Identifying the radiative ‘twilight zone’ surrounding shallow cumulus clouds from high-resolution ASTER observations

The uncertain radiative effect of shallow cumulus clouds over tropical ocean significantly contributes to the high uncertainty in climate sensitivity estimates. Radiances corresponding to clear-sky and cloudy areas can be observed in moderate resolution satellite images. To observe the radiance originating from very small clouds and from the transition zone surrounding shallow cumulus clouds, the ‘twilight zone’, high-resolution data is required. Twilight zone radiances can be higher than clear-sky radiances due to unresolved cloud fragments and/or humidified aerosols. The area of the twilight zone depends on the resolution of the underlying data. If we think of the twilight zone in terms of partially cloudy pixels, such an area results in a high uncertainty in cloud and aerosol retrievals, as they are based on cloudy and clear-sky assumptions respectively. A precise knowledge of radiances from clouds and their twilight zone is decisive in terms of the total cloud reflectance and subsequently the shallow cumulus cloud radiative effect, which climate models struggle to properly simulate.

We therefore investigate the abundance and importance of such a twilight zone from high-resolution satellite images from ASTER recoded previously and during the EUREC4A field campaign. We use radiative transfer simulations to model the contribution of background clear-sky radiances including aerosols. Subtracting known clear-sky radiances from observed cloud field radiances leaves us with a precise knowledge of non-clear-sky radiances originating from shallow cumulus clouds and their surrounding twilight zone.