The tricky but crucial question of how to model small-scale physical processes to help produce numerical forecasts is the subject of a training course being run by ECMWF from 11 to 21 May.
Numerical weather prediction (NWP) divides the atmosphere into millions of grid boxes and uses the laws of physics to work out how conditions in those boxes change over time.
A number of equations are applied to each box to predict large-scale changes in the atmosphere. But for a meaningful forecast it is also necessary to consider the complex processes that occur within each box and at the boundary between the atmosphere and the surface of the Earth.
The techniques used to do this are commonly referred to as ‘parametrization’ since they involve the definition of suitable parameters and equations to describe these small-scale processes.
“Parametrizations are absolutely vital for a weather forecast,” says Richard Forbes, a Senior Scientist in ECMWF’s Research Department and one of the trainers on the course.
Take the example of solar radiation, which influences key features of the weather, such as near-surface temperature. To account for its effects, the complicated ways in which radiation is absorbed or reflected by clouds, atmospheric gases, aerosols and the surface of the Earth need to be modelled.
“We need to be able to accurately represent all these processes so that we can forecast their impact on weather at the local scale all the way to the global-scale circulation," Richard explains.
The training course on the ‘Parametrization of sub-grid physical processes’ is a mix of lectures and practical sessions using a single-column version of ECMWF’s Integrated Forecasting System. Topics covered include radiation, clouds, convection, turbulence, land surface, atmospheric composition and numerical aspects.
The course is part of a wider NWP training programme run by ECMWF.
“I’m finding this course very useful, in particular working with the model,” says Jan Kamieniecki, who hopes that some of the insights gained will help him with his research project on tropical convection at the University of Reading.
Emanuela Pichelli, who works on weather modelling at the University of L’Aquila in Italy, particularly values the “broad overview” provided by the course, “especially since we use ECMWF data to initiate our model. So it’s nice to see how the parametrizations applied here influence the analysis we use to start the model.”
The course, attended by 30 participants from 12 countries, is part of ECMWF’s NWP training programme, which also covers ‘Data assimilation’ (16–20 March), ‘Predictability and ocean-atmosphere ensemble forecasting’ (20–29 April) and ‘Advanced numerical methods for Earth-system modelling’ (1–5 June).
ECMWF’s Annual Seminar, which will take place from 1 to 4 September this year, is devoted to the role of parametrization as numerical weather prediction moves towards ever finer grid-box resolutions. Under the title 'Physical processes in present and future large-scale models', this well-known event in the European meteorological calendar will focus on the understanding, observation and representation of processes across scales.
For more details on parametrization, visit the Atmospheric physics page.