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The current version of the NASA Surface meteorology and Solar Energy Datasets, SSE (Rel. 6.0), derived its monthly mean direct normal irradiances (DNIs) directly from the NASA GEWEX SRB (Rel. 3.0) monthly mean global horizontal irradiances (GHIs) and the employed methodology relates the DNI to GHI in terms of monthly mean clearness index, latitudinal range, and solar geometry on the monthly average day. The coefficients of the relations were empirically derived from regression analysis of the BSRN data over the period from 1992 to 2005. While the derived monthly mean SSE (Rel. 6.0) DNIs show reasonable agreement with their BSRN counterparts, improvements in the quality of the dataset and DNIs on shorter time scales were realized through the application of the D1RINDEX model to the 3-hourly GHIs of the NASA GEWEX SRB (Rel. 3.0) data to produce 3- hourly DNIs. The DIRINDEX model is a combination of the DIR1NT model and a simplified version of the SOLIS clear-sky model, and the model was modified to accommodate a wider range of input parameters than originally designed. Additional input parameters include solar geometric, geographic and atmospheric parameters. The results are 3-hourly, daily, and monthly mean DNIs spanning 2000 to 2005 on a quasi-equalarea grid system of 44016 grid cells covering the entire globe. The data are then re-gridded onto a l°x l° grid system for the convenience of users. The resulting monthly mean DNIs show better agreement with the BSRN data than the SSE (Rel. 6.0) DNIs do. To further improve the results, we assimilated the ground-based BSRN data into the DNIs using the inverse-distance interpolation in the latitude-longitude-cosine of solar zenith angle phase space. [ABSTRACT FROM AUTHOR]

ASHRAE Technical Committee 4.2, Climatic Information, publishes a quadrennial update of climatic design information in Chapter 14 of the ASHRAE Handbook--Fundamentals (ASHRAE 2009a). The design information for 5564 locations around the world is based upon hourly values of dry-bulb temperature, wet-bulb temperature, dew-point temperature, wind speed and direction, surface pressure, and solar radiation. The 2009 design conditions provided a significant enhancement over the 2005 design conditions with respect to the global coverage--564 locations in 2009 versus 4422 locations in 2005. The ASHRAE design conditions, based on meteorological data, are calculated using hourly surface data from stations having a minimum of 8 years of observations, but more stations typically span 25 years of observations; although frequently the time series are discontinuous. The ASHRAE solar-related design conditions are based on model-derived solar radiation. A potential source for both global and time contiguous meteorological and solar data is NASA's POWER (Prediction of Worldwide Energy Resource)web portal (NASA 2013a). This includes the recently available meteorological data based on an improved reanalysis model--Modern Era Retrospective-analysis for Research and Applications (MERRA). MERRA yields global, hourly surface meteorological parameters for the years 1981 to present. An initial evaluation of the MERRA daily maximum, minimum, and averaged temperatures indicates accuracies sufficient to warrant their use to supplement existing surface observations. In this paper, we present an evaluation of the accuracy of the MERRA daily temperatures, followed by an assessment of the applicability of the MERRA hourly temperatures in the development of annual dry-bulb climate design criteria and annual heating and cooling degree-days over the continental United States. [ABSTRACT FROM AUTHOR]