The Effects of Carbon Black Dust on Cumulus-Scale Convection

Abstract

The effect of seeding with carbon black dust in the tropical atmosphere is simulated in a two-dimensional time-dependent cloud model which covers a region 6.4 km × 3.2 km in the x and z directions with 200 m grid intervals. Initial conditions are taken from the mean “hurricane season” soundings for the West Indies area (Jordan, 1958). An equation for the continuity of carbon black dust is added to the model. Scavenging of the carbon black dust by precipitation and cloud water content and diffusion of carbon dust to the ocean surface are included in the model. Radiative heating of the atmosphere by the carbon black is simulated. The heating rate is set proportional to the concentration of carbon black dust at a grid point. Air motion is simulated by horizontal density differences caused by horizontal gradients of the heating rate. The circulations lead to cooling and moistening in updrafts and warming and drying in downdrafts in a conditionally stable atmosphere. In the meantime, the carbon black is redistributed. Three cases are used to test the concept of carbon black dust seeding. The first is a case having a horizontally homogeneous concentration of carbon black, namely, an “Even” case. This case shows that the atmosphere is heated by carbon particles but that no air motion occurs. Changing the initial distribution of carbon black dust to a “rectangular” pattern with gradations of carbon black concentration results in a second case called the “Layer-A” case. In this case, the vertical velocity decreases very rapidly with time after it passes its maximum value of 27 cm s⁻¹ at 10 min. Eighteen percent of the original carbon black dust is lost in 100 min due primarily to diffusion to the ocean surface. Changing the dew-point curve in the mean sounding and using the initial carbon black dust pattern of the layer-A case results in a “Layer-B” case, the third case. A small cloud forms in this case. Comparison of the total amount of carbon black in the Layer-A and Layer-B case shows that small amounts of cloud and rain are not an efficient mechanism to deplete the carbon particles; these water contents do, however, decrease the solar energy absorbed by carbon black particles and hence weaken air motion. The maximum value of the vertical velocity increases by 15 cm s⁻¹ if solar insolation can pass through cloud and rain without any loss. Results of this numerical study are not encouraging for the direct formation of cloud lines by the spread of carbon black dust in the tropical atmosphere, unless the atmosphere is much more humid than normal. No conclusions concerning mesoscale effects of the solar heating and indirect formation of cloud lines are possible within the framework of this cloud-scale model.