The Impact of Low-level Moisture Errors on Model Forecasts of an MCS Observed during PECAN

This research investigates forecast errors of the environment near an elevated mesoscale convective system (MCS) in Iowa on 24 June 2015 during the Plains Elevated Convection at Night (PECAN) field campaign. The eastern flank of this MCS produced an outflow boundary (OFB) and propagated southeastward along this OFB as a squall line.  The western flank of the MCS remained quasi stationary approximately 100 km north of the system's OFB and produced localized flooding.  16 radiosondes were launched near the MCS's eastern flank and 4 near the MCS's western flank during this event.

Horizontal moisture advection primarily contributed to a substantial increase in convective available potential energy (CAPE) and decrease in convective inhibition (CIN) during the 4 hours prior to the arrival of the squall line.  Parcels encountering the southwestern OFB required approximately 100 km of northeastward travel past the OFB to be lifted to their LFCs and to enter convective updrafts.  Numerical simulations that placed the MCS at varying distances too far to the northeast were analyzed, and MCS displacement error was found to have been connected to under-prediction of low-level moisture and subsequently over-predicted convective inhibition.  The over-predicted CIN in models resulted in northwestward moving air parcels requiring unrealistically long residence times and northeastward travel in regions of gradual meso-alpha scale lift before these parcels initiated convection.  These results suggest that erroneous MCS predictions by NWP models may sometimes result form poorly analyzed low-level moisture fields.