Abstract
AbstractMajor river flooding affected the United Kingdom in late September 2012 as a slow-moving extratropical cyclone brought over 100 mm of rain to a large swath of northern England and north Wales, with local accumulations approaching 200 mm. The cyclone developed on 20–22 September following the interaction between an equatorward-moving potential vorticity (PV) streamer and Tropical Storm Nadine, near the Azores. A plume of tropical moisture was drawn poleward ahead of the PV streamer over a low-level baroclinic zone, allowing deep convection to develop. Convectively driven latent heat release reduced upper-tropospheric PV near the streamer, causing it to fracture and cut off from the reservoir of high PV over the United Kingdom. Simulations using the Weather Research and Forecasting Model with 4-km horizontal grid spacing in which microphysical heating and cooling tendencies are set to zero, alongside calculations of instantaneous diabatic heating rates and PV tendencies along trajectories, reveal that deposition heating contributed strongly to the fracturing of the PV streamer into a discrete anomaly by directly reducing upper-tropospheric PV to the streamer’s east. Condensation heating contributed to lower-tropospheric PV generation along the cold front as the cyclone developed, while cooling due to sublimation, evaporation, and melting modified the PV much less strongly. The results of this case study show that the collocation of strong deposition heating with positive absolute vorticity in the upper troposphere can lead to substantial PV modification and a very different cyclone evolution to that when deposition heating is suppressed.
Highlights
Major river flooding affected the United Kingdom in late September 2012 as the deepest September cyclone to cross the United Kingdom in over 30 years (Met Office 2012) brought over 150 mm of rain to parts of northern England and north Wales between 23 and 26 September, resulting in widespread travel disruption and damage to homes and businesses
Given the high level of uncertainty associated with the parameterization of ice-phase microphysical processes (e.g., Forbes and Clark 2003; Dearden et al 2014), the full-physics control simulation (CNTRL) was rerun using the Morrison double-moment scheme (Morrison et al 2005), alongside two sensitivity simulations investigating the dynamical response of the cyclone to deposition heating and sublimation cooling, respectively
To complement the instantaneous heating profiles and tendencies in the previous sections we present integrated heating and potential vorticity (PV) modification along trajectories in CNTRL, to identify the most important physical processes changing u and PV in an air parcel
Summary
Major river flooding affected the United Kingdom in late September 2012 as the deepest September cyclone to cross the United Kingdom in over 30 years (Met Office 2012) brought over 150 mm of rain to parts of northern England and north Wales between 23 and 26 September, resulting in widespread travel disruption and damage to homes and businesses. The dynamical impacts of microphysical processes other than condensation heating on cyclogenesis are less well understood Addressing this lack of understanding, recent studies (e.g., Joos and Wernli 2012; Igel and van den Heever 2014; Martínez-Alvarado et al 2014; Dearden et al 2016) have shown that deposition heating provides an additional source of strong latent heating within extratropical cyclones. Contributed only weakly to PV modification in a region of small absolute vorticity in previous extratropical cyclone case studies (Joos and Wernli 2012; Martínez-Alvarado et al 2014), PV tendencies are instead calculated directly over and immediately east of the upper-level PV streamer, where absolute vorticity is expected to be large, as the streamer fractures into a discrete anomaly.
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