Jul 30, 2015

Assessment of precipitation anomalies in California using TRMM and MERRA data

Andrey K. Savtchenko, George Huffman, Bruce Vollmer
Journal of Geophysical Research: Atmospheres
  • Reassesses certain aspects of the precipitation climate in California from the past decades using modern satellite (Tropical Rainfall Measuring Mission, TRMM, 1998–2014) and reanalysis (Modern‐Era Retrospective Analysis for Research and Applications, MERRA, 1979–2015) data
  • States that California has a well‐pronounced rain season that peaks in December–February; however, the 95% confidence interval around the climatological precipitation during these months imply that deviations on the order of 60% of the expected amounts are very likely during the most important period of the rain season
  • States that while these positive and negative anomalies alternate almost every year and tend to cancel each other, severe multiyear declines of precipitation in California appear on decadal scales
  • Finds that the 1986–1994 decline of precipitation was similar to the current one that started in 2011 and is apparent in the reanalysis data
  • Finds that, in terms of accumulated deficits of precipitation, that episode was no less severe than the current one
  • States that while El Niño (the warm phase of the El Niño–Southern Oscillation, ENSO) is frequently cited as the natural forcing expected to bring a relief from drought, this assessment finds that ENSO has been driving at best only 6% of precipitation variability in California in the past three decades
  • Shows that the likelihood of losing the most intensive precipitation events drastically increases during the multiyear drying events using fractional risk analysis of precipitation during typical versus drying periods
  • States that storms delivering up to 50% of the precipitation in California are driven by atmospheric rivers making landfall
  • States, however, that these phenomena can be suppressed and even blocked by persistent ridges of atmospheric pressure in the northeast Pacific
  • Concludes by noting that the reanalysis and satellite data are proven to be reliable to the extent where they yield information on developing conditions and observed precipitation anomalies