The Potential for Discriminating Microphysical Processes in Numerical Weather Forecasts Using Airborne Polarimetric Radio Occultations

Michael Murphy,Margaret Morris,Jennifer Haase,Shu-Hua Chen,Ramon Padullés

doi:10.3390/rs11192268

Abstract

Accurate representation of cloud microphysical processes in numerical weather and climate models has proven challenging, in part because of the highly specialized instrumentation required for diagnosing errors in simulated distributions of hydrometeors. Global Navigation Satellite System (GNSS) polarimetric radio occultation (PRO) is a promising new technique that is sensitive to hydrometeors and has the potential to help address these challenges by providing microphysical observations that are relevant to larger spatial scales, especially if this type of observing system can be implemented on aircraft that can target heavy precipitation events. Two numerical experiments were run using a mesoscale model configured with two different microphysical parameterization schemes for a very intense atmospheric river (AR) event that was sampled by aircraft deploying dropsondes just before it made landfall in California, during the CalWater 2015 field campaign. The numerical experiments were used to simulate profiles of airborne polarimetric differential phase delay observations. The differential phase delay due to liquid water hydrometeors below the freezing level differed significantly in the two experiments, as well as the height of the maximum differential phase delay due to all hydrometeors combined. These results suggest that PRO observations from aircraft have the potential to contribute to validating and improving the representation of microphysical processes in numerical weather forecasts once these observations become available.

Highlights

Accurate representation of cloud microphysical processes has been challenging for numerical models of the atmosphere
The sampling from in situ observations has been limited to direct aircraft and surface observation platforms, they have proven crucial in validation of microphysical parameterizations and satellite retrievals, as well as in the development of forward operators for polarimetric radar
While the follow-on COSMIC-2 radio occultation (RO) mission will have good equatorial coverage, at 40◦N latitude, which is more typical of ARs, COSMIC-2 will provide only approximately six occultations in 24 h in a region of this size, so other RO and polarimetric radio occultation (PRO) missions with openly available data should be a priority for the community. For this atmospheric river event, the choice of microphysical parameterization scheme used in the mesoscale model had a large effect on the characteristics of the simulated convection, the vertical distribution of hydrometeors, and the resulting spatial distribution of accumulated precipitation in mountainous regions

Summary

Introduction

Accurate representation of cloud microphysical processes has been challenging for numerical models of the atmosphere Overcoming this challenge is critical because accumulated precipitation at the ground is dependent on microphysical processes in the atmosphere above. The available observations of cloud microphysics used to validate numerical models and the microphysical parameterizations that they depend on have come from two main sources: (1) in situ observations and (2) radar reflectivity. In situ observations can provide detailed measurements of individual hydrometer types and size distributions. Polarimetric radar observations can provide characteristics of individual hydrometeor types (e.g., the work by the authors of [9]) and drop size distributions can be derived from these observations (e.g., the work by the authors of [10]). More recently the full vertical structure of simulated convection has been compared to radar observations [13], and dual polarization observations have been utilized to distinguish different types of hydrometeors in numerical simulations [14]

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Discussion

Conclusion