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International Conference ‘Wind Effects on Trees’ September 16-18, 2003, University of Karlsruhe, Germany WINDSTORM LOTHAR: OROGRAPHIC INFLUENCES ON STORM DAMAGE IN THE BLACK FOREST J. Schmoeckel (1,2), Ch. Kottmeier (1), E. Aldinger (3), D. Seemann (3) (1) Institut für Meteorologie und Klimaforschung, Universität Karlsruhe (TH), (2) Graduiertenkolleg “Naturkatastrophen“ (3) Forstliche Versuchs- und Forschungsanstalt Baden-Württemberg, Freiburg Abstract In the black forest (Schwarzwald) the extraordinarily strong storm 'LOTHAR' on December 26, 1999 caused large damage. An empirical analysis of the storm damage is given with the objective to derive the orographical influence on the wind field from the damage pattern. This was recorded in summer 2000 by an airborne survey with a digital line scanner. Georefer- enced distributions of the vegetation index are calculated at high resolution, 2 2 m² pixel size, from the data. The damaged forest areas appear with a lower vegetation index than areas with intact vegetation. To distinguish between damaged forest areas and populated or differently used areas a land use model is utilised. Mapping of the storm damages and their combination with a digital elevation model and land use data is performed in a Geographic Information System (GIS). It is shown that the damage pattern is significantly affected by orographic factors. Large damage occurred e.g. at the location of saddles between single mountains, on mountain flanks facing to the North and Northwest, and at the windward (west) flanks of extended mountain ridges. Little damage is found in areas that presumably were protected against the wind, i.e. on the leeside (eastern) mountain flanks, in dells and niches as well as in valleys perpendicular to the mean west to southwest winds. The spatially complex distribution of damages not only depends on orographic effects but also on characteristics of the forest and of the soil. For a more fully explanation of the storm damages these characteristics must be considered. The gained knowledge can be profitable for future afforestation in mountain areas to stabilize forests against severe storms. Introduction The extraordinarily strong wind storm LOTHAR on December 26, 1999 caused large damage in France, Switzerland and Germany. In Germany, especially the Black Forest (Schwarz- wald) in the federal state Baden-Württemberg in Southern Germany was concerned. The damage in forests in Baden-Württemberg exceeded the average annual harvest by 300%. Also lots of damages on buildings and infrastructure occurred and accidents happened. The danger potential for buildings, forests, infrastructure and humans by strong storm events among other parameters is influenced by the shape of the terrain. Depending on orographi- cal characteristics (height, slope, orientation of the terrain) and on meteorological conditions (atmospheric layering, wind velocity in the free atmosphere) the airflow passes around or overflows the mountains. Thereby the wind velocity increases on hilltops or in narrow valleys or on saddles. Simulation of these processes with three-dimensional numerical models is possible, but so far there is no good data to evaluate their results. Thus the idea is to use the damage pattern caused by the storm LOTHAR for a comparison and calibration of numerical models.

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Page 1: WINDSTORM LOTHAR: OROGRAPHIC INFLUENCES ON STORM … · WINDSTORM LOTHAR: OROGRAPHIC INFLUENCES ON STORM DAMAGE IN THE BLACK FOREST ... size, from the data. The damaged forest areas

International Conference ‘Wind Effects on Trees’ September 16-18, 2003, University of Karlsruhe, Germany

WINDSTORM LOTHAR: OROGRAPHIC INFLUENCES ON STORM DAMAGE IN THE BLACK FOREST J. Schmoeckel (1,2), Ch. Kottmeier (1), E. Aldinger (3), D. Seemann (3) (1) Institut für Meteorologie und Klimaforschung, Universität Karlsruhe (TH), (2) Graduiertenkolleg “Naturkatastrophen“ (3) Forstliche Versuchs- und Forschungsanstalt Baden-Württemberg, Freiburg Abstract In the black forest (Schwarzwald) the extraordinarily strong storm 'LOTHAR' on December 26, 1999 caused large damage. An empirical analysis of the storm damage is given with the objective to derive the orographical influence on the wind field from the damage pattern. This was recorded in summer 2000 by an airborne survey with a digital line scanner. Georefer-enced distributions of the vegetation index are calculated at high resolution, 2 ⋅ 2 m² pixel size, from the data. The damaged forest areas appear with a lower vegetation index than areas with intact vegetation. To distinguish between damaged forest areas and populated or differently used areas a land use model is utilised. Mapping of the storm damages and their combination with a digital elevation model and land use data is performed in a Geographic Information System (GIS). It is shown that the damage pattern is significantly affected by orographic factors. Large damage occurred e.g. at the location of saddles between single mountains, on mountain flanks facing to the North and Northwest, and at the windward (west) flanks of extended mountain ridges. Little damage is found in areas that presumably were protected against the wind, i.e. on the leeside (eastern) mountain flanks, in dells and niches as well as in valleys perpendicular to the mean west to southwest winds. The spatially complex distribution of damages not only depends on orographic effects but also on characteristics of the forest and of the soil. For a more fully explanation of the storm damages these characteristics must be considered. The gained knowledge can be profitable for future afforestation in mountain areas to stabilize forests against severe storms. Introduction The extraordinarily strong wind storm LOTHAR on December 26, 1999 caused large damage in France, Switzerland and Germany. In Germany, especially the Black Forest (Schwarz-wald) in the federal state Baden-Württemberg in Southern Germany was concerned. The damage in forests in Baden-Württemberg exceeded the average annual harvest by 300%. Also lots of damages on buildings and infrastructure occurred and accidents happened. The danger potential for buildings, forests, infrastructure and humans by strong storm events among other parameters is influenced by the shape of the terrain. Depending on orographi-cal characteristics (height, slope, orientation of the terrain) and on meteorological conditions (atmospheric layering, wind velocity in the free atmosphere) the airflow passes around or overflows the mountains. Thereby the wind velocity increases on hilltops or in narrow valleys or on saddles. Simulation of these processes with three-dimensional numerical models is possible, but so far there is no good data to evaluate their results. Thus the idea is to use the damage pattern caused by the storm LOTHAR for a comparison and calibration of numerical models.

Page 2: WINDSTORM LOTHAR: OROGRAPHIC INFLUENCES ON STORM … · WINDSTORM LOTHAR: OROGRAPHIC INFLUENCES ON STORM DAMAGE IN THE BLACK FOREST ... size, from the data. The damaged forest areas

Area of Interest and Collection of Data About five months after the winter storm LOTHAR the damage pattern was measured by an airborne survey with a high resolution Color Line Scanner (CLS) (see Kottmeier et al. 2002) The survey region, the north-western part of the Black Forest, is shown in figure 1.

Fig. 1: The flight tracks show the ove

With a CCD sensor used in the Cis measured in three different chainfrared (720 – 830 nm) (see alsotion shows a steep slope in the rachlorophyll content of the plants, t

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The normalized difference vegetasities of the reflected radiation in sum:

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Page 3: WINDSTORM LOTHAR: OROGRAPHIC INFLUENCES ON STORM … · WINDSTORM LOTHAR: OROGRAPHIC INFLUENCES ON STORM DAMAGE IN THE BLACK FOREST ... size, from the data. The damaged forest areas

Mapping of Windthrow Areas The aim of the airborne survey is to derive the damage pattern caused by windstorm LOTHAR. For further analysis, the calculated NDVI has to be georeferenced and projected. Therefore, in addition to the spectral measurement, also the absolute position of the airplane (via differential GPS), the altitude, and the angels of attach, pitch, and roll are recorded. Also the position of the CLS in relation to the axes of the airplane are taken into account. After all necessary corrections, projection and georeferencing, an area wide distribution of the NDVI is achieved. For the whole overflown area (4908 km²) the NDVI was calculated with a resolution of 2 ⋅ 2 m² on the ground. Figure 3 shows an example of this analysis. The pseudo colours are chosen in the way that areas with healthy vegetation, like forests or grasslands, appear green. The more vegetation the darker is the colour. Areas with no or little vegetation, like settlements or windthrow ar-eas, appear yellow and brown. (In greyscale, healthy vegetation appears in dark grey and less vegetation appears brighter). Contour lines are added to this image, highlight relations of windthrow areas and orographic characteristics. In Fig. 3 a saddle (517 m) between the two mountains Merkur (678 m) and Kleiner Staufenberg (623 m) is shown. The whole forest on the saddle is damaged, also the trees on the north westerly flank of the smaller mountain. The paths crossing the saddle can be only seen, because there are no more trees left. Also a small damage area on the hilltop of Merkur is obvious and some windthrow areas along the paths at the south westerly flank of the Kleiner Staufenberg. The mean wind directions during the storm LOTHAR were westerly.

Fig. 3: NDVI and contour lines (50 m) for a small scene (~3.0 ⋅ 2.5 km²) (left side) and map of the re-gion (right side)

A trained observer can easily detect windthrow areas in NDVI-distributions as shown in figure 3. But in order to obtain a well defined classification of the whole overflown region it is nec-essary to detect the damaged areas automatically. As a first step is all regions without any forest are masked. This is done by means of land use data (horizontal resolution of 30 m). With this information all regions without any forest before LOTHAR can be excluded from further analysis of windthrow areas. For the remaining area some general criteria are needed as described below:

• Minimum size of the windthrow areas. This was chosen to 104 m² because there is no interest in small scale effects leading to windthrow of single trees.

• Threshold of the NDVI.

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• Variance of the NDVI in windthrow areas. There are some clouds in the data which have very low variance in the NDVI. Also there are some parts of streets or of fields which are smaller than the resolution of the land use data. The variance of their NDVI is also low whereas windthrow areas show high variances in NDVI.

In figure 4 the result of the automatic detection is shown. Areas with no forest in the land use data are masked (black colour), whereas windthrow areas are marked white. In the right part of the picture clearly a cloud can be seen, which is correctly not marked as windthrow area.

Fig. 4: Example of the detection of windthrow areas (4 ⋅ 6 km²). Areas with no forest (information of the land use data) are masked in black, windthrow areas are marked white. In the remaining area the

NDVI is shown.

After detecting all damaged areas larger than 104 m² a mapping of all windthrow areas in the whole overflown region results (see figure 5). Now these detected areas can be correlated with characteristics of the terrain.

Fig. 5: Mapped windthrow areas (blue/dark spots) within a subregion of 12 ⋅ 13 km² of the total overflown region. In the background the height of the terrain is displayed.

In Fig. 5 e.g. most of the damages (blue spots, respectively in grey scale: dark spots) are found at the windward (westerly) parts of the first mountain range of the black forest. But also some damages can be found on slopes within bends of narrow valleys.

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Analysis of Windthrow Areas Related to Terrain- and Soil-Characteristics Attempts to isolate of the orographic effects leading to storm damages additionally need to take soil characteristics into account. Ideally, also the characteristics of the forest itself, like height, age and tree species, should be included. But these data isn’t available in digitally form and area wide. Digital information about soil characteristics is available (a GIS data set) and for a large area from the Forstliche Versuchs- und Forschungsanstalt Baden-Württemberg, FVA. There, a classification of soil characteristics was made, with regard to the storm stability of spruce and beech, the predominant tree species in the black forest. The stability depends on water balance, soil layers, and soil skeleton. The soil is classified any-way, if it is forested or not. Orographic characteristics were not taken into account. Three stability classes resulted: stable, indifferent, unstable. Digital data with soil characteristics cover 1310 km² (little more than one quarter of the whole overflown area). The wooden areas within these classes are about the same size. Around 90% of each class is forested. Around 1% of the forests were damaged by LOTHAR (with each windthrow area having at least the size of 104 m²).

Fig. 6: Left side: distribution of the forest and the total area over the height classes, additionally: wooded part of total area. Right side: Ratio of damaged area to the forest area distributed over height classes.

Looking on the wind damages per forest area in each height class, it can be seen that there are two maxima in damages, one at heights of 250 m to 450 m and the other at heights above 800 m. It has to be considered, that at heights above 950 m less than 1% is forested, so these data is not relevant. At heights around 600 m to 650 m clearly a minimum of dam-age occurs. The height profile of the black forest from west to east offers one possibility for this minimum. The first hill range with exposed areas on the flanks and hilltops rises up to about 500 m. The second hill range has exposed areas up from 800 m, 900 m. Between these two ranges, there aren’t many exposed areas, and very few in heights of about 650 m.

Fig. 7: Example of a typical profile from the black forest from west (Rhine valley) to east.

This coarse explanation needs to be confirmed by more specific studies of the characteristics of the orography like valleys, hilltops, etc. as well as by numerical flow modelling.

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Also a correlation between damages and the slope of the mountains can be seen. Most dam-ages occur at flanks with slopes between 10° and 25° (see figure 8), while the forests on steeper slopes seem to be more stable.

Fig. 8: Left side: distribution of the forest and the total area over the slope classes, additionally: wooded part of total area. Right side: Ratio of damaged area to the forest area distributed over slope classes

The distribution of damages over the stability classes is also of interest. On soil classified as stable and unstable, about 0.8% of the forest is damaged. On soils classified as indifferent, around 1.4% of the forest is damaged. For understanding this, additionally the water content of the soil has to be taken into account, because this was the main criterion for the stability of the trees growing there. The December 1999 was twice as wet compared to the climate mean rainfall in Baden-Württemberg (Witterungsreport 1999). One week before LOTHAR, it was snowing and it rested cold, until two days before LOTHAR it began dewing. Hence, the soil was almost saturated with water, when the storm occurred. Thus, also locations which were classified as indifferent (or even stable), because normally the storage of water in the soil is low, under these circumstances could have been becoming unstable. On the other hand, wet soil often is found in sinks or on the bottom of valleys, which are not as exposed to the wind as flanks or hilltops. Additionally, trees on these sites are adapted to the normally high water content of the soil. Damages from disrooted and broken trees are not differenti-ated in this work. Lots of trees were broken because they could not resist the high wind ve-locities. Hence also forest on soil classified as stable and without an effect of water satura-tion was damaged by this heavy storm. Conclusions and Outlook At the actual state of the research work, mapping of damages caused by the severe winter storm LOTHAR is completed (for windthrow areas larger than 104 m²). We are now able to analyse various correlations between the windthrow areas and orographic characteristics, regarding soil characteristics. First results of analysis have been shown in this paper. Further analysis will be done, for emphasizing the orographic effects which affected and modified the wind field so these large damages could happen. Additionally the wind field will be reconstructed from meteorological station data. The windthrow areas and the recon-structed wind field will be compared in detail. Numerical simulations of the wind flow over mountainous terrain (Adrian et al. 1991, Kalthoff et al. 2003) provide a promising tool to get a consistent analysis of damage patterns and flow modifications by mountain shape.

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Acknowledgements This research project is being performed at the Institut für Meteorologie und Klimaforschung, Universität Karlsruhe (TH) in collaboration with the Forstliche Versuchs- und Forschungsan-stalt (FVA), Freiburg. It is supported by the DFG in the context of the Graduiertenkolleg “Naturkatastrophen”, Universität Karlsruhe (TH). The survey flights were funded by the Re-search Center Karlsruhe. References Adrian, G. and F. Fiedler (1991): Simulation of Unstationary Wind and Temperature Fields over Com-plex Terrain and Comparison with Observations. Beiträge zur Physik der Atmosphäre, Vol. 64, No. 1, pp. 27 - 48 Bochert, A., J. M. Hacker and K. Ohm (2000): Color Line Scanner as imaging NDVI sensor. Second EARSEL Workshop on Imaging Spectroscopy, Enschede, 11-13 July 2000 Hildebrandt, G. (1996): Fernerkundung und Luftbildmessung für Forstwirtschaft, Vegetationskartierung und Landschaftsökologie. Herbert Wichmann Verlag, Hüthig GmbH, Heidelberg Kalthoff, N., I. Bischof-Gauß, and F. Fiedler (2003): Regional effects of large-scale extreme wind events over orographically structured terrain. Theroretical Applied Climatology Vol. , 74, pp. 53 – 67 Kottmeier, C. J. Schmoeckel, C. Schmitt, and A. Bochert (2002): Sturm Lothar: Schadenanalyse und Risikkokartierung aus meteorologischer Sicht. In G. Tetzlaff, T. Trautmann, and K. S. Radtke (ed.), Zweites Forum Katastrophenvorsorge, pp. 421 – 427. Institut für Meteorologie, Universität Leipzig. Lyon, J. G., D. Yuan, R. S. Lunetta and C. D. Elvidge (1998): A Change Detection Experiment Using Vegetation Indices. Photogrammetric Engineering and Remote Sensing, Vol. 64, No. 2, pp. 143 – 150. Witterungsreport (Express) 1999, Deutscher Wetterdienst. ISSN 1436 - 6789