONSOLIDATING PERFORMING SCIENTIFIC INVESTIGATIONS WITH AND PROVIDING VALUE ADDED INFORMATION FROM THE USE OF IN SITU MICROPHYSICAL AND REMOTE SENSING DATA COLLECTED DURING THE GPM GROUND VALIDATION (GV) FIELD CAMPAIGNS. THE PROPOSED ANALYSIS WILL ADDRESS THE A PRIORI ASSUMPTIONS IN GPM ALGORITHMS FOCUSING ON IMPROVING ASSUMPTIONS IN THE DUAL-FREQUENCY PRECIPITATION RADAR (DPR) ALGORITHM ACROSS ALL GLOBAL PRECIPITATION REGIMES UPON WHICH ALL OTHER CORE AND CONSTELLATION ALGORITHMS ARE BASED. ACCURATE RETRIEVALS OF ICE WATER CONTENT (IWC) AND PRECIPITATION RATE (R) ARE CRITICAL FOR A HOST OF APPLICATIONS INCLUDING QUANTIFYING THE GLOBAL WATER CYCLE LATENT HEATING RETRIEVALS EVALUATING ATMOSPHERIC MODELS AMONG OTHERS. THUS IT IS IMPERATIVE THAT WE USE ALL INFORMATION AVAILABLE TO IMPROVE GPM REFERENCE RETRIEVALS FROM THE DPR. GPM GV FIELD CAMPAIGNS PROVIDE THE CRITICAL INFORMATION NECESSARY TO MAKE THESE IMPROVEMENTS. OUR GROUP AT ILLINOIS HAS BEEN HEAVILY INVOLVED IN THE PARTICLE SIZE DISTRIBUTION (PSD) WORKING GROUP CO-LED BY THE PI UNDER THE CURRENT PRECIPITATION MEASUREMENT MISSIONS (PMM) FUNDING CYCLE AND HAS BEEN HEAVILY INVOLVED WITH THE CONDUCT OF THE GPM GV FIELD CAMPAIGNS. FURTHERMORE OUR UNIQUE COLLABORATION BETWEEN THE UNIVERSITY OF ILLINOIS AND UNIVERSITY OF OKLAHOMA PROVIDES THE SCIENCE TEAM WITH DEEP EXPERTISE IN REMOTE SENSING AND IN SITU MICROPHYSICS FOR THE IMPROVEMENT OF GPM ALGORITHMS THROUGH TAILORED ANALYSIS OF GPM FIELD CAMPAIGN DATA. THE SPECIFIC AIMS OF THE PROPOSED PROJECT INCLUDE: 1. DEVELOP ICE- AND MIXED-PHASE PRECIPITATION PSD AND PARTICLE MASS PARAMETERIZATIONS (WITH UNCERTAINTIES) THAT USES Z KU-KA DUAL FREQUENCY RATIO (DFR) AND ENVIRONMENTAL CHARACTERISTICS AS INPUTS FROM GPM FIELD CAMPAIGN DATA. THIS PARAMETERIZATION CAN BE EXTENDED TO SINGLE-FREQUENCY RADAR RETRIEVALS. 2. USE GPM GV DATA TO DEVELOP AND TEST A NEW OPTIMAL ESTIMATION-BASED MICROPHYSICAL RETRIEVAL OF R EFFECTIVE DENSITY AND PARTICLE MEDIAN MASS DIAMETER USING OUR NEW PARAMETERIZATIONS. 3. APPLY THE RETRIEVAL GLOBALLY USING DPR INNER SWATH DATA TO STUDY THE ICE MICROPHYSICAL PROPERTIES OF ICE- AND MIXED-PHASE CLOUDS. RESULTS FROM THE STUDY WILL BE CONTINUOUSLY UPDATED WITH RELEVANT PMM RADAR AND COMBINED ALGORITHM DEVELOPERS THROUGH DIRECT COLLABORATION AND THE PMM PARTICLE SIZE DISTRIBUTION WORKING GROUP.