Tropical Anvil Clouds: Radiative Driving Toward a Preferred State

Sokol, A. B., and D. Hartmann (2020), Tropical Anvil Clouds: Radiative Driving Toward a Preferred State, J. Geophys. Res., 125, e2020JD033107, doi:10.1029/2020JD033107.

The evolution of anvil clouds detrained from deep convective systems has important implications for the tropical energy balance and is thought to be shaped by radiative heating. We use combined radar-lidar observations and a radiative transfer model to investigate the influence of radiative heating on anvil cloud altitude, thickness, and microphysical structure. We find that high clouds with an optical depth between 1 and 2 are prevalent in tropical convective regions and can persist far from any convective source. These clouds are generally located at higher altitudes than optically thicker clouds, experience strong radiative heating, and contain high concentrations of ice crystals indicative of turbulence. These findings support the hypothesis that anvil clouds are driven toward and maintained at a preferred optical thickness that corresponds to a positive cloud radiative effect. Comparison of daytime and nighttime observations suggests that anvil thinning proceeds more rapidly at night, when net radiative cooling promotes the sinking of cloud top. It is hypothesized that the properties of aged anvil clouds and their susceptibility to radiative destabilization are shaped by the time of day at which the cloud was detrained. These results underscore the importance of small-scale processes in determining the radiative effect of tropical convection. Plain Language Summary Clouds play an important role in Earth's energy balance, especially in the tropics. Thick clouds cool the climate by reflecting sunlight, while thinner clouds located high in the atmosphere warm the climate due to their strong greenhouse effect. Tropical thunderstorms generate expansive cloud systems (“anvil” clouds) that initially exert a cooling effect but evolve over time to produce a warming effect. Their net impact on the climate system depends on how much time they spend in their cooling and warming stages. In this study, we use satellite measurements and a radiation model to examine how anvil clouds evolve. We find that anvil clouds with a climate-warming effect are pervasive in the tropics and can be found far from any thunderstorm that would have produced them, suggesting that they are maintained in their warming stage for long periods of time. We observe some unique characteristics of these clouds that provide clues about the processes that maintain them. Our findings provide real-world support for previous hypotheses that, until now, relied on computer simulations. They also highlight the importance of small-scale processes in shaping the large-scale tropical energy balance and underscore the need to consider these processes in projections of future climate change.

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Research Program: 
Climate Variability and Change Program
Funding Sources: 
NASA FINESST GRANT 80NSSC20K1613, Additional support from NSF Grant AGS-1549579