In an effort to improve understanding of the role of the low-level jet (LLJ) within extratropical cyclones and associated orographic precipitation processes in quantitative precipitation forecasting, the California Landfalling Jets experiment (CALJET) was conducted along and offshore of the California coasts during the winter of 1997/98, a year that was characterized by a very strong El Niño. The follow-on Pacific Land falling Jets experiment (PACJET-01) in the winter of 2000/01 extended this effort to more of the U.S. West Coast and to a season influenced by weak La Niña conditions. Dropsonde data collected offshore by NOAA’s P-3 research aircraft in 17 storms during these two winters are the focus of this paper.

As orographically induced vertical motion depends critically on the upstream atmospheric stability, a determination of the latter, including effects of moisture, is important to examine. In spite of its importance, there have been only a few studies that have reported even limited measurements of the thermodynamic profiles upwind of coastal orography associated with enhanced rainfall. In the present study the extensive set of dropsonde measurements made over the Pacific Ocean in the field experiments CALJET and PACJET-01 is used to analyze the thermodynamic stability of the air masses upwind of the California coastal orography that are responsible for intense coastal rainfall, i.e., the LLJ in extratropical cyclone warm sectors. Because CALJET and PACJET-01 targeted the LLJ using the NOAA P-3 aircraft over two winters, the dropsonde data provide a unique opportunity to examine conditions in this critical airmass.

In addition to its importance in terms of orographic precipitation, the LLJ plays a critical role in the global water cycle because it is also the region of greatest meridional water vapor transport. Because the region of strongest water vapor transport is very narrow (roughly 500 km wide), and yet it is responsible for almost all of the meridional water vapor transport at midlatitudes, this region is referred to as an “atmospheric river.” Although a recent study clearly documented several key attributes of atmospheric rivers by compositing satellite data, the mean vertical profile of conditions in atmospheric rivers was not quantifiable using such data and only one season was examined. The present paper fills this gap by using dropsonde observations from 17 flights where aircraft data were collected when a LLJ was present offshore. In addition to the full 2-year composite, means derived from only the El Niño winter of 1997/98 and from the La Niña winter of 2001 are compared in an effort to document interannual variability.