Present-day and future lightning, and its impact on tropospheric chemistry
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Date
03/07/2017Item status
Restricted AccessAuthor
Finney, Declan Luke
Metadata
Abstract
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.