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This data set contains output from the NOAA National Weather Service's National Operational Hydrologic Remote Sensing Center (NOHRSC) SNOw Data Assimilation System (SNODAS). SNODAS is a modeling and data assimilation system developed by NOHRSC to provide the best possible estimates of snow cover and associated variables to support hydrologic modeling and analysis. The aim of SNODAS is to provide a physically consistent framework to integrate snow data from satellite and airborne platforms, and ground stations with model estimates of snow cover (Carroll et al. 2001). SNODAS includes procedures to ingest and downscale output from Numerical Weather Prediction (NWP) models, and to simulate snowcover using a physically based, spatially-distributed energy- and mass-balance snow model. SNODAS also includes procedures to assimilate satellite-derived, airborne, and ground-based observations of snow covered area and Snow Water Equivalent (SWE).
These data are not suitable for snow fall events or totals for specific regions. For snow fall data, please see the state climatology reports for a particular state. These are gridded data sets for the continental United States at 1 km spatial resolution and 24 hour temporal resolution. Data are stored in flat binary 16-bit signed integer big-endian format with header and metadata files, and are available from 1 October 2003 to present via FTP.
To broaden awareness of our services, NSIDC requests that you acknowledge the use of data sets distributed by NSIDC. Please refer to the citation below for the suggested form, or contact NSIDC User Services for further information. We also request that you send us one reprint of any publication that cites the use of data received from our Center. This helps us to determine the level of use of the data we distribute. Thank you.
The following example shows how to cite the use of these data sets in a publication. List the principal investigators, year of data set release, data set title and version number, dates of the data you used (for example, March to June 2004), publisher: NSIDC, and digital media.
National Operational Hydrologic Remote Sensing Center. 2004. Snow Data Assimilation System (SNODAS) Data Products at NSIDC, [list the dates of the data used]. Boulder, Colorado USA: National Snow and Ice Data Center. Digital media.
| Category | Description |
|---|---|
| Data format | Flat binary 16-bit signed integer big-endian format. |
| Spatial coverage and resolution | Contiguous United States (27.9300° N, 52.0000° N, 124.7300°
W, 66.9400° W)
30 arc seconds resolution |
| Temporal coverage and resolution | 1 October 2003 to present. 24-hour temporal resolution. See the table Daily NOHRSC SNODAS Products at NSIDC for more information. |
| Grid type and size | Grid values are 16-byte signed integers big-endian. Grids are 6,935 columns by 3,351 rows. |
| File naming convention | Daily files: SNODAS_YYYYMMDD.tar Monthly files: SNODAS_YYYYMM.tar |
| File size | Compressed file sizes vary greatly depending on snow coverage; uncompressed size is 46.5 MB with a 3.6 - 3.8 KB header file. |
| Parameters | Snow Water Equivalent (SWE)
Snow Depth Snow Melt Runoff at the Base of the Snow Pack Sublimation from the Snow Pack Sublimation of Blowing Snow Solid Precipitation Liquid Precipitation Snow Pack Average Temperature |
| Procedures for obtaining data | Data are available via FTP. For subsetting options, please see the Graphical Interface for Subsetting, Mapping, and Ordering (GISMO) Web site. |
1. Contacts
2. Detailed Data Description
3. Data Access and Tools
4. Data Acquisition and Processing
5. References and Related Publications
6. Acknowledgments
7. Document Information
Andrew P. Barrett
National Snow and Ice Data Center
Cooperative Institute for Research in Environmental Sciences
University of Colorado
449 UCB
Boulder, CO 80309
USA
Thomas Carroll (Retired)
National Weather Service
National Operational Hydrologic Remote Sensing Center
1735 Lake Drive W.
Chanhassen, MN 55317
USA
NSIDC User Services
National Snow and Ice Data Center
CIRES, 449 UCB
University of Colorado
Boulder, CO 80309-0449 USA
phone: +1 303.492.6199
fax: +1 303.492.2468
form: Contact NSIDC User Services
e-mail: nsidc@nsidc.org
NOHRSC, located in Minneapolis, Minnesota, provides snow information in a variety of products and formats to meet operational forecasting needs. Most of these products are available from the NOHRSC Web site. NSIDC and NOHRSC have agreed that NSIDC will archive and distribute selected parameters from the NOHRSC SNODAS. These output files are valuable for hydrologists, hydrologic modelers, climatologists, ecologists, and land surface modelers. This documentation draws heavily on an assessment of SNODAS products by Barrett (2003) and on material provided by NOHRSC. Consult these sources for additional information.
The SNODAS product is model output and should not be confused with actual observations. For information on snow fall events or snow fall totals, please contact one of the climate centers listed below:
Eight driving, state, and diagnostic variables are archived by NSIDC. Driving variables are ingested from the Rapid Update Cycle 2 (RUC2) NWP model and used to force the snow model. State variables are defined here as variables that the snow model keeps track of and that describe the state of the model snow pack. State variables are modeled snow pack characteristics that are also required to initialize the model. Diagnostic variables are model output but do not describe the internal state of the model. The variables archived by NSIDC are listed in Table 1 and can be used to compute snow water balance.
SNODAS data files are supplied to NSIDC as flat binary 16-bit signed integer big-endian grids. A header file is also supplied to NSIDC as a text file, which includes metadata. The data files can be read by user-written routines such as Fortran and C programs, off-the-shelf image processing packages such as ENVI, IDL, MATLAB, and ERDAS IMAGINE, and by GIS and other mapping packages such as GMT, GRASS, and ARC/INFO. For instructions, such as importing files into ENVI, refer to Barrett (2003). The header files contain information to georegister grids contained in the flat binary files. They also contain information about creation and modification of each file, data type of each file, georeferencing data, maximum and minimum values, calibration/scaling information, and a time stamp for each field.
If you are using ArcMap to display and analyze SNODAS products, the data will not display correctly and values may be modified. This modification occurs because SNODAS data are signed integers and ArcMap reads unsigned integers. You will need to perform the Map Algebra function in ArcMap in order to correct the data. For instructions on how to perform the Map Algebra function in ArcMap, go to the Importing SNODAS Data into ArcGIS document.
If you are using SNODAS products in ArcMap, the data will display improperly and the values will be off because these data are signed integers and ArcMap can only read unsigned integers. You will need to perform the Map Algebra function in ArcMap in order to byte swap the data. For instructions on how to perform the Map Algebra function in ArcMap, go to the Importing SNODAS Data into ArcGIS document.
The data and header files that are received from NOHRSC have an extension .grz. However, NSIDC tars and zips these files and renames them with the extension .tar.gz. Renaming is done in order to make the files easier to uncompress. The tarred and zipped files are then added to the FTP site where they are placed into a daily .tar file. Every two months, all of the daily tar files are compressed into a monthly tar file. However, the daily tar files remain in the same directory with the monthly tar file for approximately two months before they are deleted.
There are two ways to obtain data, either through the regular FTP access link or through the Graphical Interface for Subsetting, Mapping, and Ordering (GISMO). Use the GISMO ordering interface if you want to subset the data. When obtaining data through the regular FTP access link, you will receive a regular file such as us_ssmv11034tS__T0001TTNATS2003102305HP001.dat, and when obtaining data through GISMO, you will receive a subsetted file such as subset_us_ssmv11036tS__T0001TTNATS2005122605HP001.dat.bil.Z.
The data are available through the FTP site as a daily tar file with the following naming
format: SNODAS_YYYYMMDD.tar
Where:
YYYY = the year
MM = the month
DD = the day
Figure 1. Example of FTP Site Daily Tar Files
Every two months, all of the daily tar files are compressed into a monthly tar file with the following naming format: SNODAS_YYYYMM.tar
Where:
YYYY = the year
MM = the month
Figure 2. Example of FTP Site Monthly Tar Files
Note: The daily tar files remain in the same directory with the monthly tar file for approximately two months before they are deleted.
Each FTP daily or monthly .tar file contains tarred and zipped data and header files with a .tar.gz extension. The .tar.gz filenames have the following format:
Figure 3. Example of FTP Site Data and Header Tarred and Zipped Files
Each .tar.gz file contains a data and a header file. The data and header files share the same filename with the exception of the file extension. Data files have the extension .dat and header files have the extension .Hdr. The.dat and .Hdr filenames have the following format:
Figure 4. Example of FTP Site Data and Header Files
Table 2 explains the values for the data and header file variables listed in the example below.
rr_mmmffppppSvvvvtttttooooddddddddddddIOOOO.xxx
Where:
rr |
The region of the file. This code should always be U.S. |
|---|---|
mmm |
The model used to generate the estimates. Currently, this is ssm for simple snow model. |
ff |
Signifies if the file contains snow model
driving data or model output: v0 is driving data such as precipitation. v1 is operational snow model output. |
pppp |
Product code: 1025 - precipitation 1034 - snow water equivalent 1036 - snow depth 1038 - snow pack average temperature 1039 - blowing snow sublimation 1044 - snow melt 1050 - snow pack sublimation |
S |
Denotes driving variables have been downscaled from NWP model resolution to the resolution of SNODAS. This code only appears in filenames for driving variables. |
vvvv |
A vertical integration code that denotes
what type of snow pack data are being collected. For precipitation data: For snow model outputs: |
ttttt |
A time integration code: T0024 - a quantity integrated over 24 hours (generally used for mass and energy fluxes). T0001 - a one-hour snapshot (generally used for states, such as SWE). |
oooo |
A detail of snow modeling operations and will always be TTNA. |
dddddddddddd |
A time-stamp in the form of TSyyyymmddhh. The time refers to the start of the last time step of the integration period for which these data apply. For example, the time integration T0024 and time step code TS2003102305 are for the time integral 2003-10-22 06 to 2003-10-23 05 |
I |
Denotes hourly H and daily D. In general, H is associated with rasters with a time integration code T001 and D is associated with rasters with a time integration code T0024. |
000O |
Is an offset code of the form P{offset} and
refers to where the data applies during a snow model time step in the snow model's
differencing scheme. P001 denotes that a field represents a total flux for the entire time step such as precipitation, or that a field represents data at the end of a time step. P000 denotes a field of data from the start of a time step. |
xxx |
File type, either .dat for a data file, or .Hdr for a header file. |
| Example: us_ssmv11034tST0001TTNATS2003102305HP001.dat is modeled snow water equivalent (ssmv11034) summed for all the layers of the snow model (tS__). It is the output from just one hour (T0001), representing the time step starting at 2003-10-23, 0500h (TS2003102305). It is potentially generated for every hour of the day (H), and represents the snow an hour after the start of the time step (P001), that is, SWE at 0600h. | |
The following is a sample FTP header file associated with the file us_ssmv11034tS__T0001TTNATS2003102305HP001.Hdr.
Note: This header information may be different than that provided if data
are obtained through GISMO. Refer to the GISMO Header File Naming Convention section of this guide document for more information.
Format version: NOHRSC GIS/RS raster file
v1.1
Data source: RUC2, NESDIS, etc.
Created by module: sm_products
Created by module comment: number BARD codes: 0000000018 BARD codes:
58131678 58131679 75931875 37109162 58131676 18681689 87298325 5777000
58131674 68726564 9160380 87295989 12222 92757697 90881019 88954851
88955144 12229
Created year: 2003
Created month: 10
Created day: 23
Created hour: 8
Created minute: 53
Created second: 21
Last modified by module: sm_products
Last modified by module comment: Not applicable
Last modified year: 2003
Last modified month: 10
Last modified day: 23
Last modified hour: 8
Last modified minute: 53
Last modified second: 21
Satellite data: no
Satellite name: Not applicable
Satellite channel: 0
Satellite data calibrated: no
Description: Modeled snow water equivalent, total of snow layers
Thematic: no
Theme file: Not applicable
Data units: Meters / 1000.000000
Product code: 96687381
Attribute table: Not applicable
Data file pathname: us_ssmv11034tS__T0001TTNATS2003102305HP001.dat
Data type: integer
Data bytes per pixel: 2
Data intercept: 0.000000000000000
Data slope: 1.000000000000000
Minimum data value: 0.000000000000000
Maximum data value: 504.000000000000000
No data value: -9999.000000000000000
Number of columns: 6935
Number of rows: 3351
Geographically corrected: yes
Projected: no
Projection file: Not applicable
Horizontal datum: WGS84
Horizontal precision: 0.008333333333333
Elevation above datum: no
Vertical datum: Not applicable
Vertical precision: 0.000000000000000
Benchmark column: 0
Benchmark row: 0
Benchmark x-axis coordinate: -124.729583333331703
Benchmark y-axis coordinate: 52.871249516804028
X-axis resolution: 0.008333333333333
Y-axis resolution: 0.008333333333333
X-axis offset: 0.003749999996645
Y-axis offset: -0.003750483193850
Minimum x-axis coordinate: -124.733749999998366
Maximum x-axis coordinate: -66.942083333334011
Minimum y-axis coordinate: 24.950416183471813
Maximum y-axis coordinate: 52.875416183470698
Start year: 2003
Start month: 10
Start day: 23
Start hour: 6
Start minute: 0
Start second: 0
Stop year: 2003
Stop month: 10
Stop day: 23
Stop hour: 6
Stop minute: 0
Stop second: 0
Compressed: no
Compression file: Not applicable
Number of color tables: 0
Color table 01 descriptor: Not applicable
Color table 02 descriptor: Not applicable
Color table 03 descriptor: Not applicable
Color table 04 descriptor: Not applicable
Color table 05 descriptor: Not applicable
Color table 06 descriptor: Not applicable
Color table 07 descriptor: Not applicable
Color table 08 descriptor: Not applicable
Color table 09 descriptor: Not applicable
Color table 10 descriptor: Not applicable
Color table 11 descriptor: Not applicable
Color table 12 descriptor: Not applicable
Color table 13 descriptor: Not applicable
Color table 14 descriptor: Not applicable
Color table 15 descriptor: Not applicable
Color table 16 descriptor: Not applicable
Color table 01 file: Not applicable
Color table 02 file: Not applicable
Color table 03 file: Not applicable
Color table 04 file: Not applicable
Color table 05 file: Not applicable
Color table 06 file: Not applicable
Color table 07 file: Not applicable
Color table 08 file: Not applicable
Color table 09 file: Not applicable
Color table 10 file: Not applicable
Color table 11 file: Not applicable
Color table 12 file: Not applicable
Color table 13 file: Not applicable
Color table 14 file: Not applicable
Color table 15 file: Not applicable
Color table 16 file: Not applicable
Histogram: no
Histogram file: Not applicable
The header file information created by GISMO for data subsetted and retrieved through GISMO is different than the header file obtained through the regular FTP access link. For an example of the header file information obtained through the regular FTP access link, refer to the Sample FTP Header File in this guide document. While the original FTP header file contains very detailed information about the file, such as:
The GISMO header file contains:
Note: The units in the GISMO header file indicate dd for grids described in decimal degree, and should not be confused with the units of the SNODAS variables such as m/1000. Those units are given in Table 1.
Table 3 is a sample header file for subsetted data from GISMO. Currently GISMO creates one sample header file per request unless the user requests GIS output for data in a format suitable for reading into GIS systems. In that case, GISMO creates one header file per data file.
Other differences between a regular FTP access link file and a GISMO subsetted file are:
For more information about setting options for GISMO output, see GISMO Help. If you have any questions, please contact NSIDC User Services.
Table 4 shows the size of uncompressed header and data SNODAS files, and compressed tarred and zipped archive SNODAS files for a sampling of data collected 2003-10-23. The total size of all files is also given. Compressed file sizes for other dates will vary depending on how many cells are snow covered.
The data represent snow cover in the contiguous United States, extending into Canada for certain drainage basins. A bounding box is defined by 27.9300° N, 52.0000° N, 124.7300° W, 66.9400° W. Figure 5 is an example of the SNODAS SWE field displayed as an image.
Figure 5. Sample SWE Output from SNODAS
Grids are 6,935 columns by 3,351 rows. Grid values are 16-bit signed integer big-endian format. Fields are given with latitude and longitude coordinates. Grid cells have a 30-arc second spacing (nominally 1 km on the ground). Model output and precipitation variables are point estimates for the center of each grid cell and not an areal estimate. However, for the purposes of hydrologic and snow cover forecasts, these point estimates are assumed to represent average conditions in each grid cell.
SNODAS fields are grids of point estimates of snow cover. Estimates of SWE and snow depth, as well as other parameters, have no real areal extent. Therefore, projecting SNODAS output to a particular projection may not be necessary. Moreover, different users prefer different projections. For example, federal agencies are likely to use the Albers Equal Area projection, while researchers may prefer an alternative such as one of the projections used for the Equal Area Scalable Earth (EASE-Grid). Refer to All About EASE-Grid for more information. Given that SNODAS outputs are essentially point estimates, the decision to project the data and choice of projection can be left to individual users.
NSIDC archives fields representing the model state for 06:00 Universal Time (UTC). The time 06:00 UTC was chosen because this is closest to midnight for the United States. Snow data are for 01:00 local time for the East Coast and 22:00 for the West Coast. SWE, snow depth, and snow pack average temperature represent the state of the snow pack at 06:00 UTC. Snow melt runoff, sublimation and evaporation, and precipitation parameters that describe sources and sinks of snow pack water are integrated for the previous 24 hours, giving daily totals. Note that output for 06:00 UTC is a best estimate of snow pack characteristics. Because SNODAS only updates snow fields once a day, 18 out of 24 time steps in each day's model run do not use observations to update model estimates. Therefore, hourly data from SNODAS is model output only and does not represent the best possible estimate of the snow pack.
NSIDC has not conducted an assessment of these data. For information, please refer to Barrett (2003), or contact NOHRSC.
Data are available via FTP. For subsetting options, please see the section on Subsetting Data Using GISMO.
Note: NSIDC cannot always post data as soon as NOHRSC makes it available. Occasionally one or more files are missing from the NOHRSC output. Generally, files are available from NSIDC within one day of production at NOHRSC. However, users should have no expectation that NSIDC will consistently have all files within one day of production. Occasionally, NSIDC does not receive data; thus, you should check the SNODAS Data Products at NSIDC missing files list to see which files are not available. Users needing files in a timely manner for operational use should contact NOHRSC directly to make arrangements.
For information on how to ingest SNODAS data into an image processing software such as ENVI, see Appendix B in the National Operational Hydrologic Remote Sensing Center Snow Data Assimilation System (SNODAS) Products at NSIDC Special Report.
GISMO is a tool that was developed by NSIDC for working with gridded data sets. Please note that you need to register as a GISMO user in order to obtain the user name and password necessary to log into the GISMO interface. After you have registered, you can select the SNODAS Interface link at the top of the GISMO Home page. You can subset the SNODAS data set by parameter, date, and spatial extent. For example, you might use GISMO if you want to order snow depth data for Michigan on a single date. See the Collection Summary
link
in the sidebar of the GISMO application for information on the characteristics of the SNODAS data in GISMO. For information regarding GISMO error sources, please see the Error Sources section of this document.
NOHRSC supplies NSIDC with files that only have the eight variables contained in this data set. No additional processing is done at NSIDC except for the renaming of the file extension. See the reference information on NOHRSC processing.
When ordering data through GISMO, the data set comes with a .pcf file, a .hdr file, and latitude and longitude files. However, the corners, the dx, and the dy of the latitude and longitude files do not agree with the stated corners, dx, and dy in the .hdr and .pcf files. There is a known problem with the GISMO subsetting tool where the backend processing cuts a rectangle out of the data and tries to calculate what the corner points should be. Originally, GISMO was designed to work with 25 km and 12.5 km data and the tool works fine calculating the corner points accurately. However, the SNODAS data is nominally 1 km data, which does not work as well in GISMO. Thus, NSIDC recommends that you use the latitude and longitude files instead of the .pcf file. However, there is still a slight discrepancy of approximately half a pixel between the .pcf file and the latitude and longitude files. It appears that GISMO is using the outside corners of the grid cells whereas the latitude and longitude files are referencing the center. Since 1 km is approximately 0.01 degrees, half a pixel would be about 0.005 degrees.
Barrett, Andrew. 2003. National Operational Hydrologic Remote Sensing Center Snow Data Assimilation System (SNODAS) Products at NSIDC. NSIDC Special Report 11. Boulder, CO USA: National Snow and Ice Data Center. 19 pp.
Carroll, T., D. Cline, G. Fall, A. Nilsson, L. Li, and A. Rost. 2001. NOHRSC Operations and the Simulation of Snow Cover Properties for the Conterminous U.S. Proceedings of the 69th Annual Meeting of the Western Snow Conference, pp. 1-14.
Carroll, T. 2005. Overview of the Center's Web Site and Products . National Operational Hydrologic Remote Sensing Center, 15 pp.
Jordan, R. 1990. User's guide for USA-CRREL one-dimensional snow temperature model (SNTHERM.89). USA Cold Regions Research and Engineering Laboratory (CRREL), 18 pp.
Visit the NOHRSC Web site for additional NOHRSC snow products and tools.
In Colorado, the Bureau of Reclamation and Colorado Water Conservation Board are assessing SNODAS for hydrological forecasting purpose. Visit the U.S. Department of the Interior Bureau of Reclamation Web site Snow Data Assimilation System (SNODAS) Colorado Data Plots and the Western Water Assessment for more information.
This data set and documentation were developed with the assistance of NOHRSC Director Thomas Carroll and NOHRSC staff, and NSIDC’s Andrew Barrett. The product team at NSIDC consisted of Lisa Ballagh, Florence Fetterer, Alejandro Machado, and Keri Webster. Ross Swick developed GISMO and the GISMO SNODAS interface.
Development and distribution of the data set from NSIDC is supported by funding from NOAA's National Environmental Satellite, Data, and Information Service (NESDIS) and the National Geophysical Data Center (NGDC).
This documentation was written by Keri Webster and Florence Fetterer and is based on the publication National Operational Hydrologic Remote Sensing Center Snow Data Assimilation System (SNODAS) Products at NSIDC.
November 1, 2004
May 9, 2005 - F. Fetterer added information on subsetting options.
December 19, 2005 - F. Fetterer added text advising users needing data on an operational basis to contact NOHRSC.
December 22, 2005 - F. Fetterer added text describing the renaming of .grz files at NSIDC. Renaming was instituted in December 2005 for the following reasons: 1) .grz is not a standard data type or file extension, 2) The compression and storage of the files is accomplished by tarring each set and then compressing them using the gzip compression program. This has several recognized filename extensions, but the most prevalent is .tar.gz. Changing the extension to this more recognized format will help alleviate user confusion while at the same time not altering the actual distributed data files contained within the tarred file.
February 2, 2006 - F. Fetterer added links to a Frequently Asked Questions page authored by L. Ballagh.
February 2, 2007 - F. Fetterer made the following changes: Added link to the Bureau of Reclamation and WWA Web sites, added units and product code to table, added information on using GISMO formerly found in the FAQ.
April 1, 2008 - Deann Miller reformatted and reorganized the guide documentation based on comments from User Services (Kara Gergely) to make the guide documentation easier to use. USO was receiving a lot of questions about this data set.
August 7, 2008 - Deann Miller updated guide doc with edits from Florence Fetterer and Andy Barrett.
http://nsidc.org/data/docs/noaa/g02158_snodas_snow_cover_model/index.html
