Monitoring and Projections of Climate Variability and Change

CW3E will develop a comprehensive understanding of the physics and the probabilistic and statistical characteristics of extreme precipitation events in the West to inform resource and risk management.

Key Objectives:

Raging river in a burn-scarred landscape.

1. Provide new insights from strategic weather and climate observations and from relevant research on historical and projected extreme events.
2. Enhance decision-makers’ conceptual understanding and instrumental knowledge of historical, current, and future extreme events.
3. Develop and apply new approaches for monitoring, characterizing, and predicting the changing physical processes, statistical characteristics, and associated risk profiles of extreme events.

Challenges and solutions:

Extreme events—including large swings between extreme precipitation and warm drought—are proving to be the most dangerous expressions of climate change in the West. CW3E research focuses on characterizing extreme events, including storm types, frequency, intensity, duration, and variability. Better understanding of the factors that influence extreme events can improve emergency preparedness by identifying situations that may lead to significant impacts. For example, most of the largest floods along the U.S. West Coast are caused by strong landfalling ARs.

In addition to historical diagnostics, projections of extreme events from global climate models can inform long-term planning in a changing climate, but current methods will need to be improved if that potential value is to be realized. Global climate models typically operate at resolutions too coarse to capture the most extreme aspects of weather or to be used for local-scale planning. Thus, downscaling techniques are needed. Given the large number of global climate models, it is important to evaluate which models best represent the regional climate—while also recognizing that the most accurate characterizations of future extremes will benefit from having more data points (i.e., an ensemble of models).

To integrate historical analyses and projections into long-term planning, results must be communicated to decision-makers. With the expertise of close collaborators at Scripps, the mandate from the state of California to incorporate climate change in planning, and a network of partnerships, CW3E can help broaden the reach and impact of its climate research.

Key Results:

• Gershunov et al. (2019) found that while the overall frequency of precipitation events in California is expected to decrease due to fewer non-AR-related storms affecting the region, ARs will likely account for a larger share of the total precipitation and drive an increase in the frequency of extreme precipitation events in a warming climate (Figure 1).
• Sierks et al. (2019) showed that 89% of extreme rainfall events (> 10 mm; 98th percentile of wet days) in the Lake Mead watershed during July-September are associated with anticyclonic Rossby wave breaking over the U.S. West Coast.
• Lamjiri et al. (2018) determined that on average, 10-40 and 60-120 hours of rainfall in southern and Northern California, respectively, are responsible for more than half of the total annual rainfall. Approximately 10-30% of the total annual precipitation at locations throughout the state is provided by only one large storm (Figure 2).
• Guirguis et al. (2018) showed how the interaction between different modes of climate variability influence important characteristics of ARs making landfall in Northern California, including synoptic evolution, orientation, and precipitation.
• Pierce et al. (2018) and Kalansky et al. (2018) assessed regional and local climate changes, and found that precipitation in California and San Diego County is projected to undergo a regime shift toward fewer precipitation days and more dry years.

Figure 1. Future changes in daily precipitation frequency binned by percentile of daily intensity (% of historical climatology) in the (A) Russian and (B) Santa Ana River basins. Darker grey indicates AR related precipitation and light grey non-AR precipitation. AR precipitation is going to increase, especially as it relates to extremes, the highest percentiles. The map (C) shows the percent increase in AR related precipitation throughout the West Coast of the US. Adapted from Figure 2, Gershunov et al. (2019)

Figure 2. (A) Median number of hours generating 50% of annual total rainfall, 1995–2016, and (B) median fraction of annual total rainfall from the largest rainfall event, 1995–2016. Adapted from Figure 2, Lamjiri et al. (2018).