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Berlin Environmental Atlas

04.10 Climate Model Berlin - Analysis Maps (Edition 2016)

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Map 04.10.2 Air Temperature (02:00 p.m. and 04:00 a.m.)

General comments

Decisive for the temperature distribution are the land-use dependent soil and surface characteristics, as well as their interactions with the atmospheric processes in the near-ground boundary layer. Within the soil, heat and temperature conductivity are of importance in this regard. The greater the heat conductivity of the soil, for example, the faster and more deeply heat can penetrate into the corresponding material - but also: the faster it can escape again.

The surface composition of natural and artificial areas determines, via the albedo (reflection capacity) and the emissivity, the quantity of energy available in the short and long-wave ranges of radiation for warming/cooling. Finally, the turbulence condition of the near-ground atmosphere plays a major role in the transportation of perceptible and latent energy to and from the ground (Map 04.06 Surface Temperatures Day and Night, Edition 2001).

All processes mentioned are interconnected via the energy balance of the soil, and determine the temperature of the surfaces and the layers of air above them.

Map contents

The temperature conditions of the ground level atmosphere are similarly mapped based on grid and block at different times of the day as levels of the main map.

In general, although the night temperature distributions are more expressive for evaluating the climatic potentials of relief and loading of areas, they also show the characteristic differences in the noon hours (02:00 p.m.) according to area distribution.
Sealed areas as well as open spaces with lawns are heated strongly during the day, the reason for which is the intensive solar irradiation, the lack of shading as well as the strong heating of the ground level air layer. The temperatures that occur here can lie between 30 °C and 32 °C, which represent the highest values in the scope of the modelled summer situation.

The forest areas as well as larger inner-city green areas, like the Great Zoo show at this time about 3 K lower temperatures in their parts having trees.

Areas with pronounced construction are although higher in their overall temperature level, but here a differencing of the temperature behaviour reflecting the respective small-scale situation can be seen here in the grid display. This is an event of further detailing of data basis and model grid associated with this version of climate modelling. This now makes it possible, for instance, to differentiate grid cells with trees or grass from the sealed areas in their temperature behaviour and to evaluate them accordingly. The block-related aggregations smoothens the differences built by the not weighted mean value formation.

The lowest values are encountered over water surfaces owing to their specific heat capacity, they behave very homogeneously and act for compensating the climate during the day.

Depending upon the individual surface attributes of the different land uses, the earth surface cools during the night in different intensities, the temperature distribution at 04:00 a.m. in the morning reflects the time of the strongest cooling.

While this cooling is very low for bodies of water, due to their good heat-accumulating qualities, open areas like fields and meadows show a strong drop in temperature. In wooded areas, the crowns of the trees protects the near-ground atmosphere below from cooling off strongly; therefore, forests stand out in the temperature distribution as relatively warm areas.

In the urban areas, cooling is reduced considerably by the presence of heat-storing materials like concrete and stone. For one thing, the quantity of heat stored during the day causes the temperature not to decline so strongly. Moreover, the low wind speeds of turbulent and latent heat currents, which might otherwise remove warm air, are reduced. The urban areas thus continue to remain warmer on the whole. The temperature differences at the unbuilt city limits or the surroundings can be more than 8 K in the early morning hours. These high horizontal differences are not quite achieved in the neighbourhood to the inner-city open spaces, sometimes, there is also a negative effect on the green areas from the built areas.

Map 04.10.3 Radiation Temperature (02:00 p.m. and 04:00 a.m.)

General comments

The Radiation Temperature is an important component for calculating the bio-climatic evaluation indices, like the indicator PET used here (see Map 04.10.5), because it has a high impact on the heat balance of the humans.
It is defined as the "uniform temperature of a black radiating enclosed area, which leads to the same radiation energy yield of a human being as the current short and long-wave radiation flows" (Matzarakis, A., Rutz, F., Mayer, H., 2000). SIt takes into account the different radiation flows on the humans, which include the direct (short wave) solar radiation, the diffuse sky radiation, the short-wave reflex radiation, the counter-radiation of the atmosphere as well as the infra-red radiation emanating from the surfaces. However, the parameters calculated in °C may not be equated with the air temperature values of the observed grid / block owing to this complex composition of the values. The different daily course of the radiation and air temperature is shown in Figure 10.


Fig. 10: Diurnal cycle of air temperature Ta, mean radiation temperature Tmrt (from radiation measurements: Tmrt,meas.; calculated via RayMan: Tmrt,calc.) and ground surface temperature Ts (from radiation measurements: Ts,meas.; calculated via RayMan: Ts,calc.) on a pleasant summer day over treetops (degree of crown cover: 0.88) on a green area in the northern city centre of Freiburg;
reference height for Ta and Tmrt: 1.1 m height above ground
(vgl. Matzarakis, A., Rutz, F., Mayer, H., 2000)

Map contents

The radiation temperature at 02:00 p.m. shows that the characteristic of this parameter is controlled mainly by the solar irradiation. In doing so, arable land and pastures as well as sealed areas show the highest values. In forest areas the lowest radiation temperature is present owing to the shadow effect of the crown cover. In the settlement areas, a small-scale mosaic of high and low temperatures is calculated owing to the buildings, sealed areas and trees being present close to one another. The value level over water bodies lies between that of the forest and settlement areas. The reason for this is the high specific heat capacity of water, its special kind of radiation absorption and the turbulent mixing processes taking place in the water body.

At the time of 04:00 a.m. the radiation temperature is primarily controlled via the (long-wave) heat radiation from the different surface structures. While doing so, the highest values are determined within the densely built settlement areas, which can be traced back to the high construction volume and its heat emission in the night hours. A little less is the radiation temperature over the water surfaces, which are now partly emitting the heat stored during the day. The lowest values are present over the pastures and arable lands, because their surface simultaneously also show the highest night cooling. Similar to air temperature, the radiation temperature is higher in the forest areas in the night hours than over open spaces, but still less than in the settlement areas.
The possible total difference between the highest (dense block construction) and the lowest values (open spaces) for the radiation temperature lies in the night hours is around 9.5 ° Kelvin (K).

Map 04.10.4 Nocturnal Cooling Rate between 10:00 p.m. and 04:00 a.m.

Map 04.10.4 shows the nightly cooling surfaces of the individual structures between the time sections of 10:00 p.m. and 04:00 a.m. for each grid cell or as block mean value in Kelvin (K) per hour. In doing so, the extent of cooling – depending upon the physical soil and surface attributes according to the land use – can show high differences. To this effect, the city structures become apparent in a characteristic way. Owing to their high heat conductivity and capacity, the lowest cooling lies over water bodies as well as settlement areas with high construction density. A moderate night cooling is encountered in a major part of the remaining development. Forest areas and settlement types with high greenery, on the other hand, show clearly higher cooling rates. This is most strongly pronounced over arable land and pastures.

Map 04.10.5 Evaluation Index of Physiological Equivalent Temperature (PET) (02:00 p.m. and 04:00 a.m.)

General comments

Meteorological parameters do not act upon humans independent of one another. The evaluation of the thermal effect complex has a special significance. All climatic parameters, which directly influence the heat balance of the humans, play a role here. Air temperature, air humidity, wind speed and thermal-physiologically effective radiation. For evaluating the thermal heat complex, the three methods

  • Universal Thermal Climate Index (UTCI)
  • Predicted Mean Vote (PMV) and
  • Physiological Equivalent Temperature (PET)
were modelled and compared.

For evaluating the day situation, the PET index was included in the result (Höppe and Mayer 1987). As compared to the similarly calculated indices as also the PMV used earlier, the advantage of PET is that it can also be understood better by non-experts owing to its °C unit. Moreover, PET is also a parameter, which has now become a kind of "quasi standard" in the technical world and more strongly takes into account the environmental medicine aspects so that the results from Berlin can also be compared with those from other cities (even from outside of Germany).

The PET was derived from the Munich's Energy Balance Model MEMI and, like other methods, is based on the heat exchange of the human being with his environment (Höppe 1984). In Table 3 - with reference to only the day hours - the thermal sensitivity (derived from the behaviour of a "standard person", which represents an average thermal sensitivity) and the physiological load level are compared with the PET index. An optimum comfort sets in at 20 °C. At higher values, a heat load is present, whereas lower values give rise to a cold stress.

Tab. 3: Assignment of threshold values for the evaluation index PET during the day hours
PET in °C Thermal sensitivity Physiological load level
4 Very cold Extreme cold stress
8 Cold Strong cold stress
13 Cool Moderate cold stress
18 Slightly cool Weak cold stress
20 Comfortable No heat load
23 Slightly warm Weak heat stress
29 Warm Moderate heat stress
35 Hot Strong heat stress
41 Very hot Extreme heat stress
Tab. 3: Assignment of threshold values for the evaluation index PET during the day hours (Matzarakis, A., Mayer, H., 1996; VDI 2008)

Map contents

The PET values at 02:00 p.m. show a strong dependency of the heat loads occurring during the day on the local shadowing situation. A moderate heat load on cloudless Summer Days with strong solar irradiation is shown, accordingly, by forest areas as well as by areas with pronounced trees and groves. The reduced direct solar irradiation through shadow formation by the vegetation and the evaporation of water contributes here to comparatively low load potential. As a result of its quality of stay, especially in the neighbourhood of strongly overbuilt quarters, a very important role is thus ascribed to the inner-city green areas. These are contrasted by strongly sunny areas, where the heat load shows the highest values during the day. In doing so, similar high temperatures are achieved over grass fields as well as over sealed areas.

The situation in the early morning for the time section 04:00 a.m. shows that fields and meadows - especially in the outskirts and the surroundings of Berlin - cooled down considerably shortly before sunrise, whereas the built-up urban spaces remain at a significantly higher value level. This is only interrupted by the large inner-city green spaces like the Tempelhofer Feld, the Gleisdreieck or the Great Tiergarten, which have adjusted to the PET values of the open spaces in the outskirts or the surroundings according to the vegetation structure.

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