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  • Author or Editor: Michael P. Meyers x
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Michael P. Meyers
and
William R. Cotton

Abstract

A prolonged orographic precipitation event occurred over the Sierra Nevada in central California on 12–13 February 1986. This well-documented case was investigated via the nonhydrostatic version of the Colorado State University (CSU) Regional Atmospheric Modeling System (RAMS). The two-dimensional, cross-barrier simulations produced flow fields and microphysical structure, which compared well with observations. The feasibility of producing quantitative precipitation forecasts (QPF) with an explicit cloud model was also demonstrated.

The experiments exhibited a profound sensitivity to the input sounding. Initializing with a sounding, which is representative of the upstream environment, was the most critical factor to the success of the simulation. The QPF was also quite sensitive to input graupel density. Decreasing the density of graupel led to increases in the overall precipitation. Sensitivities to other microphysical parameters as well as orography and dynamics were also examined.

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Michael P. Meyers
,
Paul J. DeMott
, and
William R. Cotton

Abstract

Two new primary ice-nucleation parameterizations are examined in the Regional Atmospheric Modeling System (RAMS) cloud model via sensitivity tests on a wintertime precipitation event in the Sierra Nevada region. A model combining the effects of deposition and condensation-freezing nucleation is formulated based on data obtained from continuous-flow diffusion chambers. The data indicate an exponential variation of ice-nuclei concentrations with ice supersaturation reasonably independent of temperatures between −7° and −20°C. Predicted ice concentrations from these measurements exceed values predicted by the widely used temperatures dependent Fletcher approximation by as much as one order of magnitude at temperatures warmer than −20°C. A contact-freezing nucleation model is also formulated based on laboratory data gathered by various authors using techniques that isolated this nucleation mode. Predicted contact nuclei concentrations based on the newer measurements are as much as three orders of magnitude less than values estimated by Young's model, which has been widely used for predicted schemes.

Simulations of the orographic precipitation event over the Sierra Nevada indicate that the pristine ice fields are very sensitive to the changes in the ice-nucleation formulation, with the pristine ice field resulting from the new formulation comparing much better to the observed magnitudes and structure from the case study. Deposition-condensation-freezing nucleation dominates contact-freezing nucleation in the new scheme, except in the downward branch of the mountain wave, where contact freezing dominates in the evaporating cloud. Secondary ice production is more dominant at warm temperatures in the new scheme, producing more pristine ice crystals over the barrier. The old contact-freezing nucleation scheme overpredicts pristine ice-crystal concentrations, which depletes cloud water available for secondary ice production. The effect of the new parameterizations on the precipitating hydrometeors is substantial with nearly a 10% increase in precipitation across the domain. Graupel precipitation increased dramatically due to more cloud water available with the new scheme.

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Michael P. Meyers
,
Paul J. Demott
, and
William R. Cotton

Abstract

Ice initiation by specific cloud seeding aerosols, quantified in laboratory studies, has been formulated for use in mesoscale numerical cloud models. This detailed approach, which explicitly represents artificial ice nuclei activation, is unique for mesoscale simulators of cloud seeding. This new scheme was applied in the simulation of an orographic precipitation event seeded with the specific aerosols on 18 December 1986 from the Sierra Cooperative Pilot Project using the Regional Atmospheric Modeling System (RAMS). Total ice concentrations formed following seeding agreed well with observations. RAMS's three-dimensional results showed that the new seeding parameterization impacted the microphysical fields producing increased pristine ice crystal, aggregate, and graupel mass downstream of the seeded regions. Pristine ice concentration also increased as much as an order of magnitude in some locations due to seeding. Precipitation augmentation due to the seeding was 0.1–0.7 mm, similar to values inferred from the observations. Simulated precipitation enhancement occurred due to increased precipitation efficiency since no large precipitation deficits occurred in the simulation. These maxima were collocated with regions of supercooled liquid water where nucleation by man-made ice nucleus aerosols was optimized.

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