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Binyamin U. Neeman
,
George Ohring
, and
Joachim H. Joseph

Abstract

A parameterization of quasi-geostrophic eddy transport that takes into account the time variation of the eddy transfer coefficients according to Green's theory is studied. A relation proposed by Green connects the vertical integral of the meridional heat flux at 50°N with the second power of the 500 mb temperature difference between the boundaries of baroclinic activity. It is found that the fourth power in this relation, rather than the original second power, is obtained from analysis of zonal/monthly-mean observational data at 500 mb. For the temperature difference at 1 000 mb, however, the same analysis yields a power of 1.5.

The differences in the seasonal simulation of different powers in the eddy transfer relation are explored in a two-level statistical dynamical zonally averaged model (SDZAM), and it is found that an appropriate choice of power may be of special importance if the model is devised to simulate the seasonal climate cycle, or to test astronomical changes inducing different seasonalities. With the second power in 500 rob, the particular SDZAM being tested simulates an oversensitivity in the high latitude temperature response to the seasonal cycle/astronomical changes, due to its undersensitivity in the simulation of changes in the meridional eddy heat flux. A comparison of the results of a second power at the surface level vs a fourth power at 500 mb is difficult due to the need to retune the model, but a certain advantage to the latter model is detected.

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Binyamin U. Neeman
,
Joachim H. Joseph
, and
George Ohring

Abstract

An efficient longwave scheme for climate models originally suggested by Sasamori is modified to correctly simulate the water vapor-temperature feedback mechanism. It is found that the modified scheme with a fixed cloud-top altitude (FCA) correctly simulates the longwave sensitivity to surface temperature, F / T s , over the clear portion of the sky, but that over the cloud portion of the sky F / T <s remains too high. The fixed cloud-top temperature (FCT) method is similarly reviewed and tested. Comparisons with observational Budyko-type correlations are shown to be indecisive over the question of FCA vs FCT.

A direct observational correlation between the effective cloud-top and surface temperatures, based on annually averaged cloud statistics data, suggests a variable cloud-top temperature (VCT) model. In such a model, the temperature of the elective cloud-top layer is varied according to changes in the surface temperature at a rate which is intermediate between that of the FCA and FCT models. This model results in a reasonable F / T s over the cloud portion of the sky.

The modified longwave scheme is implemented into a zonally averaged dynamic climate model. It is shown that when the VCT mechanism is invoked, climate sensitivity is doubled compared to that simulated with the FCA model. The importance of simulating not only the correct longwave flux, but also the correct longwave sensitivity to temperature changes is therefore stressed for radiation schemes in studies involved with climate change.

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