Impacts of mesoscale cloud organization on aerosol-induced cloud properties

Submitter

Feingold, Graham — NOAA Earth System Research Laboratory
Zhou, Xiaoli — NOAA/CIRES

Area of research

Cloud-Aerosol-Precipitation Interactions

Journal Reference

Zhou X and G Feingold. 2023. "Impacts of Mesoscale Cloud Organization on Aerosol‐Induced Cloud Water Adjustment and Cloud Brightness." Geophysical Research Letters, 50(13), e2023GL103417, 10.1029/2023GL103417.

Science

Mesoscale cellular convection cell size significantly regulates aerosol-induced cloud albedo changes via its effect on cloud water adjustment. We find notable intra-cell co-variability between cloud liquid water and drop concentration within cells. This variability varies with cell size.

Impact

This study finds for the first time that stratocumulus organization at mesoscale has an impact on aerosol cloud interactions. These findings carry far-reaching implications for aerosol-induced climate forcing and future climate predictions. This study also quantitatively reports intra-cell co-variability between cloud liquid water and drop concentration within cells, shedding light on the perilous consequences of erroneously attributing this co-variability as a response of cloud water to drop concentration. This discovery underscores the need to rectify such misinterpretation.

Summary

Low clouds over the ocean often exhibit organized patterns with characteristic cell sizes, but the effect of this organization on cloud-aerosol interactions is not yet fully understood. Here we group seven years of satellite-measured low cloud cells over the North Atlantic Ocean by their cell size to investigate how cell size influences the response of cloud water and cloud brightness to cloud droplet number concentration perturbations. Large-scale cells are found to have less efficient depletion of cloud water in response to increasing drop number compared to small-scale cells. This leads to nearly an order-of-magnitude-stronger increase in cloud brightness with increasing drop number. Furthermore, we show that the spatial distributions of liquid water path and drop concentration vary with cell size. Mistakenly assuming the local correlation between cloud water path and drop concentration to be a causal response can cause a significant positive error, especially for small-scale cells.

Figure 1. (Left panel) Median and interquartile range of scene-average liquid water path (LWP) of cloudy pixels in 10% percentile bins of scene-average droplet concentration (Nd) of cloudy pixels. Values are shown on logarithmic scales. The black dashed line corresponds to an adiabatic volume-mean droplet radius at cloud top of 14µm (adiabatic condensation rate of 2.14 x 106 kg m-4). Shade of gray background color represents a general indicator of likelihood of precipitation. L0 is estimated by performing linear regression on median values of LWP in the quartile bins of Nd, L0np is the slope for the quartile bins of Nd on the right of the dotted line. (Right panel) Same as the left panel but for mean cloud albedo (Ac). S0 is estimated by performing linear regression on median values of Ac in the quartile bins of Nd. Both panels are plotted for mesoscale cellular convections (MCCs) of 8 km (black), 16 km (blue), 32 km (green), and 64 km (red). From journal.

Figure 2. Aerosol-induced LWP perturbation computed from scene-average LWP and Nd of the cloudy MODerate-resolution Imaging Spectroradiometer (MODIS) pixels for entire cloud deck (green stars) and for non-precipitation dominant MCCs (L0np, black crosses). Red and blue lines indicate interquartile ranges of LWP adjustment computed on MODIS (~1km; LMODIS) and the Clouds and Earth’s Radiant Energy System (CERES, ~0.2º; LCERES) footprints respectively via the slope of linear regression within satellite snapshots of 2ºx2º. Dots represent the median values. Results are presented for MCCs of 8, 16, 32, and 64 km. From journal.