NARCCAP data is organized into groups based on its spatial and temporal structure: 2-D vs 3-D, daily vs 3-hourly, etc. The groups are listed in the table below. Each variable name links to the corresponding description in the CF Standard Name Table.

Notes:

The "screen height" or "reference height" is 1.5-2 m above the surface.

Quantities that can be used for storage terms in budgets are instantaneous values. Quantities that are fluxes in budgets are average values for the 3-hr period prior to the reporting time.

3-hourly values (Tables 2, 3, & 5) are reported at 03 UTC, 06 UTC, … 24 UTC. (UTC stands for Universal Coordinated Time; it is equivalent to GMT, Greenwich Mean Time, in this context.)

The "day" for daily min/max values (Table 1) is from 06 UTC - 06 UTC, with the date stamp referring to the beginning time.

Table 1

Minimum and maximum daily temperatures [tasmax, tasmin] are recorded from instantaneous screen height temperatures sampled every time step during the course of the day.

Table 2

Winds [uas, vas] are true west-east and south-north winds, i.e., rotated from the model's grid to the earth's longitudinal and latitudinal directions.

Surface air temperature [tas] and specific humidity [huss] are recorded from screen height values, 2 meters above the surface.

Surface winds [uas, vas] are recorded from "anemometer level" values, 10 meters above the surface.

Units for precipitation [pr] and surface pressure [ps] are MKS, following CF standards. See below for unit conversion.

Table 3

Snow amount was originally provided as the variable snow depth [snd]. However, there was some confusion as to whether snd meant the depth of the snow itself or the amount of liquid water equivalent in the snow. In some cases snd was provided in thickness units, and in some cases in kg/m² (in which case it was published as the variable [snw]). To resolve this confusion and make the meaning unambiguous, [snd] and [snw] have been retracted, and in their place we have published snow water equivalent as the variable [swe]. (Exception: the MM5I-ncep run has actual snow depth values that could not be converted to swe.)

Snow water equivalent [swe] is the thickness of a uniform layer of water with the same water content as the snowpack. It is an average depth value over the entire grid box, even if the model allows snow to cover only a fraction of the grid box.

Table 4

Land-cover type [landtyp] is a vegetation index linked to a table supplying land-cover descriptions.

Because the Table 4 data is the same for each run and the files are small, it is not served from ESG, but instead is available here.

Table 5

3-D fields are provided every every 25 hPa from 1050 hPa up to 700 hPa, and every 50 hPa from 650 hPa up to 050 hPa. Standard pressure levels are 950, 850, 700, 600, 500, 300, and 250 hPa; when it is not feasible to provide all pressure levels at once, these levels will be provided first.

Note that according to the CF conventions, ice and liquid cloud water [cli, clw] are each reported as a ratio: (cloud-water mass)/(mass of gas+condensed constituents), i.e., more like specific humidity than mixing ratio.

Unit Conversion

Precipitation and other water fluxes are given in MKS units of kg/m^2/s. This is an instantaneous flux rate averaged over the previous 3 hours. This can be converted to more familiar units as follows:

Water has a density of 1 gram per cubic centimeter, so 1 kg of water forms a cube 1000 cubic cm in volume. Distributed evenly over 1 square meter, this forms a layer 1 mm in depth. Multiply by 10800 seconds in a 3-hour period to get total mm of precipitation per 3 hour timestep.

Similarly, soil moisture and related variables given in kg/m^2 can be converted to the more familiar cm by simply dividing by 10.