The Influence of the Chicago Metroplex on an Extreme Precipitation Event

A major precipitation event occurred on 2 July 2023 in Chicago causing significant local flooding as nearly 9 inches (225 mm) of rain was observed in the DOE Community Research on Climate and Urban Science (CROCUS) observing network in less than a 12-hour period. The heaviest rain occurred just northwest of Midway Airport (KMDW). The event is analyzed from the perspective of multi-scale (synoptic-urban scale) observations, a WRF-ARW 1 km resolution nested-grid simulation, and a WRF-UCM (Urban Canopy Model) 1 km resolution nested-grid simulation. The focus of the presentation is the inter comparison of the two simulations to determine the city’s impact on precipitation. The multi-scale process that organized the extreme precipitation event culminated in a long-lived mesoscale convective vortex (MCV) that was responsible for the most extreme rainfall as it traversed the metroplex for several hours. In this presentation we will show how the urban scale forcing modified the precipitation by impacting the structure of the MCV. The urban effects caused by the drag of the buildings and differential surface sensible heating between the metroplex, and rural areas affected both the static stability and surface blocking of the airflow hence modifying the vertical wind shear, moisture transport and static energy thus controlling the strength and trajectory of the MCV and its organization of convective precipitation. By so doing substantial differences in precipitation resulted in the two simulations which were caused by the Chicago metroplex as parameterized in the UCM’s urban canopy model. Both simulations were initialized several hours earlier (1200 UTC 1 July 2023) at the coarse resolution (4 km scale) from the ECMWF ERA-5 analysis which provided lateral boundary conditions and both simulations employed the identical physical parameterization schemes including the Thompson microphysical scheme, MYJ TKE PBL scheme, and NOAH land surface model with the UCM employing a single-level urban canopy scheme. They were subsequently nested to 1 km horizontal resolution and integrated for 27 hours of real time starting at 2100 UTC 1 July 2023 and ending at 0000 UTC 3 June 2023.

The WRF-UCM simulation produced some subtle but not insignificant differences from the WRF-ARW simulation. The UCM generated a maximum of nearly 200 mm of rainfall as opposed to 150 mm for the ARW and, as is consistently found when urban forcing is significant, shifted the precipitation maximum somewhat to the downwind (south-southwestern) side of the metroplex with predominantly north-northeasterly surface flow. The sequence of precipitation events started similarly in both simulations but subsequently evolved differently with the UCM exhibiting more complexity. As an upstream mid-upper tropospheric trough, jet streak, and isentropic potential vorticity (PV) maximum approached the Mississippi River Valley, two massive mesoscale convective systems (MCS) developed, one over southwestern Iowa and an extremely massive one over southcentral Illinois. The latter system: 1) created strong mid-upper tropospheric outflow, subsequently 2) enhancing the divergence aloft across central-northern Illinois, resulting in 3) an arc-shaped squall line, that 4) unified the two MCS and propagated towards the Chicago metroplex along with 5) an advected air mass with substantial deep moist and moderately unstable air including > 2 inches of precipitable water and > 1000 J/Kg of moist unstable layer convective available potential energy (MUCAPE). This squall line propagated poleward through the metroplex and produced some rainfall but was not explicitly responsible for the main 9-12-hour extreme event which occurred from ~1100 UTC – ~2100 UTC 2 July. It also generated cool outflow at low levels which turned the surface flow equatorward towards the south-southwestern part of the metroplex under the south-southwesterly flow in the lower-middle troposphere thus insuring very substantial vertical wind shear to strengthen any convective system that subsequently developed. As the squall line propagated through the metroplex and northward it stalled, further bowed, and bifurcated into two smaller MCS or mini-squall line systems. One subsequently rotated over Lake Michigan back towards the metroplex and another strengthened over the southwestern periphery of the metroplex and propagated east-southeastwards. Eventually both systems generated mini MCV, and both grew upscale as they merged over the area just northwest of KMDW followed by extreme rainfall. After more than 9 hours the heavy rain propagated downstream with the major single rotating MCV over and east of Lake Michigan.

The main difference between the ARW and UCM simulations was in the intensity of the blocking by the metroplex resulting in the complexity of MCV evolution. The UCM generated a much more massive area of blocked quiescent flow over the southwestern part of the metroplex prior to multiple MCV genesis. This resulted in more pronounced bifurcation of the original squall line resulting in two regions of low-level rotation and heavy precipitation that merged and built upscale as they collided near the region between KMDW and the lake front. The merger was likely enhanced by the upper IPV maximum which transported moist air poleward over the metroplex in conjunction with the outflow from the upstream MCS described earlier in southcentral Illinois. Prolonged heavy rainfall was more pronounced in the UCM than the ARW as the merger of the two small MCVs into a massive MCV occurred in comparison to the ARW simulated single rather fast moving MCV. Additional rainfall followed over the city as wrap-around precipitation as the MCV propagated over Lake Michigan. The CROCUS city scale observations and NWS Doppler Radar along with airport METARS substantiate the complex evolution of this extreme event when used to validate the UCM simulation. The main finding, which is consistent with other published studies on urban forcing of major precipitation events, is that city-scale flow blocking can both shift downstream and enhance precipitation events over a massive metroplex. In the future we plan to extend this study by using the UCM simulation to initialize a true city-scale model with horizontal resolution ~ 10 m.