Cluster Analysis of Vertical Polarimetric Radio Occultation Profiles and Corresponding Liquid and Ice Water Paths From GPM Microwave Data

Abstract. The polarimetric phase difference between the horizontal and vertical components of GNSS radio signals is correlated with the presence of ice and precipitation in the propagation path of those signals. This study evaluates the ability of k-means clustering to find relationships among polarimetric phase difference, refractivity, liquid water path (LWP), ice water path (IWP), and water vapor pressure using over two years of data matched between the Global Precipitation Measurement (GPM) mission and Radio Occultations through Heavy Precipitation demonstration mission onboard the Spanish Paz spacecraft (ROHP-PAZ). A cluster hierarchy is introduced across these variables. A potential refractivity model for polytropic atmospheres is introduced to ascertain how different types of vertical thermodynamic profiles that can occur during different precipitation scenarios are related to changes in the polytropic index and thereby vertical heat transfer rates. The clustering analyses uncover a relationship between the amplitude and shape of deviations from the potential refractivity model and water vapor pressure and confirm the expected positive correlation between polarimetric phase difference and both LWP and IWP. For certain values, the coefficients of the potential refractivity model indicate when a profile has little to no moisture, and the study reveals a similar relationship between the clustering for these coefficients and different water vapor pressure profiles. The study also confirms the relationship between the integrated polarimetric phase difference and water vapor pressure columns, known as the "precipitation pickup," globally (ρ_s=0.971 after averaging) and over different latitudinal ranges (>50°, ≥20°, and <20°, with different ρ_s for each).