Kinematic and thermodynamic structure of the Sierra Barrier Jet during a land-falling winter storm in northern California

Atmospheric rivers (ARs) are narrow corridors of enhanced water vapor transport within extratropical cyclones. Although focused research during the last few years has yielded quantitative linkages between ARs and both the regional water supply and extreme precipitation events, questions remain regarding the modification and redistribution of precipitation in ARs by California's coastal mountains and Sierra Nevada. For example, rain rates are strongly correlated with the magnitude of the cross-barrier flow (and hence water vapor flux and uplift) at upper windward slope locations. However, at lowland locations they are poorly correlated due to the influence of low-level barrier airflows within the atmosphere. The most important of these barrier airflows is likely the Sierra Barrier Jet (SBJ), which is a dominant feature in Sierra Nevada storms. Because the atmosphere typically exhibits stable stratification in the region, winds reaching the mountain range often are partially stalled and, in part, diverted along the range rather than directed up and over. Knowledge of the impact of the resulting terrain-parallel barrier jets on orographic storms remains elusive. An incomplete understanding of the physics of and joint interactions between ARs and blocked flows limits our ability to identify and predict the factors affecting the distribution and intensity of precipitation in California's Central Valley and along the Sierra's windward slopes. This study addresses these issues with data collected during the NOAA Hydrometeorology Testbed (HMT).

HMTs first regional demonstration, HMT-West, has deployed an extensive and varied network of in situ and remote sensing instrumentation to northern California during each cool season since 2003, with an emphasis on the American River Basin of the northern Sierra Nevada since 2005. HMT-West observational capabilities were enhanced during the 2010-2011 cool season due to collaboration with the CalWater project sponsored by the California Energy Commission. Detailed documentation of SBJs and associated ARs was facilitated during the 2010-2011 cool season by a suite of scanning and profiling Doppler radars (precipitation structure and one, two and three-dimensional wind retrievals), balloon soundings (in situ thermodynamic and wind measurements) and GPS receivers (integrated water vapor retrievals).

This presentation will provide an overview of the kinematic and thermodynamic structure of the SBJ and AR observed during 14-16 February 2011 as part of the third Intensive Operating Period of HMT-West/CalWater 2011. A unique aspect of the analysis is that the multiple-Doppler derived winds resolved both the SBJ and the incoming pre-frontal low-level jet associated with the AR, which has contributed to several new findings that include:

• The SBJ extends westward across the entire Central Valley.

• The AR rides over stable air associated with the SBJ and slopes upward from southwest to northeast.

• The height of the SBJ increases from southwest to northeast, especially over the windward side of the Sierra where it has a terrain-following character.

• The height of the SBJ increases from south to north in moving up the Sacramento Valley.