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Caption: MPAS grid mesh (black hexagonal contours) overlayed on Kerbin terrain (grayscale) and oceans (blue).

Model Initialization

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The global forecasting system (GFS) was used to initialize atmospheric fields such as temperature, humidity, pressure, and wind. Since Kerbin's atmosphere is similar to Earth's in composition, atmospheric fields from an Earth atmospheric model can provide a useful first guess for the atmospheric conditions on Kerbin.

Terrain and biome data from KSP were used to classify land use, vegetation type, green fraction, leaf area index, soil type, and surface albedo. This modification to the land surface facilitates simulations of Kerbin's atmosphere. The initial atmosphere, from the GFS, quickly adjusts to the new land surface producing weather unique to Kerbin. 

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Caption: MPAS Land use classification (B) adapted from Kerbin Biomes (A).

In addition to land surface modifications, several changes to MPAS were made to enable more realistic simulations of Kerbin's atmosphere. These changes are listed below:​

1. Axial obliquity was set to zero.

2. Orbital eccentricity was set to zero.

3. The solar constant was set to 1360 W/m

4. The day length was set to 6 hours (21600 s)

5. ​The Coriolis parameter was multiplied by a factor of 4.​

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(1-3) modify orbital parameters per the Kerbin wiki. As a result of these modifications, Kerbin's weather has no seasonality. (4) and (5) allow MPAS to account for the rotation rate of Kerbin, which is approximately four times faster than that of Earth. The faster rotation rate produces a stronger Coriolis acceleration which impacts the temperature and wind fields on Kerbin (Kaspi and Showman, 2015). A second Earth-Like simulation was performed, in which changes 1-5 were not applied. This simulation was used to compare how Kerbin's unique orbital and rotational characteristics impact the climatology of the planet.

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Note that the planetary radius was not changed in MPAS simulations of Kerbin's weather. While experiments were performed with different planetary radii, it was determined that reducing the radius to ~600 km produces a climate that is inconsistent with the observed biomes on Kerbin.  Reducing the planetary radius by a factor of ten results in significant instability in the model requiring modifications to the model dynamics that may be dubious and unrealistic (i.e. MPAS wasn't designed to do this out of the box). Furthermore, a planet with a much smaller radius has dramatically different weather than Earth (especially if gravity remains 9.8 m/s  ). With a 600 km radius and a day length of 6-hours, MPAS simulations produce an entirely tropical planet with temperatures at the poles well above freezing (i.e no ice sheets). From a gameplay perspective, the resulting climate is rather monotonous as there is little variation in weather around the globe. 

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Model Simulation
 

MPAS simulations were spun up over the course of one Kerbin year (~2556 hours), allowing the model to reach a steady-state. This spin-up period allowed the atmosphere to adjust to the land-surface and changes to model constants. After the spin-up period, MPAS was run for an additional five Kerbin years. The model was run with 55 vertical levels that are close together near the surface and further apart near the model top. The upper boundary of the model was set to 70-km per the atmospheric boundary in KSP. To maintain numerical stability a damping layer was used to prevent vertical and horizontal wind speeds from becoming unrealistically large near the model top. 

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After the simulation, atmospheric fields (e.g. temperature, humidity, pressure, etc.) were interpolated to a 1 x 1-degree rectangular grid. A five-year, hourly, climatology of Kerbin was produced by averaging this gridded weather data over time. Since the data is averaged by the hour the climatology captures the diurnal cycle of weather on Kerbin. To reduce the mod size, the vertical resolution of the hourly climatology was reduced from 55 to 11-levels. All climatological data was written in binary format. 

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At select Kerbal launch sites, weather time-series were extracted from the 1x1 degree gridded weather dataset. These year-long time-series allow players to experience dynamic weather that changes each day. Since these time-series were retrieved from a point locations, the weather varies temporally but not spatially. This is in contrast to the climatology discussed above, wherein the weather varies spatially but does not change from day to day.