Local Weather Package - v1.0
+Local Weather Package - v1.18
1. Introduction
@@ -15,18 +15,19 @@ The aim of a local weather system is to simulate weather phenomena tied to speci This is in contrast to the current (v.2.0.0) global weather system of Flightgear where weather changes affect the weather everywhere in the simulated world and are (with few exceptions) not tied to specific locations. In such a system, it is impossible to observe e.g. the approach of a rainfront while flying in sunshine.-The local weather package aims to provide the functionality to simulate such local phenomena. In version 1.0, the package supplies various cloud placement algorithms, as well as local control over most major weather parameters (wind, visibility, pressure, temperature, rain, snow, thermal lift, turbulence...) through interpolation routines and effect volumes. The dynamics of the different systems is tied together - for instance clouds and weather effects drift in the specified wind field. The package also contains a fairly detailed algorithm to generate convective clouds and thermals with a realistic distribution over the various terrain types. There is a simulation of the interaction of the convective cloud system with the terrain as a function of time. Clouds drifting in the wind flow over obstacles, i.e. they change their altitude dynamically. Convection is implemented with a life cycle model of Cumulus clouds - they are generated, evolve for a given lifetime dependent on the underlying terrain and decay at the end of their life cycle. Thermals associated with the clouds follow the same pattern. In particular, in the presence of wind favourable spots for convection generate 'alleys' of dense cloud cover downwind, or thermals and clouds generated over land decay rapidly once they reach open water.
+The local weather package aims to provide the functionality to simulate such local phenomena. In version 1.18, the package supplies various cloud placement algorithms, as well as local control over most major weather parameters (wind, visibility, pressure, temperature, rain, snow, thermal lift, turbulence...) through interpolation routines and effect volumes. The dynamics of the different systems is tied together - for instance clouds and weather effects drift in the specified wind field. The package also contains a fairly detailed algorithm to generate convective clouds and thermals with a realistic distribution over the various terrain types. There is a simulation of the interaction of the convective cloud system with the terrain as a function of time. Clouds drifting in the wind flow over obstacles, i.e. they change their altitude dynamically. Convection is implemented with a life cycle model of Cumulus clouds - they are generated, evolve for a given lifetime dependent on the underlying terrain and decay at the end of their life cycle. Thermals associated with the clouds follow the same pattern. In particular, in the presence of wind favourable spots for convection generate 'alleys' of dense cloud cover downwind, or thermals and clouds generated over land decay rapidly once they reach open water.
For long-range flights, the system provides an offline weather system with plausible transitions between different large-scale weather patterns like fronts and low and high pressure areas, as well as the optional use of live METAR data.
2. Installation
-The package needs to be unpacked in the Flightgear root directory. It writes content into the Nasal/, gui/, gui/dialogs/, Shaders, Effects/, Docs/, and Models/Weather/ subdirectories. The installation does not overwrite any of the default Flightgear files, but to be accessible from the menu for Flightgear 2.0.0, one must copy gui/menubar.xml.alt to the default menubar.xml or copy the three lines of the environemnt menu calling local_weather, local_weather_tiles and local_weather_config into the default file. More recent versions of Flightgear already provide the necessary menu items.+The package needs to be unpacked in the Flightgear root directory. It writes content into the Nasal/local_weather/, gui/, gui/dialogs/, Shaders, Effects/, Docs/, Environment/ and Models/Weather/ subdirectories. The package requires run-time loadable Nasal submodule functionality and is not compatible with 2.0.0.
-This adds the items Local Weather, Local Weather Tiles and Local Weather Config to the Environment menu when Flightgear is up. Most of the basic functionality is contained in local_weather.nas which is loaded at startup and identifies itself with a message.
+This adds the items Local Weather and Local Weather Settings to the Environment and Local Weather (test) to the Debug menu when Flightgear is up. Most of the basic functionality is contained in local_weather.nas which is loaded at startup and identifies itself with a message. A compatibility layer (compat_layer.nas) tests for hard-coded support and ensures that, dependent on the version, hard-coded or Nasal-coded fallback functions are used.
+ +The Local Weather Nasal modules need to be loaded at runtime using the checkbox in Environment/Local Weather Settings, but once this is specified, the setting is remembered and the package will be automatically loaded upon startup. Unless asked to do so from the menu, Local Weather does not run any process in the background. Upon loading, the package does not set any properties already existing, but only generates properties necessary for the menu entries in its own subdirectory /local-weather/ in the tree. The package also does a features check on startup if particular functions are available in hard-coded form. If the features are not present, the package will largely still function properly using slower Nasal fallback code.
-Unless asked to do so from the menu, local weather does not run any process in the background. Upon loading, the package does not set any properties already existing, but only generates properties necessary for the menu entries in its own subdirectory /local-weather/ in the tree. The package also does a features check on startup if particular functions are available in hard-coded form. If the features are not present, the package will largely still function properly using slower Nasal fallback code.
3. Functionality
@@ -35,10 +36,10 @@ The general rule is that the gui is not hardened against problematic user input, Cloud placement calls may sometimes take a significant time to execute especially for large numbers of clouds tied in a complicated way to the terrain. Placing 500 barrier clouds against a small barrier may take a minute to compute. During this time, a reduced framerate is to be expected-The first menu Local Weather contains the low level cloud placement functions. Its purpose is mainly for developing cloud patterns without having to resort to re-type the underlying Nasal code every time. Currently five options are supported: Place a single cloud, Place a cloud streak, Start the convective system, Create barrier clouds , Place a cloud layer and Make a cloudbox.
+The menu item Debug/Local Weather (Test) contains the low level cloud placement functions. Its purpose is mainly for developing cloud patterns without having to resort to re-type the underlying Nasal code every time. Currently five options are supported: Place a single cloud, Place a cloud streak, Start the convective system, Create barrier clouds , Place a cloud layer and Make a cloudbox.
+
Single cloud placement
@@ -58,7 +59,7 @@ The pattern can then be randomized in x, y and altitude. Basically, specifying n The convective system places Cumulus clouds centered on the current position based on the underlying terrain. Currently it models daily variation of convective strength and the latitude variation based on a simple sinusoidal model (i.e. it produces different results when called in the morning than at noon), but it does not take into account seasonal variation (i.e. it assumes the date to be the equinox). The actual placement is chosen based on the type of the underlying terrain, with Cumulus development more likely over city areas than on water. Details of the algorithm are described in the appendix. The parameters for this need some fine-tuning and are currently rather rough, but they lead for example to pronounced differences between land and sea in coastal regions. The following picture shows the result of a call of the system in the afternoon over TNCM.
+
Unless 'Terrain presampling' is active, clouds are placed in a constant altitude alt in a tile with given size where the size measures the distance to the tile border, i.e. a size parameter of 15 km corresponds to a 30x30 km region. When 'Terrain presampling' is selected, the distribution of clouds in altitude is determined by a more complicated algorithm described in the appendix. Clouds are placed with constant density for given terrain type, so be careful with large area placements! strength is an overall multiplicative factor to fine-tune the amount of cloud generation. @@ -68,7 +69,7 @@ Unless 'Terrain presampling' is active, clouds are placed in a constant altitude The barrier cloud system places a cloud at a random point within the region centered around the current position given by size with some probability if there is a terrain barrier downwind with the elevation alt within a distance dist or less. Cloud placement probability is larger for small distances to the barrier. The system tries to place number clouds and assumes that the wind comes from direction wind. If clouds cannot be placed (because there is no barrier within the specified altitude) the algorithm exits with a warning. The picture illustrates the result for the mountains above Las Vegas.
+
Currently, the algorithm does not check for a barrier upstream - this may change in future versions. The ufo is a good way to explore the results of the algorithm by simply flying to a suitable location and calling it there for a relatively small region. Due to its large performance use, the barrier cloud system is currently not part of the large-scale weather generating system. @@ -82,7 +83,7 @@ If rainflag is set to 1, the system will also place external precipitatio The picture illustrates the result of a layer generation call for Nimbostratus clouds with precipitation models.
+
Cloudbox
@@ -92,10 +93,10 @@ The cloudbox placement is an experimental routine allowing to define a cloud corTile placement
-The second menu is used to place complete weather patterns based on low-level calls. It is intended for the user to automatically create the various weather development during flight. Unless stated otherwise, all parameters in this menu are parsed at startup time of local weather only (i.e. when the user selects the OK button, but not while the system runs.+The second menu Environment/Local Weather is used to place complete weather patterns based on low-level calls. It is intended for the user to automatically create the various weather development during flight. Unless stated otherwise, all parameters in this menu are parsed at startup time of Local Weather only (i.e. when the user selects the OK button, but not while the system runs.
+
Weather is created in a series of 40x40 km squares, called tiles. Tiles are classified by airmass, such that the sequence of tiles can describe for example the transition from a high pressure area to a low pressure area. The dropdown menu is used to select the type of weather tile to build initially and to determine the rules according to which subsequent tiles are generated.
@@ -128,13 +129,13 @@ The slider Thermal properties is mainly relevant for soaring scenarios. I The difference is apparent from the following pictures: Smooth and well-formed clouds characteristic of a calm day:
+
Rough clouds characteristic of windshear and more turbulent conditions:
+
As for the buttons, OK starts the local weather system with the selected options (note that almost all options in this menu are startup-time options, they are read once and changing them without restarting the system will not affect the behaviour of the system). Clear/End clears all clouds and ends all local weather functionality - the button brings the system back into the state before it was started. No loops or other subroutines are executed after the button is pressed. Close closes the dialog without starting the system.
@@ -142,7 +143,7 @@ As for the buttons, OK starts the local weather system with the selected The button Show winds brings up the detailed wind menu which is needed for the wind models aloft interpolated and aloft waypoints when not in METAR mode:
+
For aloft interpolated, the menu is used by inserting wind direction and speed for all given altitudes. After OK, the specified values are used. For aloft waypoints, the same info must be supplied for a series of waypoints. First, the latitude and longitude has to be inserted, afterwards the aloft winds for that point below. The button Set waypoint commits the windfield as specified in the menu for this position into memory. For orientation, the number of points inserted is counted on the lower right. Clear Waypoints removes all information entered so far. Note that OK does not commit the data for a waypoint. Entering a windfield in this way by hand is rather cumbersome, but may be useful occasionally - the main purpose of the wind model however is to work with live weather data.
@@ -153,51 +154,59 @@ For aloft interpolated, the menu is used by inserting wind direction and The following pictures show possible results of tile setups 'High-pressure-border' and 'Low-pressure':
+
+
Performance settings
-The performance setting menu is available from the main menubar. It controls the allocation of system resources to the various tasks.+The performance setting menu is available from the main menubar. It controls the allocation of system resources to the various tasks, as well as the behaviour of the offline weather system.
+ +The upper checkbox determines if the Nasal modules corresponding to Local Weather are loaded. Unless this box is checked, no Local Weather code is available and the corresponding other menu items are not functional.
+
The first part controls the creation of new weather tiles, the second part controls the ranges up to which clouds inside a generated weather tile are shown as part of the scenery (clouds created but not shown are stored and processed in a buffer array). Clouds are visible if and only if they are 1) part of a created tile and 2) closer than the buffering range. This is illustrated in the following figure: Tiles are only created if their center is within range, clouds are only visible if the cloud model itself is within range, the resulting region dependent on the two ranges in which clouds are displayed is shown in red.
+
From this, it is apparent that the two ranges should not be drastically different. Setting tile creation range to 55 km implies lots of work trying to asses the properties of faraway terrain to generate suitable cloud distributions, setting the buffering range to 15 km means that most of these clouds are never visible. Note that in low visibility, a large tile creation range can be problematic, as Flightgear may not have loaded the terrain. Note also that setting the tile creation range above 40 km means a slow startup of the system, as it needs to create 5 tiles on startup instead of just a single one.
There are additional options to do asymmetric buffering, i.e. to 'cut' out a wedge in the rear of the aircraft in which the tile creation radius or the cloud visibility radius are lower. For example, setting the buffering ratio to 0.2 and the angle to 180 degrees corresponds to a very agressive buffering in which clouds in the rear hemisphere of the aircraft are almost absent and shown only to 20% of the set distance. This may increase performance by 20-30%, but only in straight flight - in fast turns, lots of clouds have to be shuffled from the buffer into the scenery and back, which is actually slower than never trying to cut a wedge at all. Also, agressive asymmetric buffering is not nice in external views as the reference is always the aircraft course.
-The last option controls the amout of resources allocated to making clouds drift in the wind if dynamical weather is on. In order to avoid freezes, the weather dynamics system processes only a fixed number of clouds per frame inside the field of view. That means that faraway clouds do not necessarily move even with dynamical weather on. The slider indirectly controls the distance out to which clouds are being processed.
+The buffering control can also optionally be tied to a target framerate. In this case, the visible range set with the slider is ignored and a control loop tries to adjust the visible range instead such that a 5-second average of the framerate is within 10% of a specified target framerate. Currently, the subsystem only adjusts the visible range of buffering - asymmetric buffering and/or tile loading range still needs to be specified manually. Also, the system cannot respond to unsolvable problems - if the framerate in the absence of clouds falls below 30, then no setting for the cloud range will bring it to 35.
+ +The next option controls the amout of resources allocated to making clouds drift in the wind if dynamical weather is on. In order to avoid freezes, the weather dynamics system processes only a fixed number of clouds per frame inside the field of view. That means that faraway clouds do not necessarily move even with dynamical weather on. The slider indirectly controls the distance out to which clouds are being processed.
All performance setting menu-options work at runtime, but are processed over time rather than instantaneously, i.e. it takes 10-20 seconds till the new balance between buffered clouds and active clouds is reached after the slider is moved.
+The final options, weather pattern scales, deal with the long-range behaviour of the offline weather system. They are not parsed when the tile selection mode is set to METAR. The first slider controls the transition between different airmasses. In the default setting, the typical distance to encounter a different airmass when one flies in a 'High-pressure-core' tile is 200 km. The airmass slider allows to vary this distance between 200 and 800 km.
+ +Inside each tile, there are typically 4-8 different basic cloud patterns (distribution of layers) modelled, with a random pattern selected for each new tile of the given airmass classification. The cloud patterns slider adjusts the probability that the same cloud configuration is kept as long as a tile of the same airmass type is generated. Selecting a large scale in essence means that the theme of clouds will remain similar within each airmass, a small scale allows for more variation.
+
4. Cloud models
-The package contains a number of different cloud models, both static ones for Cirrus and Cirrocumulus clouds as well as rotated ones for Altocumulus, Cirrostratus, Cumulus, Cumulonimbus, Stratus and Nimbostratus cloudlet models. Neither the cloud textures, nor the models nor the transformations are perfected, and any aspect can be improved, albeit at the cost of performance consumption.+The package contains a number of different cloud models, both static ones for Cirrus and Cirrocumulus clouds as well as rotated ones for Altocumulus, Cirrostratus, Cumulus, Cumulonimbus, Stratus and Nimbostratus cloudlet models. Thin high-altitude haze such as characteristic for Cirrostratus or Altostratus clouds is approximated by colouring the skydome as a function of altitude. Neither the cloud textures, nor the models nor the transformations are perfected, and any aspect can be improved, albeit at the cost of performance consumption.
Static clouds project textures onto curved sheets into the sky. The advantage of the technique is that cloud layers consisting of thousands of cloudlets with different sizes can be modelled. However, the sheets do not look equally well from all perspectives and somewhat unrealistic from close up.
+
Rotated cloud models have the advantage that they look much better from close up and hardly unrealistic from any perspective, but the size distribution of cloudlets is somewhat restricted and they use a lot more performance than static clouds.
+
These are rendered by different techniques. While the default Cumulus models consist of multiple layers rotated around the center of the model, the detailed Cumulus clouds consist of multiple (up to 24) individual cloudlets, rotating each around its own center, randomly distributed into a box with different texture types used for the cloud bottom. This not only improves the visual appearance, but also leads to a more realistic distribution of cloud sizes and shapes in the sky. In addition, when circling below the cloud (as done when soaring) the effect of the cloudlet rotation is less pronounced. The price to pay is that rendering detailed clouds costs more performance, so they may not be suitable for all systems.
@@ -205,7 +214,7 @@ These are rendered by different techniques. While the default Cumulus models co More complex clouds are rendered in sandwitched layers of several different textures. An example are Cumulonimbus towers, which use diffuse textures on the bottom, changing to more structured textures in the upper part of the cloud. With up to 2000 cloudlets, skies with multiple thunderstorms may not render with sufficient framerates on every system.
+
The general problem is finding a good balance between spending a lot of CPU time to make a single cloud model appear perfect, and the performance degradation that occurs if hundreds of clouds are placed in the sky. The basic aim is to provide realistic appearance for clouds from a standard view position (in cockpit looking forward), to retain acceptable appearance from other positions and to allow large cloud layers.
@@ -228,6 +237,14 @@ While the station concept is designed to support easy connection with weather up Technically, the structure of the interpolation system means that while it is running, neither setting weather parameters in the GUI menu nor changing visibility using the z-key will have an effect - any setting made there will be overwritten by the interpolation loop periodically, and local weather needs to be stopped before such changes have an effect.
+In addition to true weather parameters, Local Weather 1.18 also has a model for the light propagation in the atmosphere which is handled by the same interpolation routines. This model determines the amount of light reaching the current altitude and the amount of thin haze above the current position. The first property affects to what color distant objects fade - when there is lots of light available, faraway objects appear white, however in the presence of cloud layers casting shadows, distant objects fade into dark shapes. The following screenshot illustrates the effect: + +
++ +The second property is used to simulate diffuse structureless Cirrostratus and Altostratus clouds. Details of the modelling are given in the appendix. +
Weather parameters in effect volumes
Effect volumes are 3-dim regions in space in which the weather is not set by the interpolation system but by a specific prescription. This can be to either set a parameter to a constant value inside the volume, or to specify it as a function inside the volume. Effect volume settings always override information from the interpolation subsystem.@@ -235,7 +252,7 @@ Effect volumes are 3-dim regions in space in which the weather is not set by the Effect volumes may be nested, and they may overlap. The rules determining their behaviour can be summarized as follows: 1) there is no cross-talk between weather parameters, i.e. an effect volume specifying visibility is never influenced by one specifying pressure, regardless if they overlap or not. 2) when an effect volume is entered, its settings overwrite all previous settings 3) when an effect volume is left, the settings are restored to what the interpolation routine specifies if no other effect volumes influence the weather parameter, to the values as they were on entering the volume when the number of active volumes has not changed between entering and leaving the volume (i.e. when volumes are nested) or nothing happens if the number of active volumes has increased (i.e. when volumes form a chain). This is illustrated in the following figure:
+
Volumes 2 and 3 are nested inside volume 1, therefore all settings of 2 overwrite those of 1 when 2 is entered and are restored to 1 when region 2 is left directly into 1. Region 4 is a chain, and as such ill defined: When one leaves 2 into 4, the settings of volume 3 are used (because later definitions overwrite earlier ones). When one now leaves 4 into 3 (and hence leaves 2), it would be wrong to restore to the values on entry of 2 (i.e. the properties of 1), as 3 is still active. But the active volume count has changed, so leaving 4 into 3 does not lead to any changes. The reason why 4 is still ill-defined is that what weather is encountered in 4 depends on the direction from which it is entered - when it is entered from 2, it has the properties of 3, whereas when it is entered from 3, it has the properties of 2.
@@ -246,7 +263,7 @@ Effect volumes are always specified between a minimum and a maximum altitude, an where geometry is a flag (1: circular, 2: elliptical and 3: rectangular), lat and lon are the latitude and longitude, r1 and r2 are the two size parameters for the elliptic or rectangular shape (for the circular shape, only the first is used), phi is the rotation angle of the shape (not used for circular shape), alt_low and alt_high are the altitude boundaries, vis, rain, snow, turb and lift are weather parameters which are either set to the value they should assume, or to -1 if they are not to be used, or to -2 if a function instead of a parameter is to be used and -3 if a function for wave lift is used. Since thermal lift can be set to negative values in a sink, a separate flag is provided in this case. sat finally determines the light saturation, a parameter between 0 (dark) and 1 (normal light) - it can be used to dim the light beneath cloud layers (which is not done automatically as objects don't cast shades in Flightgear, and given that most cloud models are rotated, their shade would look rather odd on any case).
-In version 1.0, thermal lift and wave lift are implemented by function (wave lift is not yet automatically placed, but can be easily from Nasal). There is no easy way to implement any weather parameter by function in an effect volume, as this requires some amount of Nasal coding.
+In version 1.18, thermal lift and wave lift are implemented by function (wave lift is not yet automatically placed, but can be easily from Nasal). There is no easy way to implement any weather parameter by function in an effect volume, as this requires some amount of Nasal coding.
Both thermal lift and saturation require a more recent version of Flightgear than 2.0.0 in order to take effect.
@@ -325,7 +342,13 @@ The first important call sets up the conditions to be interpolated:
set_weather_station(latitude, longitude, visibility-m, temperature-degc, dewpoint-degc, pressure-sea-level-inhg);
-The cloud placement calls should be reasonably familiar, as they closely resemble the structure by which they are accessible from the 'Local Weather' menu.
+The atmosphere light propagation needs to be prepared as well by a call
+ +set_atmosphere_ipoint(latitude, longitude, vis_aloft, vis_alt_aloft, vis_overcast, overcast,overcast_alt_low, overcast_alt_high, scattering, scattering_alt_low, scattering_alt_high);
+ +The meaning of these parameters is as follows: Visibility is linearly interpolated in altitude between several altitudes: the ground value is given in the weather station call. vis_aloft determines the visibility after passing the lowest inversion layer (i.e. usually the lowest cloud layer altitude) where the visibility suddenly increases. The altitude of the transition is given by vis_alt_aloft. The visibility higher up is determined by high-altitude haze, i.e. it takes the value vis_overcast at the position overcast_alt_high (the upper edge of the haze layer) and increases with constant rate from there. The amount of high-altitude haze is given by overcase, a number between 0 and 1, and the position of the layer is determined by overcast_alt_low and overcast_alt_high. Above the second value, the skydome is no longer coloured. Finally, scattering determines the amount of light reaching the surface (between 0 and 1, reasonable values range from 0.5 to 1) and the following altitudes specify where the layers casting shadow are found - there is always the full amount of light available above scattering_alt_high. See the appendix for details of the model.
+ +The cloud placement calls should be reasonably familiar, as they closely resemble the structure by which they are accessible from the 'Local Weather (Test)' menu.
If the cloud layer has an orientation, then all placement coordinates should be rotated accordingly. Similarly, each placement call should include the altitude offset. Take care to nest effect volumes properly where possible, otherwise undesired effects might occur...
@@ -360,9 +383,9 @@ With default settings, the local weather package generates a 40x40 km weather ti
-
+
-
+
@@ -370,7 +393,7 @@ With default settings, the local weather package generates a 40x40 km weather ti
-
+
@@ -379,6 +402,10 @@ With default settings, the local weather package generates a 40x40 km weather ti
+ +
Appendix A: An introduction to the algorithms
@@ -404,37 +431,37 @@ The following series of pictures, taken over KLSV (Nellis AFB, Las Vegas) illust At 7:00 am, the thermal activity is weak, and there is no lift available in thermals yet.
+
Some activity starts around 10:00 am the average available lift is 0.3 m/s, the more active clouds tend to be above city terrain.
+
At 12:00 noon, the maximal cloud number is reached. The average available lift is 1 m/s, in peaks up to 2 m/s.
+
The maximum of lift strength is reached close to 15:00 pm. The average lift is now 1.5 m/s, in peaks up to 3 m/s, and the strong convection leads to beginning overdevelopment, some clouds reach beyond the first inversion layer and tower higher up. At this point, the clouds may also overdevelop into a thunderstorm (which is not modelled explicitly by the convective algorithm as it requires somewhat different modelling, but is taken into account in the weather tiles).
+
At 17:30 pm, the lift is still strong, 1.5 m/s on average and 2.5 m/s in peaks, but compared with the situation at noon, there are fewer clouds with stronger lift.
+
At sunset around 19:00 pm, the number of clouds decreases quickly, but there is still a lot of residual thermal energy (the ground has not cooled down yet), therefore thermal lift of on average 1 m/s is still available even without solar energy input.
+
While not accurate in every respect, the model works fairly well to reproduce the actual time dependence of convective clouds and thermal lift during the day.
@@ -452,7 +479,7 @@ A cloud field is initialized with fractional lifetimes randomly distributed betw The model of the distribution of lift inside a thermal is quite complex.
+
Vertically, is is characterized in addition to height and radius by two parameters, 'coning' and 'shaping', which make it cone-shaped and wasp-waisted. From zero to 200 m above ground, the lift is smoothly fading in, above the cloudbase it is smoothly faded out to zero at 10% above the nominal altitude. Horizontally, there is an outer ring where the air becomes turbulent, followed by a region of sink which in turn is followed by the inner core of lift.
@@ -460,7 +487,7 @@ Vertically, is is characterized in addition to height and radius by two paramete The distribution of lift and sink is time dependent.
+
In a young thermal, lift starts to develop from the ground, sink is initially absent. When the lift reaches the cloudbase, sink starts to develop from the ground and rises up as well. Only in a mature thermal are sink and lift in equilibrium. When the thermal starts to decay, lift initially decays from the ground upward, till it reaches the cloudbase. At this time the cap cloud dissolves. For a time there is a residual distribution of sink decaying from bottom to top till the thermal evolution is over and the thermal (and the associate turbulence field) is removed.
@@ -474,7 +501,7 @@ In nature, what determines the altitude of various clouds are the properties of In the algorithm, various proxies for the structure of air layers and hence the condensation altitude are used. It is assumed that air layers must follow the general slope of the terrain (because there is nowhere else to go), but can (at least to some degree) flow around isolated obstacles. To get the general layout of the terrain, the algorithm first samples the altitude of an 80x80 km square around the 40x40 weather tile to be created. The choice of a larger sampling area reduces the sensitvity of the outcome to purely local terrain features and prevent pronounced transitions from one tile to the next. The result of this sampling is a distribution of probability to find the terrain at a given altitude:
+
For instance, the terrain around Geneva is mostly flat around 1000 ft (where the peak of the distribution lies) with some mountains up to 4500 ft nearby. Based on such distributions, the algorithm next determines the minimum altitude alt_min, the maximum altitude alt_max, the altitude below which 20% of the terrain are found alt_20 and the median altitude below which 50% of the terrain are found alt_med.
@@ -482,25 +509,25 @@ For instance, the terrain around Geneva is mostly flat around 1000 ft (where the Cumulus clouds are always placed at a constant altitude above alt_20. This is done to ensure gorges and canyons do not provide a minimum in otherwise flat terrain so that clouds appear down in the gorge as opposed to on the rim where they would naturally occur. Basically, layers are assumed not to trace too fine structures in the terrain, so at least 20% of the terrain are required. In the test case of Grand Canyon, the algorithm correctly places the clouds at rim altitude rather than down in the canyon:
+
However, convective clouds are given some freedom to adjust to the terrain. The maximally possible upward shift is given by alt_med - alt_20. This is based on the notion that above alt_med, the terrain is not a significant factor any more because the air can simply flow around any obstacle. However, this maximal shift is not always used - if the cloud is placed far above the terrain in the first place, it would not follow the terrain much. Thus, a factor of 1000 ft / altitude above terrain, required to be between 0 and 1, modifies the shift. As a result, a cloud layer placed high above the terrain has no sensitivity to terrain features. The result of this procedure is that clouds show some degree of following terrain elevation, as seen here in Grenoble
+
but they do not follow all terrain features, especially not single isolated peaks as seen here at the example of Mt. Rainier:
+
Finally, layered clouds have essentially no capability to shift with terrain elevation. Moreover, they are caused by large-scale weather processes, hence they do not usually shift upward over even large mountain massives. Currently, the model places them at 0.5 * (alt_min + alt_20) base altitude in order to retain, even in mountains, the sensitivity to the flat terrain surrounding the massiv. usually this works well, but may have a problem with gorges in flat terrain. The following picture shows a Nimbostratus layer close to Grenoble:
+
The offline large scale weather pattern
@@ -510,7 +537,7 @@ The local weather package generates semi-plausible weather changes even in the a Weather tiles are classified chiefly by air pressure. What is currently in the models are three classes for a low pressure system, four different classes for the system of weather fronts and airmasses spiralling into the low pressure system and three classes for a hugh pressure system. The general rule is that low pressure tiles contain layered clouds, overcast skies and rain whereas the high pressure tiles contain clear skies and few convective clouds. The topology assumed for the weather system is apparent in the following diagram:
+
A transition between classes is possible whenever a class has a common border. However, if a transition actually takes place is probabilistic. Typically, the probability not to make a transition is about 80%. Since changes are only triggered for weather tiles one is actually in, the average distance over which weather patterns persist is 160 km. An exception to this are fronts - weather front tiles trigger changes based on direction rather than probability, so a warmfront will always be a sequence of 4 tiles, a coldfront will always be a small-scale phenomenon crossed within 30 km. @@ -529,21 +556,35 @@ Realistically, the boundary layer should also depend on terrain coverage. Due to Gusty winds are assumed to be a bounday layer phenomenon and faded out to zero at a multiple of the boundary layer thickness which is given by base wind speed [kt]/10, i.e. for 25 kt winds the gusts are absent for 2.5 times the bounday layer thickness.
-
The altitude model of visibility
+Light propagation in the atmosphere
Relatively low visibility (as reported by METAR) is usually confined to low altitudes, due to the lower density of the upper atmosphere and the reduced level of dust and water the visibility at airliner cruise altitude is frequently in excess of 100 km. This means that an altitude model of the visibility is needed.-Local weather assumes that haze and dust are confined to the lowest air layer. As a proxy for the transition to a higher layer, the altitude of the lowest cloud layer (or for 9000 ft for clear skies) is used. The visibility always increases with altitude, but below the first layer transition it increases slowly at a rate of 0.2 m / ft in altitude gain. During the layer transition where the transition occurs during 1500 ft, the visibility increases rather rapidly at a rate of 5 m / ft, which means that above the lowest air layer the visibility is almost 10 km more than on the ground. The visibility then continues to grow at a rate of 1 m / ft towards the Stratosphere. In all likelihood, this dramatically underestimates the true visibility at high altitudes, but has been chosen to limit the impact on performance.
+The following schematics illustrates the essential features of the light propagation model:
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++ +The parameters to be set are overcast (the amount of colouring of the skydome, from 0 (no haze) to 1 (completely opaque)), visibility and light (the amount of light available, determining the shade of faraway objects). The assumptions underlying the model are: + +
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- The visibility increases above the lowest inversion layer quite rapidly. It can then be manually set up to a high-altitude haze layer (to account for phenomena such as weather fronts where a low-visibility airmass may be above), but increases with constant rate higher up. +
- The high-altitude visibilities are not realistic, but kept artificially low to avoid framerate problems connected with excessive terrain loading. +
- For the amount of light, a single transition region can be specified. This usually is the lowest inversion layer, but doesn't have to - for multiple layers, a more democratic choice can be made. Above the transition region, the full amount of light is always assumed. +
- Currently, a single high-altitude haze layer is supported, and it's transition altitude can be given. No haze is simulated above it's highest altitude. + +
Credits
-The model of a thermal has been developed by Patrice Poly. The shader code used to transform clouds is heavily based on prior work by Stuart Buchanan. Hard-coding of some features by Torsten Dreyer is greatly appreciated.+The model of a thermal has been developed by Patrice Poly. The shader code used to transform clouds is heavily based on prior work by Stuart Buchanan. Hard-coding of some features by Torsten Dreyer, Thorsten Brehm and Erik Hofman is greatly appreciated.
-Thorsten Renk, March 2011 +Thorsten Renk, June 2011 - - - - + + + +