METROMEX revisited (formerly paper number 6.5)

Project METROMEX (Metropolitan Meteorological Experiment) has revealed that in and downwind of St. Louis, enhanced convective precipitation and severe weather occurs. The contending hypotheses explaining these anomalies are the so-called glaciation mechanism which incorporates the dynamic seeding hypothesis and the urban land use or urban heat island effect. The latter contender we feel is more likely. Consequently, we are running simulations with the Regional Atmospheric Modeling System (RAMS) developed at Colorado State University (CSU) to first test the urban heat island effect, and in the future, the glaciation mechanism.

To test the urban heat island effect, we have built a realistic replication of the land-use over the St. Louis metropolitan area. We are using a cloud resolving grid (CRG) with 1.5 km grid spacing over the urban area and surrounding rural environs. Using synoptic data characteristic of daytime convection during the summer, simulations of the diurnal evolution of deep moist convection in the St. Louis area with and without land-use anomalies are being analyzed.

In addition to the daytime anomaly in precipitation, Changnon and Huff (1986) found a 58 percent increase in nocturnal rainfall northeast of the city, associated with MCSs. Thus we will perform RAMS simulations of nocturnal mesoscale convective systems (MCSs) passing over the St. Louis area. We will examine whether or not the city can act to couple the boundary layer to the storms at night thus altering the precipitation characteristics of these highly organized storms.