The Importance of Resolving Mesoscale Latent Heating in the North Atlantic Storm Track

Abstract

Theoretical, observational, and modeling studies have established an important role for latent heating in midlatitude cyclone development. Models simulate some contribution from condensational heating to cyclogenesis, even with relatively coarse grid spacing (on the order of 100 km). Our goal is to more accurately assess the diabatic contribution to storm-track dynamics and cyclogenesis while bridging the gap between climate modeling and synoptic dynamics. This study uses Weather Research and Forecasting model (WRF) simulations with 120-and 20-km grid spacing to demonstrate the importance of resolving additional mesoscale features that are associated with intense precipitation and latent heat release within extratropical cyclones. Sensitivity to resolution is demonstrated first with a case study, followed by analyses of 10 simulated winters over the North Atlantic storm track. Potential vorticity diagnostics are employed to isolate the influences of latent heating on storm dynamics, and terms in the Lorenz energy cycle are analyzed to determine the resulting influences on the storm track. The authors find that the intensities of individual storms and their aggregate behavior in the storm track are strongly sensitive to horizontal resolution. An enhanced positive feedback between cyclone intensification and latent heat release is seen at higher resolution, resulting in a systematic increase in eddy intensity and a stronger storm track relative to the coarser simulations. These results have implications for general circulation models and their projections of climate change.

Can we trust climate models to realistically represent severe European windstorms?

“…This result is in general agreement with the work by Jung et al (2006) and Willison et al (2013) as discussed in Sect. 1.…”

“…This behaviour suggests that important physical processes in the development of the storm are not well reproduced by the coarse resolution model. The work by Willison et al (2013) suggests that the lack of diabatic contributions along insufficiently resolved frontal rain bands is a likely cause. This may be particularly important for Jeanette, as it occurred in October when baroclinicity is usually still relatively low, but sea surface temperatures are high.…”

“…This low confidence is related to concerns over the skill of many models in realistically representing large-scale features such as the stratosphere (Scaife et al 2012;Manzini et al 2014) and ocean circulation as well as the dynamics of individual storm systems, for example due to insufficient horizontal resolution (Willison et al 2013). Consequently, the consensus between different models on a climate change signal remains rather low (Harvey et al 2012;Zappa et al 2013b).…”

“…They find that the effect of dynamics, physics and orography dominates over the truncation effect for intense cyclones, whereas the truncation effect dominates for shallow cyclones. Willison et al (2013) argue that coarser-resolution models are not capable to reproduce aspects such as the diabatic generation of potential vorticity along fronts, leading to feedbacks on the evolution of individual cyclones (see also Madonna et al (2014) for effects of Warm Conveyor Belts) and of the entire storm track and its energetics. Pinto et al (2007bPinto et al ( , 2009a) compare intense winter cyclones over the North Atlantic in ECHAM5 and in NCEP re-analysis with the same horizontal resolution and find fewer and weaker systems in the model despite a similar mean jet speed.…”

Using satellite and reanalysis data to evaluate the representation of latent heating in extratropical cyclones in a climate model

“…As such, poor representation of latent heating can have a significant influence on the evolution of ETCs in models. Willison et al (2013), investigating the impact of resolving mesoscale heating features within ETCs in the Atlantic stormtrack, found that a higher resolution model which was able to better represent these features produced a more vigorous stormtrack. The representation of latent heating within ETCs may therefore impact on the representation of the wider environment in the extratropics in climate models.…”