Radar Observed Variability in Rainfall during OLYMPEx

The mountainous, complex terrain of the Olympic Peninsula is a challenging environment for remote sensing of precipitation. Beam blockage, low rain rates, and low melting layers are all difficulties that must be overcome in order to retrieve rain rates from ground-based polarimetric radars, while satellite retrievals must overcome non-uniform beam filling, clutter due to terrain, and drop-size distribution assumptions associated with complex microphysical regimes. Toward this end, a polarimetric rainfall retrieval algorithm specific to the region is developed for application to the NASA polarimetric, S-band radar NPOL deployed during the OLYMPEX NASA Ground Validation Experiment in 2015. This algorithm is developed to optimize the use of polarimetric data when available but minimize measurement uncertainties to achieve the best possible rain rates. Disdrometer data from the large ground network are used to develop rain rate relationships. The effects of orographic enhancement and pre- and post-frontal regimes on the polarimetric rainfall relationships and especially Z-R are investigated. Rain maps are generated every ten minutes, and a timeseries of the domain-averaged rain rate with an associated uncertainty estimate will be created for the project time period (November 10 – January 15).

Armed with the new rain retrieval algorithm, the spatial and temporal variability of rain will be analyzed in terms of differences between the ocean and land sectors, orographic enhancement, and pre- and post-frontal regimes. This variability will be put in larger microphysical context by examining the 3-D polarimetric data from the S-band radars (NPOL and KLGX), as well as the X-band DOW data.

These observations will underpin findings about the estimates of rain rates from GPM DPR, GMI, and IMERG in the region. The complexity of the meteorology and orography on the Olympic peninsula result in several challenges for satellite-based retrievals, including non-uniform beam filling, terrain, and distinct microphysical regimes such as abundant small drops and orographic enhancement. We will examine rain statistics such as CFADs, rain rate and reflectivity PDFs, and area averaged rain statistics from satellite measurements in comparison to degraded NPOL data. The available overpasses from GPM when there is precipitation in the GMI swath (20-25 samples) and the narrower DPR swath (8-12 samples) will be used to test GMI and DPR retrievals against ground observation in different regimes. Finally, longer term project statistics will be compared to IMERG climatologies over the area.