The International H20 Project (IHOP) was conducted May 13 - June 25, 2003 over Oklahoma, Texas, and Kansas for the purpose of examining water vapor variability, convective initiation, and atmospheric boundary layer evolution. During IHOP, instrumentation commonly deployed at the five Southern Great Plains ARM sites was supplemented with three-hourly radiosonde launches, mobile Doppler radars, dropsondes, and aircraft, ground, and space-based remote-sensing instruments. Several instruments were concentrated at an abandoned homestead near Balko, Oklahoma, including an AERI, three LIDARs, a GPS receiver, and in-situ meteorological instrumentation. This site, in conjunction with the NCAR S-POL radar located nearby was designed to provide near real-time measurement of the atmospheric boundary layer and troposphere.

On June 12, 2002 several instruments deployed at the homestead site recorded a rapid water vapor fluctuation in the lower troposphere. Over the six-hour period between 0700 and 1300 UTC, a Global Positioning System receiver measured a total precipitable water fluctuation of as much as 30% (1 cm). The collocated AERI temperature and moisture profiling system indicated a drop in lower-tropospheric water vapor mixing ratio of approximately 7 g/kg over the span of a thirty minutes beginning at 0700 UTC, with a similarly rapid increase at 0930 UTC followed by subsequent rapid drying at 1230 UTC. This significant fluctuation in lower-tropospheric water vapor was only weakly reflected in the surface in-situ observations. Data from all of the available IHOP observations will be presented documenting the magnitude and short time scale of the water vapor fluctuation. With the aid of the PSU/NCAR MM5 numerical model, an attempt will be made to diagnose the character and origin of the feature, which was the primary focal point for intense convection later in the day.