ESC 305. 01 Global Climate Change Chapter 5 ESC 305.01 Global Climate Change Chapter 5. Climate Change in the Future Part 1

Weather Forecasting, Climate Modelling & Prediction Weather forecasting, climate modelling and prediction require a mathematical description of the way in which energy from the sun enters the atmosphere, some being reflected by the surface or by clouds, and some being absorbed at the land and ocean surfaces or in the atmosphere. All these require powerful computer support. The difference between weather and climate is a measure of time. Weather is what conditions of the atmosphere are over a short period of time, and climate is how the atmosphere "behaves" over relatively long periods of time.

Predictions of future climate rely on numerical computer models, referred to as General Circulation Models (GCMs), which simulate the climate system of the Earth.

Weather Forecasting and Climate Prediction are based on solution of the governing physical laws expressed as basic equations: Basic gas laws Newton’s laws of motion Conservation of energy: thermodynamics Conservation of mass: dry air components, moisture, other species (plus sources and sinks)

What Weather Means Weather is the state of the atmosphere at some place and time (NASA). Weather is basically the way the atmosphere is behaving, mainly with respect to its effects upon life and human activities. Most people think of weather in terms of temperature, humidity, precipitation, cloudiness, brightness, visibility, wind, and atmospheric pressure, as in high and low pressure.

What Weather Means Because of weather's importance, meteorologists (scientists who study the atmosphere and the weather) have developed ways to forecast weather conditions. Forecasts for the next 12 to 24 hours are correct more than 80 percent of the time. Long-range forecasts for the next week or month are less accurate. These forecasts indicate general trends, such as whether the temperatures are expected to be warmer or colder than normal.

Weather Forecasting Weather forecasting was based on the experience with past situations, which developed into changed weather patterns that experts could either remember or look up from past records (Analogue forecasting). Analogue forecasting: Using the past as an analogue for the future.

Weather Forecasting During and after the Second World War, things started to change with the use of growing theoretical understanding of how weather disturbances grow, move and decay. Quantitative calculations of how the atmosphere changes were pioneered by Lewis Richardson, who was an English school teacher and scientist.

Weather Forecasting Lewis Richardson calculated solutions, by hand, governing equations of motion and continuity of matter in order to produce the first numerical weather forecast. He carried out his work during the First World War, which was later published in 1922.

Weather Forecasting However, Richardson‘s calculation was not very accurate and his method had to wait for the development of larger electronic computers for application. John von Neumann and Jules Charney faced the challenge at Princeton University after the Second World War, using one of the first electronic computers. The first computer-based weather forecasts were issued in 1955.

Weather Forecasting Edward Lorenz from MIT (Massachusetts Institute of Technology) and others showed that the limit of predictability of particular weather is about three weeks: the skill of numerical weather forecasting decreases rapidly with time into the future. The accuracy of forecasts declines due to the growth of errors present in the initial input data.

Weather Forecasting Weather forecast is an initial value problem; in which the eventual outcome depends on how accurately the initial conditions are known. Any error in specifying the initial conditions is amplified and eventually leads to a breakdown in predictability of the exact outcomes.

Weather Forecasting Progress towards improved weather forecasts is partly due to better observations of initial conditions, and more accurate calculations that minimize any rounding off errors. Other improvements in weather forecasting have come from the incorporation of heat and moisture exchange between the atmosphere and the oceans, assisted by satellite observations of sea surface temperatures.

Weather Forecasting The growing understanding of how the atmosphere and oceans combine to produce weather and how this is incorporated into computer models are the facts showing how the skill of numerical weather prediction has improved over the years.

Weather Forecasting Weather forecasting for a few days ahead requires a model covering the whole globe. Such a model would consist of grid points along the Earth’s surface and extending vertically to cover a part of the atmosphere (such as 30 km).

Weather Forecasting To this model, values of parameters such as pressure, temperature, wind velocity, humidity are fed at an initial time for every grid point. The model would contain descriptions in appropriate computer form of basic dynamics and physics of the different components of the atmosphere.

Weather Forecasting These are numerical representations of dynamic equations, such as the equation of continuity, equation of momentum, transport in the horizontal, and the hydrostatic equation. Other equations such as the equation of state, the thermodynamic equation, equations describing convective processes and heat transfer are also incorporated.

Weather Forecasting A mathematical model begins with the current state of the atmosphere, as determined by the most recent weather observations. The model uses these data to predict the state of the atmosphere for a specific time interval - for example, the next 10 minutes. Using this predicted state as a new starting point, the model then forecasts the state of the atmosphere for another 10-minute period. This process repeats over and over again until the model produces short-range weather forecasts for the next 12, 24, 36, and 48 hours.

Weather Forecasting The accuracy of weather forecasts generated by mathematical models declines steadily over time for two main reasons. First, the weather observation data initially fed into the model can never provide a complete picture of the present state of the atmosphere. Not all the data are reliable, due to both technical and human error, and data are missing from vast stretches of the atmosphere over the oceans.

Weather Forecasting Second, mathematical models of the atmosphere are only approximations of the way the atmosphere actually works, and errors in the models tend to grow with each repetition. Meteorologists also use mathematical models to produce long-range weather forecasts, such as 6- to 10-day forecasts and monthly (30-day) and seasonal (90-day) outlooks. Long-range forecasts and outlooks typically are less accurate than short-range forecasts, but they can provide an indication of general trends.

What Climate Means In short, climate is the description of the long-term pattern of weather in a particular area. Some scientists define climate as the average weather for a particular region and time period, usually taken over 30-years. It's really an average pattern of weather for a particular region (NASA).

What Climate Means When scientists talk about climate, they're looking at averages of precipitation, temperature, humidity, sunshine, wind velocity, phenomena such as fog, frost, and hail storms, and other measures of the weather that occur over a long period in a particular place.

Why Climate Projections are different ? When we talk about climate change, we talk about changes in long-term averages of daily weather. Numerical prediction of climate is a different problem, even though is starts with the same equations governing atmospheric motion and continuity of matter. There are two main differences:

Why Climate Projections are different ? 1) Climate projections are not about predicting the exact weather at anytime in the future, but rather about projecting the statistics (average behaviour and variability) of the future weather. This reduces the relevance of short-term chaotic behaviour in the atmosphere.

Why Climate Projections are different ? 2) Since climate projections are about the statistics of weather many months, years or even centuries into the future, much slower influences on the weather or climate must be taken into account. Therefore, the propagation of errors in initial conditions is not important, but rather the knowledge of slower internal variations in the climate system, and so called ‘boundary conditions’.

Weather Forecasting See the web pages of World Meteorological Organization (WMO-www.wmo.int) or NASA. Turkish State Meteorological Service (www.meteor.gov.tr)