**2.2 Satellite–based snow products and processing method**

#### **2.2.1 Snow extent and snow cover fraction products**

#### **a. IMS Daily Northern Hemisphere Snow and Ice Analysis at 24 km Resolution**

This data is 24 km daily Northern Hemisphere snow and ice coverage by the NOAA/NESDIS Interactive Multi-sensor Snow and Ice Mapping System (IMS) (National Ice Center, 2008). The key parameters for this type of data are listed below:

Satellite-Based Snow Cover Analysis and the






onset, duration and end of the snow appearance.


2003), which is more suitable to analyse the trend for at a long time span.


Fig. 3. The sample product from the NISE products, the SCA over China is also obvious for

its Plateau shape in the right part of the map (EASE-GRID)

Snow Water Equivalent Retrieval Perspective over China 51

The snow cover over China is showed in the right part of the EASE-GRID projection image (Fig. 3), which is also distorted for some extent. The data could be used to evaluation the

**c. MODIS/Aqua Snow Cover 8-Day L3 Global 0.05Deg Climate Modeling Grid (CMG)**  The MODIS Snow Cover 8-Day L3 Global 0.05Deg CMG (Fig. 4.) is a global map of snow cover expressed as a percentage of land, i.e. snow cover fraction, in each CMG cell for an eight-day period, which are derived from the Normalized Difference Snow Index (NDSI) of MODIS spectro-radiometer data (Hall, 1995). The percentage of snow-covered land is based on the clear-sky view of land in the CMG cell, and count the number of snow observation over land. So the amount of snow observed in a CMG cell is based on the cloud-free observations mapped into the CMG grid cell for all land in that cell (Hall, 2007). Compared with the daily snow-cover products, the eight-day SCFs products greatly reduce the percent of cloud obscured or masked pixels from near half to less than 7% over *Tibet Plateau* (Riggs,


From the sample data map in fig.2, we can find that the SCA data is distorted over China area. Over the northern hemisphere, the China area is not a dominant domain in the continent analysis. It is fit for the onset, duration and end of the snow for its daily resolution.

#### **b. Near-Real-Time SSM/I-SSMIS EASE-Grid Daily Global Ice Concentration and Snow Extent**

The Near-Real-Time SSM/I-SSMIS EASE-Grid Daily Global Ice Concentration and Snow Extent product (Near-real-time Ice and Snow Extent, NISE) provides daily, global near-realtime maps of sea ice concentrations and snow extent. They are derived from the passive microwave data from the Special Sensor Microwave Imager/Sounder (SSMIS) on board the Defense Meteorological Satellite Program (DMSP) F17 satellite (Nolin, 1998).

Fig. 2. The sample product from the 24km IMS SCA products, the SCA over China is obvious for its Plateau shape in the upper part of the map


From the sample data map in fig.2, we can find that the SCA data is distorted over China area. Over the northern hemisphere, the China area is not a dominant domain in the continent analysis. It is fit for the onset, duration and end of the snow for its daily

**b. Near-Real-Time SSM/I-SSMIS EASE-Grid Daily Global Ice Concentration and Snow** 

The Near-Real-Time SSM/I-SSMIS EASE-Grid Daily Global Ice Concentration and Snow Extent product (Near-real-time Ice and Snow Extent, NISE) provides daily, global near-realtime maps of sea ice concentrations and snow extent. They are derived from the passive microwave data from the Special Sensor Microwave Imager/Sounder (SSMIS) on board the



resolution.

**Extent** 





Defense Meteorological Satellite Program (DMSP) F17 satellite (Nolin, 1998).

Fig. 2. The sample product from the 24km IMS SCA products, the SCA over China is

obvious for its Plateau shape in the upper part of the map






The snow cover over China is showed in the right part of the EASE-GRID projection image (Fig. 3), which is also distorted for some extent. The data could be used to evaluation the onset, duration and end of the snow appearance.

#### **c. MODIS/Aqua Snow Cover 8-Day L3 Global 0.05Deg Climate Modeling Grid (CMG)**

The MODIS Snow Cover 8-Day L3 Global 0.05Deg CMG (Fig. 4.) is a global map of snow cover expressed as a percentage of land, i.e. snow cover fraction, in each CMG cell for an eight-day period, which are derived from the Normalized Difference Snow Index (NDSI) of MODIS spectro-radiometer data (Hall, 1995). The percentage of snow-covered land is based on the clear-sky view of land in the CMG cell, and count the number of snow observation over land. So the amount of snow observed in a CMG cell is based on the cloud-free observations mapped into the CMG grid cell for all land in that cell (Hall, 2007). Compared with the daily snow-cover products, the eight-day SCFs products greatly reduce the percent of cloud obscured or masked pixels from near half to less than 7% over *Tibet Plateau* (Riggs, 2003), which is more suitable to analyse the trend for at a long time span.


Fig. 3. The sample product from the NISE products, the SCA over China is also obvious for its Plateau shape in the right part of the map (EASE-GRID)

Satellite-Based Snow Cover Analysis and the

cover in red, March, 2003.(cited:

**2.2.3 Multisource satellite data processing method** 

characteristics over China by the averaging method.

**2.2.4 Onset, duration of the snow cover over China** 

C8.BP\_VIS35.png)

latitude and longitude.

Snow Water Equivalent Retrieval Perspective over China 53

Fig. 5. Northern Hemisphere average snow water equivalent (mm) from passive microwave, with additional area indicated as snow by Northern Hemisphere EASE-Grid weekly snow

http://nsidc.org/data/docs/daac/nsidc0271\_ease\_grid\_swe\_climatology/NL200303.NSID

According to the data characteristics mentioned above, the different projections and resolutions data need to be projected in the same project that could provide a same base for the later analysis. We select the equal latitude and longitude project to provide a more effective understanding for the China mid-latitude area. A tool has been developed to processing the EASE\_Grid, Polar Stereographic Projections into the 0.05 degree latitude and longitude map. Fig. 6 shows the transform scheme from the multi-projection to the equal

When all of these data products are resampled, we analyse the onset and duration of the data from the SCA products (named: IMS and NISE) using the accumulating, the first and the end day of the snow. The monthly SWE products are used to calculate the climatological

After all of the data mentioned above is projected into the same equal latitude and longitude grid. The IMS and NISE daily snow cover data are processed to the onset, duration and the end time map, the base-time for IMS product is 31/May, and the day of the year 183 (almost 31/May) for NISE products. The Global Monthly EASE-Grid Snow Water Equivalent

Fig. 4. Sample image derived from MODIS/Aqua Snow Cover Daily L3 Global 0.05Deg CMG data set 08 February 2004 (cited from http://nsidc.org/data/modis/data\_summaries/cmg\_sample.html)
