Boundary layer structure and evolution in a deep valley as viewed by rawinsonde observations during the T-REX field campaign

About two hundred high-resolution rawinsonde soundings launched during the T-REX field campaign in the Owens Valley, California are analyzed to determine the vertical structure of the boundary layer and its diurnal variation in this very deep and wide mountain valley on the east side of Sierra Nevada Range. The soundings revealed that the daytime boundary layer can be grouped into two broad categories: those with a relative shallow mixed layer capped by a stable layer in the upper part of the valley; and those characterized by a deep mixed layer extending to the crest level approximately 3000 m above the valley floor. The synoptic conditions responsible for each of the two categories will be discussed. The nighttime inversion is found to be weak when compared to the observations in other deep valleys, which is attributed to turbulent mixing associated with thermally driven down-valley flow at night and the channeling flow in this valley. A sharp temperature jump was seen in many sounding profiles around the crest level. Several hypotheses to explain the frequent formation of this elevated inversion were tested and the results will be discussed.