A tropical mesoscale convective system (MCS) during the Transport and Atmospheric Chemistry Near the Equator--Atlantic (TRACE A) experiment and a midlatitude squall line during Preliminary Regional Experiment for Storm-scale Operational and Research Meteorology (PRESTORM) were simulated with the National Center for Atmospheric Research/Pennsylvania State University (NCAR/PSU) MM5 model. For the TRACE A case the model simulation predicted locations of convection systems fairly accurately compared with satellite images and the rainfall pattern from rain gauge data. For the PRESTORM case the model simulation captured many observed features such as the rainfall field and wind circulation pattern. Model simulated wind fields were used to transport an inert tracer (CO) in the mesoscale systems. Tracer transport in the two convective systems (TRACE A-tropical MCS versus PRESTORM midlatitude squall line) are substantially different over 24-hour simulation periods and over the regional scale (about 2000¿2000 km domains). The time-evolving tracer fields in the upper troposphere are different in the tropical MCS and midlatitude squall line regimes as a consequence of different propagation speeds of the two convective systems. The nearly ''stationary'' tropical MCS produced regions of large upper tropospheric CO mixing ratios that had moved very little in the horizontal by the end of the 24-hour simulation. The enhanced upper tropospheric CO region propagates with the ''fast-moving'' squall line in the midlatitude case. Model transport and redistribution of CO agrees well with aircraft measurements in the TRACE A case. Significant subgrid-scale (parameterized) upward transport (48% with Grell scheme, 41% with Kain-Fritsch scheme) occurred in the TRACE A case. The subgrid upward transport in the PRESTORM event was even greater, accounting for 64% of total upward transport. Downward transport in both events was dominated by grid-scale motions. ¿ American Geophysical Union 1996