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A More Realistic Cirrus Cloud Climate Intervention Experiment
AdvisorMitchell, David Lancaster
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This MS thesis research investigates one of the three most researched ideas regarding radiation management (RM) climate engineering or climate intervention (CI), which is the deliberate modification of the Earth’s radiation balance to avoid the worst effects of global warming. To date, all cirrus cloud thinning (CCT) studies have been numerical modeling experiments, and those explicitly treating the cloud macro- and microphysical processes affecting CCT (i.e. ice nucleation) make various assumptions regarding these processes (which are poorly constrained). These different assumptions produce different conclusions regarding the viability of CCT, with some studies showing the CCT cloud radiative effect (CRE) is negligible while others show it is substantial. The CCT study here is the first to avoid the complexities of ice nucleation physics by using satellite retrievals from cirrus clouds to constrain ice nucleation. The cirrus cloud ice particle size distribution (PSD) is fundamentally characterized by three properties; the ice water content (IWC), the effective diameter (De) and the ice particle number concentration (N). Knowing IWC and De, N is directly solved for through conservation of mass using PSD moment relationships.¬¬¬In this CCT experiment, the climate model provides reasonable estimates of IWC and the satellite retrievals provide De in the climate model, allowing N to be determined analytically through conservation of mass. One 10-year climate simulation uses observed global distributions of De which are affected by N contributions from both homo- and heterogeneous ice nucleation (henceforth hom and het, respectively). This simulation is named CALIPSO or CAL. Another 10-year simulation is based on De characteristic of het (i.e. N ~ 50-100 L-1) and is named HET. Differences between these simulations reveal the impact of hom cirrus clouds on the radiative fluxes, heating rates and the general circulation. Conversely, it also reveals the CCT impact.Our CCT experiment defined by HET-CAL showed a significant increment in De where cirrus clouds exit (T < 235 K) and even within the mixed phase cloud region (235 < T < 273 K), and a decrement in IWC and N within those same temperature zones at high latitudes. An increment in De and decrement in IWC imply that optical path has decreased, resulting in more outgoing longwave radiation (OLR) from the surface and lower levels of the troposphere. Cloud fraction decreases at cirrus levels and increases at lower levels around 600 to 700 hPa. This tendency stands out in winter. These changes produce a significant radiative cooling effect with more OLR from the lower atmosphere. As a consequence, higher layers around 300 hPa in winter are significantly warmed up and lower layers around 700 hPa and the surface in winter are significantly cooled down. However, some of the regions (e.g. the west coast of the USA at 700 hPa and the western part of the North American continent on the surface) exhibit significant warming. We attribute this increased temperature to increased geopotential height in this region at 300 hPa and increased downwelling longwave flux due to increased cloud fraction at lower levels resulting from CCT.