Rain Relations Inferred from Microphysical Data in TOGA COARE and Their Use to Test a Rain-Profiling Method from Radar Measurements at Ku-Band

Abstract

The first part of this paper is dedicated to establishing relations among rain-integrated parameters representative of west Pacific precipitation. This is achieved by using airborne microphysical data gathered within a rain event on 6 February 1993 during the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). The relations between the rain rate R, the reflectivity factor Z, and the attenuation coefficient K are calculated for moderate to heavy precipitation at 13.8 GHz. They give twice as much attenuation for a given Z than the relations obtained for an exponential distribution with N₀ = 8 × 10⁶ m⁻⁴. This effect is related to the large number of small size particles observed in TOGA COARE convective systems. In the second part of the paper, these relations are used to check the reliability of a rain-profiling method applied to ARMAR (airborne radar-mapping radar) observations at 13.8 GHz in the same rain event. This method provides a bulk correction factor that can be interpreted primarily in terms of a change of the initial Z–K relation. Then, the algorithm provides modified Z–R and K–R relations while assuming a gamma or an exponential-shaped distribution for raindrops with a constant N₀. For the selected case study, the adjusted relations agree very well with those derived from the microphysical measurements. An exponential shape model with constant N₀ for the DSD is found to provide results that are consistent with the microphysical measurements. Moreover, the derived N₀ value is close to that inferred from the radar algorithm. The impact of modifying the initial rain relations in the radar algorithm on the rain-rate estimates is also discussed. The retrieved rain rates are not very sensitive to the choice of initial relations except for very high values. Finally, the results are found more representative of convective rain than stratiform precipitation.