GNSS Radio Propagation through Trapped Atmospheric Lee Waves in the San Bernardino Valley, CA

Time series from many GNSS stations throughout western North America show intermittent skewed outliers with weak seasonality. Many of these stations lie in the immediate lee of major mountain ranges and display position outliers that are consistently skewed towards the adjacent mountain range. Previous studies have implicated inadequate modeling of tropospheric gradients as the source of these skewed outliers, but the precise origin of the signals has not been studied in detail.

We focus on a cluster of GNSS stations in the San Bernardino Valley in southern California that show position outliers and model the local meteorological conditions at a 1km scale during select days showing skewed horizontal GNSS position time series outliers in excess of 1 cm. We use the WRF-ARW model with one nested domain and hourly boundary and initial conditions from the NOAA HRRR 3km model and investigate the results of ray tracing the incoming GNSS radio signals through this high-resolution weather model output to identify the source of the outliers.

We show that outliers in GNSS time series in the San Bernardino valley occur during times of enhanced jet stream activity immediately above southern California coupled with a favorable low-level wind direction. The narrowness of the mountain ranges north of the San Bernardino valley, coupled with fast upper-level winds combine to create small wavelength (<15 km) trapped lee waves above the San Bernardino valley. The jet stream location required to generate the trapped lee waves occurs primarily in boreal winter and spring during predictable large scale synoptic patterns. The predictability of the weather conditions required to generate these trapped lee waves provides ample opportunity for the future deployment of additional instrumentation for their study.