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Extreme Weather Events

Q9.

A9.

On page 6 of your written testimony you state that “I want to emphasize that one can't point to any single extreme weather event today and say for sure that global warming caused it. But we can say that such events are examples of the kinds of impacts we expect to occur with greater frequency in a warmer world.”

Please provide documentation of your statement that "such [extreme weather] events are examples of the kinds of impacts we expect to occur with greater frequency in a warmer world.

Attached is a paper that was presented at the GCOS/CLIVAR climate extremes meeting in Asheville North Carolina last June. The paper is in press and will be published in Climatic Change and as part of a special collection of papers addressing climate extremes.

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CONCEPTUAL FRAMEWORK FOR CHANGES OF

EXTREMES OF THE HYDROLOGICAL CYCLE
WITH CLIMATE CHANGE

KEVIN E. TRENBERTH

National Center for Atmospheric Research1, P. O. Box 3000, Boulder, CO 80307

Abstract. A physically based conceptual framework is put forward that explains why an increase in heavy precipitation events should be a primary manifestation of the climate change that accompanies increases in greenhouse gases in the atmosphere. Increased concentrations of greenhouse gases in the atmosphere increase downwelling infrared radiation, and this global heating at the surface not only acts to increase temperatures but also increases evaporation which enhances the atmospheric moisture content. Consequently all weather systems, ranging from individual clouds and thunderstorms to extratropical cyclones, which feed on the available moisture through storm-scale moisture convergence, are likely to produce correspondingly enhanced precipitation rates. Increases in heavy rainfall at the expense of more moderate rainfall are the consequence along with increased runoff and risk of flooding. However, because of constraints in the surface energy budget, there are also implications for the frequency and/or efficiency of precipitation. It follows that increased attention should be given to trends in atmospheric moisture content, and datasets on hourly precipitation rates and frequency need to be developed and analyzed as well as total accumulation.

1. Introduction

The character of precipitation, with highly variable rain rates and enormous spatial variability, makes simply determining mean precipitation difficult let alone how it will change as the climate changes. For instance, a detailed examination of spatial structure of daily precipitation amounts by Osborne and Hulme (1997) shows that in Europe the average separation distance between climate stations were the correlation falls to 0.5 is about 150 km in summer and 200 km in winter the more convective nature of summer precipitation is responsible for the difference. In addition, this complexity also makes it difficult to model precipitation reliably, as many of the processes of importance can not be resolved by the model grid (typically 200 km) and so sub-grid-scale processes have to be parameterized. Yet 1 The National Center for Atmospheric Research is sponsored by the National Science Foundation.

Climatic Change 36: pp-pp, 1998

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there are some overall aspects of precipitation related to the hydrological cycle that can be clarified and for which expectations as to how they will change are physically based. Here the processes involved that influence precipitation and link it to evaporation and heating are outlined along with the importance of dealing not just with accumulated amounts, but also precipitation rates (or intensity) and precipitation frequency. The relative roles of moisture stored in the atmosphere, its advection, and resupply have been examined in detail in Trenberth (1998), and only a brief summary of those aspects are included here.

The term "global warming" is often taken to refer to global increases in temperature accompanying the increases in greenhouse gases in the atmosphere. In fact it should refer to the additional global heating (sometimes referred to as radiative forcing, e.g., by the IPCC (1996)) arising from the increased concentrations of greenhouse gases, such as carbon dioxide, in the atmosphere. Increases in greenhouse gases in the atmosphere produce global warming through an increase in downwelling infrared radiation, and thus not only increase surface temperatures but also enhance the hydrological cycle, as much of the heating at the surface goes into evaporating surface moisture. This occurs in all climate models regardless of feedbacks, although the magnitude varies substantially (see section 3).

Temperature increases signify that the water-holding capacity of the atmosphere increases and, together with enhanced evaporation, the actual atmospheric moisture should increase, as is observed to be happening in many places (Hense et al. 1988, Gaffen et al. 1991, Ross and Elliott 1996, Zhai and Eskridge 1997). Of course, enhanced evaporation depends upon the availability of sufficient surface moisture and over land, this depends on the existing climate. However, it follows that naturally-occurring droughts are likely to be exacerbated by enhanced potential evapotranspiration. Further, globally there must be an increase in precipitation to balance the enhanced evaporation but the processes by which precipitation is altered locally are not well understood.

It is shown that precipitating systems of all kinds feed mostly on the moisture already . the atmosphere at the time the system develops, and precipitation occurs through convergence of available moisture on the scale of the system. Hence, the atmospheric moisture content directly affects rainfall and snowfall rates, but not so clearly the precipitation frequency and thus total precipitation, at least locally. Thus, it is argued that global warming leads to increased moisture content of the atmosphere which in turn favors stronger rainfall events, as is observed to be happening in many parts of the world (Karl et al. 1995), thus increasing risk of flooding. It is further argued that one reason why increases in rainfall should be spotty

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arguments assembled here imply the need for new observations, datasets, and ways of analyzing both model and observed data. Trenberth (1998) discusses these aspects more fully.

2. Atmospheric moisture cycling

New estimates of the moistening of the atmosphere through evaporation at the surface and of the drying of the atmosphere through precipitation are given in Trenberth (1998). These are simple estimates based on the precipitable water and average local evaporation and precipitation rates, which ignore transport. Overall for the global annual mean, the e-folding residence time (the time for amounts to fall by a factor e = 2.718) for atmospheric moisture is just over 8 days. For precipitation, local values of e-folding residence time of the atmospheric depletion rate of moisture are less than a week in the tropical convergence zones but they exceed a month in the dry zones in the subtropics and desert areas. Time constants for depletion and restoration rates of atmospheric moisture are fairly similar overall, but this conclusion does not take account of the fact that rain falls only a small fraction of the time. In midlatitudes precipitation typically falls from zero to 30% of the time, and so rainfall rates, conditional on when rain is falling, are much larger than evaporation rates. The depletion rate time scale is about 4 hours in the tropics when rain is falling. In middle latitudes, typical unconditional rainfall rates are 3 mm/day, but with rain falling about 10% of the time and precipitable water amounts of 15 mm, the depletion rate time scale of 5 days drops conditionally on rain falling to about 12 hours (Trenberth 1998). This inferred imbalance in the drying versus moistening of the atmosphere implies that most of the moderate and heavy rain that falls comes directly from the precipitable water already in the atmosphere at the time the storm responsible for the precipitation developed, not directly from evaporation, and so the lifetime of moisture in the atmosphere and its availability to rain systems is a limiting factor. However, atmospheric depletion of moisture by light rain could easily be restored by evaporation.

These above aspects do not take moisture transport into account. Therefore new estimates have also been made of how much precipitated moisture comes from evaporation from within versus transport from outside a domain, called recycling. Approximate values of recycling are computed following the approach of Brubaker et al. (1993), as detailed in Trenberth (1998). Equilibrium conditions are assumed, so that there are no changes in atmospheric moisture content but changes in moisture storage in the atmosphere do not impact the results for seasonal or longer averages. A domain of length L aligned along the trajectory of the air is considered. An

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Figure 1. The recycling, for annual mean conditions, for length scales of 1,000 km, and using E and moisture flux from the NCEP reanalyses (Kalnay et al. 1996) and P from CMAP (Xie and Arkin 1997). Values are set to missing (white) where the surface pressure is less than 800 mb.

of precipitation that falls arising from advection versus local evaporation is equal to the ratio of average advected to evaporated moisture in the air. While interest has often been on recycling estimates for large drainage basins, the heterogeneity of the land surface is such that the recycling clearly varies substantially over the basins. The regions of mountains (where surface pressures are less than 800 mb) are screened out from the calculation, as those are regions where the moisture flux is small and there are huge variations over short distances owing to orographic effects on rainfall.

In Trenberth (1998) recycling results for annual means are presented for L= 500 km. Here results presented for L= 1000 km (Fig. 1) reveal recycling percentages of about 8 to 20% over land typically. For 500 km scales the global mean is 9.6%, consisting of 8.9% over land and 9.9% over the oceans and for 1000 km scales the mean recycling is 16.8% globally, 15.4% over land and 17.3% over the oceans. Over the Amazon, the average is about 10% and over the Mississippi basin about 12%. These values prove to be compatible with most previous extimates (e.g., Brubaker et al. 1993) once the different scales of the basins are taken into account. It is worth pointing out that the larger values previously obtained for the Amazon versus the Mississippi are mostly a result of the scale of the domain.

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