The Tern Nest: Simulating MCS Propagation Across the Great Plains

The Lincoln-Omaha area in eastern Nebraska has a pronounced nocturnal maximum in thunderstorm activity due to the propagation of mesoscale convective systems (MCS) which initiate near the Rocky Mountains in the middle to late afternoon. One such MCS struck Lincoln on 21 April 2019 at 23 LST, but the operational NAM forecast initialized that morning did not produce convection over Lincoln until nearly 04 LST on 22 April. An attempt to improve the simulation of this event with an unusual nesting strategy is presented here.

The WRF-EMS was used for this experiment. Obtaining a baseline simulation similar to the NAM run was not successful with the default WRF-EMS or NAM physics parameterizations, due to the MCS quickly dissipating over western Kansas. However, with explicit convection on a 9-km grid and the NAM microphysics scheme, the model did produce this MCS, though it did not initiate early convection north of the Nebraska border, similar to the operational run.

Since the initial convection was forced by terrain far upstream, the sensitivity simulations attempted to improve the forecast for Lincoln by placing a 3-km nest over eastern Colorado. For a larger 3-km domain, the MCS did produce advance convection similar to observations, though it dissipated before reaching Lincoln. Despite the improvement, there appeared to be little difference over the larger terrain of the Rockies, but notable differences were observed in far eastern Colorado. A smaller 3-km domain placed over this area did not result in the same improvement, but when the nest was expanded toward the northwest, the more accurate modeled MCS again appeared.

The improvement of the forecast in this case due to increasing the resolution far upstream appears to result from the more accurate simulation of a leeside convergence zone caused by more modest topography to the northeast of Denver. The significance of this result with respect to using this type of nest in an operational setting is discussed, in addition to other important considerations and necessary research.