This set of files includes downscaled historical estimates of decadal means of annual day of freeze or thaw for each decade from 1910 - 2006 (CRU TS 3.0) or 2009 (CRU TS 3.1) at 771x771 meter spatial resolution. Each file represents a decadal mean of an annual mean calculated from mean monthly data.
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Units are ordinal day 15-350 with the below special cases.
For Day of Freeze (DOF)
0 = Primarily Frozen
365 = Rarely Freezes
For Day of Thaw (DOT)
0 = Rarely Freezes
365 = Primarily Frozen
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The spatial extent includes Alaska.
Each set of files originates from the Climatic Research Unit (CRU, http://www.cru.uea.ac.uk/) TS 3.0 or 3.1 dataset. TS 3.0 extends through December 2006 while 3.1 extends to December 2009.
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Downscaling:
These files are bias corrected and downscaled via the delta method using PRISM (http://prism.oregonstate.edu/) 1961-1990 (for 2km Alaska and Western Canada data) or 1971-2000 (for 771 meter Alaska data) as baseline climate. Absolute anomalies are utilized for temperature variables. Proportional anomalies are utilized for precipitation variables.
For more detailed information on base input data (GCMs, historical data), emmission scenarios, the downscaling process, or uncertainty, please go to http://www.snap.uaf.edu/
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Day of Freeze, Day of Thaw, Length of Growing Season calculations:
Estimated ordinal days of freeze and thaw are calculated by assuming a linear change in temperature between consecutive months. Mean monthly temperatures are used to represent daily temperature on the 15th day of each month. When consecutive monthly midpoints have opposite sign temperatures, the day of transition (freeze or thaw) is the day between them on which temperature crosses zero degrees C. The length of growing season refers to the number of days between the days of freeze and thaw.
This amounts to connecting temperature values (y-axis) for each month (x-axis) by line segments and solving for the x-intercepts. Calculating a day of freeze or thaw is simple. However, transitions may occur several times in a year, or not at all. The choice of transition points to use as the thaw and freeze dates which best represent realistic bounds on a growing season is more complex. Rather than iteratively looping over months one at a time, searching from January forward to determine thaw day and from December backward to determine freeze day, stopping as soon as a sign change between two months is identified, the algorithm looks at a snapshot of the signs of all twelve mean monthly temperatures at once, which enables identification of multiple discrete periods of positive and negative temperatures. As a result more realistic days of freeze and thaw and length of growing season can be calculated when there are idiosyncrasies in the data.
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File naming scheme:
While it’s hard to have a completely static file naming scheme, we make every attempt to keep some consistency across our various datasets. This naming scheme outlined below is our general guide, although it does vary depending upon each dataset.
[variable]_[metric]_[units]_[format]_[assessmentReport]_[groupModel]_[scenario]_[timeFrame].[fileFormat]
some examples of these file names parts include:
[variable] pr, tas, logs, dot, dof, veg, age, dem
[metric] mean, total, decadal mean monthly mean
[units] mm, C, in, km
[format] optional, if layer is formatted for special use
[assessmentReport] ar4, ar5 (assessment report, for projected data only)
[groupModel] cccma_cgcm31, mpi_echam5, gfdl_cm21, ukmo_hadcm3, miroc3_2_medres, 5modelavg, cru_ts31
[scenario] sresb1, sresa2, sresa1b
[timeFrame] yyyy or mm_yyyy or yyyy_yyyy or mm_yyyy_mm_yyyy
[fileFormat] tif, txt, png, pdf
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full file name examples:
tas_mean_C_ar4_cccma_cgcm3_1_sresb1_05_2034.tif
This file represents May 2034 mean temperature in degrees Celsius from the 4th Assessment Report on Climate Change from the CCCMA modeling group, using their CGCM3.1 model, under the B1 climate scenario.
pr_decadal_mean_DJF_total_mm_cru_TS31_historical_1920_1929.tif
This file represents mean total winter(Dec, Jan, Feb) precipitation from 1920-1929 from downscaled CRU TS 3.1 data.
tas = near-surface air temperature
pr = precipitation including both liquid and solid phases
dof = day of freeze
dot = day of thaw
logs = length of growing season