Investigation of microphysical assumptions in TRMM radiometer's rain profile algorithm using KWAJEX satellite, aircraft and surface datasets

This research intercompares in situ aircraft microphysical measurements directly with the assumed microphysical parameters of the level 2 Tropical Rainfall Measuring Mission (TRMM) facility radiometer algorithm where retrievals are obtained from reconstructed and coincident TRMM overpasses during the 1999 Kwajalein Atoll field experiment (KWAJEX). The analyzed differences are used to diagnose weaknesses in the assumed profile microphysics and to guide a physically-based reformulation of the microphysical representations in the 2A-12 algorithm which would mitigate the weaknesses. The analysis process includes the introduction of a passive microwave emission---scattering (E-S) coordinate system that permits concurrent mapping and quantitative comparison of radiometer brightness temperatures, radar reflectivities, rainrates, and bulk microphysical parameters; and application of a hypothesis test which asserts that 2A-12 to ground radar-derived rainrate differences are correlated to differences in assumed versus observed liquid-ice bulk microphysical properties. Thus, the main scientific objective of this research is to identify and overcome the foremost model-generated microphysical weaknesses in the TRMM 2A-12 facility algorithm through analysis of (1) in situ aircraft microphysical observations; (2) ground-, aircraft-, and satellite-based radar measurements; (3) aircraft- and satellite-based radiometer measurements; (4) synthetic satellite radar reflectivities and radiometer brightness temperatures; (5) radiometer-only (2A-12) satellite retrievals; and (6) radar-only (2A-25) algorithm retrievals.;Results indicate the assumed 2A-12 microphysical profiles differ most from aircraft observations where ground-aircraft radar and 2A-12 rainrate differences are greatest. Analysis of the E-S coordinate system intercomparisons reveals a lack of mixed-layer ice hydrometeor scatterers in the model-generated profiles as compared to observed profiles. Direct comparisons between aircraft measured and model-generated 2A-12 microphysics suggest that, on average, the radiometer algorithm's microphysics database retrieves liquid and ice water contents ∼3 times smaller than observed at levels less than 10 km. The 2A-12 rainrate retrievals are shown to be strongly influenced by the algorithm's convective fraction specification. Modification of this factor is proposed as a means to improve 2A-12 rainrate retrievals, however fundamental changes to the cloud-radiation model's ice parameterization would be necessary to physically correct the algorithm's diagnosed mixed-layer ice hydrometeor deficits.