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Technical description of the Wind disturbance module

# wind module

## wind module

The wind module is a sub module of iLand. See disturbance modules for a list of available modules.
Scientific background of the spatially explicit wind disturbance simulation is available on the wind disturbance page.

## wind module input data

Users of the iLand wind module need to provide several types of input data. These types are:

• static parameters: either global parameters or variables per tree species. Examples are species specific turning moments or crown shape parameters. While global parameters are directly provided in the project file, species specific parameters are provided in an own parameter table in the input database of iLand.
• parameters in space: this is currently the topoModifier which scales the global wind pattern to a local value. Values per resource unit can be assigned using the mechanism to set up the simulation extent.
• parameters in time: basically a time series of wind events (with accompanying properties like wind direction and storm duration). This time series is provided with a time events file.

## species parameters

Species specific parameters for the wind module are stored in a table specific to the module. The database table needs to be stored in the same database as the rest of the species parameter (see project file for more details).
The following table provides an overview:

 name datatype description shortName string short name of the species. This key must be identical to the keys used for general species parameter. CReg numeric Nm/kg, critical turning coefficient derived from tree pulling experiments crownAreaFactor numeric empirical factor for the crown shape (fraction of area of the projected crown shape compared to an rectangle) (0..1) crownLength numeric crown length of the tree given as fraction of tree height (0..1). See also competition for light for details on iLand crown shapes. MOR numeric modules of rupture (MPa) wetBiomassFactor numeric conversion factor between dry and wet biomass (wet = dry*factor).

The SQL statements to create the parameter table and to populate with default values:

-- SQL statement
CREATE TABLE wind (shortName integer, CReg real, crownAreaFactor real, crownLength real, MOR real, wetBiomassFactor real);
insert into wind select shortName, 111,0.65,0.6,30.6,1.85 from species;

## project file parameters

the general behavior of the wind module is configured by settings in the XML-project file in the section modules.wind. Especially settings related to characteristics of the storm (speed, direction, date) may be set using the time event mechanism.
The following settings are available:

 key datatype description general enabled boolean switches the module on (true), or off (false). speciesParameter string table name of the species parameter table for the wind module (see #species_parameters). triggeredByTimeEvent boolean if true, then wind events are triggered by time events (i.e. defined in an external input file). LRITransferFunction string expression to compute the Hegyi concurrency index from the trees' LRI. Default: max(min(3.733-6.467*LRI,3.41),0.5) LINK MISSING soilFreezeMode string switch to define how to determine the state of the soil. Possible values are the strings yes: always frozen, no: never frozen, auto: the state is determined by calculating the soil temperature; if the temperature in 10cm depth is below 0°, the soil is considered as frozen. edgeDetectionThreshold double threshold for the edge detection algorithm. An edge is defined as a height difference greater than edgeDetectionThreshold between neighboring pixels in the height grid. storm characteristics speed numeric hourly mean above-canopy wind speed (10m above canopy, m/s). This general wind speed is modified by topography and may be modified during a event. duration numeric duration of the wind event in minutes. Internally, duration is transformed to a number of iterations (see also durationPerIteration). direction numeric main cardinal direction of the storm in degrees (0°: north, 90°: east, 180°: south, 270°: west). The direction of the wind may be modified for individual pixels (see directionVariation) dayOfYear numeric day of the year (0..365) of the storm event (used to calculate soil temperature). modifiers topoModifier numeric multiplier accounting for differences in local wind speed. The modifier is either resource unit specific (see simulation extent) or defined by 10m pixel (see setting topoGridFile). The actual wind speed for a pixel is calculated by multiplying the modifier with the speed. topoGridFile string specifies a ASCII grid file containing the topoModifier. Modifier values are sampled for 10m pixels. File paths are relative to the project root folder. topexModifierType string specifies the calculation mode for values for the topoModifier. Possible values are "additive" and "multiplicative", which is the default. If "additive", the 10m-wind speed is calculated as (speed + topoModifier) * gustModifier. gustModifier numeric defines the variation of the global wind speed between iterations of the model. The gustModifier is given as a fraction (e.g. 0.05). The base wind speed is modified for each iteration with a value from the range +/- gustModifier (e.g.: multiplied by 0.95 - 1.05). durationPerIteration numeric duration (minutes) of one iteration of the model, i.e. how long it takes to advance the frontline of the storm to the next (10m) pixel. The factor is used to scale the duration to a number of iterations used internally by the model. directionVariation numeric variation in wind direction (°) for fetch calculation; current wind direction = direction + rnd(-x, +x) factorEdge numeric (constant) ratio between the maximum turning moments at the stand edge and conditions well inside the forest (see Byrne 2011) (default: 5) other settings onAfterWind Javascript Javascript function that is called after every wind event (e.g. for postprocessing). For example, you can provide myTestFunction() in XML, and defined a myTestFunction within the iLand Javascript engine.

## GUI visualization and grid description

The wind module operates for most calculations on a 10x10m grid. iLand provides a variety of input and output variables on that level. These grids can either be viewed in the GUI or exported via Javascript to ASCII grids.
Available grids are:

 name description height height of the tallest tree (m) on a pixel. edge values <>0 are "edges". The numeric value is the current "fetch" from this pixel, i.e. the distance in meters in the current wind direction to the next sheltering pixel. cwsUproot critical wind speed for uprooting (m/s) calculated on the pixel. If pixels are cleared (i.e. trees died) on a pixel, the value is kept. cwsBreak critical wind speed for stem breakages nKilled the number of trees killed by the wind on a pixel (tree>4m height). basalAreaKilled the sum of basal area of all trees killed on the pixel. iteration a number indicating the iteration (1..n) when a pixel is "killed" windSpeedCrown current wind speed in the crown (calculated) topo topoModifier for wind speed calculations. (if the variable is provided on the resource unit level, it is shown here scaled to the 10m pixel resolution) isFrozen 1 if soil is frozen on pixel, 0 otherwise. Note: resource unit level variable is scaled to 10m pixel resolution. Resource units that are not affected by the storm are "not frozen" by default. nEvents cumulative event counter, counting whenever trees on a particular pixel are killed (for the whole simulation).

## Javascript

The wind module provides also a Javascript binding. Whenever the module is active, a Javascript object Wind is available in global scope.

### Functions

Wind.windEvent(windspeed, winddirection, max_iteration, simulate, current_iteration)
trigger a wind event from javascript.
windspeed: average wind speed (m/s)
winddireciton: wind direction (0°=N..180°=S..270=W°)
max_iteration: maximum number of iterations
simulate: if true, trees are not really affected (default: false)
current_iteration: if given a value >=0, then only one iteration is calculated ("interactive mode") (default: -1)

// start a single wind event
Wind.windEvent(40 /*m/s*/, 90 /*east*/, 10 /* iterations */);

// code for a interactive "movie"
for (var i=0;i<10;i++) {
Wind.windEvent(40,  132.4, 10,
false, // do not simulate
i); // iteration
Globals.wait(500); // wait half a second
Globals.repaint(); // repaint the GUI
}

Wind.gridToFile(gridtype, filename)
This function saves a wind specific grid (specified by gridtype) in ESRI raster textfile format to filename. For possible values for gridtype see #GUI_visualization_and_grid_description.

// save the form of the fire in ESRI raster format
Wind.gridToFile('edge', 'temp/fetch.txt'); // default storage location: project root folder

Grid Wind.grid(gridtype)
This function returns a wind specific grid (specified by gridtype). For possible values for gridtype see #GUI_visualization_and_grid_description.
Returns a Grid.

var g =  Wind.gridToFile('nKilled');
g.name = "k"; // set the internal name of the variable
g.apply("if(k>0, 1, 0)"); // create a binary mask of damage pixels
// further processing, e.g. g.save()

Wind.initialize()
The function resets and reloads the wind module. Very helpful for repeated simulations as this allows to change model settings.

function manyEvents() {
Globals.set("modules.wind.topoGridFile", "gis/topo_grid_1.txt");
Wind.initialize(); // reload the wind module
Wind.windEvent(20,235,100);
// output processing omitted
Globals.set("modules.wind.topoGridFile", "gis/topo_grid_2.txt");
Wind.initialize();
Wind.windEvent(20,235,100);
}

## Wind disturbance module output

Table Name: wind
Wind related output generated by event (typically per year). The outputs are created after each year (or spread event) and contain information about bark beetle generations, spread and damage for the total landscape.
For spatially explicit outputs, see also the script functions for extracting gridded data.

 caption datatype description year integer simulation year iterations integer Number of iterations of the storm event. Translate to duration by multiplying with 'durationPerIteration'. windspeed_ms double Wind speed of the event in m/s (note that windspeed varies with topography and between interations). direction double Main wind direction of the event in degrees (0°: north, 90°: east, 180°: south, 270°: west). area_ha double Total area affected (sum of area of pixels with affected trees) killedTrees double total number of trees that were killed in the event. killedBasalArea double Total Basal Area (m2) of killed during the event. killedVolume double Sum of tree volume (m3) killed during the event.

citation

Seidl, R., Rammer, W., Blennow, K. 2014. Simulating wind disturbance impacts on forest landscapes: Tree-level heterogeneity matters. Environ. Model. Software 51, 1-11.

Created by werner. Last Modification: Thursday 09 of July, 2020 16:15:16 CEST by werner.