Title: Representing thunderstormelectrification for lightning forecastevaluation
Abstract: Forecasts of lightning in the Met Office’s Unified Model (MetUM) are known to overforecast the total number of lightning flashes in the UK. One of the difficulties in
understanding why this happens comes from the dependence of the lightning forecast
on the convective forecast. This problem can be mitigated using a more physically
representative forecasting method to compare against and by comparing lightning production in modelled thunderstorms to that in observed thunderstorms.
In order to provide a more physically representative forecasting method a new,
explicit thunderstorm electrification and lightning scheme is implemented within the
MetUM. This scheme uses non-inductive collisional charging to represent the charge
generated on hydrometeors and produce a charge density distribution. From this, the
magnitude of the electric field is calculated, with appropriate thresholds selected to
allow initiation of lightning events. It is shown that this scheme accurately represents
observed thunderstorm charge magnitude and structure.
Results from the new electrification scheme are compared with those from the existing lightning parameterisation within the MetUM, and to natural lightning observations in two case studies. The new electrification scheme performs well in both a
scattered, fair weather convection case study in the UK and an organised, deep convection case study in the US. It shows realistic lightning coverage and reproduced the daily
lightning flash accumulation relatively accurately. The collision-separation efficiency
is found to be a key parameter and therefore a potential source of uncertainty in the
scheme. Through comparison with the new scheme, the existing MetUM parameterisation is shown to be producing lightning in a manner that is too closely dependent
on the rainfall accumulation, which it is suggested is related to its poor performance
in the UK case study.
Observations of single cell thunderstorms are used to investigate the production of
lightning in thunderstorms in the UK. It is found that prior to the onset of lightning
production, single cell thunderstorms show an increase in storm core area. Model simulations of similarly intensifying thunderstorms show that, during these intensifications,
the updraft velocity and area both increase, as does the graupel mass in the storm
core. It is shown that the new electrification scheme can reproduce the increase in
updraft area and graupel mass in intensifying storms, whereas the existing lightning
parameterisation does not reproduce any of these parameters. The use of this new
electrification scheme, whilst not operationally feasible with existing computer power,
provides a research tool with which to further improve lightning forecasting.
Publication Year: 2020
Publication Date: 2020-07-31
Language: en
Type: dissertation
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