African Lightning and its Relation to Rainfall and Climate Change in a Convection‐Permitting Model

Abstract

Global climate models struggle to simulate both the convection and cloud ice fundamental to lightning formation. We use the first convection-permitting, future climate simulations for the lightning hot spot of Africa, at the same time utilizing an ice-based lightning parametrization. Both the model and observations show that lightning over Africa's drier areas, as well as the moist Congo, have more lightning per rainfall than other regions. Contrary to results in the literature, the future projection shows little increase in total lightning (~10 7 flashes (or 2%) per degree warming). This is a consequence of increased stability reducing the number of lightning days, largely offsetting the increased graupel and updraft velocity driving an increase in lightning per lightning day. The next step is to establish if these results are robust across other models and, if combined with parametrized-convection models, whether ensemble-based information on the possible responses of lightning to climate change can be investigated. Plain Language Summary Lightning depends on ascending air in thunderstorms and the collision of cloud ice particles, which charge the thundercloud. Many climate models have too coarse a resolution to reliably capture these processes. We focus on Africa, which has some of the most frequent lightning in the world. We use a model that is much higher resolution than usual, and this allows us to explicitly simulate the deep convection associated with thunderstorms as well as provide more detailed representation of the distribution of cloud ice particles. Our results show that in drier regions, as well as the much wetter Congo, there is relatively more lightning per kilogram of surface rainfall than there is in other parts of the continent. Lightning does increase across the continent under climate change, but by a relatively small amount. This is despite the number of days with lightning decreasing as the lower atmosphere becomes more stable. On days with lightning, there are more lightning flashes because there is an increase in cloud ice and intensity of convection. This study gives much more detailed information about African lightning than previous work. However, it is a single simulation. Future research should look at these results across other climate models.

Challenges and outlook for convection-permitting climate modelling

“…Beyond short-duration precipitation extremes, there have been several studies looking at future changes in tropical cyclones at convection-permitting scale [39,56], severe convective weather [57][58][59], snow pack [22,23,60] and rain-on-snow events [61]. Studies are also now starting to emerge looking at changes in lightning [62,63], hail [59,64] and wind extremes [21]; and CPMs are increasingly being used to provide more reliable projections of future changes over cities [25,26]. As yet it is not possible to identify whether there is any consensus on the impact of convection-permitting resolution for future changes in these high-impact events.…”

Contrasting lightning projection using the lightning potential index adapted in a convection-permitting regional climate model

“…Generally, the present study agrees with previous findings. Indeed, while the literature considering convective activity as a proxy for lightning suggested a global increase in lightning activity under a warmer climate ranging from 5 to 10% increase per degree of warming (Price and Rind, 1994;Price, 2009;Krause et al, 2014;Romps et al, 2014;Rädler et al, 2019), the few studies using a scheme, which include microphysics related parameters showed little increase or even a global decrease in lightning activity (Jacobson and Streets, 2009;Finney et al, 2018Finney et al, , 2020. However, the literature also highlights heterogeneous lightning changes around the globe.…”

Predictive Proxies of Present and Future Lightning in a Superparameterized Model

“…As mentioned in Section 3.2, the portion of the McCaul proxy was modified to include an air density factor . While this may seem like an afterthought when developing a lightning predictor for the present, it becomes important when considering long‐term changes, especially given the small magnitudes of change (3%/K) found in previous modeling studies (Finney et al., 2018, 2020; Romps, 2019). In a warmer climate a given isotherm is higher in the atmosphere and thus at a lower density.…”

“…We also note what appear to be some second-order effects over the ocean that lead to even larger differences (6%/K) than what would be expected considering changes in density alone. Finney et al (2020) applied the McCaul proxy to output from CP4A simulations and predicted a 2%/K increase in flash rates over the African continent. The authors cited their use of a CRM in explaining the disparity between their results and the projected decrease by IFLUX in a conventional, coarse-resolution model (Finney et al, 2018).…”

“…Finney et al. (2020) took advantage of the so‐called CP4A (convection‐permitting 4.5 km over Africa) experiments (Kendon et al., 2019; Stratton et al., 2018) to study African lightning.…”