Aronne J Merrelli — University of Wisconsin
David D. Turner — NOAA- Global Systems Laboratory
Daniel Vimont — University of Wisconsin, Madison
Eli Jay Mlawer — Atmospheric and Environmental Research, Inc.
Category
Modeling
Description
Zonal, annual average differences between the two CAM integrations: clear-sky longwave heating, introduced by the water vapor continuum absorption update (left); total heating, including shortwave, cloud forcing, and latent heating terms (right).
Water vapor is the dominant contributor to atmospheric absorption in infrared wavelengths. Thus, knowledge of longwave radiative heating and cooling and its connection to atmospheric dynamics is dependent on accurate knowledge of water vapor absorption properties. The total water vapor absorption is typically divided into a spectral absorption line component and a smoothly varying continuum absorption component. The ARM-supported Radiative Heating in Underexplored Bands Campaigns (RHUBC) were conducted to directly measure the water vapor absorption in dry conditions, in order to better constrain the continuum absorption model in far infrared (FIR) wavelengths (17–50 μm). Data collected during RHUBC-I led to an update in the water vapor continuum absorption model (MT-CKD v2.4). Compared to the previous version of the continuum model (CKD v2.4), the self and foreign model coefficients changed by up to 50%, which results in net longwave flux changes of up to 0.8 W/m^2. To investigate the climate impact of this change in longwave heating, we ran climate simulations with the Community Atmosphere Model (CAM) version 5, which is part of the NCAR Community Earth System Model (CESM). CAM 5 uses the RRTMg band model for the longwave radiative transfer, which contains the water vapor continuum model before the RHUBC-I updates (CKD v2.4). We modified the CAM 5 RRTMg code to use the RHUBC-I updated continuum model (MT-CKD v2.4) and ran the modified and unmodified CAM in parallel from the same starting condition. The change in longwave heating from the water vapor continuum update introduced small changes in the cloud amount, latent heating, temperature, and humidity. These thermodynamic changes produced diabatic heating that compensated the longwave heating change introduced by the water vapor continuum absorption change. No changes in the large-scale dynamical fields were observed.
