The History and Future of the La Porte Anomaly "Invited"

The precipitation anomaly near La Porte, Indiana represents one of the early cases of possible urban weather modification. Changnon (1980) noted that precipitation in La Porte increased significantly 1930-1960, while precipitation in other areas downwind of Chicago remained approximately constant. The study also noted that the increase in downwind precipitation corresponded with an increase in hazy days in Chicago, an urban aerosol effect. Subsequent research, highlighting post-1970 precipitation, cast doubt on the causes or even the existence of the La Porte Anomaly, as the precipitation maximum ceased to be centered over La Porte. However, considering contemporary research on urban precipitation processes, it is likely that a combination of urban-land surface and aerosol effects from Chicago result in anomalously increased precipitation downwind of the city. Using 100 years of historical precipitation data from the Indiana State Climate Office, this study reconstructs the appearance of the anomaly beginning in the 1930s. Between 1930 and 1950, the precipitation anomaly downwind of Chicago moved from near Valparaiso to near LaPorte and intensified. After 1970, the anomaly weakened and moved westward back to near Valparaiso. Using high resolution (4km), AHPS combined radar/rain-gauge observations (Seo, 1998), the contemporary anomaly is located south of Valparaiso. The current “Valpo Anomaly” is most pronounced on days with southwesterly storm motion and convective precipitation. Under these conditions, the anomaly is highly visible on weekdays, but almost disappears on weekends. To test the historical location and weekly cycle of the precipitation anomaly, this study uses a numerical model to simulate precipitation modification under different Chicago land-cover and aerosol conditions. The RAMS model, coupled with an air quality model allows for dynamic urban sources of aerosols from Chicago. For systems with a high proportion of convective precipitation, the aerosols downwind of Chicago invigorate precipitation. The invigoration is particularly intense for storms moving from the southwest, which encounter the downwind aerosols but not the urban-rural boundary. The concentration of downwind aerosols determine the location of the downwind precipitation maximum. Under current conditions, similar to the 1950s, the anomaly is located near Valparaiso. Under high aerosol conditions, similar to 1960-1970, the anomaly is located near La Porte. Under low aerosol conditions, similar to pre-1930, the anomaly weakens as it located closer to Chicago.

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