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June 2006 Abstracts

Observations and their analysis for WCRP/COPES

The World Climate Research Program has, through the Joint Scientific Committee, established a new framework to guide its development and progress. As a part of the implementation of this framework, called “Coordinated Observation and Prediction of the Earth System” or COPES, new overarching panels have been established on modeling (WCRP Modeling Panel) and observations (the WCRP Observations and Assimilation Panel, WOAP). On WOAP membership includes representatives from all the other WCRP projects, panels and working groups, chairs of GCOS (Global Climate Observing System): AOPC (Atmospheric Observations Panel for Climate), OOPC (Ocean Observations Panel for Climate), and the TOPC (Terrestrial Observations Panel for Climate); a representative of the GEOSS (Global Earth Observing System of Systems), the Chair of the WCRP Modelling Panel, representatives from major reanalysis centres, and possibly other experts as necessary and appropriate. I am the chair of this panel. I also have the perspective of being Coordinating Lead Author of the Intergovernmental Panel on Climate Change (IPCC) chapter on observations of the atmosphere and the surface.

In 1985 the Earth System Science Committee of NASA wrote:

"To advance our understanding of the causes and effects of global change, we need new observations of the Earth. These measurements must be global and synoptic, they must be long-term, and different processes must be measured simultaneously.

• Long-term continuity is crucial. A 20-year time series of the crucial variables would provide a significant improvement in our understanding.
• Now we are on the verge of establishing a global system of remote sensing instruments and Earth-based calibration and validation programs. Together, these space- and Earth-based measurements can provide the necessary data."

Here the bold emphasis is mine. Now 20 years later we arguably still do not have reliable time series of observations from space. It is an opportunity lost. And it is a challenge for us now to place our observing and processing systems on a track so that we will not say this 20 years from now.

Authored by Kevin E. Trenberth
National Center for Atmospheric Research ¹, Boulder CO 80307, email: trenbert@ucar.edu, phone: (303)497-1318
Submitted 14 June, 2006

¹ The National Center for Atmospheric Research is sponsored by the National Science Foundation.

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A comparison of forecast errors in CAM2 and CAM3 at the ARM Southern Great Plains Site

Figure. We compare short forecast errors and the balance of terms in the moisture and temperature prediction equations which lead to those errors for the Community Atmosphere Model versions 2 and 3 (CAM2 and CAM3). The comparisons are made at the ARM Southern Great Plains site for the April 1997 and June/July 1997 Intensive Observing Periods. The goal is to provide insight into parameterization errors in the CAM which ultimately should lead to model improvements. The atmospheric initial conditions are obtained from ERA40 reanalyses. The land initial conditions are spun up to be consistent with those analyses. We identify the differences between the model formulations that are responsible for the major differences in the forecast errors and/or parameterization behaviors. We perform a sequence of experiments, accumulating the changes from CAM3 back toward CAM2, to demonstrate the effect of the differences in formulations.

In June/July 1997 the CAM3 temperature and moisture forecast errors were larger than those of CAM2. The terms identified as being responsible for the differences were 1) the convective time scale assumed for the Zhang-McFarlane deep convection, 2) the energy associated with the conversion between water and ice of the rain associated with the Zhang-McFarlane convection parameterization, and 3) the dependence of the rainfall evaporation on cloud fraction. The latter two were not included in CAM2. In April 1997 the CAM2 and CAM3 temperature and moisture errors are very similar, but different tendencies arising from modifications to one parameterization component were compensated by responding changes in another component. CAM3 includes detrainment of water by the Hack shallow convection to the prognostic cloud water scheme that was not included in CAM2. This gives a different total parameterization tendency that is balanced by a difference in the advective tendency to yield the same total moisture tendency. The time scale assumed for the Hack shallow convection was halved in CAM3. Thus the convection is relatively weaker in CAM2 but this was compensated by the prognostic cloud water parameterization tendency to give very similar total parameterization tendency. CAM3 also had a variety of changes to the cloud fraction parameterization. These affect the radiative heating which in turn modifies the stability of the atmospheric column and affects the convection. But again, the changes in convection tendency are balanced by changes in the prognostic cloud water parameterization tendency, yielding a similar total parameterization tendency.

Authored by David L. Williamson and Jerry G. Olson
¹ National Center for Atmospheric Research, P.O. Box 3000, Boulder CO 80307
Submitted to Journal of Climate, 9 June 2006

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The water and energy budgets of hurricanes: Case studies of Ivan and Katrina

Figure. To explore the role of hurricanes in the climate system, a detailed analysis is made of the bulk atmospheric moisture budget of several simulated storms, with detailed results given for Ivan in September 2004 and Katrina in August 2005. The simulations are with the Weather and Research Forecasting (WRF) model at 4 km resolution without parameterized convection. As the initial state is from global analyses, the vortex spins up over about 12 hours, and the intensity does not match observed but the track forecast is excellent. It is demonstrated that the heavy precipitation in the core of the storms, with rainfall rates exceeding 20 mm/h, greatly exceed - by an order of magnitude - the surface flux of moisture through evaporation within 100 km of the center of storm, even though surface latent heat fluxes can exceed 1000 W m-2 (note 1 mm/h is equivalent to 696 W m-2). Hence vertically-integrated convergence of moisture into the tropical storms, which occurs mainly in the lowest 1 km of the atmosphere, is by far the dominant term in the moisture budget, and transports of moisture from distances up to 1600 km from the storm center are required to balance the precipitation. Although the moisture convergence is driven by the storm dynamics and local surface fluxes, this highlights the importance of the larger-scale environment in which the storms are embedded.

Simulations are also run for the Katrina case with sea surface temperatures (SSTs) increased by +1°C and decreased by -1°C, and we focus on statistics for hours 42 to 54 after the start of the simulation. Maximum surface winds in the eye increased about 4.5 m s-1 (9%) and sea level pressure fell 11.5 hPa per 1°C increase in tropical SSTs. As SSTs rise, not only does the environmental atmospheric moisture increase at close to the Clausius-Clapeyron equation value of about 6% K-1 in the tropics but, in the model simulation over a domain from the eye to 400 km radius, the surface moisture flux goes up at about 25% per K change in SST, relative to the control run. Precipitation is also enhanced by 19% per K and the moisture convergence increases through latent heat feedback by 22% per K change in SST. Overall the hurricane expands in size as SSTs increase and the outer circulation varies more than the inner core, with increased convergence into the storm. The hurricane is free to change its structure as the environment changes, and the area- integrated fractional changes can be much larger than one might infer from simply the change in maximum surface wind. The surface flux changes mainly depend on Clausius-Clapeyron effects and 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%.

Kevin E. Trenberth, Christopher A. Davis and John Fasullo
Submitted to J. Climate, 15 June, 2006

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The water and energy budgets of hurricanes and implications for climate change

Figure. To explore the role of hurricanes in the climate system, a detailed analysis is made of the bulk atmospheric moisture and related energy budgets using results based on several simulated storms, in particular, Katrina in August 2005. The simulations are with the Weather and Research Forecasting (WRF) model at 4 km resolution without parameterized convection. It is argued that the fundamental role of tropical cyclones in the climate system is to pump heat out of the tropical summer ocean and disperse it into the atmosphere mainly in the form of latent energy, providing evaporative cooling for the ocean. Surface fluxes of latent energy can exceed 1000 W m-2 in hurricanes. Based on simulations with Katina, 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, which includes 6-hourly estimates of the maximum winds, is used to estimate the frequency that storms of a given strength occur over the globe after 1970. Sea surface temperature (SST) observations are used for all the 6-hourly observations to estimate the saturation specific humidity dependency as a key component of the surface latent heat flux. As SSTs rise, not only does the environmental atmospheric moisture increase at close to the Clausius-Clapeyron equation value of about 6% K-1 in the tropics, following approximately constant relative humidity, but a component of the surface latent heat flux goes up at the same rate.

By estimating the surface latent and sensible heat fluxes and precipitation as a function of maximum wind speed in the model simulations, and making use of best track data on frequency of hurricanes at various strengths for 1990-2005, the total 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 0.53´1022 J per year (0.17 PW) which, if redistributed over the tropical ocean area from 20°N to 20°S, amounts to 1.3 W m-2 heat loss, or equivalently 1.0°C/year cooling over a 10 m thick layer. The enthalpy loss due to hurricanes computed based on precipitation is about a factor of 3.4 greater than these numbers (0.58 PW), owing to the addition of the surface fluxes outside 400 km radius and moisture convergence into the storms typically from as far from the eye as 1600 km. 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 SSTs. The implied much heavier rainfalls (1.4% per year increase), escalate risk of flooding over land. This highlights the importance of surface energy exchanges in global energetics of the climate system and indicates that climate models are markedly deficient by not adequately representing tropical cyclones.

Authored by Kevin E. Trenberth and John Fasullo
National Center for Atmospheric Research, P.O. Box 3000, Boulder CO 80307
Submitted to J. Climate, 15 June, 2006