Thermodynamic Gradients Across Mountain Ranges from Mobile Observational Transects

Multi-scale measurements of temperature and humidity across topographic barriers in Washington, Oregon, and California are compared and analyzed in relation to standard meteorological datasets. Cases include a “rain shadow” air mass contrast under weak high pressure, strong static stability variations at the end of a Santa Ana wind event, and interactions with coastal boundary layers. The weakening Santa Ana event of 20 October 2016 was characterized by dramatic moisture pools (RH nearly 50% in mid-day) in interior valleys under strong thermal stratification. Areas more exposed to cross-barrier gap flows inland of Ventura and Gaviota showed fairly constant potential temperature indicative of greater mechanical mixing, temperatures up to 35 C, and RH dipping below 5%. Sudden changes of temperature and dewpoint were also observed within a few km of the coast in association with marine air penetration. A 26 January 2017 case of stagnant high pressure over Washington State revealed a rapid drop of potential temperature approximately 15 km east of Snoqualmie Pass, suggestive of radiatively driven frontogenesis. A relatively gradual gradient of dewpoint extended from west to east across the Cascades, with orographic cloud cover and light rain on western slopes changing to mostly sunny conditions in the interior. On the following day, measurements between Corvallis, OR and the Pacific Ocean were characterized by dissipating fog in the Willamette River valley and cloudy conditions elsewhere. Potential temperature suddenly increased about 1 K when leaving the foggy area and jumped again shortly before the coast. Overall thermal change was about 5 K. Highest dewpoints were located about 5 km inland. Additional transects are planned across the Southern California Peninsular Ranges during summer 2017.