CGD's Dr. Kevin Trenberth

Abstract

The trends of the surface water and energy budget components in the Mississippi River basin from 1948 to 2004 are investigated using a combination of hydrometeorological observations and observation-constrained simulations of the land surface conditions using the latest version of the Community Land Model version 3 (CLM3). The atmospheric forcing data for the CLM3 were constructed by adding the intra-monthly variations from the 6-hourly National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis to observation-based analyses of monthly precipitation, surface air temperature and cloud cover. Our model-based analysis suggests that for the surface water budget, the observed increase in basin-averaged precipitation is compensated by increases in both runoff and evapotranspiration. For the surface energy budget, the decrease of net shortwave radiation associated with observed increases in cloudiness is compensated by decreases in both net longwave radiation and sensible heat flux, while the latent heat flux increases in association with wetter soil conditions. Both the simulated surface water and energy budgets support the view that evapotranspiration has increased in the Mississippi River basin from 1948-2004. Sensitivity experiments show that the precipitation change dominates the evapotranspiration trend, while the temperature and solar radiation changes have only small effects. Large spatial variations within the Mississippi River basin and the contiguous United States are also found. However, the increased evapotranspiration is ubiquitous despite spatial variations in hydrometeorology.

Figure caption: Basin-averaged trends in the water and energy budget components for the Mississippi River basin. M is the long-term (1948-2004) annual (water-year) mean (in mm for water components and W m for energy components) and b is the annual linear trend during 1948-2004 (in mm century for water components and W m century for energy components, proportional to arrow shaft width). Note that the downward arrow means that the flux increases the trend of dW/dt or G.

Support: NSF Grant ATM-0233568 and NCAR TIIMES Water Cycle Program.

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Rasmussen, R., A. Dai, K. E. Trenberth, 2007: Impact of climate change on precipitation. Chapter 16. Large-scale Disasters: Prediction, Control and Mitigation, Gad-el-Hak, Ed., Cambridge University Press, 453-472.

Summary

The above discussion highlights some of the key issues needed to be addressed in order to properly simulate the water cycle in climate models. These include:

1. The proper simulation of the frequency, intensity, duration, timing, and phase of precipitation.
2. The proper treatment of local evapo-transpiration vs. large scale moisture advection.
3. The proper treatment of water runoff and soil infiltration in order to properly model soil moisture and its impact on latent and sensible heat fluxes.

It is critical that scientists address these issues in order to provide natural disaster managers and other users of climate information the proper guidance to make the difficult decisions facing our global society in the near future.

Figure caption: Example of two hypothetical surface weather stations receiving the same total amount of precipitation, but with different frequency and intensity.

Support: NCAR TIIMES Water Cycle Program.

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Trenberth, K. E., 2006: Observed changes to the climate and their causes. Chapter 9. Confronting Climate Change: Critical Issues for New Zealand, R. Chapman, J. Boston, and M. Schwass, Eds., Victoria University Press, 93-102, doi: ISBN 0 86473 546 4.

Figure caption: Time series of annual global mean temperature departures for 1861-2005 from a 1961-90 mean (bars), left scale, and the annual mean carbon dioxide from Mauna Loa after 1957 linked to values from bubbles of air in ice cores prior to then.

Support: NSF.

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Trenberth, K. E., 2006: The role of the oceans in climate. Flotsam and Jetsam, 35, 1, 5-7.

Support: NSF.

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Trenberth, K. E., 2007: Warmer oceans, stronger hurricanes. Scientific American, July, 2007, 26-33.

Abstract

The summer of 2004 seemed like a major wake-up call: an unprecedented four hurricanes hit Florida, and 10 typhoons made landfall in Japan-four more than the previous record in that region. Daunted, scientists offered conflicting explanations for the increase in these tropical cyclones and were especially divided about the role of global warming in the upsurge. Then Mother Nature unleashed a record-breaking 2005 season in the North Atlantic, capped by the devastating hurricanes Katrina and Rita. But in 2006, as insurance rates in the southeastern U.S. soared, the number of North Atlantic storms dropped well below predictions. If global warming was playing a role, why was the season so quiet?

Figure caption: Tracks and wind speed of all tropical storms recorded through September 2005 show the regions at highest risk.

Support: NSF.

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Trenberth, K. E., and A. Dai, 2007: Effects of Mount Pinatubo volcanic eruption on the hydrological cycle as an analog of geoengineering. Geophys. Res. Lett., 34, L15702, doi:10.1029/2007GL030524.

Abstract

The problem of global warming arises from the buildup of greenhouse gases such as carbon dioxide from burning of fossil fuels and other human activities that change the composition of the atmosphere and alter outgoing longwave radiation (OLR). One geoengineering solution being proposed is to reduce the incoming sunshine by emulating a volcanic eruption. In between the incoming solar radiation and the OLR is the entire weather and climate system and the hydrological cycle. The precipitation and streamflow records from 1950 to 2004 are examined for the effects of volcanic eruptions from El Chichón in March 1982 and Pinatubo in June 1991, taking into account changes from El Niño-Southern Oscillation. Following the eruption of Mount Pinatubo in June 1991 there was a substantial decrease in precipitation over land and a record decrease in runoff and river discharge into the ocean from October 1991-September 1992. The results suggest that major adverse effects, including drought, could arise from geoengineering solutions.

Figure caption: Top: Adapted time series of 20°N to 20°S ERBS non-scanner wide-field-of-view broadband shortwave, longwave and net radiation anomalies from 1985 to 1999 [Wielicki et al., 2002a, 2002b] where the anomalies are defined with respect to the 1985 to1989 period with Edition 3_Rev 1 data [Wong et al., 2006]. Bottom: Time series of the annual water year (Oct. to Sep.); note slight offset of points plotted vs tick marks indicating January) continental freshwater discharge and land precipitation (from Fig. 1) for the 1985 to 1999 period. In both panels, the period clearly influenced by the Mount Pinatubo eruption is indicated by grey shading.

Support: NSF.

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Trenberth, K. E., C. A. Davis and J. Fasullo, 2007: The water and energy budgets of hurricanes: Case studies of Ivan and Katrina. J. Geophys. Res., in press.

Abstract

To explore the role of hurricanes in the climate system, a detailed analysis is made of the bulk atmospheric moisture budget of Ivan in September 2004 and Katrina in August 2005 from simulations with the Weather and Research Forecasting (WRF) model at 4 km resolution without parameterized convection. Heavy precipitation exceeding 20 mm/h in the storms greatly exceeds the surface flux of moisture through evaporation, and vertically-integrated convergence of moisture in the lowest 1 km of the atmosphere from distances up to 1600 km is the dominant term in the moisture budget, highlighting the importance of the larger-scale environment. Simulations are also run for the Katrina case with sea surface temperatures (SSTs) increased by +1°C and decreased by -1°C as sensitivity studies. For hours 42 to 54 after the start of the simulation maximum surface winds increased about 4.5 m s^-1 (9%) and sea level pressure fell 11.5 hPa per 1°C increase in tropical SSTs. Overall the hurricane expands in size as SSTs increase, the environmental atmospheric moisture increases at close to the Clausius-Clapeyron equation value of about 6% K^-1 and the surface moisture flux also increases mainly from Clausius-Clapeyron effects and the changes in intensity of the storm. The environmental changes related to human influences on climate since 1970 have very likely changed the odds in favor of more intense hurricanes and heavier storm rainfalls and the latter is quantified to date to be order 4 to 12% with a central best value of about 8%.

Figure caption: For 1800 UTC 28 August to 0600 UTC 29 August 2005, hours 42 to 54 of the simulation, given are (left) the azimuthally-averaged precipitation (mm/h), and (right) column integrated moisture convergence and surface latent heat flux as a function of radius for the control (red) and changes in SST of +1°C (blue) and -1°C (green). The precipitation and latent heat fluxes are area averages from the eye to the radius plotted to be compatible with the moisture convergence across that cylinder radius.

Support: Partially sponsored by the NOAA CLIVAR program under grant NA17GP1376, and by NSF.

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Trenberth, K. E., and J. Fasullo, 2007: The water and energy budgets of hurricanes and implications for climate change. J. Geophys. Res., in press.

Abstract

Based on simulations of hurricane Katina in August 2005 with the advanced Weather and Research Forecasting (WRF) model at 4 km resolution without parameterized convection, empirical relationships are computed between the maximum simulated wind and the surface fluxes and precipitation, and provide a reasonable fit to the data. The best track dataset of global observed tropical cyclones is used to estimate the frequency that storms of a given strength occur over the globe after 1970. For 1990-2005 the total surface heat loss by the tropical ocean in hurricanes category 1 to 5 within 400 km of the center of the storms is estimated to be about 0.53x10²² J per year (0.17 PW). The enthalpy loss due to hurricanes computed based on precipitation is about a factor of 3.4 greater (0.58 PW), owing to the addition of the surface fluxes from outside 400 km radius and moisture convergence into the storms typically from as far from the eye as 1600 km. Globally these values correspond to 0.33 W m^-2 for evaporation, or 1.13 W m^-2 for precipitation. Changes over time reflect basin differences and a prominent role for El Niño, and the most active period globally was 1989 to 1997. Strong positive trends from 1970 to 2005 occur in these inferred surface fluxes and precipitation arising from increases in intensity of storms and also higher sea surface temperatures. Confidence in this result is limited by uncertainties in the best track tropical cyclone data. Nonetheless, the results highlight the importance of surface energy exchanges in global energetics of the climate system and indicate the deficiencies in climate models owing to their inadequate representation of hurricanes.

Figure caption: Based on best track data for the tropical cyclones observed each year, the total surface energy loss by the global ocean is given, based on Katrina simulated fluxes within 400 km of the eye of the storms as given by (2) for latent (blue), sensible (cyan) and total enthalpy (black) flux in 10²¹ Joules per year. Also given in green (right hand scale) is the precipitation in the same units. The dotted lines are linear trends and values are given in 10²¹ Joules per decade.

Support: Partially supported by the NOAA CLIVAR program under grant NA17GP1376 and by NSF.

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Trenberth, K. E., P. D. Jones, P. Ambenje, R. Bojariu, D. Easterling, A. K. Tank, D. Parker, F. Rahimzadeh, J. A. Renwick, M. Rusticucci, B. Soden, P. Zhai, 2007: Observations: Surface and Atmospheric Climate Change. (In) Climate Change 2007. The Physical Science Basis. S. Solomon, D. Qin, M. Manning, Z. Chen, M. C. Marquis, K. B. Avery, M. Tignor, and H. L. Miller, Eds., IPCC, Cambridge University Press, 235-336.

Figure caption: Top: Annual global mean observed temperatures (black dots) are given along with simple fits to the data. The left hand axis shows anomalies relative to a 1961 to 1990 average and the right hand axis shows the estimated actual temperature in °C. The preliminary 2006 value is given by the orange dot. Linear trend fits to the last 25 (red), 50 (green), 100 (magenta) and 150 years (brown) are given, and correspond to 1981 to 2005, 1956 to 2005, 1906 to 2005, and 1856 to 2005. Note that for shorter recent periods, the slope is greater, indicating accelerated warming. The blue curve is a smoothed depiction to capture the decadal variations. To give an idea of whether the fluctuations are meaningful, decadal 5% to 95% (yellow) error ranges about that line are given (accordingly, annual values do exceed those limits). Results from climate models driven by estimated radiative forcings for the 20th century (Chapter 9) suggest there was little change prior to about 1915, and that a substantial fraction of the early 20th century change was contributed by naturally occurring influences including solar radiation changes, volcanism, and natural variability. From about 1940 to 1970 the increasing industrialization following World War II increased pollution in the NH contributing to cooling, and increases in carbon dioxide and other greenhouse gases dominate the observed warming after the mid-1970s.

Support: NSF.

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Trenberth, K. E., L. Smith, T. Qian, A. Dai and J. Fasullo, 2007: Estimates of the global water budget and its annual cycle using observational and model data. J. Hydrometeor., 8, 758-769.

Abstract

A brief review is given of research in the Climate Analysis Section at NCAR on the water cycle. A new estimate is provided of the global hydrological cycle for long-term annual means that includes estimates of the main reservoirs of water as well as the flows of water among them. For precipitation P over land a comparison among three datasets enables uncertainties to be estimated. In addition, results are presented for the mean annual cycle of the atmospheric hydrological cycle based on 1979 to 2000 data. These include monthly estimates of P, evapotranspiration E, atmospheric moisture convergence over land, and changes in atmospheric storage, for the major continental land masses, zonal means over land, hemispheric land means and global land means. The evapotranspiration is computed from the Community Land Model run with realistic atmospheric forcings, including precipitation that is constrained by observations for monthly means but with high frequency information taken from atmospheric reanalyses. Results for P-E are contrasted with those from atmospheric moisture budgets based on ERA-40 reanalyses. The latter show physically unrealistic results, because evaporation often exceeds precipitation over land especially in the tropics and subtropics.

Figure caption: The hydrological cycle. Estimates of the main water reservoirs, given in plain font in 10³ km³, and the flow of moisture through the system, given in slant font in 10³ km³/yr, equivalent to Exagrams (10¹⁸g) per year.

Support: NOAA CLIVAR and CCDD programs under Grants NA17GP1376 and NA04OAR4310073, NSF Grant ATM-0233568, and by the Water Cycle Program at NCAR.

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Serreze, M. C., A. P. Barrett, A. J. Slater, M. Steele, J. Zhang, and K. E. Trenberth, 2007: The large-scale energy budget of the Arctic. J. Geophys. Res., 112, D11122, doi:10.1029/2006JD008230.

Abstract

This paper synthesizes a variety of atmospheric and oceanic data to examine the large-scale energy budget of the Arctic. Assessment of the atmospheric budget relies primarily on the ERA-40 reanalysis. The seasonal cycles of vertically integrated atmospheric energy storage and the convergence of energy transport from ERA-40, as evaluated for the polar cap (defined by the 70°N latitude circle), in general compare well with realizations from the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis over the period 1979-2001. However, shortcomings in top of atmosphere radiation, as compared to satellite data, and the net surface flux, contribute to large energy budget residuals in ERA-40. The seasonal cycle of atmospheric energy storage is strongly modulated by the net surface flux, which is also the primary driver of seasonal changes in heat storage within the Arctic Ocean. Averaged for an Arctic Ocean domain, the July net surface flux from ERA-40 of -100 W m^-2 (i.e., into the ocean), associated with sea ice melt and oceanic sensible heat gain, exceeds the atmospheric energy transport convergence of 91 W m^-2. During winter (for which budget residuals are large), oceanic sensible heat loss and sea ice growth yield an upward surface flux of 50-60 W m^-2, complemented with an atmospheric energy convergence of 80-90 W m^-2 to provide a net radiation loss to space of 175-180 W m^-2.

Figure caption: Schematic of the energy budgets of the Arctic Ocean domain for: (a) January and (b) July, based on the information in Table 2. The width of the arrows is proportional to the size of the transports. Atmospheric and surface terms are defined as in Figure 3. Enhanced EPS.

Support: NSF and NASA contracts NNG06GB26G, NNG04GH52G, NNG04GB03G, NSF grants OPP-0229651, ARC-0531040, and ARC-0531103, and NOAA.

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Simmons, A., K. E. Trenberth, and S. Uppala, 2006: Future needs in atmospheric reanalysis. EOS Trans. Amer. Geophys. Union, 87, 583, 587.

Support: NSF.