Climatology of snow cover and snow water equivalent

• Chapter V: Building climatological time series of snow cover data


•  
• General remarks
• Cloud gap filling
• Extracting data for a particular location
• Validation techniques

Cloud gap filling

One of the main shortcomings of satellite observations is that the Earth's surface is often obscured by clouds, and it interrupts the temporal continuity of surface observations. There are several different gap filling techniques to reduce the number of cloud-covered pixels, which can also be used for other data gaps. The most common techniques are: 1) combining observations from different satellites (Wang et al., 2009; Gafurov, Bárdossy, 2009); 2) spatial filtering (Parajka, Blöschl, 2008); and 3) temporal filtering (Hall et al. 2010; Foppa, Seiz, 2012). One simple temporal filtering technique is explained here.

The temporal gap filling approach is based on assigning a cloud-covered pixel its latest cloud-free value. There are two ways to assign values to cloudy pixels: forward and backward filling. In forward gap filling the pixel is assigned its value prior to the cloudy days, while in backward gap filling the pixel is assigned the value of the first cloud-free observation following a cloudy period (Fig 1). Even though the number of consecutive days with cloud cover could accumulate over time, the gaps are filled with the same value provided from the latest cloud-free pixel.

Figure 1: Temporal data gap filling of cloud-covered pixels.

Daily "cloud-free" composites are generated using forward and backward gap-filling. This allows calculating monthly and annual numbers of snow cover days (SCD). It is assumed that exactly opposite gap-filling techniques minimizes over- and underestimations (Foppa, Seiz, 2012). The total annual number of snow days is derived by calculating the average of backward and forward composites. It should be noted, however, that this temporal gap-filling procedure is usable for data re-processing only. It is performed with the intention of deriving the total number of snow days on a yearly time resolution.