CGD's Dr. John Fasullo

Abstract

Recently observed trends in the tropical cyclone record are assessed in the context of potential sampling biases. Multi-member syntheses of the hurricane record are created and subjected to time-varying sampling rates and intensity-specific biases to assess the biases' ability to induce spurious trends. While simple linear trends in sampling frequency can be imposed that account for the observed trends in major storms, the same biases result in trends in weaker storms that are significantly at odds with observations. Moreover, it is found that intensity specific biases, which must invariably contribute to prolonged storm durations for weaker categories, are inconsistent with observations. It is concluded therefore that the proposed sampling deficiencies are unable to account fully for recently reported trends, either individually or when considered in tandem. The finding of positive trends must therefore either be robust or result from complex, and as of yet unexplained, sampling biases.

Support: National Science Foundation.

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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: 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., 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: NSF Grant ATM-0233568 and NCAR's Water Cycle Program.