Quantifying the Transport of Air into Mesoscale Convective System Updrafts

Mesoscale Convective Systems (MCSs) are important to Earth’s weather and climate systems for many reasons, including the amount of precipitation they produce and their effects on the global water and energy budgets. MCSs often have cold pools that assist in lifting near-surface environmental air ahead of the MCS to its lifting condensation level within the MCS updraft. It is this warm, moist, lower-tropospheric air that further drives the development of the MCS updraft. However, environmental air ahead of the MCS can have different characteristics at varying altitudes. It is not clear whether the source of air entering MCSs is solely the surface, or whether air from the mid- to upper-levels is also ingested into these storm systems. The goal of this study is therefore to quantify how much air from different altitudes enters MCS updrafts. This goal has been accomplished using the cloud-resolving Regional Atmospheric Modeling System (RAMS) to simulate two cases from the Midlatitude Continental Convective Clouds Experiment (MC3E). Passive tracers were placed in front of the two MCSs at different altitudes during the mature storm stages. The same number concentrations of each tracer were released at each altitudinal level in order to facilitate comparisons. The model was run for 2 hours to track the movements of the tracers which serve as representatives for environmental air.

The simulations demonstrate that low-tropospheric air ahead of the MCS appeared in the updrafts in greater amounts than air originating at higher altitudes. Specifically, in updraft regions between 7 and 8 km, the integrated tracer amount from the middle troposphere (5-7 km) was ~50% of that from the lower troposphere (0-2 km). However, even though the lower-level contributions were predominant, the ingestion of the mid-level tracers was also found to be significant. In comparing the two MC3E cases, there was a difference in the relative contribution of middle-tropospheric air to MCS updrafts, which is partially due to variations in the structure and strength of the updrafts between the two MCSs. The significant ingestion of the middle tropospheric air into both MCSs demonstrates the importance of understanding the characteristics of mid-level air and their impacts on MCS development.