Climate and Hydrologic Extremes Working Group

Objective

The central objective of the CHExWG is to produce future scenarios of extreme climate and hydrologic events grounded on past conditions that have impacts on urban infrastructure. This will promote better urban planning through the provision of realistic and city-tailored future scenarios of climate and hydrologic extremes.

Climate and Hydrologic Extreme Hazards

The WG identified several weather, climate, and hydrologic extreme hazards that can cause large impacts on urban systems. The specific types and the nature of the impacts are:

• Extreme rain (warm season): can be very intense with damaging flash flooding
• Extreme rain (cold season): usually less intense but longer lasting with longer lasting flooding
• Drought: municipal water shortages
• Heat waves: morbidity/mortality
• Snowstorms, freezing rain, winds-from winter storms (extratropical cyclones): transportation and economic disruption; power failure
• Hurricanes: injuries/death, property damage, catastrophic flooding
• Severe thunderstorms-tornadoes, hail, damaging winds: property damage, injuries/deaths
• Cold waves: morbidity/mortality
• Coastal/tidal flooding: property damage
• Haboobs/sandstorms: transportation disruptions, property damage

The city representatives in the working group evaluated the frequency of occurrence for each city in the network. They were ranked on scale of 0-4 as follows:

• 0 = Improbable (the probability of the occurrence of the hazard is zero)
• 1 = Remote (the hazard is not likely to occur in the system lifecycle, but it is possible)
• 2 = Occasional (the hazard is likely to occur at least once in the system lifecycle)
• 3 = Probably (the hazard is likely to occur several times in the system lifecycle)
• 4 = Frequent (the hazard is likely to occur cyclically or annually in the system lifecycle)

The results are displayed in Figure 1. Extreme rainfall in either the warm season or the cold season is a frequent occurrence in all of the network cities. The frequency of the other hazards varies considerably by city. In all cities, systems are likely to be affected by multiple hazards during their lifetime.

Future Changes in Hazards

As the globe continues to warm in response to increasing concentrations of greenhouse gases, the frequency and severity of these hazards is likely to change, affecting the vulnerability of urban systems. There is high confidence in the following changes:

• Extreme rainfall will increase in intensity and frequency
• Heat waves will become more intense
• Cold waves will become less intense
• Coastal/Tidal flooding will increase

There is lesser confidence in changes in the other hazards.

Quantitative estimates of the magnitude and nature of climate changes are usually obtained from simulations by global climate models under various scenarios of future changes in greenhouse gas concentrations. The spatial resolution of these simulations is too coarse for effective application to city issues. Techniques have been developed to “downscale” these simulations to higher spatial resolution. A particular downscaled dataset called Localized Constructed Analogs (LOCA) was used in the U.S. Fourth National Climate Assessment and chosen for use to develop city-specific climate scenarios. The data (available at several sites listed here) consist of daily values of maximum temperature, minimum temperature and precipitation for the period of 1950-2100. The future portion of the dataset consists of two different future emissions pathways: high emissions and low emissions scenarios. The data are derived from 32 different global climate models, which provides a rich resource for determining mean changes for the future, but the potential range of outcomes as well.

The LOCA data were used to calculate a wide range of metrics of extreme temperature and precipitation. Reports were prepared for Baltimore, Hermosillo, Miami, New York City, Phoenix, Portland, and Syracuse. Reports were not prepared for San Juan and Valdivia because the LOCA dataset does not cover those areas. These reports contain very detailed information and consist of 10 tables, over 60 figures, and accompanying explanatory text.

Assessing Drought across the Southwestern U.S.

We also conducted a study of drought in the Colorado River Basin (CRB) due to the importance of this watershed to the southwestern U.S and its vulnerability to climate change. The CRB encompasses an area of about 630,000 km² stretching across portions of seven US states (WY, UT, CO, NV, AZ, NM, and CA) and the Mexican states of Sonora and Baja California. Known as “the life blood of the southwestern US,” the CRB supplies: freshwater to nearly 40 million people, more than 4,200 MW of electrical generating capacity, and irrigation water for nearly 4.5 million acres of land including major cities of Albuquerque, Denver, Los Angeles, Phoenix, Salt Lake City, and San Diego. The CRB has experienced widespread and prolonged drought with precipitation up to 22-25% below 20^th century means and air temperature 0.8^oC warmer than average.

Models project that continued warming could significantly reduce annual flows (estimates up to 35-55% by the end of century), but the details of this transformation are still unclear, making it difficult for managers to plan accordingly. Climate extremes (e.g., heatwaves and drought) under these projections will have major consequences to urban hydrological systems. The magnitude of these effects depends directly upon surrounding and upstream watershed source areas. From this hydrologic perspective, urban resilience to extreme events in the southwestern U.S. should account for the larger context of the CRB hydraulic reach.

In this work, we analyzed the impacts of historic extreme climate events on the hydrologic responses within the CRB and its urban areas. By gaining an understanding of the historic context, we aimed to better predict the implications of future-projected climate change. For this assessment, we forced the Variable Infiltration Capacity (VIC) macroscale hydrology model with historic gridded meteorological observations (i.e., the training data for the LOCA products) to estimate water balance components in the domain modeled at 1/16^o spatial resolution and a daily time step from 1976 to 2005. We computed standardized indices of drought that were fit to hydroclimate timeseries for a range of subbasins across the domain. Results revealed an increasing importance of heatwave to magnify and prolong agricultural drought, and a strong role by catchment-scale in modulating impacts of climate change. For more detailed information, please see our first-place student poster presented at the 99^th American Meteorological Society Annual Meeting.