By Jonathen Beliera and John Monteverdi

1. The Role of Atmospheric Rivers and Precipitation-forming Mechanisms in Peak 2-Day Rainfall Events at San Francisco since 1950 By JonathenBeliera and John Monteverdi

2. Question? • What ingredients controlled precipitation amounts for the most extreme 2-day rainfall events at San Francisco since 1950? • Why is this an important question? • The presence of an atmospheric river (AR)may be a necessary, but not a sufficient condition, for extreme precipitation amounts at San Francisco. • What other mechanisms are present in these most extreme cases?

3. More Questions • Why San Francisco of all cities in CA? • San Francisco has the longest standing rainfall record in the state, dates as far back as 1850. • If 1850 is the farthest date, then why use data from only 1950 and onward? • Resources that allow for the construction of water vapor patterns in the atmosphere are only reliable up until 1950,

4. Background • Definition of atmospheric river (AR)- • An Atmospheric River is a wide (~200-300 km), deep (~6 km; 18000 feet), and long (~2000 km) plume of water vapor extending from the tropics or subtropics to the West Coast of the United States, usually in advance of a frontal system (Ralph et al., 2004) • Atmospheric Rivers transport over 90% of all water vapor in a pole-ward direction in plumes that total less than 10% of the circumference of the world at their latitude of origin. (Dettinger et al., 2015) • Contains 90% of all poleward water transport, yet only covers 10% of all zonal area on Earth.

5. Although the presence of an Atmospheric River, visualized by plumes of precipitable water, guarantees a copious amount of water vapor (the left hand box above), whether precipitation occurs or is heavy depends upon the presence of atmospheric processes leading to precipitation formation (black box). This diagram was a basis to my small case study with the idea of what other precipitation forming mechanisms go into these significant events that might not be atmospheric river events.

6. Data Needed • 2-Day rainfall climatology at SF Downtown since 1950 • Obtained from the National Climactic Data Center • Import rainfall data into Excel and stratify it on the basis of the greatest 2-day rainfall totals

7. Methods- Processing the Data • Connection between AR’s and significant rainfall • Determine recurrence interval of these events. Are they significant enough? • Based off of California highway and culvert standards • Any event with a recurrence interval of 5 years or more • Decide on the presence of an AR or not, based off of satellite imagery and precipitable water plots.

8. Data Set

9. Atmospheric River Event [(a) > 20 cm precipitable water (b) long/wide plume of water vapor stretching from tropics]

10. Non-Atmospheric River Event [(a) > 20 cm precipitable water (b) no long/wide plume of moisture from tropics]

12. 1982 (#1) Event 2008 (#14) Event Individual Case Studies • Case Study for the #1 and #14 event • Gather: • Sounding data • Analyzed Surface Charts (Temp and SLP) • Vertical Velocity data at 500 mb. • Analyze each case and find what's different. • #1 is an AR event (100 yr. recurrence) • #14 is not an AR event (5 yr. recurrence)

13. Findings • All of the top twenty events can be characterized as “extreme” since all had at least a return period of 5 years (20% chance of occurrence in any given year) • 10% of the events had a recurrence interval of 100 years (1 % chance of occurrence in any given year); 15% of the events had a recurrence interval of 50 years or greater (2% chance of occurrence in any given year); and the minimum recurrence interval of 5 years was found for 50% of the events • Atmospheric Rivers were present for 75% of these events. The two 100 y events were associated with Atmospheric Rivers • 20% of the top 20 events were not associated with Atmospheric Rivers • Although precipitable water values were largest for events with a 10 year return period or greater (23.5 kg/m^2 = 2.35 cm) than for those with 5 year return periods (21.8 kg/m^2 = 2.18 cm), rank within the top 20 was not necessarily associated with higher precipitable water values • The study shows that Atmospheric Rivers are associated with most but not all of the most extreme rainfall events at San Francisco Downtown since 1950 • The study shows that the presence or absence of an Atmospheric River does not absolutely guarantee an extreme event.

14. Case Study Findings • The 1982 and 2008 events both have water vapor values above 20 cm. However, the 2008 event lacks the presence of an AR within the designated 3x3 grid within San Francisco. Therefore, 1982 is an AR event and 2008 is not. • The case study analyses show that for the most extreme Atmospheric River events, dynamical mechanisms for the lofting of the water vapor to produce clouds and precipitation were stronger than typical, and that the non-Atmospheric River extreme events had additional lofting associated with convection/instability, which is itself NOT a feature of Atmospheric Rivers. • Skew-T Diagrams show a convective environment for the 2008 event with the possibility of moderate values of CAPE. • A warm frontal inversion is visible on both plots being supported by a veering of winds through the troposphere. • Given the presence of a warm front in the region, frontal lifting added to the strong, atypical, dynamics associated with both events. • Vertical velocity values exceeded climatological values for the West Coast for both events. However 2008 had greater values.