A document by Ehsan Erfani. Click on the document to view its contents.

A document by Ehsan Erfani. Click on the document to view its contents.

Low marine clouds are a major source of uncertainty in cloud feedbacks across climate models and in forcing by aerosol-cloud interactions. The evolution of these clouds and their response to aerosol are sensitive to the ambient environmental conditions, so it is important to be able to determine different responses over a representative set of conditions. Here, we propose a novel approach to encompassing the broad range of conditions present in low marine cloud regions, by building a library of observed environmental conditions. This approach can be used, for example, to more systematically test the fidelity of Large Eddy Simulations (LES) in representing these clouds. ERA5 reanalysis and various satellite observations are used to extract and derive macrophysical and microphysical cloud-controlling variables (CCVs) such as SST, estimated inversion strength (EIS), subsidence, and cloud droplet number concentrations. A few locations in the stratocumulus (Sc) deck region of the Northeast Pacific during summer are selected to fill out a phase space of CCVs. Thereafter, Principal Component Analysis (PCA) is applied to reduce the dimensionality and to select a reduced set of components that explain most of the variability among CCVs in order to efficiently select cases for LES simulations that encompass the observed CCV phase space. From this phase space, 75-100 cases with distinct environmental conditions will be selected and used to initialize 2-day LES modeling to provide a spectrum of aerosol-cloud interactions and Sc-to-Cumulus transition under observed ambient conditions. Such a large number of simulations will help create statistics to assess how well the LES can simulate the cloud lifecycle when constrained by the ‘best estimate’ of the environmental conditions, and how sensitive the modeled clouds are to changes in these driving fields.

Observed stratocumulus to cumulus transitions (SCT) and their sensitivity to aerosols are studied using a Large-Eddy Simulation (LES) model that simulates the aerosol lifecycle, including aerosol sources and sinks. To initialize, force, and evaluate the LES, we used a combination of reanalysis, satellite, and aircraft data from the 2015 Cloud System Evolution in the Trades field campaign over the Northeast Pacific. The simulations follow two Lagrangian trajectories from initially overcast stratocumulus to the tropical shallow cumulus region near Hawaii. The first trajectory is characterized by an initially clean, well-mixed stratocumulus-topped marine boundary layer (MBL), then continuous MBL deepening and precipitation onset followed by a clear SCT and a consistent reduction of aerosols that ultimately leads to an ultra-clean layer in the upper MBL. The second trajectory is characterized by an initially polluted and decoupled MBL, weak precipitation, and a late SCT. Overall, the LES simulates the observed general MBL features. Sensitivity studies with different aerosol initial and boundary conditions reveal aerosol-induced changes in the transition, and albedo changes are decomposed into the Twomey effect and adjustments of cloud liquid water path and cloud fraction. Impacts on precipitation play a key role in the sensitivity to aerosols: for the first case, runs with enhanced aerosols exhibit distinct changes in microphysics and macrophysics such as enhanced cloud droplet number concentration, reduced precipitation, and delayed SCT. Cloud adjustments are dominant in this case. For the second case, enhancing aerosols does not affect cloud macrophysical properties significantly, and the Twomey effect dominates.

Low marine clouds are a major source of uncertainty in cloud simulations across models from LES to global scale. To address this issue, we conducted Lagrangian LES experiments that explore the aerosol-cloud interactions for case studies covering a spectrum of observed ambient conditions, and evaluated the model against observations. Our LES benefits from a prognostic aerosol model that simulates aerosol budget tendencies such as coalescence and interstitial scavenging, surface sources, and entrainment from free troposphere. To initialize, force, and evaluate the LES, we used a combination of reanalysis, satellite, and aircraft data from the Cloud System Evolution in the Trades (CSET) field campaign in summer 2015 over the Northeast Pacific. The LES follows two Lagrangian trajectories from subtropical stratocumulus (Sc) deck region offshore of California to tropical shallow cumulus (Cu) region near Hawaii. The first trajectory is characterized by a clean, well-mixed Sc-topped marine boundary layer (MBL) on the first day, and continuous MBL deepening and precipitation onset after the first day followed by a clear Sc-to-Cu transition (SCT) and a consistent reduction of aerosols that ultimately leads to an ultra-clean layer at the top of MBL. Overall, the LES simulates general MBL features seen in observations. The runs with enhanced aerosols show distinct changes in microphysics and macrophysics such as delayed precipitation onset and SCT. The second trajectory is characterized by an initially polluted and decoupled MBL, weak or no precipitation, and no clear sign of SCT throughout the simulations. It is challenging for LES to simulate observed features, and the LES underestimates (overestimates) low cloud fraction in the first (last) day. Although enhancing aerosols among cases leads to distinct changes in microphysics (e.g., enhancement of cloud optical depth and reduction of effective radius), it does not affect cloud macrophysical properties significantly. Finally, a theoretical analysis was conducted to decompose contributions to albedo of the Twomey effect and cloud adjustments. The cloud radiative forcing due to the Twomey effect shows an enhancement with an increase in aerosol, however, the cloud radiative forcing due to cloud adjustments is strongly dependent on ambient meteorological conditions.