The Remote Role of North-American Mesoscale Convective Systems on the Forecast of a Rossby Wave Packet: A Multi-Model Ensemble Case-Study
Lojko, Alexander; Payne, Ashley; Jablonowski, Christiane
2022-12-27
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Lojko, Alexander; Payne, Ashley; Jablonowski, Christiane (2022). "The Remote Role of North-American Mesoscale Convective Systems on the Forecast of a Rossby Wave Packet: A Multi-Model Ensemble Case-Study." Journal of Geophysical Research: Atmospheres 127(24): n/a-n/a.
Abstract
North American Mesoscale Convective Systems (MCSs) have been linked to instances of poorly forecasted Rossby wave packets (RWPs). A computationally inexpensive investigation is proposed to demonstrate a dynamical mechanism by which MCSs modify a RWP associated with a high-impact weather event. Global ensemble forecast data, reanalysis and high-resolution observations are used to assess the remote role of negative potential vorticity (PV) arising from divergent outflow on RWP propagation coinciding with the 11–21 June 2017 European heatwave. In this case, synoptic-scale bands of negative PV which advect toward the jet stream arise from regions of active MCSs. The forecast data results show that the numerical misrepresentation of the anticyclonic circulation associated with negative PV can impinge on the forecast of a RWP. In each of the four forecasting models assessed, ensemble members that advected lower values of PV toward the equatorward branch of a North American ridge favored enhanced poleward amplification of the ridge and a more eastward progression of the RWP. The more eastward displacement of the RWP also coincided with an enhanced wave activity flux downstream. Although, we do not find a significant impact on the forecasted heatwave. The results urge further investigation into the role of negative PV in remotely influencing high-impact weather.Plain Language SummaryLarge severe thunderstorms over North America have been observed to occasionally precede poor forecasts over Europe. In this case-study, we use state-of-the-art weather model data and observations to provide a process-level understanding of how thunderstorms modify the evolution of the jet stream. The case selected coincides with the 11–21 June 2017 European heatwave. Our study shows that air with a signature of intense clockwise circulation arises from the cloud-tops of severe thunderstorms and rapidly expands onto synoptic-scales (∼1,000 km). In the four weather models assessed; there is a persistent bias in the representation of how this synoptic-scale, clockwise air interacts with the jet stream over North America. The erroneous interaction introduces forecast error into the jet stream and significantly modifies its down wind evolution over the Atlantic. However, we did not note a significant impact on the forecasted heatwave in this case. The results obtained in this study warrant further investigation on the role of thunderstorms in influencing the jet stream and European weather.Key PointsGlobal forecasting models struggle to realistically represent negative potential vorticity (PV) arising from mesoscale convective systemsAnticyclonic circulation errors associated with negative PV introduce rotational wind errors into the jet streamJet stream errors associated with negative PV modify the phasing and forecast skill of a Rossby-wave packetPublisher
UCAR/NCAR Earth Observing Laboratory Wiley Periodicals, Inc.
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2169-897X 2169-8996
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