Meteorology both masks and magnifies the aerosol-cloud radiative effect

 
Poster PDF

Authors

Ian B Glenn — University of California, Los Angeles/Jet Propulsion Lab.
Graham Feingold — NOAA- Earth System Research Laboratory
Jake Gristey — NOAA- Earth System Research Laboratory
Takanobu Yamaguchi — CIRES, Unviersity of Colorado and NOAA ESRL

Category

Warm low clouds, including aerosol interactions

Description

The radiative effect of shallow cumulus clouds is not a monotonic function of the fractional area covered by their shadows, as shown by one of our re-simulations with aerosol variability added to a LASSO case. The 30-minute smoothed rCRE(f_c) line is colored by the time of day, starting just before noon local time. Variation in aspects of this curve from day to day represents integrated effects of co-variability between larger-scale meteorology, cloud radiative effects, and aerosol.
We have leveraged the Large Eddy Simulation (LES) ARM Symbiotic Simulation and Observation (LASSO) project and extensive ARM observations at SGP to study the radiative signature of aerosol-cloud interactions. The 30-plus LASSO cases generate a well-characterized database of shallow cumulus observations and LES output that can be used to address the question: Does co-variability with meteorology mask or magnify the radiative impacts of aerosol-cloud interaction? To incorporate aerosol effects, we have re-simulated all LASSO cases using surface observations of spatial and temporal variability in aerosol. We use the relative cloud radiative effect (rCRE) vs cloud fraction state space as a framework for our analyses. We have found that shallow cumulus exhibit hysteresis in this space. The hysteresis has its origins in the asymmetric deepening and collapse of the surface forced boundary layer. We have shown that the deepening is closely correlated with cloud edge entrainment. Thus we have been able to connect basic cloud/convection processes to the radiative properties of the cloud field. Hysteresis in cloud radiative effect vs. cloud fraction state space has implications for the detectability of aerosol-related brightening since similar cloud fractions can have very different cloud radiative effects depending on the time of day. In other words, cloud brightening associated with cloud deepening can easily mask cloud brightening due to aerosol. We have developed a method for parsing out brightening effects into cloud deepening and aerosol/microphysical effects. We consider 2-D joint probability distribution functions (pdfs) of relevant variables. For example, a cloud area-to-height joint-pdf can be used to explain rCRE variation due to the consistent relationship of cloud deepening to widening. Compared to previous "1D" analysis methods, this 2D expansion has an order of magnitude more statistical resolution. This enhances our ability to address our research question: Does co-variability with meteorology mask or magnify the radiative impacts of aerosol-cloud interaction? Finally, we are comparing measured surface downwelling shortwave radiation with calculations based on 3-D radiative transfer applied to LES-generated cloud fields. This represents a rigorous test of the ability of our simulated cloud fields to represent the aerosol-cloud radiative effect.