Is Precipitation Becoming More Intense?
Pavel Groisman, National Oceanic and Atmospheric Administration

An overview of 12-year-long National Climatic Data Center (NCDC) studies of changes of intense precipitation during the period of instrumental observations will be presented with a focus on North America. NCDC has created a database of daily and hourly time series of high scientific quality for use in assessment of changes in precipitation characteristics over the regions where we have sufficient amount of information to answer the question outlined in the talk’s title.

Prior to 2005, NCDC constructed various time series of precipitation characteristics and analyzed their trends. Now (in addition to routine updates of these time series), we have

• analyzed the factors that control intense precipitation (e.g., CAPE and land-falling tropical cyclones trajectories),

• assessed the rainfall distribution characteristics (e.g., hourly rainfall rates), their changes, and their relationships with global and regional surface air temperatures, and

• investigated changes in “direct impact” characteristics of precipitation spectra such as prolonged no-rain periods, fire weather indices, and maximum rainfall intensity.

Our past and ongoing studies (as well as findings by other foreign researchers) embolden our opinion that in the past several decades over most of the extratropics precipitation became more intense. However, the changes in intense precipitation also occur with changes in several other precipitation characteristics and they too deserve our thorough attention.

A Process-Based “Bottom-Up” Approach for Addressing Changing Flood-Climate Relationships
Katie Hirschboeck, University of Arizona

In response to the unprecedented persistence of extreme drought conditions in the western United States, some western water managers have moved beyond conventional approaches to plan for future extreme low flow conditions in innovative ways involving paleo-records, scenarios, and climate projection modeling. In contrast, flood hazard managers are far more constrained in developing ways to incorporate climate change information operationally, in part because of existing flood policy, but also because of the short-term, localized, and weather-based nature of the flooding process itself. What is needed is information that is presented in an operationally useful format for flood managers and that describes how changes in the large-scale climatic drivers of hydrometeorological extremes will affect flooding variability in specific watersheds. This presentation outlines a framework for linking global climatic change to the gauged time series of peak flows in individual watersheds. Using a process-sensitive “bottom up” approach, each individual peak in a gauged record is associated with its flood-producing storm type and circulation pattern. This approach highlights the underlying physical reasons for



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