MMM Seminar - Thursday, April 23, 2020 - 3:30pm

Speaker: Gary Lackmann

Affiliation: North Carolina State University

Extratropical persistent flow anomalies (PAs), a subset of which are known as “blocking” events, are often associated with high-impact weather. Flooding, wildfires, heat waves, droughts, and cold-air outbreaks can be associated with PAs. These events can take place throughout the annual cycle, therefore, methods for their objective identification must be effective in all seasons. Despite their importance to society, the question of how the frequency, intensity, and duration of PAs will respond to climate change remains an open question. This is due in part to the complexity of this multifaceted phenomenon. Previous work has proposed increases in blocking and PA activity due to Arctic amplification. We can also hypothesize that the subset of blocking events resulting from latent heat release would to increase in frequency and intensity with warming owing to larger vapor content. Other GCM-based studies have found decreases in the frequency of blocking in warmer climates, while several studies have identified serious deficiencies in GCM representation of blocking. How will PAs change in a warmer climate?

Here, we present a new PA index, extending the method of Dole and Gordon (1983) to increase versatility. We apply this index to a reanalysis dataset, seeking trends in PA activity. We next examine changes in PA characteristics between the present climate and projected end-of-century conditions using high-resolution time-slice simulations from the Model for Prediction Across Scales (MPAS). In the future MPAS simulations, the Arctic Ocean is nearly devoid of autumn sea ice, making these runs ideal for studying the influence of Arctic amplification on midlatitude PA activity. Finally, we use an idealized oceanic channel model to study bivariate sensitivity of PAs to domain-average temperature and to baroclinicity (jet strength).

Using the ERA Interim reanalysis, we identify only weak trends in both positive and negative PA activity, including both annual and seasonal values. Results from the MPAS and channel model experiments will be shared at the seminar, because we would not want the abstract to give away all of the punchlines!

virtual seminar https://www.ucar.edu/live?room=fl21022

Viewers may submit their questions throughout the presentation to: Judith Berner; berner@ucar.edu

MMM Seminar - Thursday, March 5, 2020 - 3:30pm

Speaker: Andrew C. Winters

Affiliation: University of Colorado

The atmosphere often exhibits a three-step pole-to-equator tropopause structure, with each break in the tropopause associated with a jet stream. The polar jet stream (PJ) typically resides in the break between the polar and subtropical tropopause and is positioned atop the strongly baroclinic, tropospheric-deep polar front at ~50°N. The subtropical jet stream (SJ) resides in the break between the subtropical and the tropical tropopause and is situated on the poleward edge of the Hadley cell at ~30°N. On occasion, the latitudinal separation between the PJ and the SJ vanishes, resulting in a vertical jet superposition. Prior case study work indicates that jet superpositions are often attended by a dynamical and thermodynamic environment that is particularly favorable for the development of high-impact weather. Furthermore, this prior work suggests that there is considerable variability among antecedent environments conducive to jet superpositions. These considerations motivate a comprehensive examination of the relative importance of dynamical processes that operate within the double-jet environment to produce jet superpositions.

This study focuses on the identification of North American jet superposition events in the Climate Forecast System Reanalysis dataset during November–March 1979–2010. Jet superposition events are classified into three characteristic types: “Polar Dominant” events consist of events during which only the PJ is characterized by a substantial excursion from its climatological latitude band; “Subtropical Dominant” events consist of events during which only the SJ is characterized by a substantial excursion from its climatological latitude band; and “Hybrid” events consist of events characterized by an excursion of both the PJ and SJ from their climatological latitude bands. Following their classification, frequency distributions are constructed to highlight the geographical locations most often associated with jet superpositions within each event type. Composite analyses are also constructed in an effort to illuminate the dominant dynamical processes that support the production of jet superpositions for each event type. Predicated on the results from the preceding analyses, the relevance of jet superpositions to the development of high-impact weather within a changing climate is discussed.

Refreshments: 3:15 PM

*MMM Seminar - Thursday, February 20, 2020 - 3:30pm

*Please note special location - FL2-1001/Small Auditorium

Speaker: Samuel Childs

Affiliation: Colorado State University

Eastern Colorado is one of the most active hail regions in the U.S., and recent damaging events have affirmed the need for improved prediction of hailstorm characteristics and effective warning communication.  This work offers a multidisciplinary synthesis of eastern Colorado (37-41°N, 102-105.3°W) hailstorms.  Climatologically, severe (1.0 in+) hail reports and days are increasing across the domain since 1997, although hail is preferentially reported where people live or drive.  Still, the upward trend in hail days is not seen in the national record.  To estimate how the frequency and spatial distribution of hailstorms across eastern Colorado may change by the end of the 21st century, threshold exceedances of convective proxies for severe hail reports are compared between control and future high-resolution dynamically-downscaled WRF simulations.  An increase of up to 3 severe hail days per year is projected by the period 2071-2100, with the highest increase across the north-central eastern Plains.  These projections, paired with high-resolution population projections taken from the Shared Socioeconomic Pathways, are input into a Hail Monte Carlo model, which predicts an increase in human exposure up to 178% by 2100.  Results are sensitive to the overlap between future population and meteorological projections, however, and simulations that predict decreasing human exposure have a corresponding increase in agricultural exposure due to hailstorm frequency increasing most in places where population is not expected to grow.  An interview study conducted in Summer 2019 with eastern Colorado agriculturalists revealed feelings of anxiety and dejection from hailstorms due to the financial losses incurred.  Farmers also highlighted that small hail, either in large volume or driven by a strong wind, is most damaging to crops, which is contrast to the 1.0 in severe threshold used by the NWS.  Results from this work are bringing awareness of the vulnerabilities faced by the agricultural sector and inspiring continued research into hail prediction.

Refreshments: 3:15 PM

MMM Seminar - Thursday, February 6, 2020 - 3:30pm

Speaker: Ian Bolliger

Affiliation: UC Berkeley

Hurricanes are one of the costliest natural disasters, imparting over $50 billion in losses per year in the U.S. alone. Because each event can cause significant immediate damage, as well as long-lasting negative impact, the resilience of coastal regions depends crucially on quantifying the present and future risk of hurricane-driven economic losses. In this study, we construct and apply an open-source, physical-econometric catastrophe model that directly simulates high-resolution storm surge and wind from >100,000 real and synthetic storm events, accounting for probabilistic local sea level rise. We merge this hazard model with a property sale price dataset for all U.S. properties and insurance claims data from historical storms to empirically derive a damage function for both of these hazards. We then project damages from 1980-2100 using seven climate models, two emissions scenarios, and 100 stochastic realizations of each hurricane season. We use the distribution of realized losses to quantify how hurricane risk has changed across the Atlantic and Gulf coastlines over the past 35 years and how it may change over the next century. Preliminary results suggest that the U.S. economic risk from hurricanes may have grown by as much as 100% relative to a 1980s baseline and may further grow another 300% by the end of the century, under RCP 8.5. These results have informed reports and regulation from Blackrock Investment Institute, the First Street Foundation, the American Flood Coalition, and the Bank of England, each of which seek to better account for changing hurricane patterns in assessing risk to their assets and/or population.

Refreshments: 3:15 PM