Weather prediction is said to be the ultimate goal of atmospheric research. It is also described as the most advanced area in meteorology. The nature of modern weather forecasting is not only highly complex but also highly quantitative.
The various procedures adopted in weather analysis and weather prediction are certainly beyond the scope of a textbook of Climatology.
An attempt, therefore, is made to highlight the different methods used in modern weather forecasting. The various methods used in forecasting the weather are as follows:
(1) Synoptic weather forecasting,
(2) Numerical methods, and
(3) Statistical methods.
Each of these methods aims at not only projecting the location and possible intensification of existing pressure systems, but also determining the creation of new storm centers.
(1) Synoptic weather forecasting:
It is the traditional approach in weather prediction. This primary method continued to be in use until the late 1950s. Here it would be in the fitness of things that the meaning of the word “synoptic’ should be made clear to the reader. Synoptic” means that the observation of different weather elements refers to a specific time of observation.
Thus, a weather map that depicts atmospheric conditions at a given time is a synoptic chart to a meteorologist. In order to have an average view of the changing pattern of weather, a modern meteorological centre prepares a series of synoptic charts every day.
Such synoptic charts form the very basis of weather forecasts. As stated earlier, the task of preparing synoptic charts on a regular basis involves huge collection and analysis of observational data obtained from thousands of weather stations.
From the careful study of weather charts over many years, certain empirical rules were formulated. These rules helped the forecaster in estimating the rate and direction of the movement of weather systems.
For example, when the type of weather generated along a front is known to a forecaster, and the rate and direction of the moving storm is also known, a rather accurate forecast for the area in question can be made.
Because of the sudden changes in the cyclonic system, these forecasts are valid only for a shorter period of time, say, a few hours or a day. In the initial stages of weather forecasting, for cyclonic development prediction much reliance was placed on the analysis of surface fronts.
In those days, neither the exact relationship between the surface weather conditions and flow aloft was known, nor were the upper-air data available.
Since the discovery of the relationship between atmospheric conditions in the upper-air and the surface weather and also the availability of weather data in the upper atmosphere the forecaster supplements the surface charts with the upper-air data.
However, the early meteorologists had no scientific guidelines to know as to why weather systems moved at specific time of the year and followed certain paths.
In those early days the sequence of events on a weather map could be interpreted subjectively depending on the experience and skill of the individual forecaster. In the synoptic weather forecasting there was no scientific basis and there was little quantification.
(2) Numerical methods:
More recently it has been realised that other methods can more accurately predict the future weather than was possible by the traditional synoptic approach. The numerical method involves a lot of mathematics.
Modern weather forecasting is now using the techniques of Numerical Weather Prediction (NWP). This method is based on the fact that gases of the atmosphere follow a number of physical principles.
If the current conditions of the atmosphere are known, these physical laws may be used to forecast the future weather.
Since the late 1940s there has been a steady growth as regards the use of mathematical models in weather forecasting. These procedures have been made possible because of advancement in the formulation of mathematical models.
A series of mathematical equations is used to develop theoretical models of the general circulation of the atmosphere. These equations are also used to specify changes in the atmosphere as the time passes on.
For these equations certain weather elements like air movements, temperatures, humidity, evaporation at the ground, clouds, rain, snow and interactions of air with ground and oceans are taken into account.
The National Weather Service for its daily weather prediction makes use of a numerical model in which the atmosphere is divided into 6 distinct layers. In certain cases, the atmosphere is divided into as many as 11 layers.
The use of mathematical models makes it necessary that the initial state of the atmosphere is completely known. This is made possible by making observations of the atmosphere by means of radiosonde stations all over the world.
Radiosonde data are supplemented by means of radiometric observations from satellites which also provide data on humidity and cloud cover.
With the help of these quantities, maps showing the atmospheric conditions are constructed. At present automated techniques are used to draw weather maps depicting the pattern of these quantities.
In the development of numerical method of weather forecasting, valuable contributions were made by Prof. J. Charney and Academician Obukhov (of former Soviet Russia) who derived a consistent set of mathematical equations for weather prediction.
When we consider the ever-changing atmosphere a large number of variables must be taken into account. This will be a very difficult task. To solve these problem numerical models were prepared which ignore some of the variables on the assumption that certain aspects of the atmosphere do not change with time.
These models, in fact, represent a hypothetical atmosphere, but their usefulness in weather forecasting cannot be questioned. Most of the modern methods try to forecast the flow pattern of the upper atmosphere which helps in projecting the possible sites for cyclone development.
However, even the most simplified mathematical models involve incredibly large number of calculations to be made. The use of such models could only be possible after the advent of high-speed electronic computers.
There are several considerations involved in the preparation of numerical (mathematical) models: (a) The equations should be simplified to the maximum possible extent depending upon the accuracy desired in the forecasts and on the range of applicability, (b) The system of equations should be so designed that they guarantee the conservation of air mass, momentum, water vapour, and total energy for the entire volume for all time, (c) The region for which weather is predicted, is covered by a grid that is rectangular in shape.
The grid consists of a number of nodal points. The equations are solved by the computer at each nodal point for a very short period of time, say 10 minutes.
By repetitive calculations for every next 10 minutes, forecast is obtained for 24, 48 or 72 hours ahead. Remember that the finer grid yields better results. But computer limitations impose restrictions.
Limitations of Numerical Method of weather forecasting have to be taken into account. “The physical assumptions adopted and the degree of sophistication of each physical process incorporated differs from model to model.” Numerical forecasting is beset with a large number of problems.
These problems are related with the need for collecting requisite amount and type of data so that initial condition of the atmosphere may be known. Further, there is the problem of determining and allowing for changes in the conditions at the boundary of the model.
The buddies of various sizes and forms produced in the atmosphere also pose problems. As stated earlier, calculations are repeated for weather prediction. If there is any error in the initial stage, the errors go on increasing each time.
Since billions of arithmetical operations like addition, subtraction, division etc. have to be performed for only one numerical prediction, only the faster computers on the market could do it.
Another problem inherent in this method of weather prediction is that there are numerous violent weather phenomena which adversely affect human activity, but they are so small in extent that they cannot be resolved by current operational numerical models.
Such weather phenomena are called meso-scale systems and include tornadoes, thunderstorms, snow and ice-storms etc.
(3) Statistical methods:
Statistical methods are used along with the numerical weather prediction. This method often supplements the numerical method. Statistical methods use the past records of weather data on the assumption that future will be a repetition of the past weather.
The main purpose of studying the past weather data is to find out those aspects of the weather that are good indicators of the future events. After establishing these relationships, correct data can be safely used to predict the future conditions.
Only overall weather can be predicted in this way. It is particularly of use in projecting only one aspect of the weather at a time. For example, it is of great value in predicting the maximum temperature for a day at a particular place.
The procedure is to compile statistical data relating temperature to wind velocity and direction, amount of cloudiness, humidity, and to the specific season of the year. Thereafter these data are depicted on charts. These charts provide an estimate of the maximum temperature for the day from the data of the current conditions.
Statistical methods are of great value in long-range weather forecasts. The National Weather Service prepares monthly and weekly weather outlooks. In fact these are not weather forecasts in the strict sense of the term.
They are mere estimates or projections of the rainfall and temperatures that may be expected during these periods. These estimates only give an idea whether the temperature and rainfall in the region will be above or below the normal.
In the United States of America, the National Weather Service constructs a mean 700- millibar contour chart for the next month. Statistical records for the season of the past years are compiled.
Then the effects of ocean temperature and snow cover are studied. After compiling such charts the relationship between upper air movements and the surface weather pattern are I established. This helps in predicting weather aspects for a particular region of the country.
Another statistical approach to weather prediction is called the Analog Method. In this method an attempt is made to identify in the past weather records such weather conditions as are almost similar to the current conditions.
Once such analogous conditions are located, it is presumed that currently the same sequence of weather events will follow as was shown in the past records. But despite its simplicity, this method has its own drawbacks.
Because of a large number of variables, it is not necessary that the past and the future periods of weather will be identical.
Even when two periods are matching to a considerable degree the sequence of weather may be dissimilar in each case. With this method also, as in the case of numerical method, the main problem is the lack of complete information about the weather conditions.
It should be borne in mind that for the medium-range forecasts, the numerical methods I supplemented by the traditional synoptic method are unquestionable.
Now, the meteorologists have realised that there is a strong correlation between the wavy flow in the upper regions of the atmosphere, particularly in the westerlies and the cyclonic disturbances in the lower troposphere.
If a comparison is made between the surface charts and upper-air charts, it becomes clear that the upper-air patterns of the winds are rather simpler than the surface flow. Because of these and other facts, modern weather forecasting predicts the weather from the changes in the upper-level flow.
Therefore first the future trend of the flow pattern in the upper-air is decided, and then this information is used to forecast the changes in the pressure systems near the surface of the earth.
Because the circulation pattern in the upper-air is irregular, long-range forecasts are still beyond the reach of the forecaster. However, the weather men are engaged in finding ways and means to predict changes in the flow pattern aloft on a long- term basis.
Let us hope that in future the researches being carried on about the behaviour of the circulation pattern aloft will enable the forecaster to answer such questions with a considerable degree of accuracy as whether the next summer will be warmer, or whether there will be a drought in a particular region next year.