willem a. landman & francois engelbrecht. nowcasting: a description of current weather...
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“I GET ALL THE NEWS I NEED ON THE WEATHER
REPORT”Willem A. Landman
&Francois Engelbrecht
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WMO FORECAST TIME RANGES 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
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A NUMERICAL MODEL:OPERATIONAL ORGANIZATION (1)
A typical run:
modeltime t time t + 1
Initial condition
forecast
boundarycondition
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Physical laws(thermodynamic,Fluide mechanics
etc...)
TIME EVOLUTIONEQUATIONS
(P, T, U, V)
We get equationsusing parameters like
- pressure (P)- Temperature (T)
- Humidity (U)- Wind (V)
Starting with
A NUMERICAL MODEL:OPERATIONAL ORGANIZATION (2)
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Then, using the time evolution equations
One can compute the new state after a time ” D t”
(time step)
For a specific time P, T, V, U
(initial state)
New state att = t0 + D t
Initial statet0
TIME EVOLUTIONEQUATIONS
(P, T, U, V)
A NUMERICAL MODEL:OPERATIONAL ORGANIZATION (3)
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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 +2 D t
etc...
Final statet = t0 + range of
the forecast
Initial statet0
A NUMERICAL MODEL:OPERATIONAL ORGANIZATION (4)
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An example for the topography of the model
Real topography profile
Topography profile in the model using a
doubling of horizontal and
vertical resolutionTopography profile in the model
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CONVERGENCE OF NWP SKILL
ECMWF
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SHORT-RANGE WEATHER FORECAST MODELS
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A RECENT RAINFALL FORECAST
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TYPICAL PERFORMANCES OF WEATHER FORECAST MODELS AND THEIR IMPROVEMENT OVER TIME
Forecast error over the European domain(500 hPa geopotential heights)
1998199019801975persistenceclimatology
0 1 2 3 4 5 6 7 8 9 10lead-time (days)
0
20
40
60
80
100
120
140
erro
r in
gpm
climatology: average taken over a long time; the forecast is that the average value will happen.persistence: ‘today’s weather is what will happen tomorrow’.
Progress! Forecast errors made by a 1998 model after 5 days, are similar to errors made after 2 days by a 1975 model.
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LIMITS OF LONGER RANGE FORECASTS
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
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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)
Sea-surface temperature (SST) anomalies of November 1988 (La Niña of 1988/89)
Anomaly: departure from the mean or average
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DYNAMICAL FORECASTS: MONTHLY FORECASTS
Daily forecast skill
+
Monthly running mean skill
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DYNAMICAL FORECASTS: SEASONAL FORECASTS
Daily forecast skill
+
Seasonal running mean skill
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DYNAMICAL FORECASTS: SEASONAL FORECASTS
Daily forecast skill
+
Ensemble forecast, seasonal running mean skill and skilful SST forecasts
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DYNAMICAL FORECASTS: SEASONAL FORECASTS
Daily forecast skill
+
Ensemble forecast, seasonal running mean skill and skilful SST forecasts
+
Post-processing
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A TYPICAL EXAMPLE OF A SEASONAL FORECAST
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CLIMATE CHANGE PROJECTIONS
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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
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FINAL THOUGHTS
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
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Where to find our forecasts:
http://rava.qsens.net/themes/climate_template