Monthly Covariability of Amazonian Convective Cloud Properties and Radiative Diurnal Cycle

Abstract:

The diurnal cycle of convective clouds greatly influences the top-of-atmosphere 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. We use CloudSat and CERES data to further examine the nature of these connections.

First, we quantify covaraibility in multiple Amazonian cloud properties, in particular the deep convective cores (DCCs) and associated convective anvils (DCAs). We find a large significant inverse relationship between DCC frequency and updraft intensity, the latter of which being estimated with radar reflectivity. There is also a significant positive relationship between intensity and DCC top height. DCAs have a similar inverse relationship of frequency to convective intensity, but the DCA top height is mostly insensitive to intensity. Ice water content increases in both DCCs and DCAs as intensity increases, but the magnitude of DCA sensitivity decreases with distance to the convective core. The microphysical properties of DCCs core also carry into DCAs, and the influence decreases with increasing distance from the core.

Second, we examine the role that cloud variability has on radiative variability. Previous research has found that anomalous atmospheric states that enhance (reduce) deep convection tend to shift the daily outgoing longwave radiation and cloud albedo maxima earlier (later) in the day by 2-3 hr. We find a similar sensitivity of the radiative diurnal cycle to anomalous cloud properties. In particular, increased anomalous DCC/DCA frequency shifts the longwave diurnal cycle almost identically to increased convective instability, and accounts for 1 W m^-2 variability per standard deviation of frequency. Increased DCC/DCA top height shifts the longwave diurnal cycle in an opposing manner to frequency, likely an effect of the inverse relationship between cloud top height and frequency.