Methods

In this work, we compare the ability of the MODIS LST and SSM/I products to detect melt on the Greenland ice sheet, using several measures. The focus of this study is the melt

seasons of 2006 and 2007, which includes the months May through October. Those years were chosen because a record is available of the SSM/I and MODIS products, and also because of the large difference in the amount of melt that occurred during those seasons.

Attention was first given to the differences in scale of the two products. The SSM/I product from Mote [2007] used in this study is available in a 25km grid in a polar stereographic created by the National Snow and Ice Data Center (NSIDC) for use with passive microwave data sets in polar regions [Maslanik and Stroeve, 1990]. All of the SSM/I melt products used here are archived in hexadecimal format in a 61x111 subset of the NSIDC polar grid, covering Greenland and surrounding areas. The MODIS MOD11A1 Daily LST product used by Hall [2008a and b, 2009a] is in a sinusoidal 0.928 x 0.928km grid (nominally 1km) in a flat binary format,

converted from Hierarchical Data Format (HDF). The MODIS product was resampled using the nearest neighbor method to a polar stereographic projection at 1km resolution to nest within the NSIDC 25km grid.

For this work the threshold of LST ≥ -2ºC was used to signify the detection of melt by MODIS LST, after reviewing the results of Koenig and Hall [2010]. To assess how the products’

different spatial resolutions affected their detection of melt, the percentage of 1km MODIS cells detecting melt was determined for cases when the corresponding 25km SSM/I cell had a TB greater than the modeled threshold, thus indicating melt.

Time series were constructed to compare the spatial extent of melt indicated each day by the products using only non-cloud-contaminated areas. A 25km MODIS cell was considered non-cloud-contaminated if fewer than 30 1km cells had cloud cover otherwise the MODIS cell was considered cloud-contaminated and its data were not used. MODIS LST was considered to indicate melt when a majority (>50%) of 1km cells were detecting melt in the 25km cell. A time

series was also constructed showing the spatial extent of melt indicated by SSM/I for all cells and was then compared to the non-cloud-contaminated series to gain a better understanding of the amount of melt that is missed by MODIS due to cloud contamination.

The onset and duration of melt detected by each product were compared, as done by Hall et al. [2008a and 2009a] using MODIS and Quick Scatterometer (QuikSCAT) melt products.

Onset was simply indicated by the first day that melt was detected at each cell, while duration was calculated by subtracting the onset day from the last day of melt that was detected at each cell. The difference between the two products’ onset and duration was then calculated for each cell. Histograms were constructed, with positive values representing SSM/I with an earlier (longer) onset (duration). The products’ annual melt extents were then compared, showing all areas where melt was indicated for two or more days of the season. Daily melt extent was compared using the time series mentioned above as well as by creating maps for a few select days with large melt extent.

The data were stratified by the eight drainage basins, as defined by Ohmura and Reeh [1991] (Figure 3.1). Contingency tables were constructed for each region to evaluate the

agreement between the two products within the regions. These tables show the number of 25km cells with or without melt for MODIS and SSM/I as well as the number of cloud-contaminated cells indicated by MODIS. The significance of each table was assessed using Pearson’s chi-square test.

The data were also stratified by 500m elevation bands, with the same contingency table methods as discussed above, to determine how the two products’ sensitivities differed over the multiple elevation bands. The bands were used in place of ice sheet zones, such as those described and mapped by Benson [1962], which include the ablation zone, soaked-snow zone,

percolation zone, and the dry-snow zone. The upper bound of the ablation zone, which is the equilibrium line, is assumed to generally lie between 1200 and 1500m a.s.l. [Zwally, 1989] in the southern portion of the ice sheet, and on the eastern margin can range from 900m a.s.l. in the south to 500m a.s.l. in the north [Blatter and Ohmura, 1991]. However, the lines may have changed since the publication of these papers. Contingency tables were once again constructed for the entire ice sheet for both seasons to assess agreement between the two products’ melt detection and SSM/I’s detection of melt during periods of MODIS cloud cover. The contingency table data were mapped to show the fraction of time both products agreed (either both products indicating melt or both products indicating no melt). Areas where MODIS indicated melt while SSM/I did not were specifically identified and assessed.

Blended maps for days with maximum melt extent from each season were created using both the SSM/I and MODIS LST melt products to show areas of agreement or disagreement, as was done in Hall et al. [2009a] using MODIS and QuikSCAT. Categories of melt on the blended maps (1) areas where only SSM/I indicated melt, referred to as “SSM/I melt only” and shown as blue; (2) areas where only the MODIS LST product indicated melt, called “MODIS melt only”

and shown as red; (3) areas where the products agreed and both indicated melt, referred to as

“both melt” and shown as purple; and (4) areas that were considered cloud-contaminated and therefore had no MODIS data, called “MODIS cloud cover” and shown with hatch marks.

GC-Net AWS surface temperature records were retrieved for the stations with available records during 2006 and 2007. Hall et al. [2008b] established criteria for using the AWS air temperature as an approximation for surface temperature in order to compare MODIS LST to AWS temperature. These criteria include a wind speed greater than 4 m s^-1, a downward solar radiation of less than 240 W m^-2, and a temperature below freezing. Under these conditions,

temperatures are assumed to be vertically uniform within a few meters above the surface.

However, this research aims to distinguish when each product indicated melt, therefore the temperature criterion is not useful. Also, downward solar radiation is greater than 240 W m^-2 during peak heating on clear days. Because we are also comparing to the microwave data, it would be inappropriate to use a time of day when temperatures are likely substantially below the daily maximum. Instead, daily AWS temperatures from 1900 UTC were used, which was generally the warmest hour of the day. A threshold of 0º C was used to indicate melt. The number of days that melt was indicated at each GC-Net AWS location was compared with the number of days that MODIS and SSM/I indicated melt for the encompassing grid cell during non-cloud-contaminated times. Comparisons were also made by extracting simple time series from the 25km cell of SSM/I TBs (also the melt threshold TB for that cell), percentage of MODIS LST≥-2ºC, and the AWS air temperatures.