Covariability in the monthly mean convective and radiative diurnal cycles in the Amazon

English Abstract:

The diurnal cycle of convective clouds greatly influences the radiative energy balance in convectively active regions of Earth, through both direct presence and the production of anvil and stratiform clouds. Previous studies show that the frequency and properties of convective clouds can vary on monthly timescales as a result of variability in the monthly mean atmospheric state. Furthermore, the top-of-atmospheric radiative budget in convectively active regions varies by up to 7 Wm^-2. These facts suggest that convective clouds connect atmospheric state and radiation variability beyond clear sky effects alone.

Previous research has identified monthly covariability between the diurnal cycle of CERES-observed top-of-atmospheric radiative fluxes and multiple atmospheric state variables (ASVs) from reanalysis over the Amazon region. ASVs that enhance (reduce) deep convection, such as convective available potential energy (lower tropospheric stability), tend to shift the daily outgoing longwave radiation and cloud albedo maxima earlier (later) in the day by 2-3 hr. We first test the analysis method using multiple reanalysis products for both the dry and wet seasons to investigate the robustness of the previous results. We find general qualitative agreement between multiple reanalysis products, though the amplitude of the effect can vary by 50%. The seasonal results show the importance of both the cloud and clear-sky effects in determining the shift in the radiative diurnal cycle.

We then use CloudSat as an independent cloud observing system to further evaluate the relationships of cloud properties to variability in radiation and ASVs. While CERES can decompose OLR variability into clear sky and cloud effects, it cannot determine what variability in cloud properties (e.g. cloud cover, height, and microphysics) lead to variability in the radiative cloud effects. CloudSat observes these cloud properties, as well as the presence and variability of deep convective cores responsible for anvil clouds. While CloudSat cannot sample the full diurnal cycle, it can observe changes between early morning and early afternoon, such as the higher convective intensity of daytime versus nighttime. We use these capabilities to determine the covariability of convective cloud properties, ASVs, and the radiative diurnal cycle.