Impact of horizontal resolution on the numerical simulation of a midlatitude squall line: Implicit versus explicit condensation

The relative roles of implicit and explicit condensation schemes in the num erical representation of a squall line that occurred on 7-8 May 1995 over t he southern Great Plains are examined in this study using Mesoscale Compres sible Community model integrations at 2-, 6-, 18-, and 50-km resolution. Re sults from the 2-km model in which condensation is explicitly represented a gree best with observations and are used as "synthetic'' data to evaluate t he performance of lower-resolution configurations. It is found that the representation of the squall system greatly deteriorat es as resolution is decreased and that the relative roles of the implicit a nd explicit condensation schemes change dramatically. At 6-km resolution, t he leading convective band is barely resolved by the model, and the implici t-explicit partition of precipitation is ambiguous because both implicit an d explicit schemes are active simultaneously at the leading edge of the sys tem. In spite of this ambiguity, it is found that use of a deep convection scheme is still beneficial to the squall-line simulation. At 18 km, the con vective line is not resolved by the model, and its effect is completely due to the implicit scheme. The mesoscale circulations in the trailing anvil r egion of the squall system are generated at the small end of the model reso lvable scales and are exaggeratedly intense. There is no ambiguity concerni ng the partition of condensation into implicit and explicit components at t his resolution, but the relative intensity of precipitation produced by the two cloud schemes is opposite to what is observed, considering that the im plicit scheme is supposed to represent subgrid-scale convection at the lead ing edge of the system, and the explicit scheme the grid-scale condensation in the trailing anvil. At 50 km, both the leading convection and the mesos cale circulations in the trailing anvil have to be parameterized because th ey are not resolved at the model grid scale. The precipitation and internal structures associated with the squall line are thus not well represented a t this resolution. The results also show that all the configurations produce precipitation acc umulations that are much larger than observations. This problem is most imp ortant at 18-km resolution. Grid-scale condensation is mostly responsible f or this rainfall overestimation. It is suggested that this problem is linke d to a misrepresentation of convective-scale processes.