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Snow Water Equivalent

Snow Extent

Specifications for the Snow Extent product of GlobSnow

Overview

The GlobSnow Snow Water Equivalent (SWE) product covering the years 1978 - 2010 will be produced and released by October 2010. Currently a prototype data set for SWE is available covering the years 1994 - 1997 and 2003 - 2008. The data are available through the GlobSnow FTP-server (access details below).

The SWE prototype dataset v0.9.1 (released on December 2009) contains snow information derived for the Northern Hemisphere for the years 1994 - 1997 using SSM/I data and 2003-2008 from AMSR-E data. The prototype data are provided in HDF4-format: a single file contains the data for a single day; a single file contains two fields 1) the SWE estimate and 2) an error estimate. The snow water equivalent describes the amount of liquid water in the snow pack that would be formed if the snow pack was completely melted. The goal of the ESA GlobSnow project is to eventually produce daily SWE estimates for the Northern Hemisphere for the years 1978- 2010. The final SWE dataset spanning 30+ years is expected to be produced by October 2010 and will be made available to all interested parties.

The GlobSnow SWE methodology is based on the algorithm developed by Prof. Pulliainen and published in Remote Sensing of Environment - journal in 2006. The SWE product is projected to Equal-Area Scalable Earth Grid (EASE-Grid) and provides the daily SWE estimates for whole Northern Hemisphere (lambert's equal-area azimuthal - projection) in a single file. Although the EASE-Grid projection can represent data almost to the equator the product is limited between latitudes 35 and 85 for physical reasons (extent of seasonal snow cover). The input data for the SWE prototype products are from AMSR-E sensor acquired from National Snow and Ice Data Center, Boulder Colorado, U.S.A. (NSIDC).

The prototype data are publicly accessible through a FMI hosted FTP-server; access information to the server can be acquired through Dr. Kari Luojus (firstname.lastname@fmi.fi).

Specifications for the Snow Water Equivalent product of GlobSnow

Based on the discussions at the Requirements Engineering Review (3 February Geneva) and the User Workshop 1 (12 - 13 January 2010 in Innsbruck), the following specifications have been set for the Snow Water Equivalent Product:

Construction of the 30 years historical data set will be carried out using SMMR, SSM/I and AMSR-E data along with ground-based weather station data. The data are utilized for the the different years as follows:

  • 1978 - 1987 SMMR (Scanning Multichannel Microwave Radiometer onboard Nimbus-7 satellite)
  • 1987 - 2002 SSM/I (Special Sensor Microwave/Imager onboard the DMSP satellite series)
  • 2002 - 2010 AMSR-E (Advanced Microwave Scanning Radiometer-EOS onboard Aqua satellite)

For the operational near-real time (NRT) product AMSR-E data will be utilized. If AMSR-E data become unavailable (it is not an operational sensor and is currently operating beyond the planned life span), we will revert to SSM/I data for the NRT product.

A thematic accuracy goal of 30-40 mm for conditions with less than 150 mm of SWE is proposed for the global GlobSnow SWE product, depending on the availability of weather station data. This is seen as both a realistic goal and an improvement over currently available datasets when the performance of current algorithms at a continental scale is considered.

The spatial resolution of the product will be 25 km on EASE-grid projection. The SWE product will be generated using a daily temporal resolution.

The SWE product shall cover the Northern Hemisphere, excluding the mountainous areas, Greenland, the glaciers and snow on ice (lakes/seas/oceans).The spatial resolution of the product will be 25 km on EASE-grid projection. The SWE product will be generated using a daily temporal resolution, with a weekly and a monthly aggregated products.

An overview of the GlobSnow SWE product relative to the state of the art is shown in Table 1.

Table 1
Overview of the GlobSnow SWE product relative to the state of the art.

Snow water equivalent

State of the Art

Proposed GlobSnow product

Coverage

Regional (high uncertainty at hemispheric/global scales)

 

Northern Hemisphere (excluding mountainous areas)

Time period

1978 - present

1987-2010

Temporal frequency

Monthly/pentad starting from 1978

Daily starting from 2002

Weekly (7 days) and monthly products, starting from 1987

Spatial resolution

Global:

25 km EASE-grid

Northern Hemisphere:

25 km EASE-Grid

Geometric accuracy

sub pixel location error

sub pixel location error

Thematic accuracy

40-200 mm

Areas with dense weather station network: 25-35 mm

Areas with sparse weather station network: 30-40 mm

Accuracy characteristics (Error statistics)

NA

Each sample has an error estimate

The primary areas of added value for the GlobSnow SWE product are:

  • Improved accuracy characterization on a grid cell by grid cell basis for the SWE product. This type of information is not currently available for existing products (e.g. characterization for different land use and snow classes).

  • Improved SWE retrieval accuracy is expected when compared with the currently available hemispheric products, achieved via the first hemispheric (non-mountainous) application of the data assimilation retrieval approach described in (Pulliainen J. 2006. Mapping of snow water equivalent and snow depth in boreal and sub-arctic zones by assimilating space-borne microwave radiometer data and ground-based observations. Remote Sensing of Environment, Vol. 101, pp 257-269.)


Figure 1 Example of SWE product

Finnish Meteorological Institute