Spatial and Temporal Drought Patterns Derived from High-Resolution Daily SPI and SPEI Datasets

A suite of gridded daily satellite (CMORPH, IMERG) and in-situ (NClimGrid) precipitation datasets are used to compute a near-real time standardized precipitation index (SPI) over various time scales (from 1-month to 36-month). Over CONUS, the Standardized Precipitation Evapotranspiration Index (SPEI) is also computed using daily potential evapotranspiration (PET) derived from NClimGrid daily temperature estimates. The drought indices: CMORPH-SPI (global; 1998-present; 0.25x0.25deg.), IMERG-SPI (global; 2000-present; 0.1x0.1deg.), NClimGrid-SPI and NClimGrid-SPEI (CONUS; 1951-present; 0.05x0.05deg.) are used to perform a historical analysis of drought events and derive long-term statistics on drought occurrences, duration, and severity at the local, national, regional, and global scales. The impact of precipitation and temperature (i.e., PET) changes is assessed by considering several reference periods such as different durations (i.e., from a decade to the full period of record) and different time frames (i.e., 1961-1990, 1971-2000, etc.). The evolution of the distribution parameters (Gamma, Pearson III) computed for an ensemble of reference periods allows to account for long-term change in temperature and precipitation patterns. In addition to the drought indices (SPI, SPEI), the year-to-date rainfall deficit is estimated with respect to drought classification (abnormally dry, moderate, severe, extreme, exceptional) and the impact of isolated or multi-day rainfall events on drought conditions is evaluated. This work provides a better understanding of drought propagation across a continuum of accumulation scales and allows to estimate the likelihood of any deviations from normal rainfall conditions to evolve into meteorological drought.