A sensitivity study of a general circulation model with enhanced shortwave atmospheric absorption

A discrepancy on the order of 15--30 W/m2 of shortwave absorption in the earth atmosphere has been found in the comparisons between model calculations and measurements. In my study, I attempt to answer the following four questions: (1) what is the clear sky absorption anomaly? (2) How well are we doing with solar radiation models? (3) What should we do to enhance shortwave atmospheric absorption in the solar radiation models and general circulation model (GCM)? (4) What are the effects of doing so on the models and GCM?; Exclusively based on observational data taken at Atmospheric Radiation Measurement (ARM) Southern Great Plain (SGP) site, our comparison studies between broadband models and broadband measurements show that models generally underestimate shortwave absorption in the atmosphere by 4.5% out of the total incoming solar radiation at the top of the atmosphere. Further comparison between broadband model and spectral model demonstrates that 1% out of the 4.5% difference is due to degradation of accuracy of absorption coefficients in transition from line by line model to broadband model. Comparison between the spectral model and spectral measurement shows that another 1% difference of shortwave absorption between model and observation may be linked to water vapor and aerosol. Next, we conduct sensitivity study and enhancement test with broadband models to explore different approaches to enhance atmospheric absorption of solar radiation in the models. These studies evaluate the performance and characteristics of the different solar radiation models of calculating solar energy budget in the earth-atmosphere system. In the enhancement test, the structure of the increase of the solar heating rate shows dependence on the vertical distribution of the absorbers. The dependence is manifested again in the GCM studies and helps to explain many features of the GCM simulations. The results from these preliminary studies provide us the insight in designing enhancement schemes and analyzing data in the GCM sensitivity studies.; In the GCM sensitivity studies, two shortwave absorption enhancement schemes are designed to enhance the GCM shortwave atmospheric absorption. In both the water vapor and aerosol experiment, atmospheric absorption of solar radiation increases and surface absorption decreases. Increased atmospheric absorption leads to temperature increase in the atmosphere, changing the hydrological cycle and dynamic circulation of the earth-atmosphere system and causing a chain of reactions. At the surface, to keep the energy balance, latent heat flux and sensible heat flux decreases, leading to reduced moisture, precipitation, and turbulence in the atmospheric boundary layer. The difference in the vertical distribution of water vapor and aerosols leads to difference in the details of the changes. In the water vapor experiment, additional solar heating occurs in the upper troposphere, causing the polar frontal system to migrate to higher latitudes and forcing upper troposphere meridional circulation to transport heat from equator to higher latitudes. The upper atmospheric heating also inhibits high cloud formation and stabilizes deep convection in the tropics. In the aerosol experiment, solar heating rate increase is confined in the lowest three model layers. A particular temperature increase pattern strengthens the Hadley circulation in the lower atmosphere and weakens the polar meridional circulation. Lower atmosphere is stabilized as well, causing decreased low level cloud formation.