Northern Hemisphere EASE-Grid Annual Freezing and Thawing Indices, 1901 - 2002

Summary

This data set contains annual freezing and thawing indices for each year from 1901 to 2002 on the 25 km resolution Equal-Area Scalable Earth Grid (EASE-Grid). Annual freezing and thawing indices are defined as the cumulative number of degree-days when air temperatures are below and above 0°C. The values were calculated based upon the monthly mean air temperature from the 1901-2002, 0.5 deg. global land temperatures from the University of East Anglia Climate Research Unit (Mitchell and Jones, 2005).

Two ASCII files are available for each year for the freezing and thawing indices, respectively. Each file is approximately 5.5 MB in size. In addition, there is one 10.4 MB ASCII file defining the latitude and longitude coordinates for each grid point. The data set is available in compressed form via FTP.

Citing These Data

Zhang, T., O. W. Frauenfeld, J. McCreight, and R. G. Barry. 2005. Northern Hemisphere EASE-Grid annual freezing and thawing indices, 1901 - 2002. Boulder, CO: National Snow and Ice Data Center. Digital media.

Overview Table

Category	Description
Data format	ASCII text files
Spatial coverage and resolution	Southernmost Latitude: 0°N Northernmost Latitude: 90°N Westernmost Longitude: 180°W Easternmost Longitude: 180°E
Temporal coverage and resolution	Annual indices for 1901 - 2002
File naming convention	"ease25_XXX_indx.yyyy.txt" where "XXX" is either "frz" for freezing indices or "thw" for thawing indices, and "yyyy" is the four digit year. "E25_lon_lat_nh.txt" contains the grid coordinates.
File size	Two compressed tar files (thawing index = 40 MB, freezing index = 70 MB) containing 102 5.4 MB text files plus one 2.6 MB compressed text file. The total uncompressed volume is approximately 1 GB.
Parameter(s)	Freezing and thawing indices
Procedures for obtaining data	Data are available via FTP.

Table of Contents

1. Contacts and Acknowledgments
2. Detailed Data Description
3. Data Access and Tools
4. Data Acquisition and Processing
5. References and Related Publications
6. Document Information

1. Contacts and Acknowledgments

Investigators

Tingjun Zhang
Oliver W. Frauenfeld
Roger G. Barry
National Snow and Ice Data Center
CIRES, 449 UCB
University of Colorado
Boulder, CO 80309-0449

Technical Contact

Acknowledgments

This study was supported by the U. S. National Science Foundation (NSF) Office of Polar Programs through grants OPP-0229766 and OPP-0352910 and the International Arctic Research Center, University of Alaska Fairbanks, under the auspices of the NSF cooperative agreement number OPP-0327664.

2. Detailed Data Description

Format

ASCII text files

File and Directory Structure

Two tarred and compressed files, "ease25_frz_indx.tgz" and "ease25_thw_indx.tgz", containing 102 freezing index and 101 thawing index files, respectively, plus and one compressed text file, "E25_lon_lat_nh.gz", defining the grid points.

File Naming Convention

"ease25_frz_indx.tgz" contains 102 files in the format "ease25_frz_indx.yyyy.txt" where "yyyy" is the four digit year. "ease25_thw_indx.tgz" contains 101 files in the format "ease25_thw_indx.yyyy.txt" where "yyyy" is the four digit year. In addition, "E25_lon_lat_nh.txt" contains the grid coordinates.

File Size

Two compressed tar files (thawing idex = 40 MB, freezing index = 70 MB) containing 102 5.4 MB text files plus one 2.6 MB compressed text file. The total uncompressed volume is approximately 1 GB.

Spatial Coverage

Northern Hemisphere

Southernmost Latitude: 0°N
Northernmost Latitude: 90°N
Westernmost Longitude: 180°W
Easternmost Longitude: 180°E

Spatial Resolution

Input 0.5° gridded data (Mitchell and Jones 2005) were regridded to the 25 km Northern Hemisphere EASE-Grid using a Cressman interpolation.

Projection

Lambert-Azimuthal Equal Area projection on a spherical geoid. (See All About EASE-Grid)

Grid Description

25-km Northern Hemisphere EASE-Grid (See All About EASE-Grid)

Temporal Coverage

1901 - 2002

Freezing indices are based on a cold season year of July-June, and thawing indices are based on a warm season year of January-December. Therefore, the freezing indices are available only through 2001.

Temporal Resolution

Annual

Freezing indices are based on a cold season year of July-June, and thawing indices are based on a warm season year of January-December. Therefore, the freezing indices are available only through 2001.

Parameter or Variable

Parameter Description

Freezing and thawing index calculated as the total number of freezing or thawing days per cold or warm season respectively. The cold season is defined as July-June and the warm season is January-December.

Sample Data Record

The following values come from "ease25_thw_indx.2002.txt":

      347.231
      361.910
      478.075
      507.947
      506.071
      461.000
     -9999.00
     -9999.00
     -9999.00
     -9999.00

These values correspond to the latitudes and longitudes of the grid points listed in "E25_lon_lat_nh.txt":

      69.7866     -127.255
      69.9683     -127.648
      70.1491     -128.047
      70.3288     -128.454
      70.5074     -128.868
      70.6849     -129.289
      70.8613     -129.719
      71.0366     -130.156
      71.2107     -130.601
      71.3836     -131.055

-9999.00 indicates a grid cell in the ocean.

The figures below show the 1901-2001 climatology for the freezing index (left) and the 1901-2002 climatology for the thawing index (right) in °C-days. From Frauenfeld et al. (2006).

Errors and Uncertainty

Missing values in the source data were replaced with the 1961-1990 climatological values as described in Mitchell and Jones (2005). The data providers compared analyses using the source data both with and without the added climatological values and found little difference in the results (Frauenfeld, et al. 2006). The data here are based on the complete source data including the climatological values. There is also some uncertainty in the source data prior to the 1950s, due to the scarcity of station records.

Quality Assessment

Rigorous quality assessment was applied by the investigators in creating this data set as described in Frauenfeld, et al. (2006). They conducted an initial analysis to assess the accuracy of using monthly data to calculate a freezing or thawing index. They compared indices derived from daily and monthly average 2 m temperatures from the European Centre for Medium-Range Weather Forecasts 40+ year reanalysis (ERA-40) available from the National Center for Atmospheric Research (NCAR; data set ds117.0). In general, they found that using monthly data introduced a relative error in the freezing index of less than 10% and often less than 5% for most of the Northern Hemisphere above 30°N with the notable exception of Europe. The thawing index had a low relative error for the entire Northern Hemisphere with the exception of Greenland. Furthermore, these relative errors agree well with and were verified with station observations. The investigators, therefore, conclude that freezing and thawing indices derived from monthly data are adequate for broad scale analysis and for approximating permafrost and seasonal frozen ground distribution at high northern latitudes.

The investigators then compared freezing/thawing index variability for various regions for three different gridded temperature data sets: ERA-40, Mitchell and Jones (2005), and Willmott and Matsura (2003). ERA-40 showed a warm bias as has been documented before (Simmons et al., 2004). Mitchell and Jones (2005) and Wilmott and Matsura (2003) were generally comparable, but the investigators chose Mitchell and Jones (2005) because of its longer and more consistent time series. See Frauenfeld, et al. (2006) for details.

3. Data Access and Tools

Data Access

Data are available via FTP

Volume

One 112.6 MB compressed tar file contains 203 5.4 MB text files plus one 10.4 MB text file. The total uncompressed volume is approximately 1 GB.

Related Data Collections

• See All About EASE-Grid

4. Data Acquisition and Processing

Theory of Measurements and Processing Steps

The annual freezing or thawing index from daily temperature values is calculated as the cumulative degree days below or above 0°C for the cold or warm season, respectively. The cold season is defined as July-June, and the warm season is defined as January-December. Because the source data were monthly averages, the monthly average temperature was multiplied by the number of days in that month, and the monthly estimates were summed for the cold or warm season to obtain an annual freezing or thawing index.

The source data was first regridded to the 25-Km Northern Hemisphere EASE-Grid using a Cressman before calculating the indices.

Data Source

These data were derived from the 1901-2002 0.5° gridded monthly global land temperatures from the University of East Anglia Climatic Research Unit (Mitchell and Jones, 2005).

5. References and Related Publications

Frauenfeld, O. W., T. Zhang, and J. L. McCreight. 2007. Northern Hemisphere Freezing/Thawing Index Variations over the Twentieth Century. International Journal of Climatology 27(1), 47-63. doi:10.1002/joc.1372.

Mitchell T. D. and P. D. Jones. 2005. An improved method of constructing a database of monthly climate observations and associated high-resolution grids. International Journal of Climatology 25, 693-712.

Simmons A. J., P. D. Jones, V da Costa Bechtold, A. C. M. Beljaars, P. W. Kållberg, S. Saarinen, S. M. Uppala, P. Viterbo, and N. Wedi. 2004. Comparison of trends and variability in CRU, ERA-40, and NCEP/NCAR analyses of monthly-mean surface air temperature. ERA-40 Project Report Series No. 18, 38.

Willmott C.J. and K. Matsura. 2001. Terrestrial air temperature and precipitation: Monthly and annual time series (1950-1999). Newark, DE: Center for Climatic Research, Department of Geography, University of Delaware, Newark.

6. Document Information

List of Acronyms

The following acronyms are used in this document:

ASCII: American Standard Code for Information Interchange
EASE-Grid: Equal-Area Scalable Earth Grid
ERA-40: European Centre for Medium-Range Weather Forecasts 40+ year reanalysis
FTP:File Transfer Protocol
NCAR: National Center for Atmospheric Research
NSF: National Science Foundation
NSIDC: National Snow and Ice Data Center
URL: Uniform Resource Locator

Document Creation Date

7 November 2005

Document Review Date

7 November 2005

Document URL

nsidc.org/data/docs/fgdc/ggd649_freeze_thaw_nh/index.html