CHELSA high-resolution land surface temperature and precipitation

The following was contributed by Dirk Karger (Swiss Federal Research Institute, October, 2017):

Key Limitations:

• Low-level temperature inversions are not sufficiently represented in the data.
• orographic precipitation might be slightly overestimated in flat terrain.
• Bias correction of precipitation over the oceans is not reliable for now. Therefore only land surface areas can be represented.

Key Strengths:

• Better representation of orographic rainfall patterns in complex terrain at 30 arc sec. resolution. E.g. dry mountain valleys and wet mountain slopes.
• The combined approach of mechanistical + statistical downscaling of GCMs allows to apply the algorithm to future and past climatic scenarios.
• Better suited for species distribution modelling and ecological applicaiton than previous high resolution products.

Chelsa is especially built for applications in ecological, agricultural, and meterological studies which rely on very high spatial resolution data. The first validation results in show that including orographic precipitation effects can improve existing climatologies and reanalysis to a degree that the derived analysis (such as species distribution models, SDMs) show increasing accuracies. While CHELSA is an improvement over existing very high-resolution climatologies, it still exhibits errors which have been quantified in several ways (see Karger et al. 2017, Sci. Dat.). The validation of the main correction step in the precipitation algorithm that includes the orographic wind effects and boundary layer shows that the precipitation at the stations is better captured after the correction than before the downscaling to 30 arc sec resolution. The improvement varies by region and month with the majority of months showing an improvement, but also reducitions (increased noise) can occur. Most importantly, the better prediction with regard to SDMs in which precipitation and temperature data are used already indicates that CHELSA might be a substantial improvement over existing products which are currently being employed for such purposes.

All CHELSA products are in a geographic coordinate system referenced to the WGS 84 horizontal datum, with the horizontal coordinates expressed in decimal degrees. The CHELSA layer extents (minimum and maximum latitude and longitude) are a result of the coordinate system inherited from the 1-arc-second GMTED2010 data which itself inherited the grid extent from the 1-arc-second SRTM data.

Note that because of the pixel center referencing of the input GMTED2010 data the full extent of each CHELSA grid as defined by the outside edges of the pixels differs from an integer value of latitude or longitude by 0.000138888888 degree (or 1/2 arc-second). Users of products based on the legacy GTOPO30 product should note that the coordinate referencing of CHELSA (and GMTED2010) and GTOPO30 are not the same. In GTOPO30, the integer lines of latitude and longitude fall directly on the edges of a 30-arc-second pixel. Thus, when overlaying CHELSA with products based on GTOPO30 a slight shift of 1/2 arc-second will be observed between the edges of corresponding 30-arc-second pixels.

Areas just west of the dateline in Siberia are missing due to a bug in the code.

The dataset is in GEOtiff format.

Grid extent:

Resolution (decimal degrees): 0.0083333333

West extent (minimum X-coordinate, longitude): −180.0001388888

South extent (minimum Y-coordinate, latitude): −90.0001388888

East extent (maximum X-coordinate, longitude): 179.9998611111

North extent (maximum Y-coordinate, latitude): 83.9998611111

Rows: 20,800

Columns: 43,200

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