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Comments on “Development and Evaluation of a Convection Scheme for Use in Climate Models”

Comments on “Development and Evaluation of a Convection Scheme for Use in Climate Models” In a recent paper Emanuel and Živković-Rothman (1999) proposed a new convection scheme. Evaluating the performance of their scheme, Emanuel and Živković-Rothman used the Betts–Miller–Janjić scheme ( Betts 1986 ; Betts and Miller 1986 ; Janjić 1994 ) as one of the reference schemes. The authors noted that the Betts–Miller–Janjić scheme performed well in the tests using the Global Atmospheric Research Programme (GARP) Atlantic Tropical Experiment (GATE) data and stated that the original version of this scheme was tuned to the GATE data quoting the work of Betts and Miller (1986) . The Betts–Miller–Janjić scheme differs from the Betts–Miller scheme in several important aspects. First, in the deep convection algorithm the parameters that define the moisture profile and the relaxation time are not fixed. They are variable and depend on the “cloud efficiency,” a parameter that characterizes the convection regime ( Janjić 1994 ). The cloud efficiency is proportional to a nondimensional combination of the entropy change over the time step, precipitation over the time step, and the mean temperature of the cloud. Second, in the shallow convection algorithm, the moisture profile is defined from the requirement that the entropy change be small and nonnegative ( Janjić 1994 ). The Betts–Miller–Janjić scheme has been optimized over years of operational application in the National Centers for Environmental Prediction’s Meso Eta model for precipitation forecasts over North America. Thus, in addition to the described conceptual differences between the Betts–Miller scheme and the Betts–Miller–Janjić scheme, the values of the parameters used in the Betts–Miller–Janjić scheme do not coincide with those recommended by Betts and Miller (1986) . Therefore, the good performance of the Betts–Miller–Janjić scheme in the tests with the GATE data carried out by Emanuel and Živković-Rothman (1999) cannot be explained by having tuned the scheme on the same dataset that was used for the tests. On the contrary, this was an unbiased, independent test of the Betts–Miller–Janjić scheme as well. Editor’s Note: Emanuel and Živković-Rothman requested the following be included here: “The authors agree with Dr. Janjić’s comments and thank him for pointing out our error in describing the tuning of the Betts–Miller–Janjić scheme.” http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of the Atmospheric Sciences American Meteorological Society

Comments on “Development and Evaluation of a Convection Scheme for Use in Climate Models”

Journal of the Atmospheric Sciences , Volume 57 (21) – Apr 3, 2000

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Publisher
American Meteorological Society
Copyright
Copyright © 2000 American Meteorological Society
ISSN
1520-0469
DOI
10.1175/1520-0469(2000)057<3686:CODAEO>2.0.CO;2
Publisher site
See Article on Publisher Site

Abstract

In a recent paper Emanuel and Živković-Rothman (1999) proposed a new convection scheme. Evaluating the performance of their scheme, Emanuel and Živković-Rothman used the Betts–Miller–Janjić scheme ( Betts 1986 ; Betts and Miller 1986 ; Janjić 1994 ) as one of the reference schemes. The authors noted that the Betts–Miller–Janjić scheme performed well in the tests using the Global Atmospheric Research Programme (GARP) Atlantic Tropical Experiment (GATE) data and stated that the original version of this scheme was tuned to the GATE data quoting the work of Betts and Miller (1986) . The Betts–Miller–Janjić scheme differs from the Betts–Miller scheme in several important aspects. First, in the deep convection algorithm the parameters that define the moisture profile and the relaxation time are not fixed. They are variable and depend on the “cloud efficiency,” a parameter that characterizes the convection regime ( Janjić 1994 ). The cloud efficiency is proportional to a nondimensional combination of the entropy change over the time step, precipitation over the time step, and the mean temperature of the cloud. Second, in the shallow convection algorithm, the moisture profile is defined from the requirement that the entropy change be small and nonnegative ( Janjić 1994 ). The Betts–Miller–Janjić scheme has been optimized over years of operational application in the National Centers for Environmental Prediction’s Meso Eta model for precipitation forecasts over North America. Thus, in addition to the described conceptual differences between the Betts–Miller scheme and the Betts–Miller–Janjić scheme, the values of the parameters used in the Betts–Miller–Janjić scheme do not coincide with those recommended by Betts and Miller (1986) . Therefore, the good performance of the Betts–Miller–Janjić scheme in the tests with the GATE data carried out by Emanuel and Živković-Rothman (1999) cannot be explained by having tuned the scheme on the same dataset that was used for the tests. On the contrary, this was an unbiased, independent test of the Betts–Miller–Janjić scheme as well. Editor’s Note: Emanuel and Živković-Rothman requested the following be included here: “The authors agree with Dr. Janjić’s comments and thank him for pointing out our error in describing the tuning of the Betts–Miller–Janjić scheme.”

Journal

Journal of the Atmospheric SciencesAmerican Meteorological Society

Published: Apr 3, 2000

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