Climatology of Orographic Precipitation Gradients in the Contiguous Western United States

Abstract

Orographic precipitation gradients (OPG) relating to the increase or decrease in precipitation amount with elevation are not well studied or analyzed except for case examples. A quality controlled daily OPG data set for the western United States based on a linear regression framework of gauge precipitation observations and elevation for a 39-year time period was created and analyzed to identify spatial and temporal patterns and variability in OPG and some of the drivers of variability on seasonal, annual, interannual, and climatological timescales. Most locations in the western United States experience positive OPG during most of the year, exhibiting an annual cycle with the highest magnitude of OPG in the winter season and lowest magnitude of OPG in the summer season. Coastal locations tend to have higher magnitude OPG and larger variability in OPG than interior locations during cool seasons. Empirical orthogonal function (EOF) analysis identifies two principal components that account for 33 percent of the variability in a subset of the OPG data set, and these modes of variability are related to precipitation amount and atmospheric circulation over the Pacific Ocean. Comparison of daily OPG with similarly calculated three-day and monthly OPG identifies that OPG magnitudes are sensitive to the length of the precipitation accumulation period chosen.

Connecting Hydrometeorological Processes to Low‐Probability Floods in the Mountainous Colorado Front Range

“…The results of Section 4.1 suggest that extreme rainfall is approximately homogeneous over the transposition domain, thus giving the “green light” to the employment of SST in Sections 4.5 and 4.6. Our finding is consistent with several recent studies (Bohne et al., 2020; Mahoney et al., 2015; Rossi et al., 2020), showing that intense precipitation events can also occur at high elevations in the region. All these findings point to the fact that elevation‐based constraints on heavy rainfall need to be investigated on a case‐by‐case basis.…”

Precipitation Estimates and Orographic Gradients Using Snow, Temperature, and Humidity Measurements From a Wireless‐Sensor Network

“…To investigate the hydrologically important precipitation gradient with elevation, we used the OPG ( α unit of km −1 ) following Lundquist et al. (2010), instead of the precipitation lapse rate (β, cm km −1 ) that varies with precipitation totals (Bohne et al., 2020) and event duration, thus hindering the comparison of orographic enhancement across different events. We defined the OPGs for the lower and upper basins separately.…”

Precipitation gradients across the Continental Divide in the southern Canadian Rockies

“…Westerly flow is associated with greater amounts of orographic enhancement since it is perpendicular to the mountains (Luce et al 2013). Summertime convection and increased low-elevation precipitation due to blocking from wintertime high pressure ridging patterns, in addition to other types of blocking patterns, could explain the larger precipitation/elevation ratio at lower elevations (Shea & Marshall 2007;Bohne et al 2020). Low-level atmospheric blocking caused by anticyclonic conditions at any time of year could create a barrier between incoming precipitation and the topography, forcing precipitation upwind, or it may smooth topographical features diminishing orographic enhancement of precipitation, since orographic enhancement can be partially dependent upon orographic elevation gradients (Neiman et al 2002;Barry 2008, p. 171;Ramelli et al 2021).…”

“…According to existing literature, there is a strong positive precipitation/elevation relationship for precipitation events on the western side of the mountain ranges in western North America. This is due to interaction between moisture advection from the Pacific Ocean and orographic enhancement on mountain ranges oriented roughly perpendicular to the direction of this advection (Anders et al 2007;Siler et al 2013;Mass et al 2015;Bohne et al 2020).…”

“…Overall, the absence of a positive precipitation/elevation relationship on the western side of the Continental Divide in this study for the annual period (Figure 4.15) was unexpected in our study. Geographic areas with a negative precipitation/elevation relationship are rare in North America (Bohne et al 2020).…”