Mesoscale moisture transport by the low-level jet during the IHOP field experiment

Previous studies of the Low-Level Jet (LLJ) over the Central Great Plains of the U.S. have been unable to determine the role that mesoscale circulations play in the transport of moisture. To address this issue, two aircraft missions during the International H2O Experiment (IHOP) were designed to closely observe a well-developed LLJ over the Great Plains (primarily Oklahoma and Kansas) with multiple observation platforms. In addition to standard operational platforms (most importantly radiosondes and profilers) to provide the large-scale setting, dropsondes released from the aircraft at 50 km intervals and a pair of onboard lidar instruments (HRDL for wind and DIAL for moisture) observed the moisture transport in the LLJ at greater resolution. Using these observations, we investigate the existence and physical origins of mesoscale moisture and wind structures in the LLJ, and assess the significnce of transports by these structures. First we briefly present the qualitative multi-scalar structure of the LLJ. In particular, we describe and diagnose mesoscale undulations at the top of the LLJ with horizontal scales near 20 km, and multiple moist layers near the core of the jet with depths about 250 m. We then focus attention on the bulk properties and effects of scales of motion by computing vertically-integrated fluxes through sections that bracket the LLJ. From these computations, we are also able to compute Reynolds averages within the screens and around the bracketing rectangular domains, from which we estimate the bulk effect of so-called ”prime-prime” terms that are interpreted as integrated estimates of the contribution of small-scale (meso- to convective-scale) circulations to the overall transport.