no way to compare when less than two revisions

Differences

This shows you the differences between two versions of the page.

@@ Line 1: / Line 1: @@
+====== Averaged View Factor (AVF) vs Integrated View Sphere (IVS) ======
+===== Secondary radiation in ENVI-met =====
+Accurate simulation of radiative transfer is a very important aspect in climate modeling. For microclimate models in particular, it is not only important to simulate primary but also secondary radiative fluxes in great detail, i.e. emitted longwave and reflected shortwave radiation. As there are always limitations regarding computational effort and memory, these radiative fluxes are commonly implemented using simplified approaches \\
+The complexity in modeling these radiative fluxes lies in the multiple interactions between the different elements within the view range of the grid analyzed. For instance, radiation reflected by a surface will contribute to the incoming radiation received by other surfaces. These surfaces will again also re-reflect parts of this radiation and distribute in vicinity \\
+ENVI-met offers two approaches to simulate these radiative fluxes: AVF (default mode) and IVS:
+===== Averaged View Factor (AVF)  =====
+To describe the radiative situation and solve the interactions between different elements, ENVI-met can use a generalized visibility concept based on Averaged View Factors (AVF). In the AVF concept, first, a three-dimensional ray tracing analysis is performed for every cell. Starting from a grid cell’s center, rays are being shot for every 10° height and 10° azimuth angle creating a sphere consisting out of 18 x 36, thus 648 individual view facets. While calculating these 648 individual view facets for every grid cell can be very time consuming, it only needs to be performed once for a model simulation as the objects seen by a cell do not change over the course of a simulation. Based on the object type (sky, building, plant, ground surface) seen by the view facet / hit by the ray and the total number of view facets, averaged view factors of sky σ_Sky, vegetation σ_Veg, buildings σ_Bldg and ground surfaces σ_Grnd are stored as single values for each grid cell. By only saving the view factors for the different object types instead of the actual façade elements, ground surfaces, and plant sections seen in the facets, a lot of memory can be saved. \\
+However, since no information is stored about which individual elements are in radiative exchange with the cell, the calculation of secondary radiative fluxes cannot take into account the radiation exchanges between actual objects seen but rather has to resort to an approximation. Instead of individual information about radiation received from particular objects, the secondary radiation is approximated by combining a grid cells’ view factor for buildings, plants, and ground surfaces with averaged values of reflected and emitted longwave radiation for all buildings, plants, and ground surfaces over the entire model domain. \\
+By using AVF a lot of memory is saved as the individual cells "seen" by a cell are not stored. This might however come at a downside as well: Effects of different materials onto the secondary radiation are only averaged and not taken into account explicitly.
+===== Integrated View Sphere (IVS) =====
+To overcome this simplification, a new algorithm called the Indexed View Sphere (IVS) was developed and implemented into ENVI-met. The main idea behind IVS is to efficiently save the results of the individual facets making up the view factor and thus enable back-linking the contributing elements of the urban environment. By storing not only the type of element seen by the cell, but also a reference pointer to the exact building surface, plant cluster and ground surface is saved. Using this pointer, the actual state of the objects seen by the grid cell can be used to calculate the received secondary radiative fluxes. Furthermore, a visibility factor that alters the radiation received has been added. This visibility factor accounts for partial obstruction caused by leaves.
+Similar to the former AVS routine, the pointer calculation of the IVS only needs to be performed once during a model run. When the visibility analysis of the facets is completed and the reference pointers are stored, only actual reflected shortwave radiation and longwave radiation emitted by the objects needs to be updated in order to receive secondary radiative fluxes of the analysed point.
+Storing reference pointer and visibility information for every facet of a 3d ray tracing leads to immense memory demand. Even for smaller model areas of 200 x 200 x 35 grids around one billion data records need to be stored when using a 10° height and 10° azimuth angle calculation. To reduce the number of data records we offer the possibility to adjust the resolution of the height and azimuth angles above and user-defined threshold. You can do so in by enabling "height cap" in the Radiation Settings in the ENVI-guide.
+For more information about this topic, see: [[https://www.mdpi.com/2076-3417/11/12/5449/pdf|here]]
+----
+<fs smaller>[[kb:start|← go back to Knowledge Base Index]] \\
+**See also:** ()
+</fs>
+----