Friday, 31 October 2014

OSTIA water/land mask and new inland water bodies dataset

Laura Carrea and I have been looking at the OSTIA water/land mask in comparison with the new higher resolution dataset from the Landcover CCI project. The OSTIA mask is at 0.05 deg lat-lon resolution, and is used for the SST CCI Analysis product (gap filled daily SSTs); its precise heritage (since being created several years ago) hasn't been re-traceable, so it is useful to check its nature.

The plot below shows for a small area (Baltic Sea) the prevalence of water (according to the LC CCI product) within each 0.05 deg cell which is labelled in OSTIA as land. Red colours indicate the presence of a small fraction of water in an OSTIA "land" cell, and rivers and the many lakes on land are obvious. Dark blues indicate total or near-total water in an OSTIA "land" cell. Some of these are inland lakes no resolved in OSTIA, which is no surprise. There are a few cells around the Baltic coasts that are apparently water-filled (dark blue) yet are "land" cells in the OSTIA mask. Globally, however, such cases are exceptions. In general the coasts don't have a fringe, which suggests that the OSTIA mask is designed such that a cell tends to be labelled "water" if there is any significant fraction of sea within the cell. (If the design were such that a cell was labelled "water" if it contains >50% of sea, then there would be a fringe of intermediate values of %water-in-land-cell all coastlines.) So OSTIA must use a "fat" water mask, rather than a mask that is neutral with respect to land and water.

This next plot shows the prevalence of land in cells labelled water in OSTIA. Here, blue colours indicate a small amount of land, and red a lot of land in the cell. The fact that the whole coastline tends to be fringed with colour confirms that the OSTIA mask is "fat" with respect to water. (That is, truly mixed cells tend to be labelled as sea, so there is a fringe of land-in-water-cell cases along coasts.) However, it is also clear that there are many "water" cells that are in LC CCI completely land (dark red in this picture). These appear preferentially on northern coasts. This is consistent globally, and not just around the UK. This suggests that there is an offset in the N-S direction, roughly half an OSTIA cell in size, in the OSTIA mask relative to the Landcover CCI data set.

Where an OSTIA cell is flagged as sea and is in fact filled with land, there will be (or should be) no satellite SST retrievals ever available for that cell -- the data will always be provided by the gap-filling procedures associated with creating the L4 SST CCI analysis.

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