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 2a

The variables in Table 2a are not part of the required output from the regional models, but have been calculated from the submitted data during the QC and archiving process. Every effort has been taken to ensure these calculations are correct, but because they are secondary transformations, the odds of an error sneaking in are necessarily higher.

NOTE: Because these variables are created by extra processing during QC, they may not be available until after the source data has been published. In particular, there is a backlog for datasets published before autumn of 2009.

Total precipitation [prtot] is derived from precipitation flux [pr] using the formula prtot = pr*3600*3. This converts the rate flux into the more familiar accumulation over time. When units are simply 'mm', it is implicit that the rate is 'per (3-hour) timestep'. For time-averaged summary variables, the units will be mm/day.

Relative humidity [hurs] is derived from specific humidity [huss], surface pressure [ps], and surface air temperature [tas] using the NCL function relhum(tas,x,ps), where x is the mixing ratio, calculated from huss using the formula x = h/(1-h). Dew point temperature [dewpt] is then derived from hurs and tas using the NCL function dewtemp_trh(tas,hurs*100).

Wind speed [spd] and direction [dir] are derived from u-wind [uas] and v-wind [vas] using the formulae spd = sqrt(uas^2 + vas^2) and dir = atan2(uas, vas). Following meteorological convention, direction is the direction the wind is blowing from in degrees clockwise from north.

Table 3

Snow depth [snd] is snow-water equivalent and is the average depth over the entire grid box, even if the model allows snow to cover a fraction of the grid box.

Table 4

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

Table 5

3-D fields are provided every 25 hPa from 1050 hPa to 25 hPa.

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.