Documentation of future climate projections over NJ
Bill Sacks (sacks@ucar.edu)
March, 2012
I obtained 21st century climate projections from the RCP8.5 run performed by CCSM4. RCP8.5
is the most extreme of the four RCP climate scenarios (i.e., greatest warming). I used the MOAR
("Mother Of All Runs") ensemble member, which is the only ensemble member that contains
high temporal resolution (sub-daily) output. This run is approximately 1° lat/lon resolution.
I obtained data from NCAR's HPSS system, here </CCSM/csm/b40.rcp8_5.1deg.007>. These
data are also publicly available here <http://www.cesm.ucar.edu/experiments/cesm1.0/>. I
obtained the variables of interest from the CAM (Community Atmosphere Model) output, using
files that had been processed into time series of individual variables.
I also obtained 20th century simulation results from the 1850-2005 transient simulation
performed by CCSM4. Again, I used the MOAR ensemble member. I extracted years 1970-2005
from this simulation. The spatial resolution of this run matches the RCP8.5 run. Specifically, the
simulation I used was </CCSM/csm/b40.20th.track1.1deg.012>.
Note that year 2005 occurs in both runs.
For both runs, I subset the data spatially to include only the six grid cells that overlap NJ, as
shown in the following example image:
I extracted the 2x3 rectangle (containing two longitudes and three latitudes) that overlaps NJ.
Notice that the southeastern-most point barely (if at all) overlaps NJ.
I then created spreadsheets containing the variables of interest, as well as some additional,
similar variables. The naming convention of the files is as follows:
Run_TemporalResolution_Latitude_Longitude. e.g., for the file
<rcp8.5_3hr_39.11N_285E.csv>: this is from the RCP8.5 run, giving all variables that are
available at 3-hourly temporal resolution, over the grid cell centered at 39.11 N, 285 E.
The columns of these spreadsheets are as follows:
Each file contains the following temporal and spatial information:

Time variables (yyyymmdd, year, month, day, sec): give the year, month, day and
number of seconds since midnight for the given data point. Note that all times are
expressed in Greenwich Mean Time (GMT). For instantaneous variables, the time gives
the time at which the value applies. For averaged variables, the time gives the end of
the averaging period over which the value applies (for example, for a 3-hour averaged
variable, the time step labeled 2005-01-01 at 10800 sec gives the average from 0 sec to
10800 sec on that date).

Coordinate variables (lat, lon): give the coordinates of the center of the grid cell (degrees
N, degrees E) for the given data point
The 3-hourly file contains variables from CAM that are available at 3-hourly temporal
resolution, as well as variables I have derived from these outputs. Unless otherwise stated, the
variables here are direct outputs from CAM. For each variable, I list whether it is an
instantaneous quantity (i.e., its value in the spreadsheet is the instantaneous value at the time
given) or a time-averaged value (i.e., its value in the spreadsheet is the average of this variable
over the time period ending at the time given – see the documentation of time variables, above,
for more details):

Temperature-related variables:
o TREFHT: Reference height temperature (K) (instantaneous): This is probably the
temperature quantity of interest, providing the near-surface temperature
o TS: Radiative surface temperature (K) (instantaneous): Essentially the
temperature of the ground surface. This is probably of less interest than TREFHT.

Precipitation-related variables:
o PRECC: Convective precipitation rate, liquid + ice (m/s) (time-averaged)
o PRECL: Large-scale (stable) precipitation rate, liquid + ice (m/s) (time-averaged)
o PRECIP: Total precipitation rate (m/s) (time-averaged) (derived quantity): I
derived this quantity by adding PRECC+PRECL. If you are interested in
precipitation, this is the variable of interest. Note, however, that it does not
distinguish between rain and snow.

Solar radiation-related variables:
o FSDS: Downwelling solar flux at surface (W/m2) (time-averaged): This is the
variable to use if you want to know solar incidence
o FSNS: Net solar flux at surface (W/m2) (time-averaged): This variable subtracts
the upwelling (i.e., reflected) solar flux from FSDS

Humidity-related variables:
o QREFHT: Reference height specific humidity (kg/kg) (instantaneous): This
provides near-surface humidity in terms of the mass of water vapor per mass of
air
o PS: Surface pressure (Pa) (instantaneous): Probably not of interest by itself, but
needed to derive relative humidity
o RHREFHT: Reference height relative humidity (%) (instantaneous) (derived
quantity): Relative humidity near the surface. See Appendix A, below, for details
on how this was derived.
The 6-hourly file contains variables from CAM that are available at 6-hourly temporal
resolution, as well as variables I have derived from these outputs. Unless otherwise stated, the
variables here are direct outputs from CAM:

Wind speed-related variables:
o UBOT: Lowest model level zonal wind (m/s) (instantaneous)
o VBOT: Lowest model level meridional wind (m/s) (instantaneous)
o WSPDBOT: Lowest model level wind speed (m/s) (instantaneous) (derived
quantity): Wind speed near the surface, computed as sqrt(UBOT^2 + VBOT^2)
-------------------------------------Appendix A: Computation of relative humidity
Although CAM can compute relative humidity, it was not available at the desired temporal
resolution, so I had to derive it myself. There is no standard way to convert specific humidity to
relative humidity, because this conversion depends on an empirically-derived function to
determine the saturation vapor pressure (which can take the form of an exponential, a
polynomial, etc.).
I used a formula for saturation vapor pressure (esat) from Lowe (1977) ("An approximating
polynomial for the computation of saturation vapor pressure", Journal of Applied Meteorology,
16, 100-103). Lowe gives separate polynomial fits for esat over water and esat over ice. As in
CAM, I used a 20°C transition region: for temperatures greater than 0°C, I used the formula for
esat over water; for temperatures less than -20°C, I used the formula for esat over ice; in
between, I used a weighted average of the two, with weights varying linearly as a function of
temperature. I used TREFHT for the temperature.
Given this value of esat (mb), I computed qsat (saturation specific humidity, kg/kg) as:
qsat = EPSILON*( esat / (ps - (1 - EPSILON)*esat) )
where ps is surface pressure (mb), and EPSILON = H2OMW/AIRMW, where H2OMW = 18.01
and AIRMW = 28.97 (EPSILON gives the ratio of the molecular weight of water to the
molecular weight of air).
Finally, RHREFHT = (QREFHT / qsat) * 100.
From a quick test, I found this resulted in relative humidity values within about 0.25% of those
computed by CAM (RMS error 0.059% relative humidity).
---------Appendix B: Other miscellaneous notes
This section contains notes about things that are not important for using the data, but could trip
you up if you try to repeat these analyses, e.g., for a different CCSM simulation.
B.1 Time convention
The first time step in a CAM run is an initialization time step, denoted by time bounds of 0
length. Unfortunately, the processed time series that were created from the MOAR runs include
this initialization time step and do not include the final time step of the run. Thus, in my
processing, I had to (1) exclude the initialization time step (or, for the 20th century run, where
the data start in the middle of the run, I had to exclude the first time step in the first processed
file, for related reasons), and (2) extract the last time step of the run from the unprocessed history
files.
B.2 Wind speed from 20th century run
For the RCP8.5 run, UBOT and VBOT were output at 6-hourly resolution. However, for the 20th
century run, only U and V (multi-level variables) were output at this resolution. Thus, I had to
extract the lowest model level from these multi-level variables.
B.3 Location of code
Mostly a note to myself: Scripts used to process the data, and a README file containing more
detailed notes about the processing steps, can be found in the subversion repository here:
<https://svn-user-sacks.cgd.ucar.edu/easm_misc/nj_future_high_temporal_res/trunk>. However,
currently I am the only one with access to this repository.