Abstract
Mesoscale convective systems (MCSs) are complexes of thunderstorms that become organized and cover hundreds of kilometres over several hours. MCSs are prolific rain producers in the tropics and mid-latitudes and are the major cause of warm-season flooding. Traditionally, climate models have difficulties in simulating MCSs partly due to the misrepresentation of complex process interactions that operate across a large range of scales. Significant improvements in simulating MCSs have been found in kilometre-scale models that explicitly simulate deep convection. However, these models operate in the grey zone of turbulent motion and have known deficiencies in simulating small-scale processes (e.g. entrainment, vertical mass transport). Here, we perform mid-latitude idealized ensemble MCS simulations under current and future climate conditions in three atmospheric regimes: hydrostatic (12 km horizontal grid spacing; Δx), non-hydrostatic (Δx = 4, 2 and 1 km) and large eddy scale (Δx = 500 m and 250 m). Our results show a dramatic improvement in simulating MCS precipitation, movement, cold pools, and cloud properties when transitioning from 12 km to 4 km Δx. Decreasing Δx beyond 4 km results in modest improvements except for up- and downdraft sizes, average vertical mass fluxes, and cloud top height and temperature, which continue to change. Most important for climate modelling is that Δx = 4 km simulations reliably capture most MCS climate change signals compared to those of the Δx = 250 m runs. Significantly different climate change signals are found in Δx = 12 km runs that overestimate extreme precipitation changes by up to 100%.This article is part of a discussion meeting issue ‘Intensification of short-duration rainfall extremes and implications for flash flood risks’.
Highlights
Mesoscale convective systems (MCSs) play an important role in the earth’s energy balance [1,2] and are essential for the water cycle in the tropics [3] and mid-latitude regions [4,5]
The convective inhibition (CIN) properties do not change significantly between current and future climate scenarios, and the same behaviour holds for the relative humidity (RH; figure 2d)
Constant relative humidity means that the atmospheric precipitable water (PW; figure 2g) increases at close to Clausius–Clapeyron rates
Summary
Mesoscale convective systems (MCSs) play an important role in the earth’s energy balance [1,2] and are essential for the water cycle in the tropics [3] and mid-latitude regions [4,5]. A major bottleneck for predicting possible climate change effects on future extremes is that convective storms and storm intensity (precipitation, updraft strength) are poorly represented by state-of-the-art models [9]. These challenges are exacerbated for MCS that represent some of the largest and most impactful of convective storms. An ongoing challenge of kilometre-scale modelling is that these models operate in the grey zone of turbulent motion, wherein convection is not fully resolved [14] This causes challenges in realistically simulating cloud entrainment processes and draft characteristics [15]. We use a larger domain than previous studies, enabling realistic simulation of three-dimensional MCSs, rather than MCS sections in a channel configuration [15,24]
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