Mesoscale Moisture Transport By The Low-level Jet During The IHOP Field Experiment

Abstract

Previous studies of the Low-Level Jet (LLJ) over the central Great Plains of the United States 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 two separate well-developed LLJs over the Great Plains (primarily Oklahoma and Kansas) with multiple observation platforms. In addition to standard operational platforms (in particular, radiosondes and profilers) to provide the large-scale setting, dropsondes released from the aircraft at approximately 55 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. Two questions immediately present themselves: (1) Do focused observations at exceptionally high resolution provide details critical to our operational depiction of the LLJ; and (2) if they do, what is the physical nature of the circulations that are implied? A practical way of stating (1) is, do small-scale correlations between moisture and wind fluctuations within the LLJ significantly alter larger-scale estimates of LLJ moisture transport? To illustrate this possibility, we briefly compare the qualitative multi-scalar structure of the LLJ as revealed in point profiles and within vertical sections across the LLJ of wind, moisture, and resulting moisture transport as observed by multiple observation sets including radiosonde only, dropsondes, and simultaneous lidar measurements of moisture and wind. We then focus attention on the bulk properties and effects of scales of motion by computing layer-averaged fluxes through sections that bracket the LLJ. From these computations, we are able to compute Reynolds averages within the layers, from which we estimate the bulk effect of so-called ”prime-prime” terms, interpreted as integrated estimates of the contribution of small-scale (meso- to convective-scale) circulations to the overall transport. We then briefly describe modeling efforts that may eventually be able to describe mesoscale mechanisms that affect the LLJ moisture transport.