Climate Model Berlin - Analysis Maps 2001

Map Description

The following is a common description for all single evaluations of the model calculations. Links to the individual main emphasis areas are provided for faster orientation in the text:

• Near-ground temperatures (10 PM) in the entire area;
• Near-ground temperatures (6 AM) in the entire area;
• Near-ground temperatures (10 PM and 6 AM) in the detailed-analysis area;
• Air exchange and air-mass currents (10 PM and 6 AM) in the entire area;
• Air exchange and air-mass currents (10 PM and 6 AM) in the detailed-analysis area.

The model calculations were started every day at sundown, and continued until sunrise the following day. The time segments 10 PM and 6 AM were evaluated and represented in the form of maps for the individual climate parameters. The 10 PM point in time represents the reversal from an irradiation to an exradiation situation shortly after sundown, and stands for the beginning of a phase with great cooling dynamics in the variously structured spaces in the urban area. The 6 AM point in time stands for maximum cooling within the urban structure.

In the following single, exemplary results of the model calculations for the entire urban area or for the detailed-analysis area from the southern outskirts to the center will be briefly described. Fig. 3 provides a summary of the respective climatological parameters evaluated.

Near-Ground Temperature, General Remarks

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.

Near-Ground Temperature, Berlin and the Surrounding Countryside

The temperature conditions of the near-ground atmosphere 10 PM in the entire area are shown in Map 04.10.01. Due to the great variety of differences determined by land use of these effect quanta, a strongly structured spatial distribution of near-ground temperatures is simulated. Within the early nighttime hours (10 PM) the main land uses stand out against each other in a characteristic manner.

At this time, the wooded areas are approx. 1 K cooler than the surrounding open fields, and considerably colder than the built-up areas.

The open areas are heated up strongly during the day, and cool down after sundown just as strongly. Due to the relatively high evening base temperature, this cooling has on the whole not yet progressed sufficiently at the chosen point in time to reach the level of the cooler wooded areas.

Urban areas stand out considerably from their surroundings by an overall higher temperature level. However, the temperature distribution is spatially differentiated strongly in built-up areas, since, e.g., grid cells with detached house buildings, core areas, industrial areas and transportation facilities show strongly differing ground and surface characteristics. Moreover, the higher mean temperature level is interrupted by inner-city parks like the Great Tiergarten and the Tempelhof and Tegel airports.

Depending on the individual surface characteristics of the various land uses, the ground will cool down differently strongly during the night; Map 04.10.02 shows the temperature distribution at 6 AM.

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 remain warmer overall. While the temperature difference vis-à-vis the undeveloped surrounding countryside is typically 2 K during the evening hours, this value increases to K 6 by the early morning hours. These great horizontal differences are not quite achieved in the inner-city open areas. Here, the proximity to the relatively warm built-up areas manifests itself.

Near-Ground Temperature – Detailed-Analysis Area, Southern Outskirts to Center

With an increase in the spatial resolution, the individual use types stand out against each other still more considerably in terms of temperature, especially in built-up areas (cf. Maps 04.10.07 and 04.10.08).

Grid cells which are completely filled with built-up areas are immediately adjacent to open areas, and thus exhibit temperature differences over very short distances. These differences are reduced only insignificantly, due to the fact that wind speed is strongly reduced in the urban areas, so that they are maintained during the nighttime hours.

The different building structures in the area selected are interrupted particularly by Tempelhof Airport, the Britz Garden recreational park and the open areas to the east and west of Lichtenrade.

The thermal behavior of the individual land uses is no different from the conditions described above for the entire area of Berlin. In the evening hours, the wooded sections of the parks are the coolest areas, followed by the open areas. Bodies of water show only a very low diurnal variation, and at this point in time fit into the temperature level of the general range of temperatures for the various built-up areas.

The land-use typical temperature conditions emerge clearly by the early morning hours. While the temperature above meadows and fields declines relatively strongly in the open fields surrounding Lichtenrade, the inner-city open areas, for example in the area of Tempelhof Airport, do not cool down to the same degree. This shows that the airport area is embedded in overall warmer surroundings.

Air Exchange and Air Mass Current as Criteria for Climate Compensation Measures

Good ventilation of residential areas can lead to a reduction in human bio-meteorological burdens (cf. Moriske and Turowski 2002). During the nighttime hours, the introduction of cooler air from the surrounding countryside can reduce the temperature level of the warmer air masses stored in the city, thus leading to a reduction of anthropogenic thermal pollution during the summer months. If this introduced cooler air (fresh air) is not burdened by air pollutants, the ventilation will also lead to an improvement in the air-quality situation.

Consequently, evaluation of the ventilation situation requires an appropriate assignment of burdened areas and compensation areas, which can make the appropriate unburdened air available, as well as a circulation system to ensure the transportation of the air mass.

Climate-ecological compensation effects originate in undeveloped areas scattered throughout the urban area. They are characterized by a high vegetal proportion as well as a low degree of sealing of less than 20%, and improve the local climatic situation even in the densely built-up cores areas of Berlin (cf. Maps 04.10.03 through 04.10.06 for the entire city, or Maps 04.10.09 through 04.10.12 for the detailed-analysis area).

The compensatory effect is produced via thermally and/or orographically induced current systems. The following delimitation criteria were used to identify the open areas which provide neighboring built-up areas with fresh and/or cold air, and to be able to assign them to the various exchange processes. The self-generated compensation currents should achieve at least a speed of 0.2 m/s in climate-ecologically relevant open areas during a low-exchange, cloudless summer night. The compensation currents can be described as slope or valley winds if slopes or valley-floor inclinations of > 1° occur. Thermally induced current systems can be found in the relatively flat areas (cf. Fig. 6).

Fig. 6: Areas involved in cold-air generation or cold air out-flow in the urban areas of Berlin during a low-exchange cloudless summer night (10 PM; grid resolution: 200 m)

Significant compensation can be expected from the large coherent forest and parkland areas which are very extensive, particularly on the edge of Berlin. Due to the high cooling rates during the evening and nighttime hours, these areas can be considered important cold-air provision areas. Table 1 shows the proportional space of the city area involved in the formation of corridor winds and cold-air outflows:

Tab. 1: Proportion of spaces in the city area involved in the formation of autochthonous current systems. The thermally and orographically (relief-caused) induced current systems are distinguished.

Thus, more than 30% of the city area is involved in the formation of compensation currents, with the proportional area increasing slightly over the course of the night from 30.8% at 10 PM to 32.0% at 6 AM. This increase can be explained by the fact that additional undeveloped areas are involved in the cold-air generation process, particularly around the Müggel Lake and in the Grunewald forest.

However, the result is that while in the early morning a larger undeveloped area is involved in cold-air generation with a current speed > 0.2 m/s, this occurs at a lower level than at 10 PM. A comparison of the mean air-exchange rates of all grid cells in the entire urban area shows notably that the mean cell value climbs from 7.6 at 10 PM to 8.1 at 6 AM. On the other hand, the maximum cell value of 29.47 drops to 22.8. Thus, while the mean air exchange rate climbs overall, the maximum values reached at the 10 PM time segment, and the intensity of air exchange, are no longer achieved, due to the increased leveling of temperature differences.

The compensation from the open areas reaches large parts of the built-up areas of Berlin. A city-wide balance sheet yields the following results:

• About 65.5% of the built-up areas are reached or penetrated at the 10 PM time segment by autochthonous currents with a speed of at least 2 m/s.
• Over the course of the night, the compensation from the open areas drops, due to the leveling of the temperature level – and the resulting drop in thermally induced current systems – to a spatial coverage of approx. 40% of that portion of the urban area characterized by built-up development (time segment: 6 AM).

Due to the close interaction between built-up areas and open areas, Berlin has a high climate-ecological compensation potential overall. However, cold-air out-flows account for a relatively small share of this. They occur extensively primarily in the following areas:

• the eastern Havel shore along the Grunewald;
• the eastern edge of the Grunewald; and
• south of the Great Müggel Lake, in the Berlin Bürgerheide City Forest.

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 Liepe Bay and Ruhlebener Strasse which conducts cold air toward the borough of Spandau along an approx. 7 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 air-stream channels 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.

As examples of the compensation provided by open areas, three locations are discussed in detail under Map Description/Supplementary Notes so as to elucidate the dynamics of the cold air balance in the border area of cold-air producing open area to the built-up areas.

In conclusion, let us address the cold-air balance of Berlin as a whole. For this purpose, the air mass current is to be used, in which cold-air movement is quantified on the basis of 10 PM values in an 8-hour night. Accordingly, 2.18 trillion m³ of cold air are moved in the urban area of Berlin in a low-exchange cloudless summer night. This corresponds to a flow-through rate of 0.27 trillion m³ of cold air per hour. The total mass of cold air moved, broken down by borough, is shown in Table 2.

Tab. 2: Cold-air mass currents in the Berlin boroughs during a cloudless summer night

The borough-related results coincide with the expectations with regard to size and location within the urban area. It is apparent here that such core areas as Friedrichshain-Kreuzberg or Mitte, with high degrees of built-up areas and of sealing, have relatively weak mass currents. The situation is different in the Boroughs of Pankow, Reinickendorf and Köpenick.

Here, too densely-built areas are apparent toward the city center, but this is compensated for by the large undeveloped spaces in the interaction areas with the surrounding countryside. The non-built-up, cold-air generating areas of the edge boroughs thus make the largest contribution to the cold-air mass current.

Map Description / Supplementary Notes

In the following, extensive additional information is provided on the dynamics and significance of the cold-air balance of open areas, with selected examples. The text completes the contents of the chapter Map Description.

Cold-Air Producing Open Areas and their Effect on Built-Up Areas

In Fig. 7, 3 locations are identified, which are to provide an example for discussing in a more detailed manner the cold-air balance on the basis of a selected corridor section of 9 grid cells of 1600 m of section length (200 m grids), and of 450 m (50 m grids), respectively. For the characterization of the dynamics of the cold-air balance, these examples were placed along the boundary area of cold-air-generating open areas and built-up areas. For an overall comparison of the values within the 200 m grid, a mean grid cell value was then determined based on the cells along the corridor section.

As examples of the compensation of open areas, transition zones were used: from Grunewald to Wilmersdorf (A) and in the Mahlsdorf (B) neighborhood on the eastern outskirts of Berlin. The southwestern section of Tempelhof Airport © represents the detailed-analysis area in which a higher-resolution 50 m grid was used.

Fig. 7: Situation of the test areas A, B and C for the clarification of the processes "cold air generation" and "cold-air out-flow" during a low-exchange cloudless summer night (10 PM; grid resolution: 200 m)

Example: Grunewald Forest

With over 3000 hectares, the Grunewald forest is among the largest wooded areas in the city area. Along a corridor of approx. 11 km, parts of the Boroughs of Charlottenburg-Wilmersdorf and Zehlendorf-Steglitz, located to the east of the forest, profit particularly from the high cold-air productivity. Fig. 8 shows the transition zone from the Grunewald to the detached-home areas in Wilmersdorf; here, the air interchange per grid cell and hour is relatively high, with exchange rates of over 20.

Fig. 8: Air exchange per grid cell and the autochthonous current field in the Grunewald-Wilmersdorf transition zone during a low-exchange cloudless summer night (10 PM; grid resolution: 200 m)

The correspondingly long range of the cold-air current is most strongly in evidence in Wilmersdorf at 10 PM, with up to 3000 m, which is due to the low-density detached-home structure of the area. In Steglitz, by contrast, a value of only approx. 1500 m is achieved, due to the increasing density of the built-up area. At 6 AM, the cold air only penetrates approx. 1000 to at most 2200 m into the built-up area.

For the 1600 m long section, the mean value of the air exchange per grid cell, the mass current as well as the flow speed of the corridor wind were exemplarily calculated (cf. Table 3). This corridor starts at Auerbachstrasse at the AVUS Freeway, and leads across Regerstrasse to Waldmeisterstrasse.

A decrease of the mean grid cell value over the course of the night is apparent. The air-exchange rate declines by approx. 30%, from 20.13 to 13.99. The same applies to the mass current, which is reduced by approx. 25%. The drop in current speed of the corridor wind is still more strongly apparent, at approx. 64%.

Tab. 3: Mean air-exchange rate, mass current and flow speed of the corridor wind per grid cell along the corridor section in the Grunewald-Wilmersdorf transition zone during a low-exchange cloudless summer night (grid resolution: 200 m)

Example: Mahlsdorf

The cold-air source area for this example is the open area adjacent to and north of the Dahlwitz forest. It stands out considerably from the built-up areas, with air exchange rates of over 20 per grid cell and hour (cf. Fig. 9).

The range of this air movement around 10 PM is between 1100 m north of the Federal Highway 5 in the area of the rapid-rail line (S-Bahn) and 1800 m toward Hönower Damm. There, it unites with the corridor wind from the Kaulsdorf Busch and flows northward, where the wind speed finally drops to below 0.2 m/s at the Mahlsdorf rapid-rail station.

The penetration depth remains almost unchanged until 6 AM. Only the flow direction changes, toward the southwest.

Fig. 9: Air exchange per grid cell and autochthonous current field in the Mahlsdorf area during a low-exchange cloudless summer night (10 PM; grid resolution: 200 m)

Except for the mass current, the mean grid-cell values surpass those of the area Wilmersdorf(A) slightly (cf. Tab. 4). However, the essential difference is the weaker drop in the characteristic values until the early morning.

Tab. 4: Mean air exchange rate, mass current and flow speed of corridor wind per grid cell along the corridor section in Mahlsdorf during a low-exchange cloudless summer night (grid resolution: 200 m)

Example: Tempelhof Airport

Here too, analogously to the investigation areas already described, the situation was examined along a selected corridor section of 9 grid cells. In this case, the southwestern part of Tempelhof Airport served for the analysis of the cold-air balance in the detailed-analysis area, where a 450 m long section along Tempelhofer Damm was selected (cf. Fig. 10). For the analysis of air exchange per grid cell/hr., the longitudinal distance that the air-mass flows through is decisive. To make a comparison possible between the two grid intervals, the cell value of the 50 m grid must therefore be divided by 4, to make it comparable with the 200 m value.

Fig. 10: The air exchange per grid cell and autochthonous current field in the area of Tempelhof Airport during a low-exchange cloudless summer night (10 PM; grid resolution: 200 m)

With regard to the air change rate, two areas with relatively high hourly air exchange rates of over 80 per grid cell are recognizable in the airport area, including its apron area, and the intersection of Tempelhofer Damm and the A 100 freeway. In reference to the 200 m grid, which was used for the urban area, this corresponds to a cell value of 20, and is comparable to the example Wilmersdorf (A), in terms of the characteristic value. Despite the high air-exchange rate, even in terms of broad extent, the generation of the corridor wind in the apron area is impaired by the air terminals both at 10 PM and at 6 AM. The incorporation of the structural heights into the calculation grid for the FITNAH simulation yields a mean structural height, with the result that the air flow can pass over single obstacles which are nominally higher than 5 m.

By contrast, a corridor wind which, with a western current along the freeway A 100 and a length of at most 450 m, is rather weak by comparison with the other locations, can develop along the corridor section observed at 10 PM. At this time, it is juxtaposed to an eastward cold-air current from the allotment-garden colonies of the Schöneberg South Terrain. To a certain extent, Manteuffelstrasse functions as the separation line between these two corridor winds.

By 6 AM, the latter corridor wind from the allotment-garden area has come to a virtual standstill, while the range of the corridor wind starting from Tempelhof Airport has doubled to approx. 800 m and penetrated up to 200 m to the south to Tempelhofer Damm (cf. Fig. 10).

Table 5 summarizes the results for the part of the core area observed. It is clear that the calculated values have increased slightly over the course of the night, and thus demonstrate a different trend from areas (A) and (B). This is due to the high spatial resolution of the 50 m grid, which shows a shift of the grid cells with a high air exchange rate toward the built-up area.

Tab. 5: Mean air-exchange rate, mass current and flow speed of the corridor wind per grid cell along a corridor section in the Tempelhof Airport area during a low-exchange cloudless summer night (grid resolution: 50 m)

For the area to the west of the observed corridor section and the area adjacent to it in the direction of flow of the corridor wind, the cold-air volume was determined on the basis of the air exchange. The grid cells covering an area of approx. 20 hectares adjoining the corridor section to the west of the airport and for which a wind speed of > 0.2 m/s could be proved have been taken into account.

The height of these near-ground grid cells is 5 m, which yields a volume of 12,500 m³ per cell. For the stated area, an hourly air exchange of 5.52 million m³ can be estimated. For this segment, projected to a nighttime period of 8 hours, this yields an air exchange induced by the cold-air generation of the airport near-ground layer of air (up to 5 m above ground) of a total of 43.36 million m³.