Microphysical Retrievals from Simultaneous Measurements by Airborne and Ground Radars during OLYMPEX

The Olympic Mountains Experiment (OLYMPEX), a Global Precipitation Measurement (GPM) ground validation field campaign held in the vicinity of the Olympic Peninsula of Washington during November-December 2015, provided numerous opportunities to observe precipitation systems and their interaction with complex terrain in a variety of synoptic environments. Airborne radars at X, Ku, Ka, and W band were deployed on the NASA ER-2 (nominal altitude: 20km), and Ku, Ka, and W band measurements were made from the APR-3 instrument onboard the NASA DC-8 (nominal altitude: 12 km). Meanwhile, ground radars at S (NPOL), C (Doppler on Wheels), and Ku/Ka (D3R) provided dedicated polarimetric measurements that could be coordinated with aircraft flight patterns, augmenting the US National Weather Service and Environment Canada operational weather radar networks. In addition to these radar measurements, passive microwave radiometers measuring at frequencies between 10 and 183 GHz were present on both aforementioned aircraft, and the University of North Dakota Citation aircraft provided in-situ measurements of particle size, concentration, and bulk water content from multiple probes. On the ground, disdrometer measurements were made at 15 sites, four of which also operated vertically-pointing K band Micro Rain Radars (MRRs).

To take advantage of this unprecedented combination of radar measurements at multiple frequencies and view angles, a framework for microphysical retrievals from multiple measurements with arbitrary frequency, viewing geometry, and polarization has been developed for passive and active microwave sensors. The forward model used in the retrieval is the Atmospheric Radiative Transfer Simulator (ARTS), which contains solvers for fully-polarized passive and active (with and without multiple scattering) measurements through three-dimensional gridded atmospheres. Particle scattering properties (phase and extinction matrices) are user-supplied and can represent either randomly oriented or horizontally-aligned scattering media. The retrieval framework is an ensemble filtering technique based upon a set of single-frequency Hitschfeld-Bordan retrievals with perturbed assumptions regarding the particle size distribution, particle shape (for ice), and environmental parameters such as cloud liquid water, water vapor, and temperature. The covariances between observed parameters (reflectivities, polarimetric quantities, radiances) and the physical quantities to be retrieved (e.g., water content, mean particle size, aspect ratio) can be used to refine the ensemble and provide a pdf of the retrieved parameters, along with an assessment of the information content of the measurements defined as a reduction in spread of the filtered ensemble from the initial ensemble.

This presentation will provide an overview of the retrieval method and an evaluation of selected cases during OLYMPEX with the in-situ measurements provided by the Citation aircraft. Particular attention will be paid to the ability of various oriented ice particle scattering models to reconcile multi-frequency observations at nadir view angles with ground-based ZDR at horizontal incidence and passive microwave polarization differences at oblique incidence. The ability of these measurements to identify layers of supercooled water in precipitating ice clouds will also be examined.