2  Lecture 1

In this lecture we will discuss the following topics:

2.1 Definition of climate and weather

Weather, Witterung, Climate
  • 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)
Modern definition of climate by AMS (Link)

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.

Figure 2.1: Components of the climate system. Source: NASA.
Figure 2.2: From Schönwiese (2020).
Figure 2.3: From Schönwiese (2020).

2.3 Weather forecasts, subseasonal to decadal climate predictions, and climate projections

Figure 2.4: Sources: ECMWF (left), ECMWF (middle), Bildungsserver (right).

2.4 Climate elements, climate factors and climate classifications

Figure 2.5: Essential Climate Variables. Source: GCOS.
Figure 2.6: Source: NASA Visible Earth.

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.

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.
Figure 2.7: Köppen-Geiger classification. Fig. 1 from Kottek et al. (2006).

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.

TOA energy budget

\[ dE/dt = N = I - R - L \]

  • \(dE/dt\): 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
Figure 2.8: A CERES instrument on the left and onboard the NOAA-20 satellite on the right. Sources: NASA and NOAA.
Global-mean time-mean Earth’s energy budget for July 2005 – June 2015
Table 2.1: Values from Tab. 5 of Loeb et al. (2018) in units of Wm\(^\text{-2}\).
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
Figure 2.9: Fig. 3.1 of Trenberth (2022).

\[ \alpha_p = \frac{R}{I} = \frac{99}{340} \approx 0.3. \]

\[ \alpha_p^{\text{clear-sky}} = \frac{R}{I} = \frac{53}{340} \approx 0.15. \]

Table 2.2: Tab. 3.1 of Brönnimann (2018).
Figure 2.10: Global map of annual-mean planetary albedo derived from CERES. Fig. 2.9 of Hartmann (2016).
Figure 2.11: Global map of absorbed shortwave radiation derived from CERES. Fig. 5.8 of Trenberth (2022).
Figure 2.12: Global map of annual-mean outgoing longwave radiation derived from CERES. Fig. 2.10 of Hartmann (2016).
Figure 2.13: Global map of annual-mean net radiation derived from CERES. Fig. 2.11 of Hartmann (2016).
Figure 2.14: Zonal-mean annual-mean radiative fluxes at the TOA. Fig. 2.12 of Hartmann (2016).

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:

\[ \frac{dE(\varphi)}{dt} = N(\varphi) - \text{div} F(\varphi). \]

Figure 2.15: Fig. 6.5 of Brönnimann (2018).