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Berlin Environmental Atlas

04.12 Future Climatic Change and Thermal Load (Edition 2010)

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Methodology / Supplementary Notes

Ascertainment of Background Load

The background load for the REMO data was ascertained using an area of 3 x 3 grid points in the southwest of Berlin which is largely unaffected by land use considerations (Deutschländer et al., 2009). At each grid point, time series of all physiologically relevant meteorological quanta were available for the desired periods. Evaluation with respect to thermal load days was carried out pixel by pixel for the three time periods, after which the area mean was determined. The percentage of sunny days was determined analogously.

WettReg generates results that are specific to climate stations. In the present case, the data from the Schönefeld and Lindenberg stations were evaluated separately with respect to number of thermal load days and percentage of sunny days. The arithmetical average of the values from both stations thus represents the background load that is characteristic of the Berlin area. The WettReg stations at Müncheberg and Zehdenick, which could theoretically also have been considered as additional bases for the investigation, were not used for the evaluation, since the bias corrections described below could not have been implemented there, due to very fragmentary measurement and observation data.

Unlike REMO, WettReg always provides only one value per day. In order to be able to carry out an adequate evaluation of thermal load, full-day runs and hence hourly values were generated for the calculation of the quanta required for Perceived Temperature with the aid of statistical methods specifically adjusted to the measurements from the Schönefeld and Lindenberg climate stations. The quanta used were temperature maximum, temperature minimum, daily mean air temperature, wind speed, humidity and cloud cover. This is certainly not sufficient data to permit a realistic representation for each and every day. This procedure must also be criticized in that for the calculation of Perceived Temperature a contemporaneous assignment is necessary as a matter of principle, due to the fact that the weather parameters often develop in opposite directions. However, since a thermal load day is not fixed to a single point in time, but is rather defined by three time measurements during a day (see Methodology), and also since an extended time period is used, it is certainly possible to obtain a realistic picture.

The evaluation of the measured data from the Schönefeld climate station and the REMO and WettReg data for the 1971 - 2000 period (see Table 6) indicates that thermal load days are slightly underestimated by REMO, but are overestimated by WettReg. The deviations for sunny day percentages are considerably greater.

Table 6
Tab. 6: Number of thermal load days (WB) and the percentage (strant) of sunny days, as an annual average for the period 1971 - 2000, from the measured data from the Schönefeld climate station (10385), and the corresponding time series of REMO and WettReg

Excel
[Table is also available as Excel-File (MS-Excel is required).]

Application of Statistical Methods/Bias Correction

The deviation of the model value from the expected value according to the measurements is described as model bias. Bias corrections permit the model results to be improved. For this purpose, the respective percentiles for the threshold values for Perceived Temperature, wind speed and cloud cover are determined from the distribution frequency of measurement data from Schönefeld and Lindenberg. Then, in reverse, the distribution frequency of the model data which fall at these values in these percentiles is used to define new threshold value (Deutschländer et al., 2009). The bias is thus reduced considerably for thermal load days for WettReg, and for the sunny day percentages for both models (see Table 7).

Table 7
Tab. 7: Number of thermal load days (WB) and the percentage (strant) of sunny days, as an annual average for the period 1971 - 2000, calculated using the REMO and WettReg time series, with bias correction

Excel
[Table is also available as Excel-File (MS-Excel is required).]

The bias corrections identified for the control period were also applied to the evaluations of the future periods 2021 - 2050 and 2071 - 2100. Table 8 shows the results. By the middle of the century, according to both models, thermal load days will increase by approx. 50 %. At the same time, the percentage of sunny days will also increase by 5 % according to REMO, and by 6 % according to WettReg. By the end of the century, the thermal load situations in the undisturbed surrounding countryside will almost double again, while the percentage of sunny days will not change further significantly.

Table 8
Tab. 8: Number of thermal load days (WB) and the percentage (strant) of sunny days, as an annual average for the projection periods 2021 - 2050 and 2071 - 2100, calculated using the REMO and WettReg time series, with bias correction

Excel
[Table is also available as Excel-File (MS-Excel is required).]

Application of Statistical Methods / The Confidence Method

In order to be able to better assess how well the data from the models correspond to those from the measurements, confidence intervals for the 90 % significance level were calculated based on the thermal load days ascertained during the 1971 - 2000 period (see Fig. 6). There is a 90 % probability that the value for the background load will be found in the area between the two thin cross-lines. The deviations among the three confidence intervals (the results of the measurement and of the models, respectively) are low, which permits the conclusion that the background load from the models reflects the background load observed at the stations fairly well.

Figure 6
Fig. 6: 90 % confidence intervals for the thermal load days during the 1971 - 2000 period (10385: Schönefeld climate station, C20R: control series REMO, C7100W: control series WettReg)

Fig. 7 additionally shows the 90 % confidence intervals of the future projection periods. Those of the 2021 - 2050 projection period overlap those of the control period only to an insignificant degree. This permits the assumption of a slight but significant rise in the number of thermal load days by the middle of this century. For 2071 - 2100, the increase in thermal load days is more considerable.

Figure 7
Fig. 7: 90 % confidence intervals for thermal load days during the 1971 - 2000 period (10385: Schönefeld climate station, C20R: control series REMO, C7100W: control series WettReg), and in the projection periods 2021 - 2050 (A1B2150R: REMO, P2150W: WettReg) and 2071 - 2100 (A1B7100R: REMO, P7100W: WettReg)

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