MODELING OF GEORELIEF INFLUENCE ON WIND STREAMING BY THE USE OF GIS TECHNOLOGY

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1 Geographica 37, MODELING OF GEORELIEF INFLUENCE ON WIND STREAMING BY THE USE OF GIS TECHNOLOGY Peter Pavličko 1, Miroslav Vysoudil 2 1 Department of Cartography,GIS and Remote Sensing, Faculty of Natural Sciences, Comenius University of Bratislava, Slovak Republic Head of Department: Ass. Prof. Eva Mičietová 2 Department of Geography, Faculty of Science, Palacky University of Olomouc, Czech Republic Head of Department: Ass. Prof. Miroslav Vysoudil Received June 3, 2002; accepted June 30, 2002 KEY WORDS: wind streaming, windward, terrain, wind acceleration, topoclimate, modelling ABSTRACT The paper deals with the possibilities of computer based spatial modeling of the windward/leeward sites of georelief to the selected climatic efects in GIS enviroment. It also concerns other possibilities of topoclimatic characteristics modeling, e.g. well ventilated locations with regards to the particular wind direction. Digital terrain model (DTM) with required qualitative attributes is the base for these models. In this case study, modules designed for raster data processing and their visualisation were utilized. 1. INTRODUCTION In the paper, the authors deal with the possibilities of computer based spatial modelling of the parts of georelief that are directly liable or shaded with regards to the wind streaming and thus also to the wind acceleration, rainfall totals, and/or transmission of deposition of air pollutants. On the basis of the described methods, it is possible to describe some topoclimatic characteristics by using GIS techniques, such as most exposed windward sites, well ventilated locations with regards to certain wind directions etc. We did not take into consideration the physics aspects of wind streaming (e.g. turbulence, convection). The applied model is not appropriate for description of relationship between the windward sites and atmospheric precipitation due to the physics law (physical aspects of precipitation). 2. NATURE OF THE PROBLEM The wind direction and the wind speed are significantly modified by georelief. The air streaming influences climatic characteristics and an occurrence of climatic events, e.g. fogs and inversions, evaporation, air and soil temperature, precipitation distribution, etc. These influences are very expressive in the case of convex shapes of georelief. For these reasons, it is important to examine the phenomenon of wind field in topoclimatology. Vertical heterogeneous georelief significantly modifies the air streaming. It influences its speed, direction and structure. During the contact between the air streaming and terrain obstacle is formed overflow, convergent and divergent flows. Close to the obstacle are formed eddies and turbulence is intensified. Leeward places brake the air stream, air is accumulated in this place, and eddies are formed. Above terrain obstacles wind speed increases. The influences of the obstacles on wind characteristics are observed from 10 to 20 times distance in comparison with height, often up to 20 times (P. Prošek, F. Rein, 1982). The size of the air stream deformation depends on temperature stratification of the atmosphere. In the case of labile stratification eddies are created even above the moderate slopes. In the case of stabile atmosphere eddies are not created even not on slopes that inclination is more than 15 degrees. Then overflow of terrain obstacles is fluent and it does not interfere with stronger changes of structure of wind field. As mentioned, isolated convex shapes especially modify climatic characteristics. This fact can be seen in the case of wind speed and rainfall totals. For instance, western winds cause the lowest rainfall totals that are observed at all on NE and SE slopes. On the other hand, in the study area, the highest rainfall totals are observed on NW and SW slopes. The role of very significant differences of irradiation of georelief and their impact on local circulation are important for the form of topoclimate. The differences

2 64 Acta Universitatis Palackianae Olomucensis Facultas Rerum Naturalium wind direction In the first phase, a plane perpendicular to the stream direction is established. There is a regular spacing among the vectors of the wind stream coming out of this plane. This spacing is in dimension identical to raster resolution of DTM to achieve required accuracy of the final model. The streamlines from the plane were consecutively screened on georelief. As the result of this step there is a bitmap raster file showing the sites of georelief, which are visible from the established plane by the perpendicular screening. Directly liable sites have value 1, the others value 0 (e.g. Erdas, 1997). In general, for this operation there are in GIS very appropriate modules of type viewshed working on base DTM. In the second phase there is the second partial output raster file of the slope aspect of georelief. The sites with the deviation from the direction of designated stream direction in about ± 90 o we classified as windward. The others are classified as leeward. By the use of raster map algebra there are the component outputs multiplied to the final raster file output, which shows windward sites of georelief liable to the designated wind direction and also where wind speed acceleration is supposed to be. It would be possible to model other parameters of topoclimate in case of adding the others positional aspects of georelief (slope, relative rise of georelief, forms of georelief) in this way, for example, potential conditions of wind speed acceleration by the influence of georelief. To reach intended goal of study it was used GIS software GRASS version 5.0.0pre3 (Geographical Resources Analysis Support System) working on platform of operating system UNIX/LINUX. This product offers many modules appropriate for modeling of natural phenomena, mostly modules designed for raster data proceswindward areas (wind speed acceleration) leeward areas (wind speed reduction) Fig. 1: Simplified scheme of windward/leeward sites of georelief in 2-D of irradiation cause thermic determined anabatic and katabatic streaming, which are observed up to meters above the surface. During the time of negative energy balance the cold air flows downhill along slopes, so katabatic streaming is formed. Slope aspects do not have the influence on these processes (P. Prošek, F. Rein, 1982). For the study of topoclimate is important to know that close to the active surface the wind accelerates regarding to the height. It is determined by its roughness, character and by robustness of temperature stratification of atmosphere. In the study of topoclimate can not be neglected the effects of macrocirculation, it means predominant wind direction. Therefore it is necessary to explicitly shape the windward and leeward localities on topoclimatic maps. These accomplishments with the knowledge about exposition of sites towards slope aspects allow describing topoclimate in the sense of higher values of humidity, precipitation, eventually sites exposed to the massive transport and potential deposition of atmospherics pollutants. 3. METHODOLOGY As it was mentioned above, the method of delineation of the windward/leeward sites of georelief is not based on physical, but on morphometric principles. The morphometric (geometric) approach allows the formulation of the air streaming and georelief relationships, especially those of geometric nature, such as directly liable sites of georelief to the air streaming. The wind direction should be designated as the first. After that, the process of delineation of windward sites of georelief is separated into two phases, where each phase has its own output.

3 Geographica 37, sing and their visualization. In fact, all needed information about software GRASS is located on the server For mentioned system is typical easy accessible source code of original modules (Open source GIS), also it allows forwarding an individual programs solving specific tasks in which is incorporated new scientific knowledge. wind streaming direction Fig. 2: Study area (3-D view) Fig. 3: The model of windward areas to the NW air streaming (dark-gray color)

4 66 Acta Universitatis Palackianae Olomucensis Facultas Rerum Naturalium N Fig. 4: The model of windward areas to the SW air streaming (light-grey color) 4. CASE STUDY ON PART OF THE KYSUCA RIVER BASIN The model was done for area delineated by the Kysuca river basin for that was suitable input data. We have generated digital elevation model (DTM) with raster resolution 30 m (Fig. 2). For better understanding we tested used method only for smaller part of mentioned basin with area probably 5 by 4 kilometers (see border on Fig. 3). In GRASS environment r.los (Line of Sight) was used as main module. The r.los generates a raster map output in which the cells that are visible from a user specified observer location are marked with integer values that represent the vertical angle required to see those cells (viewshed). By this module we have generated the first output. By the use of r.slope.aspect module was slope aspects computed. As already mentioned the sites with the slope aspect ± 90 o to the direction of designated stream direction were classified as windward. It was the second output. The r.mapcalc is module for raster map algebra. It was used to multiple particular outputs to the final raster output. The figure 4 shows final models to the designated air stream direction. The models were visualized in GRASS module nviz. Our first mentioned method was very similar to the methodology published by Mikloš, Matecny, Kozová et. all (1991). The authors assessed aspects of georelief and level of its shading in according to possibility of air pollution, unfortunately only in analogue form. The results were documented and compared with the empirical experiences from polluted area Jelšava-Lubeník (Slovak Republic). The sites with destroyed vegetation in that area was mostly identical with the exposed places in constructed map of pollution. The problem of windward/leeward effects, in our mind, can be solved also on the other hand, when would be changed the orientation of whole valley to the wind direction. It means, that on the base of DTM there are automatically generated subbasins (valleys) when minimally area for the calculation is matched. On the base of minimally value (frequency of aspects due to main directions in subbasin) is designated the orientation of whole basin and thus level of the windward/leeward effect. In this way we can fair to estimate possibilities of ventilation due to the individual wind direction. Figure 6 shows separated subbasins based on DTM suitable for mentioned analyze. For this step it is necessary to generate such a DTM, which fulfils the requirements of hydrological process model. This model should have correctly computed flowline field (see Fig. 7b) from the elevation raster file. In this case it is possible to delineate subbasins precisely. The solved situation is related only to some parts of the subbasin and their close surrounding (black line). Black line depicted whole subbasin. The results on others parts are not fair because for modelling we did not use whole region as can be seen on figure 3. The figure 7a displays map of the lengths of flowlines computed on the base of our DTM. In the process of interpolation there are formed local depression (are highlighted by arrows).

5 Geographica 37, Fig. 5: Raster elevation model with watercourse network Fig. 6: Subbasins generated from DTM base This fact should be sorted out in order to use mentioned DTM for other application such as modeling of katabatic streaming, the use for physical process models etc. In any case this model does not have the influence on delineation of windward areas that we described above. During the calculation and delineation the mentioned mistakes disappear. Local depressions on DTM (black arrows) are caused by inaccurate structured input points (inputs for interpolation), settings of interpolation function, etc. Therefor elevation raster file is needed repeatedly to smooth, it means that vertical accuracy of model is than not required. 5. CONCLUSION In here presented case study we have tried to demonstrate the possibilities of computer modeling of relationship between georelief and air streaming by GIS techniques. Specifically, we referred to modeling of windward/leeward parts of georelief that are directly liable or shaded with regards to the wind streaming. These parts can depict the sites of georelief that are for example, directly liable to transmission of deposition of air pollutants, or there is an assumption for wind speed acceleration. On the other hand, there are of course sites, which are shaded to the mentioned effects. Fig. 7a: Flowlines generated from DTM using module r.flow. Fig.7b: Correctly computed flowline field from DTM

6 68 Acta Universitatis Palackianae Olomucensis Facultas Rerum Naturalium In these models are not included the physical aspects of air streaming. There was used only morphometric (geometric) aspect. The base for these models was utilized DTM and some modules of GIS, which are served for working with raster data and its visualization. In this case study we have indicated that on the base of DTM with correctly modeled flowline field is possible to model also the other topoclimatic characteristics such as well ventilated locations with regards to certain wind directions etc. In physical geography is modeling of these phenomena very rare, but often it is very needed. Hopefully this case study will contribute or at least will strive to start up discussion related to problems. SUMMARY The entire nature of georelief influence very expressive the characteristics of windstream and thus quality of climate. This fact is to be relevant also in scale of topoclimate. For its detail study seems to be important to be able identify windward/leeward sites. Thanks to potential of GIS it offers to do it on the base of computer modelling. To achieve this goal the authors used GIS software GRASS, ver pres 3. The results show that for smaller area, it means in the local scale, intended methods are promising and give hopeful outcomes. SOUHRN MODELOVÁNÍ VLIVU GEORELIÉFU NA PROUDĚNÍ VĚTRU VYUŽITÍM GEOINFORMAČNÍCH TECHNOLOGIÍ Celkový charakter georeliéfu velmi výrazně ovlivňuje mj. charakteristiky proudění a tím ráz klimatu. To platí i na úrovni topoklimatu. Pro jeho detailnější studium se proto jeví jako důležitá schopnost vymezit návětrné/ závětrné polohy. Díky možnostem, které poskytují prostředky GIS, se nabízí řešit tento úkol modelováním. Ke splnění tohoto cíle autoři využili GIS software GRASS, verze pre 3. Výsledky ukázaly, že pro menší území, tj. na úrovni místního měřítka, jsou tyto postupy možné a přinášejí nadějné výsledky. REFERENCES MIČIETOVÁ, E., PAVLIČKO, P., VYSOUDIL, M.: Tvorba topoklimatických map v prostředí geoinformačních technologií. Geoinfo, č. 3/2001, str. Computer Press. a.s., str MIKLÓS, L., MATEČNÝ, I., KOZOVÁ, M.: Interpretation of the Morphometric Relief Indices for Spatial Differentiation of Microclimatic Conditions. Ecology (CSFR), 10/1991, č. 2, pp PROŠEK, P., REIN, F.: Mikroklimatologie a mezní vrstva atmosféry. SPN, Praha 1982, 237 s. ERDAS Field Guide. 4 th Edition. Erdas, Inc., Atlanta, Georgia, GRASS Manuals. Mgr. Peter Pavličko Komensky University of Bratislava Faculty of Natural Sciences Department of Cartography, GIS and Remote Sensing Mlynska dolina Bratislava Slovak Republic pavlicko@fns.uniba.sk Ass. Prof. Dr. Miroslav Vysoudil Palacky University of Olomouc Faculty of Science Department of Geography Svobody Olomouc Czech Republic vysoudil@risc.upol.cz Reviewed: Ass. Prof. Dr. Milan Trizna, Komensky University of Bratislava Faculty of Natural Sciences Department of Physical Geography and Geoecology Mlynska dolina Bratislava Slovak Republic trizna@fns.uniba.sk Acta Univ. Palacki. Olomuc. Fac. Rer. Nat. (2002), Geographica