IPCC Expert Meeting on Geoengineering

Annex 4: Poster Abstracts
Cloud-resolving modeling of marine stratocumulus cloud brightening
Hailong Wang 1 , Philip J. Rasch 1 *, and Graham Feingold 2
1
Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory, USA
2
Chemical Sciences Division, NOAA Earth System Research Laboratory, USA
Marine Stratocumulus (Sc) clouds cover vast areas of the ocean surface. They play a prominent role affecting the Earth’s
albedo and energy budget. The possibility of mitigating some consequences of global warming by brightening these clouds
(Latham, 2002) has been explored in various global modeling studies, providing important insight into some aspects of this
idea. However, the inability of global models to adequately represent cloud-scale physics and dynamics due to low model
resolution limits their usefulness for assessing the climate system response to cloud brightening. High-resolution cloud
modeling has proven to be a useful tool for process-level understanding of aerosol-cloud-precipitation interactions. It can
also provide a necessary and critical test to the assumptions and parameterizations employed by global models when
exploring the geoengineering idea.
In this study (H. Wang et al., 2011), we use the high-resolution Weather Research and Forecasting (WRF) model with
explicit treatment of aerosol-cloud-precipitation interactions to perform cloud-system-resolving simulations of cloud
brightening. The same model configuration has been used by Wang and Feingold (2009a) to examine the transition from
closed-cell Sc to open cells. The two distinct cellular structures that have starkly different reflectance patterns (also revealed
by satellite imagery; e.g.,Garay et al., 2004; Wood and Hartmann, 2006), have been shown to be associated with aerosol
effects on rain-production processes and dynamical feedbacks in a self-organizing system (Stevens et al., 2005; H. Wang
and Feingold, 2009a; Feingold et al., 2010). More relevant to cloud brightening, however, is that once the open cells are
established, they appear to be resilient to aerosol perturbations. Another key result is that gradients in aerosols and
precipitation can generate mesoscale circulations that impact clouds well beyond the conventional “aerosol indirect
effects” (H. Wang and Feingold, 2009b).
Based on these prior studies, we conducted experiments with four combinations of different meteorological and aerosol
background conditions observed in the Northeast Pacific Sc regime. Seawater particles are injected into the model
atmosphere by single, multiple or numerous sprayers. In the first two cases, sprayers are allowed to move at a speed of 5 m
s-1 in the domain approximately the size of a climate model grid box, while in the latter case sprayers are uniformly
distributed over the model domain. The average surface injection rate of 1.45×106 m-2 s-1 is close to that suggested by
Salter et al. (2008). The injection strategy is critical in influencing the spatial distribution of the additional aerosols. Both
areal coverage and local number concentration are important players in cloud brightening but neither one always emerges
as dominant. For a given amount of aerosols, the two aspects need to be balanced to optimize the enhancement of cloud
amount and albedo. In the cases where clouds are susceptible to aerosol perturbations, injection from single or multiple
sprayers can introduce a significant heterogeneous response in the domain. The study also suggests that mesoscale
circulations induced by rain suppression associated with multi-sprayers can have unforeseen influence on cloud structures
and brightness depending on the distance between the sprayers, and that diurnal variations of clouds and precipitation
may be important for the effective timing of seeding.
Results show that the effectiveness of cloud brightening depends strongly on meteorological and background aerosol
conditions and, sometimes, the injection strategy. Suppressing rain as a means of sustaining cloud water is the most
efficient way to brighten clouds in the cases considered here. Hence, it is very effective in a weakly precipitating boundary
layer in which the additional aerosols can substantially weaken the precipitation. Seeding the aerosol-limited cloud in
conditions preceded by heavy and/or persistent rain that has significantly depleted existing aerosols is also effective. The
same amount of injected material is less effective in either strongly precipitating clouds or aerosol-rich clouds. Cloud drops
grow large enough to rain out in the former case even in the presence of additional aerosols. Cloud brightening is
ineffective in a relatively dry boundary layer that supports clouds of low liquid water path. In the aerosol-rich case and the
dry case, the aerosol injection increases drop number concentrations but lowers supersaturation and liquid water path. As a
result, the cloud experiences very weak albedo enhancement, regardless of the injection method. Detailed results have
been published in Wang et al. (2011), part of which is to be presented at the meeting.
<strong>IPCC</strong> <strong>Expert</strong> <strong>Meeting</strong> on Geoengineering - 57