Seeding-Opportunity Recognition in Winter Orographic Clouds

Geoffrey E. Hill Utah Water Research Laboratory, Utah State University, Logan 84322

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Abstract

Detailed measurements of supercooled cloud water, precipitation, cloud-top temperature and vertical air motion in winter orographic clouds are used to develop criteria for the seedability of those clouds. Winter orographic clouds over the upwind mountain base with cloud-top temperatures between 0 and −22°C are found to be primarily composed of supercooled water and are therefore seedable. The supercooled water concentration is empirically found to depend on the updraft velocity. The potential precipitation yield is dependent on the flux of supercooled water over the barrier. Because the updraft velocity is approximately proportional to the cross-barrier wind, the potential precipitation yield is approximately proportional to the square of the cross-barrier wind, provided that the cloud-top temperature is in the seedable range of temperatures.

These findings are strongly substantiated by systematic use of aircraft icing reports over a full winter season (November–March 1978–79). It is shown that a cloud-top temperature of about −22°C separates clouds with a precipitation enhancement potential from those without such a potential. It is found that aircraft icing is approximately proportional to the cross-barrier wind, and that the flux of supercooled water over the barrier for cloud-top temperatures warmer than −22°C is (as derived from the research data) approximately proportional to the square of the cross-barrier wind.

About 20% of cloud episodes over the mountains of northern Utah may be expected to have a high modification potential.

Abstract

Detailed measurements of supercooled cloud water, precipitation, cloud-top temperature and vertical air motion in winter orographic clouds are used to develop criteria for the seedability of those clouds. Winter orographic clouds over the upwind mountain base with cloud-top temperatures between 0 and −22°C are found to be primarily composed of supercooled water and are therefore seedable. The supercooled water concentration is empirically found to depend on the updraft velocity. The potential precipitation yield is dependent on the flux of supercooled water over the barrier. Because the updraft velocity is approximately proportional to the cross-barrier wind, the potential precipitation yield is approximately proportional to the square of the cross-barrier wind, provided that the cloud-top temperature is in the seedable range of temperatures.

These findings are strongly substantiated by systematic use of aircraft icing reports over a full winter season (November–March 1978–79). It is shown that a cloud-top temperature of about −22°C separates clouds with a precipitation enhancement potential from those without such a potential. It is found that aircraft icing is approximately proportional to the cross-barrier wind, and that the flux of supercooled water over the barrier for cloud-top temperatures warmer than −22°C is (as derived from the research data) approximately proportional to the square of the cross-barrier wind.

About 20% of cloud episodes over the mountains of northern Utah may be expected to have a high modification potential.

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