“I get all the news I need on the weather report”

1. Willem A. Landman & Francois Engelbrecht “I get all the news I need on the weather report”

2. Nowcasting:A description of current weather parameters and 0 to 2 hours’ description of forecast weather parameters • Very short-range weather forecasting:Up to 12 hours’ description of weather parameters • Short-range weather forecasting:Beyond 12 hours’ and up to 72 hours’ description of weather parameters • Medium-range weather forecasting:Beyond 72 hours’ and up to 240 hours’ description of weather parameters • Extended-range weather forecasting:Beyond 10 days’ and up to 30 days’ description of weather parameters. Usually averaged and expressed as a departure from climate values for that period • Long-range forecasting:From 30 days up to two years • Month forecast:Description of averaged weather parameters expressed as a departure (deviation, variation, anomaly) from climate values for that month at any lead-time • Seasonal forecast:Description of averaged weather parameters expressed as a departure from climate values for that season at any lead-time • Climate forecasting:Beyond two years • Climate variability prediction:Description of the expected climate parameters associated with the variation of interannual, decadal and multi-decadal climate anomalies • Climate prediction:Description of expected future climate including the effects of both natural and human influences WMO Forecast time ranges

3. A numerical model:Operational Organization (1) A typical run: boundarycondition model time t time t + 1 Initial condition forecast

4. A numerical model:Operational Organization (2) Starting with We get equations using parameters like - pressure (P) - Temperature (T) - Humidity (U) - Wind (V) Physical laws (thermodynamic, Fluide mechanics etc...) TIME EVOLUTION EQUATIONS (P, T, U, V)

5. A numerical model:Operational Organization (3) For a specific time P, T, V, U (initial state) TIME EVOLUTION EQUATIONS (P, T, U, V) Then, using the time evolution equations Initial state t0 One can compute the new state after a time ”D t” (time step) New state at t = t0 + D t

6. A numerical model:Operational Organization (4) Initial state t0 A succession of very short range forecast are made (Dt is in the range of a few minutes to 30 minutes) The previous forecast gives the initial state for the next Dt corresponds to the ”time step” of the model t = t0 +D t t = t0 +2D t Final state t = t0 + range of the forecast etc...

7. An example for the topography of the model Topography profile in the model using a doubling of horizontal and vertical resolution Real topography profile Topography profile in the model

8. Convergence of NWP skill ECMWF

9. Short-range weather forecast models

10. A recent rainfall forecast

11. Typical performances of weather forecast models and their improvement over time climatology: averagetaken over a long time; the forecastisthat the average value willhappen. persistence: ‘today’sweatheriswhatwillhappentomorrow’. Progress! Forecasterrors made by a 1998 model after 5 days, are similar to errors made after 2 days by a 1975 model. Forecast error over the European domain (500 hPa geopotential heights) 140 120 100 80 error in gpm 60 1998 1990 1980 40 1975 persistence climatology 20 0 0 1 2 3 4 5 6 7 8 9 10 lead-time (days)

12. Great progress has been made to predict the day-to-day state of the atmosphere (e.g., frontal movement, winds, pressure) However, day-to-day fluctuations in weather are not predictable beyond two weeks Beyond that time, errors in the data defining the state of the atmosphere at the start of a forecast period grow and overwhelm valid forecast information This so called “chaotic” behaviour is an inherent property of the atmosphere Limits of Longer Range Forecasts

13. How is it then possible to predict seasonal climate anomalies? Predictions of rainfall, frontal passages, etc. for a particular day at a certain location several months ahead has no usable skill. However, there is some skill in predicting anomalies in the seasonal average of the weather. The predictability of seasonal climate anomalies results primarily from the influence of slowly evolving boundary conditions, and most notably SSTs (i.e., El Niño and La Niña), on the atmospheric circulation. Sea-surface temperature (SST) anomalies of September 1997 (El Niño of 1997/98) Anomaly: departure from the mean or average Sea-surface temperature (SST) anomalies of November 1988 (La Niña of 1988/89)

14. DynamicalForecasts: MonthlyForecasts Daily forecastskill + Monthly running meanskill

15. DynamicalForecasts: SeasonalForecasts Daily forecastskill + Seasonal running meanskill

16. DynamicalForecasts: SeasonalForecasts Daily forecastskill + Ensemble forecast, seasonalrunning meanskill and skilful SST forecasts

17. DynamicalForecasts: SeasonalForecasts Daily forecastskill + Ensemble forecast, seasonalrunning meanskill and skilful SST forecasts + Post-processing

18. A typical example of a seasonal forecast

19. Climate change projections

20. Change in yearly rainfall totals (%) over southern Africa • A generally warmer and drier climate • A shorter and more intense summer rainfall season • Cloud bands displaced westwards in spring and autumn • Drier winter rainfall region

21. Forecasts for various time-ranges are subject to different forcings Forecast models are skilful Forecast model development has resulted in improved forecasts, also of extreme events The South African modelling community, in partnership with international modellers, is constantly improving on operational forecast systems Final thoughts

22. Where to find our forecasts: http://rava.qsens.net/themes/climate_template