Impact of modifying the longwave water vapor continuum absorption model on community Earth system model simulations

Author: D. D. Turner and Eli J. Mlawer
Date: 
February 24, 2012
Type: 
Journal Article
Venue: 
Journal of Geophysical Research
Citation: 

Turner, D. D., A. Merrelli, D. Vimont, and E. J. Mlawer (2012), Impact of modifying the longwave water vapor continuum absorption model on community Earth system model simulations, J. Geophys. Res., 117, D04106, doi:10.1029/2011JD016440.

The far-infrared (wavelengths longer than 17 μm) has been shown to be extremely important for radiative processes in the earth's atmosphere. The strength of the water vapor continuum absorption in this spectral region has largely been predicted using observations at other wavelengths that have been extrapolated using semiempirical approaches such as the Clough-Kneizys-Davies (CKD) family of models. Recent field experiments using new far-infrared instrumentation have supported a factor of 2 decrease in the modeled strength of the foreign continuum at 50 μm and a factor of 1.5 increase in the self-continuum at 24 μm in the Clough-Kneizys-Davies continuum model (CKD v2.4); these changes are incorporated in the Mlawer-Tobin-CKD continuum model (MT_CKD v2.4). The water vapor continuum in the Community Earth System Model (CESM v1.0) was modified to use the newer model, and the impacts of this change were investigated by comparing output from the original and modified CESM for 20 year integrations with prescribed sea surface temperatures. The change results in an increase in the net upward longwave flux of order 0.5 W m−2 between 300 and 400 mb, and a decrease in this flux of about the same magnitude for altitudes below 600 mb. The radiative impact results in a small but statistically significant change in the mean temperature and humidity fields, and also a slight decrease (order 0.5%) of high-cloud amount. The change in the cloud amount modified the longwave cloud radiative forcing, which partially offset the radiative heating caused by the change in the water vapor continuum absorption model.