Overview of Multi-frequency and Dual-Doppler Radar Observations in the East River Watershed of Colorado during the SAIL and SPLASH Field Experiment

The production of water in mountainous watersheds is influenced by a variety of atmospheric physical processes and land-atmosphere interactions. To better understand this, two overlapping field experiments are being conducted in the East River watershed of the Colorado Rocky Mountains from 2021 to 2023. These experiments are the Study of Precipitation, the Lower Atmosphere and Surface for Hydrometeorology (SPLASH) and the Surface Atmospheric Integrated Field Laboratory (SAIL), led by the NOAA Physical Sciences Laboratory (PSL) and the U.S. Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) facility, respectively. Both experiments aim to provide complementary observations at a wide range of spatial and temporal scales, using a comprehensive, state-of-the-art observing network that characterizes precipitation, aerosols, clouds, winds, radiation, temperature, and humidity. This paper specifically focusses on the radar observations of precipitation in the domain.
The SPLASH experiment has a deployment of an X-Band dual-polarization scanning radar at the Roaring Judy Hatchery in the East River valley, while the SAIL experiment has an identical X-Band radar stationed at the Crested Butte Mountain. These scanning radars will provide detailed precipitation information that will be useful for observing embedded convective structures in wintertime storms, precipitation-type transitions, warm-season orographic convection, and transport of hydrometeors across mountain ridges. Additionally, a Ka-Band zenith pointing radar (KAZR), W-Band scanning dual-polarization radar, optical disdrometers, and a variety of other surface-based instruments are also deployed in the study domain.
This work focuses on analyses using radar-based observations from different precipitation events that impacted the East River Watershed region from the winter of 2021 to the summer of 2022. Inter-comparison and cross-validation of simultaneous observations between different radars are carried out for different precipitation conditions, addressing differences in spatial and temporal resolution, frequency, and viewing geometry. This demonstrates the feasibility of using coordinated scan strategy between different radars to obtain multi-frequency observations that can be used for detailed understanding of complex microphysical processes that dominate in complex terrain. In-situ observations are used to also develop QPE estimates for both rain and snow.
Despite having challenges posed by beam blockages due to the complex terrain, useful dual-Doppler wind syntheses can be realized at heights above terrain ridgelines and above the baseline between the two X-band radars. Dual-Doppler wind syntheses from several precipitation events are presented with an emphasis on various wave flow features. A special set of scans are used to generate co-plane scans and corresponding dual-Doppler retrievals which is a unique aspect of this program. Next, radar-based Quantitative Precipitation Estimation is performed, addressing challenges due to beam blockage and beam overshooting of low cloud tops in wintertime precipitation. X-Band radar Range Height Indicator (RHI) scans are used to aid in the Vertical Profile of Reflectivity (VPR) correction. Disdrometer measurements are used to study the variability in particle size distribution, which are further utilized in developing the dual polarization-based snow water equivalent (SWE) and rainfall estimators. QPE evaluation is performed using the ‘Santa Slammer’ snow event of late December 2021 and summer-time precipitation events from the year 2022 as case studies. The QPE product compared with surface measurements and operational products.