5. OUTLOOK

Using the new development it is possible to incorporate larger scale processes in a high resolution, microscale climate simulation. The algorithms that manage the extractionof model outputs and the downscaling of the meteorological parameters onto the nested model area’s resolution show a good performance even if large host areas are used.Using these three-dimensional boundary conditions to run high resolution model areas opens up a wide range of possibilities to analyse different larger scale meteorologies inhigh resolution microclimate models. In future versions, other parameters like pollutants will be integrated in the nesting and a NetCDF-Import will be released to run ENVI-met simulations using boundary conditions of models like the Weather Research and Forecasting Model (WRF).

4. DOWNSCALING BOUNDARY CONDITIONS TO RESOLUTION OF NESTED MODEL AREA

Since the larger scale host simulations are typically run at a coarser resolution than the nested simulations,the simulation outputs of the host simulation have to be interpolated to provide boundary conditions forthe nested model area. First, the location of the nested model area within the host’s domain needs to becalculated and, in case the model areas are not aligned, a rotation matrix is used:

𝑅 =cos 𝛼 − sin𝛼sin 𝛼 cos 𝛼

with 𝛼 as the rotation angle (counterclockwise) of the nested area within the host model domain.The interpolation is carried out using a three-dimensional inverse distance weighting (IDW) algorithm,taking into account all cells of the larger scale model area within a predefined search radius of the nestedboundary cells:

ത𝑅𝑖𝑛𝑡 =

𝑛=1

𝑚

𝑤𝑛𝑅𝑛

with ത𝑅𝑖𝑛𝑡 as the interpolated value, 𝑅𝑛 as the larger scale cell’s output value and 𝑤𝑛 as the weightcorresponding to the grid cell. The summation runs over all host cells 𝑚 within the search radius of thenested cell. The calculation of the weighting factor is carried out as:

𝑤𝑛 =𝑑𝑛−1

σ𝑛=1𝑚 𝑑𝑛

−1

with 𝑑𝑛 as the Euclidean distance calculated using the 3D Pythagorean Theorem between the boundaryposition in the nested model area and the position of the output cell of the host model. Summing up allindividual weights, the result must be equal to 1.In case the boundary cells for the nested model run are occupied by obstructions in the host simulation(e.g. buildings), the algorithm searches for free adjacent atmospheric cells in the host model domain toextract the boundary conditions. Since this data might not be located along the lateral boundary of thenested model area, the atmospheric data is interpolated again using the three-dimensional inverse distanceweighting (IDW) algorithm.Since larger differences in spatial resolutions between the host and the nested model area result in agreater loss of information due to the spatial downscaling, the downscaling ratio is advised not to exceedabout 3:1 (ASHRAE, 2001; Christensen et al., 1998). In case a higher downscaling is needed, the nesting canbe run in cascades, where intermediary simulations gradually downscale the resolution (see Fig. 1 and 2).

3. EXTRACTION OF 3-DIMENSIONAL BOUNDARY CONDITIONS

The forced boundary conditions, however, only provide the possibility to define horizontally homogenous, one-dimensional profiles at all inflow boundaries of the model area.Using this method the same vertical profiles are attached to the entire inflow border, denying horizontal alterations. However, in heterogeneous environments such as cities,where the meteorological parameters can vary greatly along the borders of the model domain, being limited to horizontally one-dimensional profiles may yield unrealisticresults. To include heterogeneous distributions of meteorological parameters at the model borders, the simulations need to be provided with horizontally and verticallydynamic boundary conditions.To obtain such dynamic, more-dimensional boundary conditions, a nesting module was implemented into ENVI-met. With the new module, coupled simulations can be run inwhich the small model area is nested into a larger model domain that provides horizontally and vertically dynamic boundary conditions for the nested model area. Whennested model runs are used, the high resolution boundary conditions are driven by previously run model results of a larger model domain that surrounds the smaller highresolution model area. This offline coupling method offers the possibility to first simulate a large, so called “host” model domain in a coarse resolution. The three-dimensionalmodel results are then being used to drive the high resolution model with three-dimensional, dynamical boundary conditions. That way, larger scale processes provided by thehost model can be incorporated in the nested simulation.To extract data from an ENVI-met simulation, the host model writes three-dimensional outputs of all data needed to perform a nested simulation within its limits. Using asmall tool, the boundary conditions of the nested area are then extracted from the datasets of the host simulation and a nesting file is being created.

2. BOUNDARY CONDITIONS IN ENVI-MET

Due to the fact that ENVI-met only simulates parts of the atmosphere,boundary conditions are required for the lateral and vertical borders of the3D model. To provide these boundary conditions, the 1D boundary modelgenerates one-dimensional profiles for meteorological parameters such asair temperature, specific humidity, wind vectors (horizontal), kinetic energyand turbulent exchange. To ensure stable laminar conditions, the boundarymodel extends to an altitude of 2,500 meters. The one-dimensional boundary model with its horizontally homogeneous verticalprofiles is then used to provide data on the borders of the 3D model (Bruseand Fleer, 1998).To include realistic boundary conditions, ENVI-met already offers theso-called “forced” boundary conditions which allow the definition of diurnalvariations of various meteorological parameters as boundary conditions forthe microclimate model (Huttner, 2012).

Introduction of nesting into the microclimate model ENVI-metHelge Simon, Michael Bruse, Tim Kropp, Francesca Sohni

Geoinformatics, Johannes Gutenberg-University Mainz

Contact: Helge Simon Website: www.geoinformatik.uni-mainz.de/simon E-Mail: h.simon@geo.uni-mainz.de

1. INTRODUCTION

Microclimate models like ENVI-met have the advantage that, thanks to their high resolutions,very little parameterization is needed to represent objects of the urban environment (Simon,2016): Trees, building materials and complex structures can be directly reproduced within themodel. However, this high resolution comes at a disadvantage as well: With increasing spatialresolutions, the differential equations guiding the model have to be solved with smaller timesteps (Courant-Friedrichs-Lewy Condition). This drastically increases the time needed to run thesimulations and limits the size of the model area that can be simulated within a reasonableframe of time. Even with the advancements of 64bit compatibility and heavy parallelprocessing, high horizontal and vertical resolutions of around 2 meters only allow thesimulation of model areas of around 1,000 meters x 1,000 meters x 60 meters on personalcomputers. With these rather small model areas, the boundary conditions driving themicroclimate simulation play a crucial role in determining the outcome and quality of thesimulation. Here a new method to obtain realistic boundary conditions that include thesurrounding meteorology in the form of nesting is presented.

REFERENCES

ASHRAE, (2001). International Weather for Energy Calculations (IWEC Weather Files), [Online], Available: https://www.ashrae.org/technical-resources/bookstore/ashrae-international-weather-files-for-energy-calculations-2-0-iwec2 [05 June 2018].Bruse, M. and H. Fleer, (1998). Simulating surface-plant-air interactions inside urban environments with a three dimensional numerical model. Environment Modelling & Software, 13(3-4): p. 373-384.Christensen, O., J. Christensen, B. Machenhauer and M. Botzet, (1998). Very High-Resolution Regional Climate Simulations over Scandinavia - Present Climate. Journal of Climate, 11(12): p. 3204-3229.Huttner, S., (2012) Further development and application of the 3D microclimate simulation ENVI-met. Dissertation, Johannes Gutenberg University Mainz.Simon, H. (2016) Modeling Urban Microclimate. Development, implementation and evaluation of new and improved calculation methods for the urban microclimate model ENVI-met. Dissertation, Johannes Gutenberg University Mainz.

BOUNDARY CONDITIONS PROVIDED BY THE HOST DOMAIN

• wind speed and direction • direct shortwave radiation

• air temperature • diffuse shortwave radiation

• specific air humidity • longwave radiation

• turbulent kinetic energy & dissipation • horizontal and vertical exchange coefficient impulse / heat

Figure 1: Cascade of nested model areas

Figure 2: Downscaling schematic of a twofold nested model