Climate Model Berlin - Analysis Maps 2014

Methodology

One important factor in the creation and formation of an urban climate are the soil and surface characteristics, which have been changed in the urban area in relation to the surrounding countryside. The result is urban overheating and also the local urban wind circulation. Wind and temperature as well as quanta derived from them are the dominant factors affecting the assessment of urban climate from the aspects of human bio-meteorology and air quality.

The investigation and recording of the urban climate can be carried out with the help of various methods. These include field measurements, long-range ascertainment methods, wind tunnel studies, and the application of numerical simulation models.

Direct numerical simulation models are particularly well capable of representing the meteorological quanta, which are spatially and temporally very strongly variable, due to the great complexity of the building structure.

The detailed calculation of the wind and temperature conditions in the Berlin area within the scope of this actualization has been carried out using the FITNAH (F low over I rregular T errain with N atural and A nthropogenic H eat Sources) model. Groß 1993 and Richter & Röckle n.d. present detailed mathematical and physical descriptions of the model. Further detailed indications for the basic structure and approach of the three-dimensional FITNAH model, and for the interpretation of the model results, can be seen herein, on the basis of an exemplary comparison with measurement data, under Methodology/Supplementary Notes.

Generally, numerical simulation models are accepted for use in many fields of meteorology, since the data obtained provide important basic information for many areas of life (cf. Overview of the Most Important Models). The weather forecast for the next 1-5 days is obtained almost exclusively by means of such complex and extensive computer models. Knowledge of possible changes of our global climate over the next decades can also be derived from such calculations. And finally, models of a similar type are used to calculate the local and the regional distributions of meteorological variables in the atmosphere (Groß 2002).

Independent from the various scales and task definitions, all models are based on the same mathematical-physical system of equations. Only in detail are there scale-specific differences.

Meteorological and synoptic general conditions for model calculation

Along with determinations internal to the model, the meteorological general conditions also play a major role. Under the complete project, two model runs were conducted with the meso-scale climate model FITNAH for a high-summer weather situation with a horizontal resolution of 10 m. The first model run is based on an allochthonous west wind weather condition occurring relatively frequently in Berlin during the summer months. The second model run is based on an autochthonous weather condition without higher-level wind influence used regularly for an analysis of the urban climate. Additional data, differentiated in space, were derived from the various climatological parameters from the resulting meteorological fields. On the whole, therefore, three supplementing extensive data sets are available.

During the high pressure weather conditions (autochthonous weather conditions) the local climatic special features of a landscape can be expressed very well. Such a weather condition is characterised by a cloudless sky and a very weak, overlaying synoptic wind. In case of the numeric simulations carried out here, the large-scale synoptic general conditions were laid down accordingly:

• Cloud cover 0/8
• no overlaying geostrophic wind
• relative humidity of the air mass 50%
• 19°C air temperature at 09:00 p.m.

The comparatively low wind speeds at a low-exchange weather condition cause a reduced exchange of air in the air layer near the ground. A simultaneously high irradiation and emission can consequently lead to local human bio-meteorological and air-hygienic polluted areas. Characteristic for this (high-pressure) weather condition is the origin of independent cold air streams (corridor winds), which are driven by the temperature gradient between the cool open spaces and the warmer settlement areas.

Fig. 6: Course of temperature and vertical profile of the wind speed at noon time for various land uses

Calculated parameters and characteristic values, final climate analysis map

During the analysis phase of the project, a large number of meteorological parameters were calculated at three points of time. Along with the day situation (02:00 p.m.), the evening as well as the early morning were also simulated at time sections 10:00 p.m. and 04:00 a.m. The observed height levels were at 2 m and 22 m above ground.

For the analysis part 04.10, grid maps and block-based maps are present, in which the individual parameters as well as also the meteorological climatic parameters are shown as mean area values. The summary Climate Analysis Map 04.10.7 builds the conclusion of the analysis as one of the main results of the study (see Map Description).

Determination of climatic parameters in the Berlin area (04.10.6)

A “climatic parameter” implies a day, on which a defined threshold value of a meteorological parameter is achieved or exceeded. In the context of the topic of urban climate, mainly the following climatic parameters are relevant, because they are closely associated with the occurrence of bio-climatic loads in settlement areas:

• Summer Days (maximum temperature ≥ 25 °C)
• Hot Days (maximum temperature ≥ 30 °C)
• Tropical Nights (minimum temperature ≥ 20 °C)

The long-term measurements of climatic parameters (SenStadtUm 2015a) show a characteristic distribution of minimum and maximum temperatures on Summer Days for the various measurement locations in the urban area. The distribution reflects the different heat behaviour of the city, which results as a whole from the different usage structures as also from the position of a location within an urban area. In case of an otherwise similar use, the spatial position within the city thus determines, to what extent a location can benefit from the cooler surroundings or is present in the influence of over-heated adjacent city areas. An open space under the influence of the urban heat island will show a lower diurnal cycle than a similar area outside of the city. Since the absolute level of the summer temperature is determined primarily by the prevalent weather condition and only a modification is done by the situation of the location in the urban area, conclusions can be drawn from the measured temperatures of a location about the level at another location on the basis of the characteristic temperature differences.

The exceeding of defined values for the daily minimum / maximum determines the occurrence of the so-called climatic parameters. Since the daily extremes and also the simultaneously measured temperature differences at the stations show a characteristic distribution among one another, by knowing the temperature difference it can be determined for a reference location, how high is the probability that the threshold values will be exceeded there. If the frequencies of climatic parameters per year are known for a reference location, the frequencies at another location can be estimated.

Reference geometry for designating the climatic parameters is the block map 1:5,000 (ISU5_UA 2010), for which the grid-based temperature values of model simulation given therein were calculated using statistical methods. In a further step, temperature differences were determined for the values calculated for the DWD station Tempelhof (measurement period 2001 to 2010). Based on these temperature differences and the functional relationship between the temperature differences and the occurring frequencies of climatic parameters, these can be interpolated extensively to the urban area.

Climate Analysis Map (04.10.7)

The results obtained by applying the climate model FITNAH led to an extensive latest stocktaking of the climatic situation in the urban area and in the nearby surroundings. According to the VDI guideline 3787, Sheet 1, the Climate Analysis Map represents “the spatial climatic attributes of a reference area, which set in based on area usage and topography. The thermal, dynamic as well as air-hygienic conditions are shown. Note: The Climate Analysis Map contains and replaces the earlier synthetic climate function map.” (VDI 2015).

This restructuring of thermal alignment and naming for preparing climate maps also follows the latest display in environment atlas by replacing the earlier climate function map 04.11.1 as part of evaluation map (SenStadt 2009) by the current Climate Analysis Map as completion of the analytical map part.

The aim of the map is to demarcate the areas of the city according to their different climatic functions i.e. their effect on other areas. Starting point is the classification of the study area in settlements with bio-climatic and/or air-hygienic loads (effective area) on one hand, and on the other, in cold-air producing, undeveloped and areas with pronounced vegetation (compensation areas) If these areas do not directly touch one another and the air-exchange processes are strong enough, linearly aligned, low developed open spaces (air channels) can connect both of them. The mutual demarcation of favourable and unfavourable areas as well as of the connecting structures results in a complex picture of process system of air-exchange streams of the compensation area-effective area structure.

The climatic function areas to be demarcated should provide information about the areas, in which

• on one hand there is a potential for relief of other (adjacent and remote) areas,
• on the other hand, the strongest additional loads are expected over and above the large-scale influence,
• preferably air-exchange areas are to be assumed i.e. an important role is assumed for the ground-level transport of fresh air.

The individual legend units are explained below.

The cold air producing areas are the open areas with pronounced vegetation, such as forests, parks and small garden areas, which are shown as ‘green areas’ according to their use. For characterising their compensation effect, the Cold Air Volume Flow is included in the Climate Analysis Map. It expresses the inflow of cold are from the adjacent grid cells in m³/s per 10 m grid cell (map 04.10.1).

The green area portions, which show a climate-ecological effective Cold Air Volume Flow of more than 90 m³/s, are highlighted with a shading. This makes it clear, which portions of a green area are to be considered as especially climatically relevant.

The ranges of the positive effects were termed as operation area of cold air origin areas in the Climate Analysis Map. These are to be considered as continuation of the Cold Air Volume Flow emanating in the settlement areas from the green and open areas. Frequently, a good aeration is present here as well as a trend towards a lower heat island effect.

A (positive) development relevant to climate shows an overall sealing degree of average less than 30 %. Because of their climatically favourable features, these areas show a more or less strong origin of cold air and favour the effect of nightly cold air in the direction of remote settlement areas.

The depiction of the heat island effect in the settlement areas and in the road space illustrates the thermal situation in the urban area during the night hours. The basis for this is the respective average night air temperature per area at 04:00 a.m., whereby the value determined over all areas is 17.8 °C. The demarcation of the evaluation classes is based on the method described for Z-transformation in VDI guideline 3785 Sheet 1 (VDI 2008), which is based on the local/regional value level of an analysis and evaluates the deviation of a parameter from the mean conditions in an investigation area. Unlike as in the planning advice map, the spatial characteristic of the nightly heat island effect is present in the foreground so that here the block area are shown differentiated with an exceeding of the area mean value (Z-value < 0). In the planning advice map, the settlement areas are evaluated as bio-climatically favourable with a Z-value of ≤ 0.

Tab. 1: Evaluation of the nightly heat island effect in settlement areas and in road space during the summer sunny weather conditions

This corresponds to a temperature value of less than 17.8 °C, at which there is no nightly over-heating. This is contrasted by the settlement / road areas with a mean value of more than 19.5 °C, which are classified as strong heat island effect. This is the same as an over-heating of more than 1.7 Kelvin (K).

The connections of different causes and characteristics in the form of lines and areas serve the air exchange at the ground level. Pathways connect cold air origin areas (compensation areas) and load areas (effective areas) with one another and hence are an elementary part of the air exchange. Four different air exchange types were worked out in the maps under consideration of the process:

• Cold air pathway, mainly induced thermally,
• Cold air pathway, mainly induced orographically (e.g. smaller river plains),
• Area-wise cold air outflow to slope ranges (in case of slope inclination >1°),
• Extensive air and ventilation pathways (lowlands of larger flowing water bodies).

The identification of cold air pathways is oriented to the autochthonous flow field of the FITNAH simulation. The indicated pathways, except the river lowlands, are areas with pronounced vegetation with a linear alignment to the effective areas.

In order to identify the area with slope inclinations >1°, on which an extensive outflow of cold air takes place, a relief analysis was carried out with the ground elevation model used in FITNAH.

For displaying the traffic-dependent air load, the data of the environment atlas map 03.11.2 “Traffic-dependent air load” was taken for information (SenStadtUm 2011). In the map, both the central traffic-dependent air pollutants of fine dust (PM10) and nitrogen oxide (NO₂) have been joined together to make an emission index.

The signature wind field changes denotes settlement areas having a potential for higher gustiness and sudden change of wind direction. These are the area types of core areas and large settlements, whose development can lead to corresponding effects on the wind field.

Moreover, the noise control structures are also shown in the Climate Analysis Maps, which were taken from noise mapping only for information (SenStadtUm 2013a). Their relevance for the climatic functions results from the fact that these are the structures, which sometimes can have a height of several metres. This results in a potential effect on nightly cold air flows. The knowledge of their position represents an important additional information in evaluating the air exchange processes, since the noise protection walls cannot be considered explicitly in the model.

Verification of the results of the climate model FITNAH

The information derived from the model runs was checked by means of data from a study of local climatic functions of open spaces in the Gleisdreieck area of Schöneberg.

On the basis of an extensive comparison, the measurement results of the study were compared with the simulation results of the model application.

As a result of this comparison, a good level of agreement can be established between the results of the measurement project and the modeling of the local air-current field in the Gleisdreieck area using FITNAH.

The self-generated, local current phenomena postulated by the model calculations can largely be confirmed by the measurements. Details on flow direction and speed are within the same range. The relevant air exchange processes – small-scale, orographically caused cold-air outflow from the Viktoriapark area of Kreuzberg and thermally induced compensation currents between the open areas of the Gleisdreieck and the adjacent buildings – are recorded and represented equally, both qualitatively and quantitatively (cf. Vogt 2002a, pp. 26 et seqq.). However, a more regionally characterized compensation flow between central Berlin and the surrounding countryside cannot be confirmed by either of the two methodological approaches (cf. Methodology / Supplementary Notes).

Approach, data basis and method of the process used for updating the climate data take into account a largest possible, simultaneously extensive detailing of the resulting statements. However, owing to the dynamic development in the city, the initial prerequisites for the evaluation on individual areas change faster than the possible update cycle of the maps in the environment atlas. Therefore, it is recommended to use the overlay function with the respectively latest air patterns in the geo-portal for an area check as well as for a comparison with the technical data of the analysis maps. From this, one can drawn inferences for the usability of the results.

Methodology / Supplementary Notes

In the following, extensive additional information on the topical complex of the methodological processing of the Berlin climate model is provided. The text thus complements the contents of the chapter Methodology.

The concept and methodology of the FITNAH climate model

The basic structure of the three-dimensional FITNAH model consists of the conservation equations for impulse, mass and internal energy, as well as balance equations for moist components and additive elements to the air. The various turbulent currents are connected to the calculable mean quanta with the aid of empirical inclusions. The turbulent diffusion coefficient appearing in that context is calculated from the turbulent kinetic energy, for which an additional equation is solved.

The warming and cooling rates in the atmosphere due to the divergence of the long-wave radiant fluxes are calculated by means of a procedure in which the emissivity of the water vapor in the air is taken into account.

For detailed simulations in real terrain, the orography, and particularly the effect of wooded areas and urban structures on the distribution of the meteorological quanta must also be taken into account in a realistic manner. For this purpose, FITNAH is provided with special parameterization.

A forest or grove will be incorporated into the model via stock-specific quanta, such as tree height, stock density and tree species. This permits, among other things, the simulation of the reduction of the mean speed in the stock, the rise of the turbulence in the crown area and the strong nightly cooling in the upper crown third, in agreement with available observations.

With consideration for the city-specific quanta building height, sealing and degree of construction coverage, and anthropogenic waste heat, the typical formation of an urban heat island at a reduced mean air current can be simulated (cf. Groß 1989).

The entire system of equations, including the parameterization, is transformed into a coordinate system which corresponds to the terrain. That permits, in particular, the formulation of the ancillary conditions of the various meteorological quanta in a problem-specific manner, specifically at the lower edge, the ground. The calculation of the earth surface temperature is carried out via an energy-flux balance, in which perceptible and latent heat current, the soil heat flow, short and long-wave radiation components, and anthropogenic heat flow are considered.

The differential equation of the equation system used is transferred to finite-difference equations and solved on a numerical grid. The spatial grid size Δx used here is 10 m in the two horizontal dimensions. The vertical grid interval is not equidistant, and the calculation areas are particularly dense in the near-ground atmosphere, in order to incorporate the strong variation of meteorological quanta realistically. The lowest calculation areas up to a height of 22 m are 2 m, beyond that 4 m. The interval Δz becomes ever greater as the height increases, and the top limit of the model is located at a height of 3,000 m above ground. This is the altitude at which it is assumed that the disturbances caused by orography and land use at ground level (cf. Fig. 7) will have faded away.

Fig. 7: Example for depicting a natural landscape in the model area character

Verification of the results of the climate model FITNAH

For checking the information levels derived from the model runs, one can fall back on the evaluation of long-term operated measurement stations in Berlin and Potsdam conducted in the scope of the EFRE project (SenStadtUm 2015a). These station data also helped in deriving maps for distribution of the mean number of meteorological climatic factors 2001-2010 (Map 04.10.6).

To check the informational levels derived from the model runs, a study of local climatic functions in the open spaces of the Gleisdreieck area can be used. The orienting investigation on current and temperature fields in the Gleisdreieck area is methodologically composed of:

• stationary measurement, summer semester 2001 (four measurement operations); and
• mobile measurements, winter semester 2001-2002 (four measurement operations).

The meteorological framework conditions seemed suitable to permit self-generated current systems to develop in the area of the Gleisdreieck (cf. Vogt 2002a and Vogt 2002b).

The following working hypotheses were to be checked by the measurement operations:

1. There is an autochthonous, regional current which transports cold air over the low-friction structures of the track area (i.e., an air-stream channel) from the Teltow area into the center of Berlin;
2. The Gleisdreieck area, which is characterized by open spaces, provides cold air to the immediately adjacent built-up neighbourhoods;
3. There is a cold-air outflow from the area of Viktoria Park in Kreuzberg, which intrudes into the open spaces of the Gleisdreieck area.

These assumptions agree with the hypotheses on the formation of autochthonous current systems between differently structured urban areas in this investigation, and therefore should be ascertainable again in the model results of the FITNAH simulations as well. The measurement data on the current field can therefore be used to check the plausibility of the model results herein.

However, a restricted applicability of this comparison must be assumed:

• The meteorological framework conditions for measurement were not in every case ideal for the clear development of self-generated current systems.
• A mobile and stationary measurement can always have only a random-sample character (spatially and temporally).
• The mobile measurements carried out during the winter semester were conducted during periods of extreme sub-freezing temperatures.
• “Virtually stationary” short-term measurements are involved, since at each of the 37 measurement points in sequence, the wind field parameters were ascertained for approx. 4 minutes. The measurement procedures for the recording of the wind field for this area probably lasted about 4 to 5 hours. Thus, what is represented is not a wind field for a defined period of time.
• In the model runs which were used for the comparison, ideal framework conditions for the formation of self-generated flow systems were assumed, i.e. the top current has a speed of 0 m/s.

Check of the working hypotheses on the autochthonous wind field

In this comparison, the results achieved within the early night time hours at 2.5 m above ground are to be considered as a matter of priority. Thus, far-reaching comparability of the model vs. the measurement results is ensured in this regard. The comparison is carried out on the basis of an established working hypothesis on flow conditions in the area under investigation:

• There is an autochthonous, regional current which transports cold air over the low-friction structures of the track area (i.e. a ventilation lane) from the Teltow area into the center of Berlin.
  Neither in the measurement procedure nor in the model calculations can any regional current be ascertained which uses the low-friction open spaces of the railway facilities as an air-stream channel.
  Any such current system would have had to be demonstrated in the measurements at the Monumentenbrücke measurement point (cf. Vogt 2002a, p. 14). However, all that could be ascertained from the measurement operations was the intervention of the top current into the relatively low-friction, vegetal areas of the Gleisdreieck. Even the mobile, winter measurement ascertained no such current (cf. Vogt 2002b, pp. 78 et seqq.).
  The model result, too, would not support any extensive exchange flow. The flow field (10 PM) shows a locally determined mosaic of small-scale air exchange cells, which are for the most part thermally induced. As a rule, the spatial extent of these “flow cells” is between 500 m and 1,200 m (cf. Fig. 5).
• The Gleisdreieck area, which is characterized by open spaces, provides cold air to the immediately adjacent built-up neighborhoods.
  The measurement provided clear indications of the existence of these local equalization currents (cf. Vogt 2002a, p. 15). However, the less-than-optimal meteorological framework conditions during many of the measurement procedures and the temporal delay at mobile measurements prevented a comprehensive illustration of these current systems.
  On the other hand, the model results generated by FITNAH provide a comprehensive picture of the spatial character of these local, primarily thermally induced current systems. In addition to the item-related statements of the measurements, the model results permit statements concerning the range (i.e., penetration depth) of the flows into the adjacent built-up areas. The area between the Lützowstraße and Kurfürstenstraße test points on the western edge of the Gleisdreieck area can be taken as an example. The cold air formed locally here penetrates approx. 500 m into the built-up area.
  The flow speeds measured or modeled achieve very similar orders of magnitude. As a rule, these thermally induced current systems are accompanied by wind speeds of from 0.1 to 0.5 m/s. The measurement projects indicate that these values were achieved both in the summer and the winter semesters (cf. Vogt 2002a, pp. 19 and 22). There is also a cold air flow at the Möckernstraße, whereas the penetration range amounts up to 150 m (cf. Fig. 8).
• There is a cold-air outflow from the area of Victoria Park in Kreuzberg.
  The measurement results on local cold-air outflow from the area of Viktoria Park in Kreuzberg confirm these FITNAH simulation calculations (cf. Vogt 2002a, p. 17). The measurements confirmed the canalization of the cold-air outflow through the Kreuzbergstraße and Großbeerenstraße. This flow was accompanied by low wind speeds of 0.7 to 0.2 m/s.

Fig. 8: Verification of the results of the climate model FITNAH 3D, application 2009 and 2015 (always right part of the figure) with the help of stationary and mobile measurements in the area of triangular junction (figure on the left, from Vogt 2002a and Vogt 2002b). In the figure on the left, the lines originating from the measurement points point to the direction, from which the wind comes; in the figure on the right, the wind arrows show the direction of flow

Result

Overall, there is good agreement between the results of the measurement procedure and the modeling of the local current field in the Gleisdreieck area using FITNAH.

The self-generated, local current phenomena postulated by the model calculations can largely be confirmed by the measurements. Data on flow direction and speed are within the same ranges. The relevant air exchange processes – small-scale, orographically-determined cold-air outflows from the Viktoriapark in Kreuzberg and thermally induced compensation currents between the open spaces of the Gleisdreieck and the adjacent built-up neighborhoods – have been ascertained and represented equally, both qualitatively and quantitatively (cf. Vogt 2002a, pp. 26 et seqq.). However, it was not possible, by either of the two methodical approaches, to support any regionally-characterized compensation current between central Berlin and the surrounding countryside.