30.2. Technical Description
The advanced thermal modeling provided by MuSES can be integrated in to DIRSIG simulations. The spatial-temperature distribution of a Humvee can be modeled in MuSES, the temperature-attributed geometry exported to DIRSIG, and the full radiometry capabilities of DIRSIG can generate hyperspectral image data.
In order to ensure that the thermal modeling assumptions are consistent between DIRSIG and MuSES, the same weather data must be used for the two models. The prn2wth tool provides a simple method of converting from the PRISM format used in MuSES to the WTH format used in DIRSIG.
30.2.1. Converting to WTH Format
Several pieces of information needed for DIRSIG's WTH format can not be directly derived from the PRISM data. For these cases reasonable default values are used. The derivation of each column in a WTH file (see Weather History File) from a PRISM input is shown in Table 30-1.
Table 30-1. WTH Data Sources
| WTH Column | Source |
|---|---|
| Relative Time | "TIME" converted from military time to decimal hours. |
| Air Temp | "AIRT", unchanged |
| Pressure | No PRISM equivalent; hard-coded to 1013.25 mbar (1 atm). |
| Relative Humidity | "HUMID" converted from percentage (0-100) to fraction (0-1). |
| Dew Point | No PRISM equivalent; modeled via Magnus-Tetens. |
| Wind Speed | "WIND", unchanged |
| Direct Insolation | "SOLAR" converted to Langleys/hr (conversion factor: 0.0858) or optionally modeled via make_weather algorithm. |
| Diffuse Insolation | No PRISM equivalent; modeled via make_weather algorithm. |
| Sky Exposure | "CLOUD", converted linearly from range (0=clear,10=full overcast) to range (0=full overcast,1=clear). |
| Cloud Type | No PRISM equivalent; hard-coded to "None" if sky exposure is clear and "Altocumulus" otherwise. |
| Precipitation Type | "RAINRATE" with a non-zero value indicates precipitation is "present", otherwise type set to "not present". |
| Precipitation Rate | "RAINRATE" converted from mm/hr to cm/hr. |
| Precipitation Temp | No PRISM equivalent; hard-coded to ambient air temperature. |
30.2.2. Modeling Insolation
The prn2wth utility provides the option of modeling direct and diffuse insolation values. This insolation model can compensate for missing diffuse data in a PRN (stemming from an all zero LWIR column).
The latitude, longitude, and date provided to the utility from the command line are sufficient to make a rough estimate of the insolation values. Figure 30-1 shows that this simple insolation model provides only a rough approximation of the actual conditions. It is generally preferable to use the actual measurements when available.
