The scientific basis for a satellite mission to retrieve CCN concentrations and their impacts on convective clouds

Abstract

Abstract. The cloud-mediated aerosol radiative forcing is widely recognized as the main source of uncertainty in our knowledge of the anthropogenic forcing on climate. The current challenges for improving our understanding are (1) global measurements of cloud condensation nuclei (CCN) in the cloudy boundary layer from space, and (2) disentangling the effects of aerosols from the thermodynamic and meteorological effects on the clouds. Here, we present a new conceptual framework to help us overcome these two challenges, using relatively simple passive satellite measurements in the visible and infared (IR). The idea is to use the clouds themselves as natural CCN chambers by retrieving simultaneously the number of activated aerosols at cloud base, N a , and the cloud base updraft speed. The N a is obtained by analyzing the distribution of cloud drop effective radius in convective elements as a function of distance above cloud base. The cloud base updraft velocities are estimated by double stereoscopic viewing and tracking of the evolution of cloud surface features just above cloud base. In order to resolve the vertical dimension of the clouds, the field of view will be 100 m for the microphysical retrievals, and 50 m for the stereoscopic measurements. The viewing geometry will be eastward and 30 degrees off nadir, with the Sun in the back at 30 degrees off zenith westward, requiring a Sun-synchronous orbit at 14 LST. Measuring simultaneously the thermodynamic environment, the vertical motions of the clouds, their microstructure and the CCN concentration will allow separating the dynamics from the CCN effects. This concept is being applied in the proposed satellite mission named Clouds, Hazards and Aerosols Survey for Earth Researchers (CHASER).

Assessment of cloud supersaturation by size-resolved aerosol particle and cloud condensation nuclei (CCN) measurements

“…Particles with a critical supersaturation equal to or lower than the peak supersaturation are activated and grow into cloud droplets. The peak supersaturation is a major determinant for the cloud droplet number and the regime of CCN activation (aerosol-vs. updraft limited; Reutter et al, 2009;Rosenfeld et al, 2012). Due to inhomogeneities of the atmospheric aerosol load and air flow pattern (turbulence, entrainment), the peak supersaturations of different air parcels in a cloud can be temporally and spatially heterogeneous.…”

“…To fully describe the process of CCN activation and cloud droplet growth in the atmosphere, however, the supersaturation of water vapor in the cloud also needs to be known (e.g., Reutter et al, 2009;Pruppacher and Klett, 2010;Rosenfeld et al, 2012;Renno et al, 2013).…”

Global and regional estimates of warm cloud droplet number concentration based on 13 years of AQUA-MODIS observations

“…Several studies also propose different and potentially more elaborate approaches for deriving CDNC. For example, Rosenfeld et al (2012) propose a spatially highly resolving satellite mission dedicated to the retrieval of cloud condensation nuclei for convective clouds and also point to limitations of the CDNC retrieval approach pursued here when applied to convective clouds. Zeng et al (2014) propose a combination of Cloud-Aerosol Lidar with Orthogonal Polarization (CALiOP) observations with MODIS observations to retrieve CDNC.…”

Satellite retrieval of convective cloud base temperature based on the NPP/VIIRS Imager

“…It also allows the calculation of the boundary layer water vapor mixing ratio with an accuracy of about 10%. Finally, the feasibility of retrieving cloud base temperature and height is an essential component that is required for retrieving CCN from satellites by using convective clouds as natural CCN chambers [Rosenfeld et al, 2012;Rennó, 2013].…”

“…Rosenfeld et al [2012] proposed to use the convective clouds as natural CCN chambers and retrieved the cloud drop number concentrations (N d ) that were nucleated near cloud base along with cloud base updraft (w b ). Knowing both N d and w b allows the calculation of the supersaturation in cloud base.…”