Forecasting meteorology and climate

 

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General

Forecasting or prediction is the process of estimation of future, unknown situations. In recent years, climatic forecasting has become a very popular phenomenon. As an example, in 2007, the Intergovernmental Panel on Climate Changes Working Group One issued its Forth Assessment Report. This report included forecasts or predictions of high increases in average global temperatures, and serious harm resulting from this, during the 21st century.

Early October 1812 Napoleon in Moscow was studying some of the first existing weather statistics, to forecast when cold winter weather was likely to set in. This told him that it would not be really cold before the beginning of December, so he did not feel any sense of urgency. The first snow in Moscow a few days later, on 13th October, showed him that this weather forecast probably was somewhat imprecise.

During the Crimean War, in November 1854, a major part of the French-English fleet was destroyed in the Black Sea by an unexpected storm. By later collecting local reports, the track of this storm could be followed across Europe all the way to the Black Sea. The French astronomer Leverrier was then given the responsibility to investigate if it was possible to forecast such weather events. With great difficulties, these developments lead to the first network of meteorological stations in France, sending information on local weather to a central weather office in Paris. From 1863 the first real daily weather maps showing pressure differences were produced for western Europe by this office. Within few years most nations in Europe and USA followed suit. By this, the time around 1870-1875 marks the beginning of widespread meteorological observations in Europe, USA, Greenland and Iceland.  

However, up to about the 2nd World War weather forecasting was more or less limited to meteorologists plotting the travel and development of individual weather systems on the weather map day by day, steered by winds at higher altitudes, until their ultimate decay or absorption in another weather system. Only when a anticyclone (high pressure area) decided to settle over a certain geographical position, outlooks or forecasts for two or three days ahead could be made. This has all changed very much after the the arrival of powerful computers.

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Principles of forecasting

The rapid development of sophisticated weather forecasts has had other consequences than just improved weather forecasts. One important effect of this and other scientific developments has been that since around 1935 there has been systematic research done on the issue of forecasting or making predictions. This rapidly evolving research branch has, among other things, summarized all useful knowledge about forecasting in what is called the Principles of Forecasting. These 140 principles are based on empirical studies that compare methods to determine which ones provide the most accurate forecast in specified situations, so the resulting principles are in essence what might be designated as evidence-based principles. A recent review of new evidence on some of the key principles of forecasting was published by Armstrong (2006).

Green and Armstrong (2007) recently attempted an evaluation of the forecasting processes described in Chapter 8 of the IPCC's WG1 Report (2007). They were able to find enough information to make judgement on 89 out of the 140 forecasting principles. Of these 89 principles 72 were violated by the forecasting procedures described in the report. The authors concluded that the scientific values of the forecast made thereby were reduced to essentially just the personal opinion of the scientist involved.

Click here to read a statisticians evaluation of the recent global temperature changes (since 1998) compared to the IPCC-temperature forecasts (located on the Public Policy Forecasting website). 

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Making forecasts

Public policy decisions related to climate change are now usually based on forecast by climate experts, especially if the experts tend to agree. It is, however, slightly disturbing to find that many previous examples of expert climate forecasts have turned out to be completely wrong. A classic example is the widespread anxiety in the 1970'es of the prospect of a coming ice age.

During the past decades the methodology for climate forecasting has shifted from surveys of experts opinions to the use of numerical climate models. Climate models are, however, only a mathematical way for experts to express their opinions about how the real world operates, so nothing has really changed. Pilkey and Pilkey-Jarvis (2007) examined long-term climate forecasts and concluded that they were based only on the opinion of the scientist, expressed in complex mathematical terms without evidence on the validity on the chosen approach. 

What remains is simply to compare published forecasts with the real world and, based on this, evaluate their degree of success. This empirical approach is also the foundation for the Principles of Forecasting. An example of this is presented at the bottom of this page.

The difficulty of making an accurate forecast increases with the time period considered. In many regions of the world there is about 57% chance of the weather tomorrow being more or less identical to the weather today. But making a forecast about the weather two or three days into the future is much more difficult.

The reason for this boils down to the initial value problem. Knowing the present weather conditions is paramount for forecasting the weather tomorrow. Numerical computer models integrating many physical laws attempt to overcome the problem of forecasting the future many days (or years) ahead by their sheer number crunching power.

Modern weather forecast is usually reliable within 2 or 3 days. Beyond 4 or 5 days the quality declines rapidly, partly because it still has not been possible to forecast the development of jet streams. So, how is it possible to forecast climate when weather forecasts have very little skill more than a week ahead?

It is not possible to predict details of weather further ahead, than a few days, a consequence of “chaos” in the atmosphere. On the other hand, the present conditions in the oceans will affect the average weather over a period of weeks, months or even years to come. So given knowledge of the present ocean state (e.g., sea surface temperature and distribution of the sea ice cover), it is perhaps possible to predict how the probability of particular future outcomes is enhanced or diminished. Chaos in the atmosphere however limits the precision of climate forecasts, which are usually therefore expressed as probabilities. Especially the state of the oceans influences the frequency statistics of certain future weather developments.

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Seasonal weather forecasting

This brings us to seasonal forecasting. Seasonal forecasting usually have a time horizon of 1-6 months. They relies primarily on information about the ocean and various other initial conditions (the current weather) as a constraint on the future climatic development. As slightly different initial values may cause very different results, the models are run several times, using slightly different initial conditions. The spread of modelling results are then taken as a kind of forecast uncertainty, e.g. that associated with chaos in the atmosphere. The generation of seasonal forecasts is now an important operational activity at various meteorological or climatic centres worldwide. The work put into the production of seasonal forecasts is impressive.

Current frontiers in climate forecasting are to improve forecast skill through more observations, to improve the use of observations, to improve climate models, forecasting regional details, and the ability of forecasting extreme events like heat waves, droughts, etc. In addition, an improved quantification of forecast uncertainty is needed.  

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Visual test of seasonal weather forecasting

Seasonal or three-monthly forecast for northwest Europe are published monthly by meteorological institutes. In Norway, you can find the latest three-month forecast here. The forecast is provided as average surface air temperatures for the three-month period in question, compared to the WMO-normal period 1961-1990. The forecast is based on 40 different model runs, using slightly different initial values. The forecast itself is generated as the average of all 40 models runs. The responsible institution for the actual calculations is the European Centre for Medium-Range Weather Forecasts (ECMWF) in Reading, UK. The Norwegian Meteorological Institute has its own useful evaluation of the performance of the forecasts, comparing measured station temperatures in Norway with temperatures forecasted. While this is a useful way of evaluation, however, a direct spatial comparison between the forecasted and the real world temperature development remain the stronger way of testing the success of the forecasts.

Below you will find a suite of three-month temperature forecast for NW Europe, compared with the real world spatial development. When comparing the different forecasts, it is useful to remember that summer seasons tend to vary little from year to year, while winter seasons may show large interannual variations. For that reason it is of cause easier to forecast a summer season compared to a winter season. Spring and autumn seasons take an intermediate position in this respect.

In the individual diagrams the forecast is shown as an insert on top of the real world temperature deviation from the 1961-1990 average. Although the colour scales used are different, a visual comparison is easy to carry out, identifying both geographical areas where the forecast performs well, and areas where it performs less well.  

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Spatial comparison of forecasting versus observations

TIME 01-03 02-04 03-05 04-06 05-07 06-08 07-09 08-10 09-11 10-12 11-01 12-02
2005-06                    
2006-07                
2007-08
2008-09
2009-10
2010-11                  

Seasonal forecasts (three-month periods); modelled (insert) versus observations. Click on each of the small diagrams to open a corresponding, but larger diagram. Reference period: 1961-1990. Latest period updated: December 2009 - February 2010. Last diagram update: 13 March 2010.

 

Comments to the spatial comparison (diagrams above)

200912-201002: Again the forecast was not successful for land areas. Once again, the forecast for ocean regions turned out to be better than for land areas, most likely because of the much greater thermal inertia of oceans, compared to land areas. For the European land areas the forecast was far from good, as the forecasted positive thermal anomaly in the real world turned out to be a pronounced negative anomaly. The lack of forecasting success for this three-month period more or less turned out to be a repeat of the previous three-month forecast (see below). 

200911-201001: The forecast was not successful, especially for land areas. As seen before, the forecast for ocean regions is much better than for land areas, presumably because of the much greater thermal inertia of oceans, compared to land areas. For the European land areas the forecast was far from good, as the forecasted positive thermal anomaly in the real world turned out to be a pronounced negative anomaly.     

200910-200912: The forecast was not entirely successful. For eastern Greenland, Iceland and the Atlantic north of Iceland the forecast was quite good. Over Europe, however, there was disagreement between the forecasted and the measured temperatures. Temperatures were forecasted to be 0.5-1.0oC above the average 1961-1990. In reality, they turned out to be quite close to the 1961-1990 average. The disagreement in this part of the study area was mainly caused by one particular cold month, December 2009. As seen before, the forecast appear to be most successful for oceanic areas, and less so for land areas.

200909-200911: The forecast correctly indicated general positive surface air temperature anomalies over the study area. However, with regard to the spatial pattern of anomalies the forecast was less satisfactorily. The forecast indicated maximum positive temperature anomalies along a ridge between Norway and East Greenland, and from there decreasing towards more northerly latitudes. In the real world, the positive anomalies increased toward higher latitudes, with a region of maximum values located north of Svalbard. 

200908-200910: The forecast again was good with regard to both absolute values of temperature anomalies and the spatial pattern of recorded anomalies. Once again, the best fit between forecast and real world development is obtained for the ocean, while the fit is somewhat less for land.

200906-200908: The forecast was good with respect to the overall configuration of temperature anomalies, especially over the ocean. Over land, the forecast deviates more from the real world development, where the forecasted maximum positive temperature anomaly over western Russia did not materialise. 

200905-200907: The forecast was correct with regard to temperatures generally being above the 1961-1990 average. The actual geographical distribution of temperatures was, however, not successfully forecasted. The area forecasted area of warming was over Europe, while in reality this turned out to be the area of minimum warming.

200904-200906: The forecast was not successful. The area of cooling forecasted between western Norway and Iceland in reality was the axis of a ridge of relative warming across the North Atlantic. In short, the real world turned out to show more or less opposite changes, compared to what was forecasted for the study area.

200903-200905: The forecast was not extremely successful. The forecasted warming area along the east coast of Greenland and the cooling area over the sea around and north of Iceland did not show up in the real world result. Actually the pattern of warming in the real world was more or less opposite to the pattern forecasted. The forecasted relative warming for northern Europe was correct, but only in very general terms. 

200902-200904: The forecast was not very successful. The overall configuration of relative warming and cooling areas within the geographical area considered was not well represented by the forecast. Along North-East Greenland, an area of forecasted above normal temperatures turned out to be an area of below normal temperatures. Likewise, around Iceland the area of forecasted below normal temperatures turned out to be an area of above normal temperatures. Over Europe, temperatures were forecasted to be above the average for 1961-1900, which turned out to be correct. The general configuration of isotherms within this part of the study area was, however, not very well forecasted.

200901-200903: The forecast was satisfactory. The overall configuration of relative warming and cooling areas within the geographical area considered was well described by the forecast. Real world warming along East Greenland and over southern UK, Germany and Denmark was, however, smaller than forecasted.

200812-200902: The forecast was excellent. The overall configuration of relative warming and cooling areas within the geographical area considered was well described by the forecast.

200811-200901: The forecast was good. The warming along East Greenland was overestimated, and the warming over the Nordic countries was underestimated, but the forecast was excellent as to the overall configuration of relative warming and cooling areas within the geographical area considered.

200810-200812: The forecast was only partly successful. The intensive warming forecasted for areas along eastern Greenland is not shown by the observations. Perhaps, this might be caused by the forecast assuming a significant reduction of seal ice along East Greenland, which did not take place? An area of general warming was forecasted to cover the region from East Greenland towards central Scandinavia. Instead warming turned out to characterise the northeastern part of the study area. The forecast is, however, satisfactorily when it comes to the temperature development over UK and Ireland, and adjoining parts of the North Atlantic. In general, as in previous forecasting periods, the forecast works better for the ocean than for the land areas. This might be due to the larger thermal thermal inertia of the ocean, compared to the land areas. 

200809-200811: The forecast was satisfactorily for the ocean north of Iceland, but the forecasted warming along East Greenland was to high. For the rest of the forecasting area cooling was expected to dominate towards east, but in reality this became an area of positive temperature anomalies. A small area of cooling is seen near the SW corner of the study area, although not forecasted in this position.

200808-200810: The forecast was satisfactorily for the ocean north of Iceland, where a ridge of relative high temperatures extends between Norway and East Greenland. The forecast is, however, less satisfactory for the land areas. Southern UK and Ireland was exposed to below average 1961-1991 temperatures, in contrast to the forecast. Also the temperature contrast between the northern and southern parts of the Nordic countries is in contrast to the forecast for the period. During the last months the forecasting model has performed better above the ocean than in the terrestrial environment. The higher thermal inertia of the oceans may perhaps contribute to this, making it more easy to forecast future (three-month) temperature changes over the ocean, compared to the land surface? 

200807-200809: The forecast is good for the marine part of the North Atlantic region, where a ridge of relative warming is seen extending from the North See towards East Greenland. Over terrestrial Europe, however, the forecast is less satisfactory. The North Atlantic area of warming was forecasted to extend across most of North Europe. This turned out not to be the case with the exception of southwestern Norway. In reality most of the land areas experienced only small temperature changes compared to 1961-1990, and easternmost Finland and NW Russia actually experienced relative cooling. 

200806-200808: The forecast is good, when compared to the real development. The warm ridge extending from Denmark towards the sea north of Iceland is correctly forecasted, including the warm peak north of Iceland. The local cooling region southwest of Svalbard is also shown in the forecast. The forecast is less satisfactory for northeastern part of the geographical region considered, where small changes or even slight cooling was experienced. The general impression remains, however, of a successful forecast.

200805-200807: The forecast is not entirely satisfactory when compared to the real development. Especially the negative temperature anomaly over northern Scandinavia, Finland and NW Russia was not forecasted at all. The maximum warm anomaly forecasted for northern Germany did not come true, either. The North Sea and the region north of Iceland instead showed the maximum positive temperature anomaly. 

200804-200806: For UK and Denmark the fit is satisfactory, although not perfect. For northern Scandinavia, the North Atlantic north of Iceland and along East Greenland, however, the forecast was opposite to the real temperature change recorded. 

200803-200805: Agreement between the seasonal MAM-forcast and the real development within the UK-Denmark region is good. However, the forecast for Greenland, Greenland Sea, Norwegian Sea and the Barents Sea was not good. In these regions the real temperature change was opposite to that forecasted.

200902-200904: Good fit for NW Europe and the Barents Sea. Bad fit for the Greenland Sea and East Greenland.

200901-200903: Good fit for NW Europe and the Barents Sea. Bad fit for the Greenland Sea and East Greenland.

200812-200902: Satisfactory fit for NW Europe and the Barents Sea, but warming in NW Russia is underestimated. Less satisfactory fit for the Greenland Sea and East Greenland.

200811-200901: Satisfactory fit for NW Europe but warming in NW Russia and in the Barents Sea is underestimated. Moderate fit for the Greenland Sea and East Greenland, where warming is overestimated.

200810-200812: Moderate fit. Warming in the regions from Svalbard to NW Russia is underestimated.

200809-200811: Unsatisfactory fit. Cooling over central Europe is not forecasted. Warming in the Barents Sea is not forecasted. Along East Greenland warming is forecasted, which is in contrast to the 'normal' conditions recorded.

200808-200810: Unsatisfactory fit. Moderate warming is forecasted for the a region across the North Atlantic extending from central Norway across Iceland to East Greenland. Observations show this region to be near or slightly below 'normal' conditions, while NW Russia and the Barents Sea region experienced warming.

 

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