Satellite Retrieval of Updrafts and Cloud Condensation Nuclei Concentrations at Cloud Bases
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Aerosol-cloud-climate interactions are the largest uncertainty in climate forcing. Narrowing down the uncertainties of the aerosol impact on Earth’s climate ultimately requires concurrent global measurements of aerosol, cloud, and key dynamic quantities. The core variables dictating cloud formation and development as well as aerosol-cloud interactions (ACI) are cloud condensation nuclei (CCN) concentrations and updrafts at cloud base (Wb). Both variables haven long been regarded as non-retrievable from conventional satellite remote sensing, a major cause of large uncertainty regarding ACI-induced climate radiative forcing. This study attempts to confront these challenges by exploiting any feasibility of satellite-based retrieval of Wb and CCN concentrations based on the most recent generation of operational weather satellite sensors.
Unlike conventional satellite remote sensing that retrieves geophysical quantities from radiance measurements, we estimate variables based on physical understanding of their interactions with conventional meteorological parameters obtained by satellite retrievals together with reanalysis data. Specifically, our methodology uses clouds as a natural analog for CCN chambers. Supersaturation (S) at cloud bases is determined by Wb and satellite-retrieved activated cloud droplet concentrations, which constitute the CCN spectrum, or CCN(S). The Wb is inferred by estimating components of energy that propels the convection. Validations of the retrieved Wb against ground-based updrafts measurements by Dopper lidar/Radar at Oklahoma, at Manaus, at Graciosa Island, and onboard a ship in the northeast Pacific show good agreements with mean absolute percentage errors (MAPE) of 21% and 22% for convective clouds and stratocumulus clouds, respectively. The retrieved Wb were applied for estimating CCN(S) with a MAPE of 30%.
In addition to advancement in satellite retrievals, we find the first robust observational evidence supporting the essential role of cloud-base height in regulating updrafts, an argument that has been used extensively to explain the contrast in lightning activities between land and sea. Additionally, we put forth a new theory interpreting the mechanism of surface coupling of marine stratocumulus clouds. This theory underscores the important role of cloud-top radiative cooling in driving surface moistures not only in well-mixed marine boundary layers but also in poorly mixed ones where the coupling is achieved via the mechanism of cumulus feeding stratocumulus. This new theory is examined and confirmed by ship-based and satellite measurements.