Climate Model Berlin - Analysis Maps 2014

Map Description

Given below is a joint description for all the individual evaluations of model calculations. Links are given to the individual focus areas for a faster orientation in the text:

• 04.10.1 Near Ground Wind Field and Cold Air Volume Flow (10:00 p.m. and 04:00 a.m.)
• 04.10.2 Air Temperature (02:00 p.m. and 04:00 a.m.)
• 04.10.3 Radiation Temperature (02:00 p.m. and 04:00 a.m.)
• 04.10.4 Nocturnal Cooling Rate between 10:00 p.m. and 04:00 a.m.
• 04.10.5 Evaluation Index of Physiological Equivalent Temperature (PET)
• 04.10.6 Number of meteorological climatic factors in the middle of the years 2001-2010
  • Number of Summer Days
  • Number of Hot Days
  • Number of Tropical Nights
• 04.10.7 Climate Analysis Map

Within the mentioned topics, the different spatial structures (grid, block) as well as the different points of time and height sections are mapped separately as further differentiations in the maps. These different pieces of information can be selected for each topic via the level switching of the Geoportal .

The model calculations were started in the evening at the time of the sunset and carried out till sunrise of the day after the next. The time sections, in which the model results are to be selected, can be selected freely in principle (minutes till seconds). The individual climatic parameters for the various points of time (MEZ) are evaluated and shown in the form of maps, which permit inferences about the climatic functions and their significance.

The date 10:00 p.m. represents the reversal from irradiation to emission situation shortly after the sunset and stands for the start of a phase with a high cooling dynamics in the differently structured partial areas in the city. The date 04:00 a.m. stands for the maximum cooling within the body of the city in a high summer clear night. Both the points of time are thus relevant mainly for characterising the nightly air-exchange. The time section 02:00 p.m. is also suitable for evaluating the bio-climatic situation during the day, because at this point of time the solar irradiation and consequently also the air temperatures are strongly pronounced. The time of 02:00 p.m. as evaluation time was also necessary for the evaluation of the bio-climatic stresses during the day newly included in the current version of the planning advice map (SenStadtUm 2016).

The analysis maps 04.10.1 to 04.10.5 are present in grid-based as well as block-based form. In doing so, the statistical, not weighted mean value of all grid cells overlapping the block / partial block areas is shown. The meteorological climatic factors (Maps 04.10.6) as well as the Climate Analysis Map (Map 04.10.7), on the other hand, are present only in the form of a block, since these were not modelled directly, but instead were derived from the grid-based results of climate modelling.

Given below is a brief description of individual, exemplary results of model calculations for the complete city area.

In chapter Map Description / Supplementary Notes , a differentiated explanation is provided for an exemplary area in the district of Charlottenburg-Wilmersdorf

Map 04.10.1 Near Ground Wind Field and Cold Air Volume Flow (10:00 p.m. and 04:00 a.m.)

General comments

The good aeration of the settlement areas can lead to a reduction of human bio-meteorological loads (see Moriske and Turowski 2002). Thus, in the night hours, the bringing up of cooler air from the surrounding areas can lower the temperature level of the warmer air masses present in the city, which leads to a reduction of heat load on the people in the summer months. If this introduced cooler air is free of air pollutants (fresh air), the aeration then simultaneously also leads to an improvement of the air-hygiene situation.

For evaluating the aeration situation, consequently, it is necessary to have the suitable assignment of load areas and compensation areas, which provide the correspondingly unloaded air as well as a circulation system, which can bring about the transportation of air masses.

The ground level temperature distribution causes horizontal air pressure differences, which, in turn, are triggers for the local, thermal wind systems. Starting point of these processes are the night temperature differences, which become set between the settlement areas and the open spaces with pronounced vegetation. At the prone areas, the cooled and hence heavy air comes in motion in the direction of the deepest point of the terrain. This gives rise to cold air outflows at the slopes (incl. Mosimann et al. 1999). The wind speed of this small-scale phenomenon is determined primarily through the temperature deficit to the surrounding air and through the inclination of the terrain.

Along with the orographically caused flows with cold air outflow, the so-called floor/structure winds i.e. a direct compensation flow from high to low air pressure are also formed. They arise, when strongly overbuilt or sealed areas get heated more strongly than the surrounding open spaces and, as a result, a thermal trough arises over the urban areas. Consequently, the resulting pressure gradient can be compensated through inflowing cooler air masses from the surrounding regions (Kiese et al. 1992). For characterising these flows, it is important that the air can be accelerated over a certain stretch and is not hindered by the available obstacles, such as the built-up area. The floor/structure winds are closely restricted, often only weakly pronounced flow phenomena, which can be overlaid by a weak overlaying wind. Their speed lies mostly below 0.2 m/s (Mosimann et al. 1999).

The temperature differences typical for land use start building shortly after sunset and can last for the whole night. In doing so, grasslands and arable regions especially prove to be productive of cold air. Depending upon the surface features and cooling rates, the rapid development of cold air streams is associated with this, which at first are very weak vertically (5-10 m layer height) and form between the multitude of areas with different temperatures.

Map contents

Climate-ecological compensatory effects potentially originate from all unbuilt and unsealed areas, inside as well as outside of the city area. To what extent can this potential unfold, depends on the respective boundary conditions, from the size of the area, the vegetation structure, the position in the city area as well as on the surroundings characterised by construction. The large number of open spaces within the city, as compared to the other metropolises, is extremely significant for a city like Berlin not classified topographically to a large extent, because here the city-climatic compensation for the core area of the city takes place primarily through circulations caused thermally, which calls for a highest possible nearness of green and built areas to each other (see Map 04.11.1 Planning Advices Urban Climate 2015, Fig. 8).

The representation of the near-ground temperature field involves the grid instrument of temperature at the near-ground layer of the atmosphere (0-5 m above ground). If several land uses with different area shares are present within a grid cell, the temperature shown is calculated from the proportional weighting. Thus, the simulated temperature values are comparable only for larger areas with a uniform or comparable land use with ground-bound measurements.

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. 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.

The grid-map levels represent ground level pronounced cold air flow field at a height of 2 m at the night evaluation times of 10:00 p.m. and 04:00 a.m. in 10 m x 10 m resolution as well as at roof level at a height of 22 m above ground. The wind field in the form of direction of flow and flow speed is mapped via the arrow direction and arrow length in the form of vectors for all cells of the model grid with a minimum speed of ≥ 0.05 m/s relevant for the climate. These grid-based data are supplemented by the display of the Cold Air Volume Flow as area value in m³/s. The term of Cold Air Volume Flow implies the product of flow speed of cold air, its vertical (layer height) and horizontal expansion of the flowed cross-section (flow width). It thus describes the quantity of cold air in the unit m³, which flows every second through – in this case – a 10 m x 10 m grid cell (see Fig. 9).

Fig. 9: Schematic diagram of cold air volume flow

The Volume Flow is thus a measure of the inflow of cold air and determines the magnitude of the aeration potential. Since it is a parameter integrated over height, there is no display for the roof level. All cells of the 10 m x 10 m grid are mapped with a value of > 60m³/s, for which a potential climate-ecological effectiveness is determined. The qualitative evaluation of this meteorological parameter is shown in Tab. 2. The classification of the grid-based Cold Air Volume Flow is oriented to the method for Z-transformation described in the VDI guideline 3785 Sheet 1 (VDI 2008). This method is based on the local/regional value level in a period of study and evaluates the deviation of a parameter from the mean conditions in this area. This method results in four evaluation categories of grading very favourable / favourable / less favourable / unfavourable.

Tab. 2: Classification of the grid-based cold air volume flow

The penetration depth of cold air in settlement areas and hence also the measure of bio-climatic favourable effect during the high pressure weather conditions in summer depends on the development structure and the intensity of cold air dynamics. In line with the increasing building height and density, single and town house complexes are flown through better than a block and perimeter development.

At the analysis time of 10:00 p.m., shortly after the sunset, the evening cooling phase starts, whose intensity depends on the respective structures and hence also influences the cold air flow field that is building up. In the context of inner-city green and open spaces, these are mostly rather a small-scale pronounced air exchange processes, in which the Cold Air Volume Flow hardly exceeds 90 m³/s. Depending upon a cold-air producing area and the surroundings, the cold air acts between 50 m and 300 m in the development. This makes it clear that mainly an adequate number and favourable position of these relief areas is significant for reducing the stresses inside the city.

High or very high volume flows in this section of time are observed only in the outlying areas and are mostly related to the local cold air outflows. These are encountered on the eastern bank of the river Havel along the Grunewald or to the south of the Great Müggelsee in the city forest of Bürgerheide.

At the time of 04:00 a.m., the cooling of the green and open spaces and hence also the production of cold air is much advanced. The span of the penetration depth varies perceptibly and lies, depending upon the structural conditions, between 100 m and more than 1000 m. In areas of settlement types with lot of greenery, their ‘independent’ production of cold air is also added, which is then reflected in the favourable bio-climatic conditions there (SenStadtUm 2016). Parts of the inner-city block and perimeter development as well as of the district centres, on the other hand, are not flown through by cold air in the second half of the night too, because their high building density and hence the higher temperature level weaken the possible cold air flows, as long as they are present in the potential activity range of compensation areas at all.

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 3K 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 8K 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). It 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 Fig. 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

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 Tab. 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

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.

Map 04.10.6 Mean Number of Meteorological Climatic Parameters

General comments

With the knowledge of the climatic parameters at a reference location, the frequencies can be estimated at other places with the help of the present temperature difference. Based on the simulated air temperatures during the day and in the night at the measurement location of Tempelhof (reference time period 2001 to 2010), the difference from the mean temperature for each ISU5 block (partial) area was determined and the frequency of climatic parameters per day was determined (see Methodology / Supplementary Notes).

The number of Hot Days represents a subset of the Summer Days and is hence included in the map of days with ≥ 25 °C.

The frequency within the settlement areas depends on their structural density and their green portion. In doing so, mainly the blocks with corresponding degree of building and less green portion, such as core areas or uses similar to core areas, commercial and industrial areas with dense building or large-area sealing as well as similar area types of residential buildings show the highest values on the summer as well as Hot Days. Similar high values are shown by large settlements and high-rise buildings owing to the high share of open areas with grass, which can get strongly heated up during the day. These are contrasted by green settlement types with a strongly pronounced tree stock e.g. in Charlottenburg-Wilmersdorf with much less number of summer and Hot Days.

Even the green and open spaces show a high range, whereby a high number of summer and Hot Days per block is present over the arable land and grass lands because of the more intensive solar irradiation. Because of the micro-climate, the frequency is the lowest within the forest areas.

In case of the nightly lowest temperatures and hence also the number of Tropical Nights, the effect of the development present and hence the urban heat island effect becomes clearly apparent in the block areas. Most of the Tropical Nights can thus be seen in City East and parts of City West. These are contrasted by the green settlement types or the development near the outskirts of the city having a low number. The remaining development takes an average position in its night temperature level and hence in the number of Tropical Nights.

Owing to a strong nightly cooling, the arable land and grass lands show a low number of Tropical Nights. This is a bit more highly pronounced in case of forest areas and somewhat similar to the well green settlement areas, such as villa construction, as far as the number is concerned.

Map 04.10.7 Climate Analysis Map

General comments

The Climate Analysis Map maps the actual state of the climate relevant for planning. To do this, the extent of the urban over-heating, the compensation effects of areas producing cold air as well as the spatial relationships between compensation and effective areas are shown. The effects of open spaces of the surrounding regions on the city area are also included.

The differentiation of spatial units “settlement area” and “green and open areas” follows a system, which derives from the area types of the Urban and Environmental Information System (ISU) (SenStadt 2010). More detailed information for deriving these spatial units is given in the accompanying text for planning advice map urban climate here (SenStadtUm 2016). Fig. 11 represents the spatial distribution over the city area.

Fig. 11: Complete urban distribution of the spatial units of the climate model on the basis of area types of the Urban and Environmental Information System (ISU)

Green and open spaces

Open spaces with vegetation having a noteworthy cold air production represent the climate and emission ecological compensation areas. A high long-wave nightly radiation during the low-exchange high pressure weather situations leads to a strong cooling of the ground-level air layer. The quantity of the cold air produced depends upon the prevalent vegetation type, the ground attributes and the related nightly cooling rate.

The total area of the potential cold-air producing green areas within the urban area is approx. 351km², which is the same as an area portion of around 39.5 % of the complete urban area and can be considered as high. In doing so, the characterisation of cold air supply within the green areas is mostly differentiated spatially. Often, the central inner-city green areas show a rather low Cold Air Volume Flow as compared to the one present at the development of the adjacent partial areas. The reason for this is that, driven by the temperature difference between the open space and construction, the cold air must first be accelerated and then the values increase in the direction of construction. In the transition area between green area and construction, the temperature gradient and hence also the intensity of the air exchange are the highest. Areas with a volume flow of >90 m³/s are, therefore, highlighted as areas with high and very high air exchange (see Tab. 2). Partly, these areas also continue as areas with valorisation “Cold air activity area in settlement space” in the constructed areas.

Large, linearly shaped open areas with relatively low surface friction function as air-stream channels for cold-air transportation. In this regard, three areas of the Havel and Spree Valleys are significant. First, the Havel section between Pichel Lake and Ruhlebener Straße which conducts cold air toward the borough of Spandau along an approx. 3 km corridor; second, Rummelsburg Lake, a part of the Spree, stands out as an area through which cold air streams from old Treptow and from the Plänterwald woods toward Rummelsburg; and finally, a section of the Dahme should be mentioned, along the Grünauer Straße-Regattastraße corridor. These results coincide with the results of a report of the German Meteorological Service (DWD 1996).

However, due to the hardly distinctive orography, such relief-determined ventilation lanes are rather rare. An essential contribution to the transportation of cold air from the countryside surrounding Berlin into the urban area cannot be ascertained; rather, only some parts of the river valleys within the urban area function as air-stream channels.

Settlement areas

As already described in the Section Method (cf. Tab.1), the night heat island effect has been determined 2 m above ground with the help of the statistical method of Z-transformation of the modelled night air temperature. With this method, a spatial sub-division of the settlement area can be done according to the criteria of the nightly over-heating as compared to open space conditions. In the scope of determining the areas with bio-climatic load during the night in the planning advice map, similarly, the distribution of the air temperature was included (SenStadtUm 2016).

Mainly the areas classified as “Heat island effect not present or weak” are more or less under the positive influence of a cold air effective area and are, in these cases, mostly marked by an adequate aeration, whose range in the construction depends on one hand on the cold air productivity (in the construction itself too), and on the other also on the obstacle effect of the respective construction type. Mainly in the densely built quarters, the blocks present in the influence area of cold-air producing areas can also be evaluated as areas with moderate to strong heat island effect in the night hours. These local phenomena indicate that in these cases the effect of the supplied cold air does not suffice to induce a clear reduction of the Air Temperature.

These are contrasted by settlement areas with a high degree of greenery, which show only a weak or no nightly over-heating. Constructed area with function relevant for climate show an open settlement structure with a total degree of sealing of less than 30 % as well as no or at the most low over-heating; they thus contribute potentially to local origin of cold air. The concrete local effect, however, depends on the respective local situation i.e. essentially on the vegetation present. Typical area types are the ones of individual, town and double houses, or, in general, the construction with garden and surrounding greenery. They often border at the cold air producing green areas and thus contribute towards aeration of further remote settlement areas having a nightly overheating.

Air exchange

Structures, which enable the air exchange and introduce cold air, are the central connecting link between compensation areas and effective areas with bio-climatic load. Pathways should generally sow a low surface roughness, whereby wood-less valley and meadow areas, larger green areas (mainly with their open areas having low vegetation) and rail lands are considered as suitable structures. Wide roads can serve only for climate compensation owing to their emission load, but not for introducing unloaded air. The pathways are subdivided in the Climate Analysis Map with respect to the process sequence. In the ‘ideal case’, an area producing cold air also represents a part area of a pathway.

There is a predominance of mainly thermally induced pathway types with a compensation flow purely caused by the usage dependent temperature differences. The small garden complex at Priesterweg can be mentioned as an example of such flow areas inside the city, which transport cold air from the cemetery at the Bergstraße in Steglitz as well as from islander to the north direction. The situation is similar with respect to the small park complexes at Heckerdamm as well as the public park Rehberge; here, a part of the cold air produced at the airport Tegel is forwarded towards the city interior.

On the whole, the recognised thermally induced pathways are concentrated in the following areas:

• to the north of the line Tegel – Lichtenberg
• to the west of the castle grounds Charlottenburg till the city limits in Staaken; partly, the cold air is introduced from the northern Gatower field as well as from the surrounding regions
• in the south to the east of city limit to Groß-Ziethen in the city areas of Rudow and Bohnsdorf.

Areas in the direct neighbourhood of greenery/construction are not indicated as part of a pathway.

Mainly orographically induced pathways are concentrated in the eastern area of the city. These are mainly valley areas like the Wuhle and the Mühlenfließ, which function as pathways owing to their alignment, width and surface condition. To this effect, the depression line Hundekehlsee – Dianasee – Koenigssee – Halensee, originating from Grunewald can be arranged in the western part of the city.

The low lands of the larger flowing waters like Spree and Havel go beyond this function and possess, in addition, an attribute as higher-level air and ventilation pathways. The favour the air exchange in the adjacent construction even in case of stronger, higher-level weather conditions.

An extensive cold air outflow is restricted on areas with slope inclination >1° and occurs rather rarely in the city area of Berlin because of the comparatively lower height differences. For this reason, this process is connected with few areas having a noteworthy slope inclination, such as those of Grunewald and the Körpernicker Bürgerheide. Furthermore, an individual cold air outflow can be assumed to the north of Tegel lake, in Kaulsdorf as well as in Forst Düppel. The cold air supply is above-average high on these sloped forest areas, since the radiation and hence the primary cooling takes place mainly from the upper crown area and not from the immediate ground vicinity. Because of the large, radiating surface of the area, the cold air also flows in and above the crown area, instead of sinking first in the area below the canopy (Groß 1989).

Other identifications

The air-hygiene situation in the main roads network is mapped via the index of air load on the basis of nitrogen dioxide (NO2) and fine dust (PM10) (SenStadtUm 2011). The spatial distribution of the load situation closely depends on the traffic volume as well as on the construction available along the road sections. The latter influences the dilution and the removal of air-hygiene air masses so that a high load is encountered mainly in the densely constructed city areas having a high traffic volume.

Wind field changes i.e. the tendency towards strong turbulences as well as up and down winds can occur in the area of bigger buildings, the way they are present in construction topologies of heterogeneous, inner-city mixed development, large settlements and high rises as well as core area uses. With these changes there are, on one hand, positive effects like stronger turbulence of air-hygiene loads, on the other hand, there are also more and more restrictions in the wind comfort. On days with heat load, a cooling function originates from the water bodies in the urban area of Berlin for the nearby surroundings. They also act as air and ventilation pathways even under weather conditions with a strong exchange.

Noise protection devices are available at sections along the noise-emitting traffic paths and correspondingly sensitive uses. They are indicated primarily along the Federal Highway A 113 as well as along the rail tracks in the southern and western parts of the city. They represent an additional information in this map, because they cannot be taken into account explicitly in the modelling with respect to their possible influence on the spread of the air masses.

Areas not evaluated include water bodies, some places, tracks including the surrounding track bed.

Map Description / Supplementary Notes

With the help of an example area with the size approx. 5 km x 3 km in the district of Charlottenburg-Wilmersdorf, the modelled climate parameters as well as the Climate Analysis Map (04.10.7) and the planning advice map (04.11.1) (SenStadtUm 2016) are explained below. This text thus complements the contents of the chapter Map Description.

General comments

The area shows a high bandwidth of the usage topologies available in Berlin and is hence especially suitable for an in-depth display of the city climatic conditions. It stretches from Grunewald lake in south-west till the Hohenzollernplatz in north-east and is characterised by the diagonal run of the highway A 100 (see Fig. 12). Grid cells that represent the buildings are shown in black.

Fig. 12: Expansion and usage structure in the transition area of Forst Grunewald - City West in Charlottenburg-Wilmersdorf

Map 04.10.1 Near Ground wind field and Cold Air Volume Flow

Since the small-scale wind field cannot be mapped in a useful way in the selected section, the display of the cold air flow field is done with the example of the grid-based Cold Air Volume Flow (see Fig. 13). On a large surface, the Cold Air Volume Flow shows a high to very high value level and reaches far in the eastern surrounding development, the reason for which is the intensive origin of cold air in the area of Grunewald. This is favoured additionally by cold air outflows, which occur at slopes of more than 1° inclination over the eastern Grunewald.

Fig. 13: Cold air volume flow at 04:00 a.m. on a sunny summer day in the transition area of Forst Grunewald - City West in Charlottenburg-Wilmersdorf

In doing so, mainly the adjacent strongly green settlement areas are favoured by a very high volume flow. Owing to their low surface roughness and their own cold air production, these development structures can already be considered in themselves as positively relevant for the climate. Towards the east, the volume flow shows an even higher value till the A 100 and after that reduces to a moderate level. The reason for this is the gradually increasing construction density and the higher temperature level, which weaken the Cold Air Volume Flow. It goes back to a lower value to the east of Brandenburger Straße.

Furthermore, surface structures are prominent in the map, which let the cold air penetrate far in the development. These are less built areas with pronounced vegetation. To the north of the section shown, the green axis Diana lake – Koenig lake – Halen lake emerges to this effect as cold air pathway with a very high Cold Air Volume Flow in the context of the adjacent track areas. To the south, the green complex Sommerbad Wilmersdorf-Friedhof Wilmersdorf/Fennsee can be recognised as the air exchange area. The high to very high volume flow continues beyond the Uhlandstraße till the public park Wilmersdorf and illustrates the pathway potential of such green structures.

Video animation cold air volume flow

In order to illustrate the effectiveness of near-ground airstream channels, a video animation was calculated with the help of the climate model FITNAH. This animation exemplifies the influence of the different land use in the borough Charlottenburg-Wilmersdorf on its neighborhood. The focus is on the transition zone between the Grunewald forest as a relevant cold air generating area and its adjacent development. Basis for the illustration is the Cold-Air Volume Flow between 22.00 p.m. and 03.00 a.m., which has been evaluated in a temporal resolution of 7,5 minutes. The Grunewald has an important function as a large cold-air production green space for the easterly adjacent development in Charlottenburg-Wilmersdorf and Steglitz-Zehlendorf respectively. The air exchange in the shown area is dominated by spatial Cold Air Flow, which develop at a slope of more than 1°. Based on this near-ground ventilation, the area in the North of the Halensee und between the train stations Grunewald and Charlottenburg can be considered as an Air Stream Channel. The outstanding climatic function of the Grunewald along of its approx. 11 km long transition zone between the forest and the development has been discussed in the accompanying text of the 2009 edition (SenStadt 2009).

The animation shall illustrate the functionality of cold air delivery areas, here with the eastern Grunewald forest in the borough of Charlottenburg-Wilmersdorf. A long wave emission within low-exchange nocturnal radiation periods leads to an extraordinary cooling of the near ground air masses. In consequence of this the cold air flows – with a slope inclination of more than 1° – in an eastward direction into the surrounding development. In this context, the surrounding area along the railway track becomes apparent as an Air Stream Channel.

The animation is based on a model calculation with FITNAH, for which overall 40 temporal cuts have been calculated. The Cold Air Volume Flow is illustrated in a qualitative occurrence in four classes. Whereas a low Volume Flow shows no color, a medium value is displayed in green. A high/very high Volume Flow is illustrated in light blue and dark blue respectively.

A second information level is presented by overlaying trajectories of several air packages, which represent the flow direction of cold air. The real time of integration of the video animation amounts to about 5 hours.

The respective zones, in which Cold Air Flow and Wind Speed rise in the course of the night, show a concentric increase. Due to the intense cold air production an extensive impact on the settlement area can be observed. As of approx. 01.00 a.m. a very high Cold Air Volume Flow emerges near Hilde-Ephraim-Straße as well as in the surroundings of the motorway junction Funkturm. The outflow of cold air intensifies in the course of the night, so that this area can be considered as an Air Stream Channel. Because of their meaning for climatic interrelation, structures like these deserve a very high level of protection.

Map 04.10.2 Air Temperature

The temperature field simulated for 04.00 a.m. in the night includes a range of about 7 Kelvin (K) between the minimum values of 13.9 °C and maximum values of 21.2 °C (see Fig. 14). In line with the structural characterisation, the temperature increases gradually starting from Grunewald in the direction of City West. The lowest Air Temperature is determined as 13.9 °C to the north of Grunewald lake over the grassland area of Hundekehlefenn. The small garden areas to the south of Forckenbeckstrasse within the development areas are highlighted as another prominent cold air region – also because of its size. Slightly weakly pronounced with respect to their cooling behaviour, the values in the cemetery Wilmersdorf lie between 14.5 °C and 16 °C; over the smaller green areas the nightly cooling is less strongly pronounced depending upon the size of the area.

Fig. 14: Air temperature at 04:00 a.m. on a sunny summer day in the transition area of Forst Grunewald - City West in Charlottenburg-Wilmersdorf

Within the built areas, the temperature characterisation is differentiated in space corresponding to the distribution of construction and greenery volumes. While the larger green inner courtyards of blocks can comparatively show lower air temperatures of 17 °C to 18 °C, in (completely) sealed courtyards of similar size as well as on broad road spaces, these are up to 1.5 Kelvin (K) higher. The greened settlement areas show values between 16 °C and 18 °C. The air temperatures above the water bodies are also to be classified at this level.

Map 04.10.3 Radiation Temperature

The characterisation of the Radiation Temperature during the day mainly through the solar irradiation is determined as an important influencing variable for calculating the bio-climatic evaluation index Physiological Equivalent Temperature (PET). In the absorption area shown in Fig. 15, values of less than 35 °C at a height of 2 m occur only in the shade in usages with pronounced trees at the simulated noon hour of 02:00 p.m. on a sunny summer day. For this reason, the comparatively lower temperatures are encountered extensively in Grunewald, whereas this is associated with the available stock of trees in settlement areas. Consequently, in the green settlements adjacent to Grunewald, there is a trend towards lower values than in the more strongly sealed development areas with less greenery to the east of A 100.

Here as well as in the broad road areas, the radiation temperature can rise to more than 50 °C in the unshaded areas. This value level can be interrupted locally by planting trees on the sides of the roads. The Kurfürstendamm at the upper border of the image as well as the Hohenzollerndamm running diagonally across the area are characterised to this extent. It is conspicuous that even the grassy areas within the green areas show high radiation temperatures owing to a lack of shading, which lie only negligibly below those of the sealed areas.

Fig. 15: Radiation temperature at 14:00 a.m. on a sunny summer day in the transition area of Forst Grunewald - City West in Charlottenburg-Wilmersdorf

Map 04.10.4 Nocturnal Cooling Rate

Fig. 16 shows the cooling during the night for each grid between 10:00 p.m. and 04:00 a.m. on a sunny summer night. The cooling of the different surface structures without existing buildings is associated with their respective thermal attributes, such as the heat flow. This influences, how much energy is absorbed by a surface during the day and is stored in the material or else is emitted again to the atmosphere at the ground during the night.

Fig. 16: Nocturnal cooling rate during the night between 10:00 p.m. and 04:00 a.m. on a sunny summer day in the transition area of Forst Grunewald - City West in Charlottenburg-Wilmersdorf

Corresponding to the structural characterisation, there is a dominance of a moderate to low night cooling of -0.75 to -0.25 Kelvin (K) per hour to the east of A 100, which can be traced back to a high percentage of sealed and built-up area. Only the green areas interspersed in the construction show a high cooling rate with -1.0 to -0.75 K. This is seen extensively mainly in Grunewald or in the upstream green settlement types, such as the small gardens at the Forckenbeckstraße as well as the cemetery Wilmersdorf. In keeping with the heat storing properties of water, a similarly low nightly cooling as in the densely constructed city areas is observed over the water bodies.

Map 04.10.5 Physiological Equivalent Temperature (PET)

For evaluating the heat load during the day, the Physiological Equivalent Temperature (PET) is included in the planning advice map (map 04.11.1, SenStadtUm 2016), which is based on the heat exchange of humans with their environment (Höppe 1984). In doing so, the radiation-related energy flows play a central role so that the spatial distribution of the PET is linked closely with the characterisation of the radiation temperature. While the lowest values of less than 30 °C are observed over the water bodies, a PET of about 30 °C to 31 °C occurs in the shade provided by the crown cover of Grunewald (see Fig. 17). Depending upon the stock of trees, this temperature level also continues in the adjacent green settlement areas. For this reason, these areas show favourable bio-climatic conditions during the summer weather conditions with potential heat load even during the day over and above the night conditions.

Fig. 17: Behaviour of the evaluation index PET (Physiological Equivalent Temperature) 14:00 a.m. on a sunny summer day in the transition area of Forst Grunewald - City West in Charlottenburg-Wilmersdorf

In the remaining city structures, values between 33 °C and 35 °C are encountered, which can also go beyond the bigger, strongly sunny portions of the areas. To this effect, larger, ground-level sealed areas as well as sports areas become apparent in the course of A 100, which have abundant sunlight. On the roads, the PET is lower by 1 to 2 below the trees so that the quality of staying is considerably improved here and walking or cycling in the shade is always more pleasant. The portions of green and open spaces with grass, on the other hand, show similar high values as a major part of the city area. The range of temperatures occurring in the section shown is thus approx. 13 K. For classifying the values of the PET with respect to the thermal sensitivity and the physiological stress, see Tab. 3.

Map 04.10.6 Mean Number of Meteorological Climatic Parameters

In the scope of the urban climate analysis, different climatic parameters were determined for block and partial block areas of ISU5 (see Method) and the average frequency of the occurrence per year (with respect to the period 2001-2010) was calculated (water bodies and road areas were not evaluated).

The characterisation of the Summer Days (T_max ≥25 °C) in the example range is shown in Fig. 18. Areas with a high portion of greenery have a low number of Summer Days. Whereas the number here is less than 42 days/year, it increases with the degree of construction and sealing increasing towards the East. Thus, within the perimeter development in the area of Hohenzollerndamm, a maximum temperature of at least 25 °C occurs on 42 to 44 days/year. Moreover, a number of 46 to 48 Summer Days/year is seen frequently, such as in the area of the stadium Wilmersdorf/Sommerbad; it becomes clear here that the grass areas with an abundant sunlight have an increasing effect on the number of Summer Days with increasing heat during the day (on the other hand, represent areas of intensive cooling in the night) (see Fig. 14 and Fig. 16). To the east of A 100, the number of Summer Days increases small-scale also over 50 days/year.

Fig. 18: Number of Summer Days (maximum temperature ≥ 25(o)C) per block on an average in the years 2001-2010 in the transition area of Forst Grunewald – City West in Charlottenburg-Wilmersdorf

The range of the Hot Days/year that occur lies between less than 5 in Grunewald and more than 12 over the larger, ground-level sealed areas e.g. surrounding the Messedamm or in the area of the thermal power plant Wilmersdorf (see Fig. 19). Furthermore, the track areas along the A 100 also show a similarly high number of Summer Days. With respect to the values, the settlement areas lie between these extremes and behave – in a moderated form – similar to the characterisation on Summer Days. The range of the Hot Days occurring here lies mostly between 7 and 11 days/year.

Fig. 19: Number of Hot Days (maximum temperature ≥ 30(o)C) per block on an average in the years 2001-2010 in the transition area of Forst Grunewald – City West in Charlottenburg-Wilmersdorf

The determination of Tropical Nights per year provides information about the night temperature level and the thermal situation associated with it, since bio-climatic load conditions can occur at nightly minimum temperatures of ≥ 20 °C. The number of Tropical Nights/year per block area is closely related to the nightly temperature level. In doing so, these are mainly the areas with a low nightly cooling (see Fig. 16), which are highlighted in Fig. 20 with more than 8 Tropical Nights/year. Associated with a moderate to high night cooling, the number of Tropical Nights is also lower to the west of A 100. In the settlement areas, it is 5 to 8 nights/year and reduces over the track area to the west of the Halen lake as well as the expanded small garden area to the south of Forckenbeckstrasse to less than 4 Tropical Nights/year and thus confirms their intensive potential for cold air formation.

On the whole, therefore, the mosaic of the different uses and their climatic behaviour over the course of the day is reflected clearly in the evaluations of the climatic parameters.

Fig. 20: Number of Tropical Nights (minimum temperature ≥ 20(o)C) per block on an average in the years 2001-2010 in the transition area of Forst Grunewald – City West in Charlottenburg-Wilmersdorf

Map 04.10.7 Climate Analysis Map with Reference to the Map Planning Advices Urban Climate (04.11.1, SenStadtUm 2016)

The Climate Analysis Map represents the primary result of the analysis portion of the climate modelling with FITNAH 3D and is based on the meteorological parameters described. The demarcation of production and effective areas as well as their connecting structures results in a complex picture of process system of air exchange streams of the structure of built and green areas.

The green and open spaces are shown in the Climate Analysis Map as climatic compensation areas, whereby a major part of these also show a climate-ecological effective Cold Air Volume Flow in the observed section owing to the intensive air exchange. Only the smaller green areas to the east of Brandenburg Straße are excluded from this evaluation (see Fig. 21).

Fig. 21: Statements of the summarised climate analysis map in the transition area of Forst Grunewald - City West in Charlottenburg-Wilmersdorf

Therefore, a major part of the settlement areas lies in the effective area of these cold air origin areas, which continue till the height of the Brandenburg Straße beyond the A 100. In addition, the settlement areas with greenery are indicated as built areas with function relevant for climate. A night heat island effect is not observed here, because the air temperature at 04:00 a.m. lies below the average value of all the investigated settlement areas in the city. In the surroundings of Diana lake, Koenig lake and Hubertus lake, the heat dissipation of the water areas is mapped in a light over-heating within the block areas. This increases on the whole in the direction of Hohenzollerndamm and A 100, whereby a moderate heat island effect dominates to the east of A 100. Individual construction blocks then become apparent with a strong overheating at Kurfürstendamm.

The air exchange at the eastern Grunewald is characterised mainly by cold air outflows, which can occur at slope inclinations of more than 1°. In addition, the area to the north of Halen lake between the suburban train stations of Grunewald and Charlottenburg is to be classified as cold air pathway. The air-hygiene situation is characterised by the progression of several main roads with high traffic. To this effect, primarily Kurfürstendamm and Hohenzollerndamm show a high to very high air-hygiene stress. To the west of A 100, on the other hand, there is mostly a low to moderate stress through pollutants fine dust (PM10) and nitrogen dioxide (NO₂) caused by traffic (detailed information is provided by Environment atlas map 03.11., edition 2011).

Individual block areas show a potential for wind field changes. These are, for instance, big settlements, like the highway construction Schlangenbader Straße or the core area usage in the area of Kurfürstendamm.

Fig. 22: Statements of the main map for planning advice for urban climate in the transition area of Forst Grunewald - City West in Charlottenburg-Wilmersdorf

An evaluation of the functions shown in the Climate Analysis Map from a planning perspective (worthiness of protection / thermally favourable / unfavourable situation) is done in the planning advice map for urban climate. It represents the planning valorisation of the generated model data as well as of the Climate Analysis Map derived from it and is the central basis of information for weighing and decision-making processes from the perspective of urban climate. Fig. 22 shows the corresponding section of the example period.

The evaluation of the thermal conditions in settlement area is based on the combination of night air temperature and Physiological Equivalent Temperature (PET) during the day. For this reason, very favourable to favourable conditions are encountered in the green settlement areas to the west of A 100. The degree of construction and sealing, increasing towards the east, also increases the heat stress on the whole. Bio-climatic unfavourable conditions are encountered mainly in the surroundings of Kurfürstendamm, Hohenzollerndamm and Brandenburg Straße.

The thermal situation in the road area is based on an evaluation of the PET, it is determined as negative on sunny radiation days essentially through the degree of solar irradiation and is determined positively through the shadow effect caused by the buildings / trees. To this effect, some sections of Kurfürstendamm and Hohenzollerndamm are to be classified as favourable to very favourable. These are contrasted by intensively irradiated road spaces during the day, which show a less favourable to unfavourable quality of stay.

The worthiness of protection of green and open spaces in the example period is to be considered almost extensively as high to very high, the reason for which is the spatial nearness to bio-climatically stressed settlement areas on one hand, and the intensive night cooling effect on the constructed areas on the other. In addition, the cold area pathway in the height of Halen lake is demarcated extensively as pathway corridor. A detailed description of the method structure of the three maps for planning advice map is given in the Accompanying document .