Present-day and future lightning, and its impact on tropospheric chemistry

Lightning represents a key interaction with climate through its production of nitrogen oxides (NOx) which lead to ozone production. These NOx emissions are generally calculated interactively in chemistry-climate models but there has been little development of the representation of the lightning processes since the 1990s. In most models the parametrisation of lightning is based upon simulated cloud-top height. The aims of the thesis are: to explore existing schemes, and develop a new process-based scheme, to parametrise lightning; to use a new process-based lightning scheme to give insights regarding the role of lightning NOx in tropospheric chemistry; and to use alternative lightning schemes to improve the understanding of the response of lightning to climate change, and the consequent impacts on tropospheric chemistry. First, a new lightning parametrisation is developed using reanalysis data and satellite lightning observations which is based on upward cloud ice flux. This parametrisation is more closely linked to thunderstorm charging theory. It greatly improves the simulated zonal distribution of lightning compared to the cloud-top height approach, which overestimates lightning in the tropics. The new lightning scheme is then implemented in a chemistry-climate model, the UK Chemistry and Aerosol model (UKCA). It is evaluated against ozone sonde measurements with broad global coverage and improves the simulation of the annual cycle of upper tropospheric ozone concentration, compared to ozone simulated with the cloud-top height approach. This improvement in simulated ozone is attributed to the change in ozone production associated with the improved zonal distribution of simulated lightning. Subsequently, data from a chemistry-climate model intercomparison project (ACCMIP) are used to study the state-of-the-art in lightning NOx parametrisation along with its response to climate change. It is found that the models using the cloud-top height approach produce a very similar response of lightning NOx to changes in global mean surface temperature of +0.44± 0.05 TgNK-1, for a baseline emission of 5 TgN yr-1. However, two models using two alternative lightning schemes produce a weaker and a negative response of lightning to climate change. Finally, simulations in a future climate scenario for year 2100 in the UKCA model were performed with the cloud-top height and the ice flux parametrisations. The lightning response to climate change when using the cloud-top height scheme is in good agreement with the positive response found in the multi-model results of the cloud-top height approach. However, the new ice flux approach suggests that lightning will decrease in future. These opposing responses introduce large uncertainty into the projections of tropospheric ozone and methane lifetime in the future scenario. An analysis of the radiative forcing from these two species also shows the large uncertainty in the individual methane and ozone radiative forcings in the future. Due to the opposite effect that lightning NOx has on methane (loss) and ozone (production) the net radiative forcing effect of lightning in present-day and future is found to be close to zero. However, there is a small positive feedback suggested by the results of the cloud-top height approach, whereas no feedback is evident with the ice flux approach. These results show there are large and crucial uncertainties introduced by lightning parametrisation choice, not only in terms of the actual lightning distribution but also atmospheric composition and radiative forcing. The new ice-based parametrisation developed here offers a good alternative to the widely-used approach and can be used in future to model lightning and develop the understanding of associated uncertainties

‘Eastern African paradox’ rainfall decline due to shorter not less intense long rains

An observed decline in the Eastern African Long Rains from the 1980s to late 2000s appears contrary to the projected increase under future climate change. This “Eastern African climate paradox” confounds use of climate projections for adaptation planning across Eastern Africa. Here we show the decline corresponds to a later onset and earlier cessation of the long rains, with a similar seasonal maximum in area-averaged daily rainfall. Previous studies have explored the role of remote teleconnections, but those mechanisms do not sufficiently explain the decline or the newly identified change in seasonality. Using a large ensemble of observations, reanalyses and atmospheric simulations, we propose a regional mechanism that explains both the observed decline and the recent partial recovery. A decrease in surface pressure over Arabia and warmer north Arabian Sea is associated with enhanced southerlies and an earlier cessation of the long rains. This is supported by a similar signal in surface pressure in many atmosphere-only models giving lower May rainfall and an earlier cessation. Anomalously warm seas south of Eastern Africa delay the northward movement of the tropical rain-band, giving a later onset. These results are key in understanding the paradox. It is now a priority to establish the balance of mechanisms that have led to these trends, which are partially captured in atmosphere-only simulations

Future changes in seasonality in Eastern Africa from regional simulations with explicit and parametrised convection

The Eastern Africa precipitation seasonal cycle is of significant societal importance, and yet the current generation of coupled global climate models fails to correctly capture this seasonality. The use of convective parametrisation schemes is a known source of precipitation bias in such models. Recently, a high-resolution regional model was used to produce the first pan-African climate change simulation that explicitly models convection. Here, this is compared with a corresponding parametrised-convection simulation, to explore the effect of the parametrisation on representation of Eastern Africa precipitation seasonality. Both models capture current seasonality, although an overestimate in September-October in the parametrised simulation leads to an early bias in the onset of the boreal autumn short rains, associated with higher convective instability and near-surface moist static energy. This bias is removed in the explicit model. Under future climate change both models show the short rains getting later and wetter. For the boreal spring long rains, the explicit convection simulation shows the onset advancing but the parametrised simulation shows little change. Over Uganda and western Kenya both simulations show rainfall increases in the January-February dry season, and large increases in boreal summer and autumn rainfall, particularly in the explicit convection model, changing the shape of the seasonal cycle, with potential for pronounced socio-economic impacts. Interannual variability is similar in both models. Results imply that parameterisation of convection may be a source of uncertainty for projections of changes in seasonal timing from global models, and that potentially impactful changes in seasonality should be highlighted to users

Extreme rainfall in East Africa October 2019 – January 2020 and context under future climate change

The 2019 October-December rains over East Africa were one of the wettest seasons on record, with many locations receiving more than double the climatological rainfall, leading to floods and landslides across the region. Above average rainfall continued into January 2020. The persistently high rainfall also contributed to the locust plagues that affected much of East Africa in January 2020. Wet conditions in East Africa are typically associated with El Nino and/or positive Indian Ocean Dipole events. In October-December 2019 a warm anomaly was present in the western Indian Ocean while a cool anomaly was present in the eastern Indian Ocean (a positive Indian Ocean Dipole); conditions known to give above average rainfall over East Africa. The warm anomaly in the western Indian Ocean persisted into January 2020. Seasonal and monthly forecasts correctly predicted above average rainfall during the October-December season. January rainfall is found to be correlated with sea surface temperatures over the western Indian Ocean. Climate model projections suggest that strong positive Indian Ocean Dipole events and wet October-December seasons may become more frequent under future climate change, with associated increased risks of floods

Extratropical forests increasingly at risk due to lightning fires

Fires can be ignited by people or by natural causes, which are almost exclusively lightning strikes. Discriminating between lightning and anthropogenic fires is paramount when estimating impacts of changing socioeconomic and climatological conditions on fire activity. Here we use reference data of fire ignition locations, cause and burned area from seven world regions in a machine-learning approach to obtain a global attribution of lightning and anthropogenic ignitions as dominant fire ignition sources. We show that 77% (uncertainty expressed as one standard deviation = 8%) of the burned area in extratropical intact forests currently stems from lightning and that these areas will probably experience 11 to 31% more lightning per degree warming. Extratropical forests are of global importance for carbon storage. They currently experience high fire-related forest losses and have, per unit area, among the largest fire emissions on Earth. Future increases in lightning in intact forest may therefore compound the positive feedback loop between climate change and extratropical wildfires

Representation of precipitation and top-of-atmosphere radiation in a multi-model convection-permitting ensemble for the Lake Victoria Basin (East-Africa)

The CORDEX Flagship Pilot Study ELVIC (climate Extremes in the Lake VICtoria basin) was recently established to investigate how extreme weather events will evolve in this region of the world and to provide improved information for the climate impact community. Here we assess the added value of the convection-permitting scale simulations on the representation of moist convective systems over and around Lake Victoria. With this aim, 10 year present-day model simulations were carried out with five regional climate models at both PARameterized (PAR) scales (12–25 km) and Convection-Permitting (CP) scales (2.5–4.5 km), with COSMO-CLM, RegCM, AROME, WRF and UKMO. Most substantial systematic improvements were found in metrics related to deep convection. For example, the timing of the daily maximum in precipitation is systematically delayed in CP compared to PAR models, thereby improving the agreement with observations. The large overestimation in the total number of rainy events is alleviated in the CP models. Systematic improvements were found in the diurnal cycle in Top-Of-Atmosphere (TOA) radiation and in some metrics for precipitation intensity. No unanimous improvement nor deterioration was found in the representation of the spatial distribution of total rainfall and the seasonal cycle when going to the CP scale. Furthermore, some substantial biases in TOA upward radiative fluxes remain. Generally our analysis indicates that the representation of the convective systems is strongly improved in CP compared to PAR models, giving confidence that the models are valuable tools for studying how extreme precipitation events may evolve in the future in the Lake Victoria basin and its surroundings

Scientific understanding of East African climate change from the HyCRISTAL project

Integrating Hydro-Climate Science into Policy Decisions for Climate-Resilient Infrastructure and Livelihoods in East Africa (HyCRISTAL) is a Future Climate for Africa (FCFA) project funded to deliver new understanding of East African climate change and its impacts, and to demonstrate use of climate change information in long-term decision-making in the region. Here, we briefly summarise key findings from HyCRISTAL so far on climate change, as well as key findings from the pan-African FCFA project “IMPALA” relevant to East Africa, both in the context of previous literature on the topic

Scientific understanding of East African climate change from the HyCRISTAL project

Integrating Hydro-Climate Science into Policy Decisions for Climate-Resilient Infrastructure and Livelihoods in East Africa (HyCRISTAL) is a Future Climate for Africa (FCFA) project funded to deliver new understanding of East African climate change and its impacts, and to demonstrate use of climate change information in long-term decision-making in the region. Here, we briefly summarise key findings from HyCRISTAL so far on climate change, as well as key findings from the pan-African FCFA project “IMPALA” relevant to East Africa, both in the context of previous literature on the topic

The effect of explicit convection on couplings between rainfall, humidity and ascent over Africa under climate change

The Hadley circulation and tropical rain belt are dominant features of African climate. Moist convection provides ascent within the rain belt, but must be parameterized in climate models, limiting predictions. Here, we use a pan-African convection-permitting model (CPM), alongside a parameterized convection model (PCM), to analyze how explicit convection affects the rain belt under climate change. Regarding changes in mean climate, both models project an increase in total column water (TCW), a widespread increase in rainfall, and slowdown of subtropical descent. Regional climate changes are similar for annual mean rainfall but regional changes of ascent typically strengthen less or weaken more in the CPM. Over a land-only meridional transect of the rain belt, the CPM mean rainfall increases less than in the PCM (5% vs 14%) but mean vertical velocity at 500 hPa weakens more (17% vs 10%). These changes mask more fundamental changes in underlying distributions. The decrease in 3-hourly rain frequency and shift from lighter to heavier rainfall are more pronounced in the CPM and accompanied by a shift from weak to strong updrafts with the enhancement of heavy rainfall largely due to these dynamic changes. The CPM has stronger coupling between intense rainfall and higher TCW. This yields a greater increase in rainfall contribution from events with greater TCW, with more rainfall for a given large-scale ascent, and so favors slowing of that ascent. These findings highlight connections between the convective-scale and larger-scale flows and emphasize that limitations of parameterized convection have major implications for planning adaptation to climate change