Marine and Continental Low-level Cloud Processes and Properties

Low-level Cloud processes and properties are extremely important parts of the climate system. Their treatment in climate models is one of the largest sources of uncertainty in predicting any potential future climate change. As concluded in Randall et al. (IPCC Chapter 8, 2008) “Cloud feedbacks have been confirmed as a primary source of the inter-model differences, with low clouds making the largest contribution. In this study, we have collected and retrieved low-level cloud macrophysical and microphysical properties over the ARM SGP observations (Dong et al. 2005, J Climate) and ARM AMF deployment at the Azores (Dong et al. 2014, J Climate). Based on the ARM ground-based observations and retrievals, we have investigated the similarities and differences of the formation-dissipation processes, and the seasonal and diurnal variations of MBL and continental cloud fraction and properties.

Marine Boundary Layer (MBL) clouds are formed under a strong temperature inversion at the top of the MBL, maintained by large-scale subsidence combined with cold sea-surface temperature. These MBL clouds are turbulently mixed from top to down due to LW radiative cooling at cloud top, while for continental low clouds they are usually mixed from the bottom up due to solar heating. For single-layer low clouds, the annual average CF at the Azores is 0.30, about 3 times of the SGP CF (0.11), dominated by persistent high pressure system. There is a bimodal vertical distribution of CF in both sites. MBL cloud layer is shallow, thin and warm with large LWP and LWC during summer. During winter, however, it is deep, thick and cold with less LWP and LWC. At the ARM SGP site, their low-level cloud properties are totally opposite to those at the Azores. Low-level cloud layer is deeper, thicker, and warmer with less LWP and LWC during summer than those during winter. It remains a challenge for us to understand what processes control these similarities/differences, and can we model them?

Reference: Dong et al. (2014): A 19-month Record of Marine Aerosol-Cloud-Radiation Properties derived from DOE ARM AMF deployment at the Azores: Part I: Cloud Fraction and Single-layered MBL cloud Properties. J. Clim. DOI: 10.1175/JCLI-D-13-00553.1. Dong et al. (2005): A climatology of midlatitude continental clouds from ARM SGP site. Part I: Low-level Cloud Macrophysical, microphysical and radiative properties. J. Climate. 18, 1391-1410.