A principal motivation for the International H2O Project (IHOP) over the central United States Great Plains was to assess the value of detailed observations of tropospheric water vapor for improved warm-season quantitative precipitation forecasting (QPF). Since the low level jet (LLJ) plays a critical role in transporting water vapor toward regions of precipitation, it is potentially important for QPF to determine the ability of operational observing systems to capture that transport. These operational systems cannot capture the full range of scales of these LLJ processes because of limitations of timing (for example, radiosonde observation times are only at 00 and 12 UTC), spatial density, or vertical scanning characteristics (wind profilers, for example, miss the important lowest layers of the atmosphere). When these measurements are used as initialization data in operational forecast models or as diagnostic guidance to forecasters, a considerable underestimate of moisture transport is possible.

Several IHOP aircraft missions were designed to observe mesoscale structures of moisture and winds during LLJ episodes. We describe preliminary results from analyses of these data during two such missions. Our primary measurements are dropsonde observations made at roughly 60 km intervals along box patterns that encompass the jet core and cut across the jet at locations near the core. In addition to qualitative descriptions of LLJ structure, and as a first step toward moisture budget calculations, we make vertically integrated estimates of moisture transport through these transects into and out of the box. We then compare these estimates with others produced using only operationally-available observations. Airborne lidar data collected during IHOP useful for understanding even smaller scales of moisture transport will also be shown at the conference.