Estimates of Thunderstorm Precipitation Efficiency from Field Measurements in CCOPE

J. C. Fankhauser National Center for Atmospheric Research, Boulder, Colorado

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Abstract

The precipitation efficiency (the ratio of surface rainfall to water vapor inflow) of a small sample of thunderstorms observed in the Cooperative Convective Precipitation Experiment is calculated using surface and cloud-base airflow and moisture measurements and subcloud rainout based on radar reflectivity factor. Highly-resolved vertical flux measurements from aircraft indicate that a significant amount of water vapor inflow may have been overlooked in past work of this kind, resulting in overestimates in precipitation efficiency. Trends in the mass of water vapor influx resolved at intervals of 10 to 30 min are corroborated by the evolution of water vapor flux convergence computed at 5-min intervals from objectively analyzed surface mesonetwork observations.

Fluxes of water vapor inflow and precipitated rainwater are integrated over periods exceeding an hour to obtain precipitation efficiencies applicable to the mature storm phase. Precipitated rainwater estimates from radar reflectivity-rainrate relation-ship suffer from the usual uncertainties involved in single-parameter radar rainfall estimation, but neither the choice of a particular Z-R relation nor the upper reflectivity threshold assigned to compensate for the likely presence of hail changed the order of the storms when they were ranked according to their precipitation efficiency. This permits a meaningful comparison with environmental factors even though the absolute accuracy of the precipitation efficiencies is somewhat in doubt. Results indicate that factors controlling thunderstorm precipitation efficiency are more complicated than a simple inverse dependence on vertical wind shear, as advanced in earlier work, and that other environmental parameters undoubtedly come into play. In the present analyses, for example, subcloud mixing ratio, shear kinetic energy in the lower troposphere and cloud base area all exhibit weak positive correlations with precipitation efficiency, while there was a tendency for storms with high bases to display a slight inverse correlation.

Abstract

The precipitation efficiency (the ratio of surface rainfall to water vapor inflow) of a small sample of thunderstorms observed in the Cooperative Convective Precipitation Experiment is calculated using surface and cloud-base airflow and moisture measurements and subcloud rainout based on radar reflectivity factor. Highly-resolved vertical flux measurements from aircraft indicate that a significant amount of water vapor inflow may have been overlooked in past work of this kind, resulting in overestimates in precipitation efficiency. Trends in the mass of water vapor influx resolved at intervals of 10 to 30 min are corroborated by the evolution of water vapor flux convergence computed at 5-min intervals from objectively analyzed surface mesonetwork observations.

Fluxes of water vapor inflow and precipitated rainwater are integrated over periods exceeding an hour to obtain precipitation efficiencies applicable to the mature storm phase. Precipitated rainwater estimates from radar reflectivity-rainrate relation-ship suffer from the usual uncertainties involved in single-parameter radar rainfall estimation, but neither the choice of a particular Z-R relation nor the upper reflectivity threshold assigned to compensate for the likely presence of hail changed the order of the storms when they were ranked according to their precipitation efficiency. This permits a meaningful comparison with environmental factors even though the absolute accuracy of the precipitation efficiencies is somewhat in doubt. Results indicate that factors controlling thunderstorm precipitation efficiency are more complicated than a simple inverse dependence on vertical wind shear, as advanced in earlier work, and that other environmental parameters undoubtedly come into play. In the present analyses, for example, subcloud mixing ratio, shear kinetic energy in the lower troposphere and cloud base area all exhibit weak positive correlations with precipitation efficiency, while there was a tendency for storms with high bases to display a slight inverse correlation.

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