6.2 The Air Force Surface Spatial Climatology Dataset

Friday, 13 June 2014: 8:15 AM
Church Ranch (Denver Marriott Westminster)
Raymond B. Kiess, AWS, Asheville, NC

The Air Force Surface Spatial Climatology Dataset

The U.S. Air Force's 14th Weather Squadron (14WS) provides climatology support for the Department of Defense and other national agencies. Increasingly, the 14WS has been asked to provide support in data sparse regions where authoritative climatology does not exist or to present climatology data in map or GIS compatible formats. To satisfy these requests, we have built the Surface Spatial Climatology Dataset (SSCD). The SSCD is a near global, land mass climatology dataset with a nominal resolution of 10km or .083 degrees and a latitudinal extent from 60S to 90N, which excludes Antarctica. It contains spatial layers of the 45 parameters of the 14WS Operational Climatic Data Summary (OCDS) for each monthly and hourly representation totaling over 1800 unique layers. In addition to standard climatology parameters such as mean high temperature, the SSCD and OCDS contain derived parameters such as Days of Temperature > 90 degrees F, Maximum Pressure Altitude, and ceiling conditions – parameters not available in other gridded datasets. In that regard, this makes the SSCD a unique dataset.

The SSCD is built by blending and interpolating climatology analyses from several sources. The two primary sources are the 10,000 world-wide OCDS products and analyses derived from the Climate Forecast System Reanalysis (CFSR) and continuing with the Climate Forecast System Version 2 (CFSV2) analyses. We also use the WorldClim (worldclim.org) data to augment temperature and precipitation analyses and the Air Force Weather Agency's World-Wide Merged Cloud Analysis (WWMCA) to provide mean cloud cover. Improvement efforts will incorporate Global Precipitation Climatology Centre (GPCC) data and lightning-derived thunderstorm analysis using Earth Networks Total Lightning Network Data later this year.

The process begins by creating the 10km grids from each source for each applicable parameter by applying an inverse distance weighted (IDW) interpolation using ESRI's GIS software. However, interpolating in areas of complex terrain and/or poor spatial density with an IDW often produce poor results. For example, in complex terrain, the OCDS value may not be representative for adjacent areas with different elevations. To compensate, we apply two treatments. For temperatures, which have a known change in regards to elevation, we use an elevation correction to bring the point temperatures to sea level, do the interpolation, and then apply the elevation lapse rate at the 10km grid cells. For other parameters, we compute a mask that flags grid points in which the IDW is unreliable. Those grid points will be given a greater weight by other data sources, namely the CFSR-derived analysis.

The CFSR-derived analysis begins by creating a time series of hourly pseudo-observations from the analysis and forecast fields for surface air temperature, dew point temperature, station pressure, wind direction and magnitude, and at every 6 hours, the precipitation rate from which a precipitation amount is derived. These data are then summarized into a monthly time series beginning in 1979 and extending to the present. The temperature and precipitation time series from 1979-1999 were summarized in order to produce bias correction factors derived from comparison against the WorldClim climatology. This bias correction has been reapplied to the observational time series so that bias corrected monthly time series can be computed including threshold dependent parameters such as temperature days > 90F or temperature days < 32F. The latest 25 years are summarized and then interpolated by IDW to the 10km grid.

The final climatology layers are made by blending the multiple sources. For layers in which the OCDS is the only source, we assume the quality-controlled data as truth and no changes are made. For layers in which OCDS is blended with other sources, a determination is made on whether the complex terrain mask should be applied. If it is and the grid point is flagged as complex, the OCDS analysis is set to NULL for that point. Otherwise, the available analyses are blended by using the GIS Mosaic operation, assigning equal weights to all of the input.

The grids are quality controlled by comparison. Rules are set to flag grid points and coincident OCDS points that differ by more than the threshold amount. These are examined to determine the cause and if an OCDS data error was the result, the OCDS is corrected and reprocessed into the blend.

The final grids become the SSCD and are used to support map based climatology and single point exploitation. One set of products are the 14WS country climatologies which are regional maps of primary parameters. The data are hosted on the 14WS GIS server and accessed through a web application which gives the user full control to create customized climatology maps. These maps have quickly become one of the most accessed of 14WS web-based products fulfilling many requests for climatology in the Middle East and Africa and have been used to assess conditions for the 2014 Afghanistan elections and humanitarian relief plans in and around Syria.

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