CSU-CHILL Radar Observations of Bragg Scattering Echoes in Mountain Wave Lee Flow

S-band weather radar’s general capability to detect Bragg scattering echoes arising from spatial gradients in the atmospheric refractive index field has become well established (Knight and Miller, BAMS Feb., 1993, Davison et al, JAS 2013, Richardson et al, JAOT, 2017). In an early study of this scattering mechanism, Starr and Browning (QJRMS, 1971) observed that Bragg echo layers assumed undulating patterns in RHI scans made through the region immediately downstream of mountainous terrain during strong synoptic flow conditions. To collect high time resolution observations of this phenomena, the CSU-CHILL radar conducted continuous RHI scans oriented towards the eastern face of the Rocky Mountains during several high wind / mountain wave events that occurred in the early months of 2017. Layered Bragg echoes (as well as some meteorological echoes) were detected during several of these operations. The appearance of these layers showed varying degrees of organization and stationarity. Local reflectivity enhancements apparently were related to turbulent wave breakdown (Hicks and Angell, JAM, 1968). This paper will present several of the evolutionary patterns that were documented in the RHI scan sequences conducted during two high wind cases. The radar and environmental observations will also be compared to high-resolution WRF-model simulations of these two cases.