The Terrain input field of the Building object in EnergyPlus takes the wind speed from the EPW file and alters it for the simulation to account for wind shielding by surrounding structures (buildings, trees, etc.). There are 5 options that you can enter, one of which is City. If you leave this input field blank, that tells EnergyPlus to assign a default value. According to the Input Data Dictionary (IDD), that default is Suburbs (see below).
Building,
\memo Describes parameters that are used during the simulation
\memo of the building. There are necessary correlations between the entries for
\memo this object and some entries in the Site:WeatherStation and
\memo Site:HeightVariation objects, specifically the Terrain field.
...
A2 , \field Terrain
\note Country=FlatOpenCountry | Suburbs=CountryTownsSuburbs | City=CityCenter | Ocean=body of water (5km) | Urban=Urban-Industrial-Forest
\type choice
\key Country
\key Suburbs
\key City
\key Ocean
\key Urban
** \default Suburbs **
If you want to see the impact of changing this input field, you can read about the local wind speed equations used in the Engineering Reference. As is the case with most simulations, the effect of this input field depends upon the building you are modeling. For example, it would have a higher impact on a single-story residence than a multi-story office because of the greater ratio of exterior surface area to air volume, as well as the intensity of internal gains like plug loads and lighting that require cooling.
It doesn't appear that this Terrain input field is exposed in the OpenStudio application. As such, you will need to use the Site::setTerrain() method of the OpenStudio SDK to alter this input field through scripting.
