The impact of low-level moisture errors on model forecasts of an MCS observed during PECAN

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Author
Peters, John M.
Nielsen, Erik R.
Parker, Matthew D.
Hitchcock, Stacey M.
Schumacher, Russ S.
Date
2017-09Metadata
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This article investigates errors in forecasts of the environment near an elevated mesoscale convective system (MCS) in Iowa on 24–25 June 2015 during the Plains Elevated Convection at Night (PECAN) field campaign. The eastern flank of this MCS produced an outflow boundary (OFB) and moved
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. A total of 16 radiosondes were launched near the MCS’s eastern flank and 4 were launched near the MCS’s
western flank.
Convective available potential energy (CAPE) increased and convective inhibition (CIN) decreased
substantially in observations during the 4 h prior to the arrival of the squall line. In contrast,
the model analyses and forecasts substantially underpredicted CAPE and overpredicted CIN owing to
their underrepresentation of moisture. Numerical simulations that placed the MCS at varying
distances too far to the northeast were analyzed. MCS displacement error was strongly correlated
with models’ underrepresentation of low-level moisture and their associated overrepresentation of
the vertical distance between a parcel’s initial height and its level of free convection (DzLFC, which is correlated with CIN). The overpredicted DzLFC in models resulted in air parcels requiring unrealistically far northeastward travel in a region of gradual meso-a-scale lift before these parcels initiated
convection. These results suggest that erroneous MCS predictions by NWP models may sometimes result from poorly analyzed low-level moisture fields.
Description
The article of record as published may be found at http://dx.doi.org/10.1175/MWR-D-16-0296.1
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This publication is a work of the U.S. Government as defined in Title 17, United States Code, Section 101. Copyright protection is not available for this work in the United States.Collections
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