Numerical simulation of coastal clouds when solar radiation is blocked by smoke

Abstract

Large fires ignited in a major nuclear exchange would inject a considerable amount of smoke into the atmosphere. This smoke could block the incident solar radiation from reaching the ground. Without the input of solar radiation on the ground, the temperature over the land surface would drop more rapidly than the temperature over the ocean, leading to formation of a thermal gradient at the continental coastlines. This temperature gradient has been hypothesized to induce a flow field that would initiate precipitation which could rapidly scavenge much of the smoke. This hypothesis is being investigated using an enhanced version of the Colorado State University Mesoscale Model to simulate air flow, clouds and precipitation along continental coastlines. The model has been modified to include the formation of fog and clouds, removal of moisture by precipitation, and the transport and diffusion of smoke. Also, the long wave radiation parameterization has been changed to take into account the effects of clouds. For both east and west coasts, with a moderate westerly synoptic wind and with solar radiation blocked by smoke, continental cooling leads to the formation of fog or low clouds over land. When fog forms, the layer of most rapid cooling moves to the top of the cloud while the potential temperature inside the cloud becomes nearly constant. As higher layers become saturated, the cloud expands vertically, and the liquid water content increases until precipitation falls from the cloud. Over the ocean, the atmosphere cools radiatively while the sea surface remains at constant temperature thus reducing the thermal stability and enhancing the vertical mixing of moisture which is injected into the atmosphere from the sea. Eventually a layer of precipitating cloud forms over the ocean that is very similar to the clouds over land. Once these similar cloud layers have formed, the cooling rates over land and sea become nearly identical, and there is no mechanism to produce the horizontal temperature gradients needed for vertical growth of clouds and precipitation into the upper levels of the atmosphere where they can scavenge the injected smoke.