Abstract
AbstractWe present a simple, physically consistent stochastic boundary layer scheme implemented in the Met Office’s Unified Model. It is expressed as temporally correlated multiplicative Poisson noise with a distribution that depends on physical scales. The distribution can be highly skewed at convection-permitting scales (horizontal grid lengths around 1 km) when temporal correlation is far more important than spatial. The scheme is evaluated using small ensemble forecasts of two case studies of severe convective storms over the United Kingdom. Perturbations are temporally correlated over an eddy-turnover time scale, and may be similar in magnitude to or larger than the mean boundary layer forcing. However, their mean is zero and hence they, in practice, they have very little impact on the energetics of the forecast, so overall domain-averaged precipitation, for example, is essentially unchanged. Differences between ensemble members grow; after around 12 h they appear to be roughly saturated; this represents the time scale to achieve a balance between addition of new perturbations, perturbation growth, and dissipation, not just saturation of initial perturbations. The scheme takes into account the area chosen to average over, and results are insensitive to this area at least where this remains within an order of magnitude of the grid scale.
Highlights
There has been a rapid growth in the development and use of so-called convective-scale numerical weather prediction (NWP) systems using ‘‘convection-permitting models’’ (CPMs) (Clark et al 2016)
This testing evaluates the impact of the scheme on its own; it is not expected that this source of uncertainty can account for the whole of forecast error, so evaluation is restricted in Part I of this paper to impact on ensemble spread and not on overall forecast reliability
The testing compares forecast uncertainty due to initial and boundary condition uncertainty with the uncertainty resulting from the scheme running continuously. It relies on the existence of realistic initial and boundary condition perturbations from the Met Office Global and Regional Ensemble Prediction System (MOGREPS) scheme; this only has one boundary layer scheme in operational use
Summary
There has been a rapid growth in the development and use of so-called convective-scale numerical weather prediction (NWP) systems using ‘‘convection-permitting models’’ (CPMs) (Clark et al 2016). This testing evaluates the impact of the scheme on its own; it is not expected that this source of uncertainty can account for the whole of forecast error, so evaluation is restricted in Part I of this paper to impact on ensemble spread and not on overall forecast reliability The latter is addressed in Flack et al (2021, hereafter Part II). The testing (in Part II) compares forecast uncertainty due to initial and boundary condition uncertainty with the uncertainty resulting from the scheme running continuously It relies on the existence of realistic initial and boundary condition perturbations from the Met Office Global and Regional Ensemble Prediction System (MOGREPS) scheme; this only has one boundary layer scheme in operational use.
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