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.