Summary:

## Land-Only Mode

In land-only mode, the atmospheric forcing required by the Community Land Model (CLM) is supplied by observed datasets instead of being coupled to an atmospheric model. The two primary forcing datasets used are:

1. **GSWP3**: A 110-year (1901-2010) dataset with 0.5° x 0.5° spatial resolution and 3-hourly temporal resolution. This dataset is modified to fill in missing data over oceans, lakes, and Antarctica.

2. **CRUNCEP**: A 110-year (1901-2010) dataset that combines the CRU TS3.2 monthly data (1901-2002) and the NCEP reanalysis 6-hourly data (1948-2010). This dataset has been used in various studies of vegetation growth, evapotranspiration, and gross primary production.

The forcing data is ingested into a data atmosphere model in three streams: precipitation, solar radiation, and other fields (atmospheric pressure, humidity, temperature, and wind). The solar radiation is further processed to provide a smoother diurnal cycle and ensure that all the solar radiation supplied by the 3-hourly forcing data is used.

The article also describes how the user can provide alternative atmospheric forcing data, including the option to supply relative humidity or dew point temperature instead of specific humidity, and direct and diffuse solar radiation instead of total solar radiation.