National Oceanic and Atmospheric Administration (NOAA) National Centers for Environmental Information (NCEI)
The Living Blended Drought Atlas (LBDA) is a recalibrated data series of June-July-August Palmer Modified Drought Index (PMDI) values on a 0.5 degree latitude/longitude grid, compiled by blending tree-ring reconstructions and instrumental data from the coterminous United States.
The Palmer Modified Drought Index (PMDI) is a hydroclimate index that employs temperature and precipitation information, as an operational version of the Palmer Drought Severity Index, to estimate relative dryness taking into consideration the cumulative nature of long-term drought. The Living Blended Drought Atlas uses tree-ring PMDI reconstructions to extend the instrumental record, yielding a record of drought distribution and intensity covering over 2,000 years. Tree-ring data were used up until 1978 AD, with instrumental data spliced in after that point. The reconstructed tree-ring values were recalibrated to instrumental conditions using a June-July-August PMDI derived from the GHCN 5km nClimGrid dataset. The recalibration was done using the Kernal Density Distribution Method (KDDM) outlined by McGinnis et al. (2015). When looking at PMDI values, increasing drought intensity is represented by increasingly negative numbers.
Where do these data come from?
Pre-instrumental values were taken from the Cook et al. (2010) Living Blended Drought Atlas (LBDA) summer PMDI values reconstructed on a 0.5 grid over North America. This reconstruction used 1,845 annual tree-ring chronologies to reconstruct past conditions. The regression procedures outlined in Cook et al. (1999) were used to develop the summer PMDI values from the tree ring network. The instrumental data are comprised of thousands of quality-controlled station values that go into the GHCN monthly temperature and precipitation data.