2026/05/28 by Nicholas M. Markus, William A. Gallus Jr., William A. Gallus
Earth and Planetary Sciences · Environmental Science · #Meteorological Phenomena and Simulations #Tropical and Extratropical Cyclones Research #Climate variability and models
paper · doi:10.1175/mwr-d-26-0001.1
Abstract The 10 August 2020 Midwest derecho had winds exceeding 55 m s −1 with severe intensity winds persisting up to an hour. In the present study, the Weather Research and Forecasting (WRF) Model was run using a 3-km horizontal grid with a 1-km inner nest to better understand mechanisms that might explain the extreme wind behavior. Simulations were sensitive to the microphysics used, with many schemes failing to produce a long-lived system in the area of the observed derecho. However, an unusually realistic simulation was obtained using the Thompson scheme but only when the Kain–Fritsch convective scheme was used. This scheme prevented spurious nocturnal storms that dried the lower troposphere prior to derecho development, causing the derecho to dissipate in western Iowa. The intensity and longevity of the winds in the simulated derecho were primarily due to enhanced rainwater evaporation that occurred when hydrometeor production increased at a time when the system moved into a region with drier midlevel relative humidity. The evaporation caused strong cooling which increased downward momentum transport and caused the rear inflow jet to descend over a broad area within the storm. This occasionally led to a secondary peak in surface winds behind the leading edge of the storm. Cooling due to graupel melting was maximized in the same areas as evaporation, but magnitudes were about 5 times smaller. Cooling rates due to snow melting were about half those of graupel melting. As observed, a prominent mesolow was simulated around the time of the strongest surface winds which likely contributed to peak speeds.