Guest Weblog by Barry H. Lynn, Richard Healy, and Len Druyan

Introduction by Roger A. Pielke Sr.

Climate Science has had a very productive e-mail exchange of perspectives in response to the weblog of May 14 2007. The authors of the article referred to in the weblog have graciously agreed to write a guest weblog which is given below. For background on the authors, a brief biographical summary of each scientist is:

Dr. Barry Lynn is a research scientist at the Hebrew University of Jerusalem. The research on climate change was conducted while he was an associate research scientist at Columbia University and Carnegie Mellon University. Dr. Lynn’s interests include studying the impacts of “greenhouse” gases on climate and the effect of aerosols on precipitation. Many of his papers have been published by the AMS and JGR. He is also the C.E.O of Weather It Is LTD (www.weather-it-is.com), a company that produces weather forecasts and climatological information, with an emphasis on deriving new economic applications from such products.

Rick Healy is a systems analyst at the National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS) at the Woods Hole Oceanographic Institution, developing computational methods in performing high precision radiocarbon analysis. He also collaborates with the NASA/Goddard Institute for Space Studies (GISS) climate-modeling group in New York. His interests include Regional Climate Impacts using integrated regional climate models to study climate change issues. He also collaborates with scientists at UMass Amherst in paleoclimatetracer studies using the GISS d18O tracer model. http://nosams.whoi.edu/research/staff_healy.html

Dr. Druyan is a Senior Research Scientist and the Director of the Center for Climate Systems Research which is a unit of the Earth Institute at Columbia University. Alternatively referred to as “GISS at Columbia”, CCSR is the administrative umbrella for many Columbia University research scientists based at the Goddard Institute for Space Studies. Dr. Druyan’s research interests focus on climate variability in tropical latitudes. His published work relates to a range of themes, including the Indian and West African summer monsoons, Sahel drought, African wave disturbances, climate change impacts on tropical cyclones, El Niño and other sea-surface temperature anomaly impacts on regional climates and seasonal climate prediction for Brazil. He has conducted climate simulation studies using several versions of the GISS GCM and more recently using a regional climate model (RM3) that represents variables at higher spatial resolution. Dr. Druyan’s research group is using the RM3 for collaborative research in the context of the African Monsoon Multidisciplinary Analysis (AMMA) and the West African Monsoon Modeling and Evaluation (WAMME).

The Guest Weblog follows:

This is a brief response to the posted critiques of our recent paper in the Journal of Climate. (Lynn et al., see ) We have since had a constructive dialogue with Dr. Pielke by email and we appreciate his giving us this opportunity for clarification on his blog. We hope to correct the mistaken impression that we were in any way looking to sensationalize dangers from global warming. Contrary to the impression promoted on the blog, we were diligent in our research. In addition, our paper was peer reviewed and it underwent revisions consistent with the suggestions of three (presumably) professional reviewing scientists.

The study was based on both observed data and model simulations. A significant result of the observational analysis was finding the strong inverse relationship between eastern U.S precipitation frequency and maximum surface temperatures (see figure). Dr. Pielke’s main criticism seems to focus on the poor performance of the AOGCM that provided data for driving our regional model. The AOGCM admittedly has deficiencies, as do all models. We believe that this AOGCM has comparable skill (or lack of skill) to other GCMs that formed the basis of the IPCC fourth assessment. The AOGCM is a tool, albeit imperfect, for projecting the broad scale climate consequences of increasing concentrations of greenhouse gases. It accounts for the distribution of oceans and continents and all of the major interactions between the different Earth systems affecting the climate. However, it has serious flaws regarding the simulation of regional climate.

Dr. Pielke maintains that dynamic downscaling by even the most skillful regional model cannot improve the simulation of a flawed climate simulation by a GCM. However, he concedes that the regional model solution is less sensitive to the driving GCM when the nested domain is large, as in our case. We also counter that (in our double-nested experiments) since the outer regional model domain extended from the Pacific to the Atlantic Oceans, no GCM data generated over the US was ever used to drive the regional model. In addition, we claim that crucial aspects of our climate simulations were indeed improved – by the alternative moist convection schemes operating at high horizontal resolution in the regional model. At least one version of our regional model simulated summertime precipitation frequencies that were much more realistic than the GCM.

Dr. Pielke correctly points out that the regional model cannot correct for GCM errors in the timing or trajectories of synoptic systems. We reply that radiation feedbacks that depend on the frequency of precipitation, mean ground wetness and frequency of cloudiness are far more important in determining rates of warming over the next 80 years. This downscaling produced a greater warming trend over the eastern US into the 2080s than the GCM because it did not make the mistake of “predictingâ€? rain on 65% of the summer days (see figure). Was this result adversely affected by the GCM data streaming in at the boundaries over the Pacific and Atlantic? The same GCM boundary conditions were used to drive another version of the regional model with a convection scheme that made the same mistake (as the GCM) of predicting rain too frequently. This version produced a more gradual warming trend just like the GCM. A third version that underestimated afternoon precipitation predicted the most severe warming trend. Based on all of this evidence, we are convinced that the radiation feedbacks created by the precipitation regime control the warming rates, and that our paper’s “apocalypticâ€? prediction of 5°C warming over the eastern US between the 1990s and 2080s is the most realistic prediction – a correction if you will to the underestimate of IPCC models that rain too frequently. See http://ams.allenpress.com/perlserv/?request=get-document&doi=10.1175%2FJCLI3672.1 or http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2FJCLI3672.1

fig1pielkeblog.jpg
Relationship between the JJA anomalies of mean maximum T for the eastern US vs. the percent of rainy days in the corresponding seasons, 1977-2004.

fig-2pielkeblog.jpg
Precipitation frequency (percent of rainy days during JJA) for JJA 1993-97 and JJA 2083-87 over the eastern US for observations and model versions. “Scaledâ€? observations refers to frequencies within 4° x 5° AOGCM grid elements.

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