2 Lecture 1
In this lecture we will discuss the following topics:
- How is climate defined? How is climate different from weather?
- What are the components of the climate system?
- How is climate characterized regionally?
- What are climate classifications?
- What is the energy balance at the top of the atmosphere and what factors determine it?
- How are energy balance and energy transport related?
2.1 Definition of climate and weather
- Weather is the current state of the atmosphere at a given location.
- Witterung is the weather averaged over a few days to weeks (e.g. “Altweibersommer” in September or “Eisheiligen” in May).
- The classical definition of climate is the average weather or the totality of meteorological phenomena that characterize the average state of the atmosphere at any point on the Earth’s surface. (Julius von Hann, 1883)
The slowly varying aspects of the atmosphere–hydrosphere–land surface system.
It is typically characterized in terms of suitable averages of the climate system over periods of a month or more, taking into consideration the variability in time of these averaged quantities. Climatic classifications include the spatial variation of these time-averaged variables. Beginning with the view of local climate as little more than the annual course of long-term averages of surface temperature and precipitation, the concept of climate has broadened and evolved in recent decades in response to the increased understanding of the underlying processes that determine climate and its variability.
2.2 Components of the climate system
One way to appreciate the complexity of the climate system is through a visualization provided by NASA, available at https://svs.gsfc.nasa.gov/31139.
2.3 Weather forecasts, subseasonal to decadal climate predictions, and climate projections
2.4 Climate elements, climate factors and climate classifications
Climate classifications aim to provide a typification of the characteristic geographical differences of climate, and are typically presented in the form of global maps. There are several types of climate classifications.
- Genetic classifications refer to the radiation and energy balances and the atmospheric-oceanic circulation. They hence classify regional climate based on its origin. Genesis is Ancient Greek for origin.
- Descriptive classifications are based on the typical conditions of the most important climate elements (usually temperature and precipitation), including their annual and diurnal variation.
- Effective classifications are based on the effects of climate, usually considering the potential natural vegetation and sometimes also the soil.
- There are also mixed classification forms. One example is the Koeppen-Geiger classification shown in Fig. 2.7, which is a descriptive-effective classification.
2.5 Earth’s energy budget
In climate science, the “top of the atmosphere” (TOA) is defined as the upper boundary of the atmosphere. At the TOA, the atmosphere becomes so thin that mass transport is negligible and the vertical exchange of energy is exclusively by radiation. The energy budget of Earth as a whole is hence determined by the radiative fluxes at the TOA.
\[ dE/dt = N = I - R - L \]
- \(dE/dt\): time rate of change of energy content of the Earth system (“storage term”)
- \(N\): net radiation
- \(I\): incoming shortwave radiation
- \(R\): reflected shortwave radiation
- \(L\): outgoing longwave radiation
| Contribution by clouds | |||
|---|---|---|---|
| Incoming shortwave radiation | \(I\) | 340 | 0 |
| Reflected shortwave radiation | \(R\) | 99 | 46 |
| Outgoing longwave radiation | \(L\) | 240 | -28 |
| Net radiation | \(N\) | 1 | -18 |
\[ \alpha_p = \frac{R}{I} = \frac{99}{340} \approx 0.3. \]
\[ \alpha_p^{\text{clear-sky}} = \frac{R}{I} = \frac{53}{340} \approx 0.15. \]
2.6 Meridional energy transports
The relation between Earth’s energy budget and meridional energy transports is captured by the following budget equation that expresses the conservation of energy:
\[ \frac{dE(\varphi)}{dt} = N(\varphi) - \text{div} F(\varphi). \]