Observations and Numerical Simulation of Upper Boundary Layer Rapid Drying and Moistening Events during the International H2O Project (IHOP_2002)

Abstract

On the morning of 12 June 2002, a series of upper boundary layer (UBL) rapid drying and moistening events (RDEs and RMEs, respectively) occurred at the ?Homestead? site of the International H2O Project (IHOP_2002). Over a period of 10 h, atmospheric water vapor in the UBL decreased or increased within a matter of minutes four separate times. High-temporal-resolution data of the RDEs and RMEs collected by numerous instruments deployed for this intensive observation period are presented. The results of an Advanced Regional Prediction System (ARPS) simulation of the weather conditions around the time period reproduced one of the two RDE?RME pairs with reasonably accurate amplitude and timing. Both the observational data and ARPS numerical model output indicate that the second RDE?RME pair resulted from the interaction between a dry air mass descending from the Rocky Mountains and a cold pool?internal undular bore couplet propagating over the Homestead site from a mesoscale convective complex to the north. The RDEs and RMEs, which were rarely observed during IHOP_2002, are believed to be an indirect indicator of such bores.