A.    Proposed Title of Study:


The Role of Atmospheric Rivers and Precipitation-forming Mechansims in Peak 2-Day Rainfall Events at San Francisco since 1950


B.     Context


Much research has been undertaken showing the effectiveness of Atmospheric Rivers (AR) in being associated with extreme rainfall events over regions of elevated topography in California. Precipitation occurs when the water vapor in the atmosphere is converted to clouds and precipitation by various atmospheric processes. In the concept diagram below, AR would connect to the first left hand box and precipitation-forming mechanisms in the middle black box. There is a widespread mistaken notion that the presence of AR alone controls precipitation totals. An ingredients-based approach conceptually relates precipitation totals (far right box) to the magnitude of the terms in the first two boxes. AR, in general, maximizes the left hand box, and topographic lifting is a major, but not the only, contributor to the middle box.



The 2017 Senior Project by Xiomara Aguirre showed that the peak yearly rainfall events during the period 1986-2016 at San Francisco, where rainfall mechanims are largely unaffected by topographic lifting, were also generally associated with AR. However, many of these events were unremarkable statistically. Thus, she found that the assumption that the presence of AR guarantees an extreme rainfall event was untrue, at least for the 30 years of data she examined.


In this study, we propose to find the top 20 2-day rainfall totals since 1950 at San Francisco, to determine the statistical remarkability of those totals and to determine if AR were associated with each of those totals. We also propose to do a case study analysis of the 1st, 10th, and 20th ranked events to investigate the nature of the water vapor fields and preciptiation forming mechanisms for these events.


C.      Research Question


What ingredients controlled precipitation amounts for the most extreme 2-day rainfall events at San Francisco since 1950?


D.     Hypothesis to be tested


The presence of AR maybe a necessary but not a sufficient condition for extreme precipitation amounts at San Francisco. 


E.      Background Needed


F.      Data or Analyses Needed


      Maximum daily (48 h)  precipitation total  for San Francisco Downtown (with longest record in California) for the period 1950-2017 and the date(s) over which this occurred. 


      Rainfall depth-duration-frequency tables to establish the uniqueness of the rainfall totals found.


      Charts to Visualize or Locate Atmospheric Rivers constructed from North American Reanalysis data sets.


·      Standard synoptic meteorological charts and diagrams to illustrate dynamical and thermodynamical control on vertical motion for these events

G.    Data Sources


San Francisco Daily Rainfall




Rainfall Depth Duration Frequency Tables


NOAA, 2016: Point Precipitation Frequency (PF) Estimates.  NOAA Atlas 14 Point Precipitation Frequency Estimates, Volume 6, Version 2, Avaialbe Online from NCDC.




Charts for Case Studies






NCEP Reanalysis Plotter




Sounding Site for Raw Soundings for RAOB Plotter




H.    Steps


  1. Atmospheric River Background Information (Review and One Paragraph Summary, Feb 8, 2018)
  2. Obtain Peak Two-Day Rainfall Amounts (Obtain Data, Feb 8, 2018; Rank Order of Peak 2-Day Rainfall Events, Feb 15, 2018)
  3. Determine Statistical Remarkability of Each Amount (Use DDR Calculations to Determine Return Period information for the Peak 2-Day Rainfall Events, Feb 22, 2018)
  4. Rank Order the Amounts with Return Period Information (Excel Spreadsheet Feb 22, 2018)
  5. Obtain Precipitable Water Plots Mid Event (Feb 28, 2018)
  6. Characterize the Magnitude of the PWAT for Each Event (March 7, 2018; Record in Spreadsheet made in Step 4 above)
  7. Yes/No for AR for Each Event (March 7, 2018; Record in Spreadsheet made in Step 4 above)
  8. Characterize the Ingredients: Water Vapor and Lifting Mechansims for the 1st, 10th, and 20rh Ranked Events by Case Study Analysis (Plan and Sources of Data, Due Feb 22, 2018; Completion of Preliminary Work, Feb 28, 2018; Final Graphics, March 21, 2018)
  9. Conclusions


I.     Procedure


H1 to H4 Procedural Issues Worked Out by Feb 15, 2018 (Mentor John Monteverdi)


J. Timeline (Preliminary Work Completion Date; Graphics or Table Production Completion Date; Section Summary Completion Date)


Atmospheric River Background (H1 ) (Feb 8, 2018; Feb 15, 2018; Feb March 7, 2018)

Rainfall Information and Analysis (H1, H2, and H3) (Feb 8, 2018; Feb 15, 2018; February 22, 2018)

Precipitatble Water Charts and Analyses (H5, H6, and H7) (Feb 28, 2018; March 7, 2018; March 14, 2018)

Synoptic and Mesoscale Case Studies(H8) (Feb 22, 2018; Feb 28, 2018; March 21, 2018)



K. References


General Atmospheric River Definition/Review Articles

Ralph, F.M., M. Dettinger, D. Lavers, I.V. Gorodetskaya, A. Martin, M. Viale, A.B. White, N. Oakley, J. Rutz, J.R. Spackman, H. Wernli, and J. Cordeira, 2017: Atmospheric rivers emerge as a global science and applications focus. Bulletin of the American Meteorological Society, early online release, http://dx.doi.org/10.1175/BAMS-D-16-0262.1


Neiman, P. J., M. Hughes, B.J. Moore, F.M. Ralph, and E.S. Sukovich, 2013: Sierra barrier jets, atmospheric rivers, and precipitation characteristics in northern California: A composite perspective based on a network of wind profilers. Monthly Weather Review, 141, 4211-4233.


Importance of Atmospheric Rivers to West Coast Environment


Albano, C.M., M.D. Dettinger, and C.E. Soulard, 2017: Influence of atmospheric rivers on vegetation productivity and fire patterns in the southwestern U.S. Journal of Geophysical Research: Biogeosciences122, 308-323. http://dx.doi.org/10.1002/2016JG003608.


Dettinger, M. D., 2013: Atmospheric rivers as drought busters on the US west coast. Journal of Hydrometeorology14, 1721-1732. 


Weaver, R., 1962:  Meteorology of hydrologically critical storms in California, U.S. Weather Bureau, Hydrometeorologcial Report #37, 207 pp. https://babel.hathitrust.org/cgi/pt?id=uiug.30112101032230;view=1up;seq=1


How Atmospheric Rivers Relate to Cyclones


Cordeira, J.M., F.M. Ralph, and B.J. Moore, 2013: The development and evolution of two atmospheric rivers in proximity to western North Pacific tropical cyclones in October 2010. Monthly Weather Review, 141, 4234-4255.